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Strain selection of charge and orbital ordering patterns in half-doped manganites
Abstract
Theoretical and computational results are presented clarifying the role of long-ranged strain interactions in determining the charge and orbital ordering in colossal magnetoresistance manganites. The strain energy contribution is found to be of order 20-30 meV/Mn and in particular stabilizes the anomalous 'zig-zag chain' order observed in many half-doped manganites. Comment: 4 pages, 2 figures
... A quantitative analysis [69] predicted a dramatic sensitivity of material properties to strain, particularly the shifting of T c with strain. The separation of lattice distortions into short and long wavelength strain-related modes was shown to be a very useful framework for describing the texture formation in such systems [10,[70][71][72]. The importance of strain in the thin film samples was also exemplified in a number of experimental studies [73][74][75][76][77][78]. ...
The strongly correlated magnetic systems are attracting continuous attention in current condensed matter research due to their very compelling physics and promising technological applications. Being a host to charge, spin, and lattice degrees of freedom, such materials exhibit a variety of phases, and investigation of their physical behavior near such a phase transition bears an immense possibility. This review summarizes the recent progress in elucidating the role of magnetoelastic coupling on the critical behavior of some technologically important class of strongly correlated magnetic systems such as perovskite magnetites, uranium ferromagnetic superconductors, and multiferroic hexagonal manganites. It begins with encapsulation of various experimental findings and then proceeds toward describing how such experiments motivate theories within the Ginzburg-Landau phenomenological picture in order to capture the physics near a magnetic phase transition of such systems. The theoretical results that are obtained by implementing Wilson's renormalization-group to nonlocal Ginzburg-Landau model Hamiltonians are also highlighted. A list of possible experimental realizations of the coupled model Hamiltonians elucidates the importance of spin-lattice coupling near a critical point of strongly correlated magnetic systems.
... There are indications that the relative stability of FM phase is at maximum when the film thickness is around 100 nm [52]. Relative changes in the CO phase stability of films thinner than studied in this work, on the other hand, are often explained by the strain effects, where the substrate induced tensile strain increases the Mn-O-Mn bond length and hence stabilizes the CO phase, whereas the relaxation of the strain again improves the FM ordering of the samples [53][54][55][56]. This stabilizing effect of the substrate induced tensile strain is understandable, as the melting transition of CO state is of the first order and accompanied with an actual change in crystal lattice symmetry [35]. ...
We report the effect of photonic field on the electronic and magnetic structure of a low bandwidth manganite PCMO thin film. In particular, the present study confirmed a mechanism that was recently proposed to explain how optical excitation can bias or directly activate the metamagnetic transition associated with the colossal magnetoresistance (CMR) effect of PCMO. The transition is characterized by a shift in the dynamic equilibrium between ferromagnetic (FM) and antiferromagnetic clusters, explaining how it can be suddenly triggered by a sufficient external magnetic field. The film was always found to support some population of FM-clusters, the proportional size of which could be adjusted by the magnetic field and, especially in the vicinity of a thermomagnetic irreversibility, by optical excitation. The double exchange mechanism couples the magnetic degrees of freedom of manganites to their electronic structure, which is further coupled to the ion lattice via the Jahn-Teller mechanism. In accordance, it was found that producing optical phonons into the lattice could lower the free energy of the FM phase enough to significantly bias the CMR effect.
... In general, the magnetic field required to melt the CO state at low temperature increases with the degree of the J-T distortion, and inversely with the tolerance factor and the Mn-O-Mn bond angle of the MnO 6 octahedra of PCMO [17]. In comparison with more relaxed 110 nm film, the substrate induced tensile strain can distort the MnO6 octahedra which leads to the more stable CO state in the thinnest film [18,19,20,21]. ...
We report photo-induced colossal magnetoresistive insulator-metal transition (IMT) in Pr0.6Ca0.4MnO3 thin films under much reduced applied magnetic field. The colossal effect was studied as a function of film thickness and thus with variable structural properties. Thorough structural, magnetic and magnetotransport characterization under light shows that the highest effect on the transition field can be obtained in the thinnest film (38 nm). However, due to the substrate induced strain of this film the required magnetic field for IMT is quite high. The best crystalline properties of the 110 nm film lead to the lowest IMT field under light and 109 % change in resistance at 10 K. With increasing thickness, the film properties start to move more towards the bulk material and, hence, IMT is no more observed under the applied field of 9 T. Our results indicate that for obtaining large photo-induced CMR, the best epitaxial quality of thin films is essential.
... Below T N ¼ 110 K, antiferromagnetic order also emerges. The origin of orbital order remains debated, and electronic correlations [9][10][11] , structural distortions [12][13][14][15] , or spin interactions [16][17][18] have each been suggested as the primary factor. ...
In-plane anisotropic ground states are ubiquitous in correlated solids such as pnictides, cuprates and manganites. They can arise from doping Mott insulators and compete with phases such as superconductivity; however, their origins are debated. Strong coupling between lattice, charge, orbital and spin degrees of freedom results in simultaneous ordering of multiple parameters, masking the mechanism that drives the transition. Here we demonstrate that the orbital domains in a manganite can be oriented by the polarization of a pulsed THz light field. Through the application of a Hubbard model, we show that domain control can be achieved by enhancing the local Coulomb interactions, which drive domain reorientation. Our results highlight the key role played by the Coulomb interaction in the control and manipulation of orbital order in the manganites and demonstrate a new way to use THz to understand and manipulate anisotropic phases in a potentially broad range of correlated materials.
... 28,29 However, in our case the strip-domain phase is rather governed by the cooperative Jahn-Teller effect since Mn 3þ and Fe 3þ belong to the so-called Jahn-Teller-type ions and tend to orbital ordering and charge. 30 ...
Microstructure and magnetic properties of BiMnO3 and BiFe0.5Mn0.5O3 films, prepared by rf magnetron sputtering on LaAlO3 (001) single-crystalline substrate, are investigated. The selected-area electron diffraction analysis allows us to identify the crystal structure of the BiMnO3 film as orthorhombic, while the BiFe0.5Mn0.5O3 film has a hexagonal lattice symmetry. High-resolution electron microscopy study reveals the presence of strip-domain phase with a periodic spacing of about 3c in both films. Magnetic measurements show that in addition to the basic paramagnetic phase the films exhibit Griffiths phase behavior in a wide temperature range. We argue that the observed weak ferromagnetism is due to the strip-domain layered inclusions, rather than intrinsic physical origin of the films.
... Yet it has been argued that these interactions alone do not guarantee the stability of the CE phase (Khomskii and Kugel, 2003;Calderon, Millis and Ahn, 2003;Bała, Horsch and Mack, 2004) and longer distant elastic strain and/or JT interactions are essential for the stability of the CE-OO. This claim is actually supported by the fact that CE-type orbital correlations have also been observed in the absence of antiferromagnetism, namely, in the FM metallic phase of Nd 1/2 Sr 1/2 MnO 3 (Geck et al., 2002). ...
Orbital degeneracy and strong correlations in transition-metal oxides generate spins and orbitals as low-energy degrees of freedom. Their interplay with both real-charge motion and virtual-charge excitations lead to a fascinating richness of spin–charge–orbital-ordered phases in transition-metal oxides, to striking phenomena like the colossal magnetoresistance in manganites, the switching of charge-ordered phases into metallic phases by applied magnetic field, and many other fascinating effects. Central topics of this review are the spin-orbital superexchange models which provide a concise description of spin and orbital order, effective spin interactions, spin waves, and orbiton excitations in doped and undoped Mott insulators. Particular emphasis is put on recent developments of spin-orbital superexchange models, the relation of magnetism and optical spectral weights and their temperature dependence, t2g systems with strong composite spin-orbital fluctuations, the concepts of orbital liquid, orbital polarons, and the colossal magnetoresistance.
Using density functional calculations within the generalized gradient approximation and Hubbard U framework, the electronic and the magnetic properties of SrCoO3 are investigated. The result shows that the spin-up t2g and eg states of Co shift toward the lower energy with the increase of the U value, whereas the spin-down t2g and eg states of Co shift toward the higher energy. The O 2p state almost does not shift with the increase of U value. The electronic structure of SrCoO3 changes from metal state into half-metal state around U of 7-8 eV. The magnetic moment of Co ions increases linearly with U increasing for U 7.0 eV, and almost does not change for U 8.0 eV. Compared with the experimental results, U = 8.0 eV is thought to be suited for the study of SrCoO3. The result shows that with U = 8.0 eV, the magnetic moment on Co site is 3.19 B and SrCoO3 has the half-metallic nature.
Realizing active quantum control of materials near room temperature is one of the ultimate aims for their practical applications. Recent technological breakthroughs demonstrated that optical stimulation may lead to thermally inaccessible hidden states with unique properties. However, most of the reported hidden states were induced around or below liquid nitrogen temperature. Here, we optically manipulated a manganite near its Curie temperature of 300 K, where typically complex phase competitions locate as well as opportunities for new functionality. A room temperature hidden state was formed with threshold behavior evidenced by a femtosecond paramagnetic to ferromagnetic order switching and a structural change distinct from thermal induced lattice expansion in tens of picoseconds accompanying with phonon softening. We propose that such a hidden state originates from the charge transfer between antiferromagnetic chains after strongly correlated spin-charge quantum excitation, which subsequently initiates an orbital polarization rearrangement described as \({\mathrm{Mn}}_{3x^2 - {r}^2/3y^2 - {r}^2}^{3 + }{\mathrm{Mn}}^{4 + } \to {\mathrm{Mn}}^{4 + }{\mathrm{Mn}}_{3z^2 - {r}^2}^{3 + }\) and associated non-thermal lattice change. This study started from room temperature yet near a phase transition point, which suggests a new route to create or manipulate novel phases for practical purpose.
The so-called Arrott plot, which consists in plotting H/M against M2, with H the applied magnetic field and M the magnetization, is used to extract valuable information in second-order magnetic phase transitions. Besides, it is widely accepted that a negative slope in the Arrott plot is indicative of a first-order magnetic transition. This is known as the Banerjee criterion. In consequence, the zero-field transition temperature T* is reported as the characteristic first-order transition temperature. By carefully analyzing the mean-field Landau model used for studying first-order magnetic transitions, we show in this work that T* corresponds in fact to a triple point where three first-order lines meet. More importantly, this analysis reveals the existence of two symmetrical second-order critical points at finite magnetic field (Tc,±Hc). We then show that a modified Arrott plot can be used to obtain information about these second-order critical points. To support this idea we analyze experimental data on La2/3Ca1/3MnO3 and discuss an estimate for the location of the triple point and the second-order critical points.
The influences of W doping at Mn site on the magnetic structure of the charge-ordering system La0.3Ca0.7MnO3 are studied by the measurements of M-T curves, M-H curves and ESR spectra of polycrystalline samples of La0.3Ca0.7Mn1-xWxO3 (x = 0.00, 0.04, 0.08, 0.12, 0.15) are studied. The results show that when the doping amount is 0.00 <= x <= 0.08, the charge-ordering(CO) phase exists in the system, the AFM/CO states coexist below the transition temperature, and the charge-ordering temperature T-co goes up with the increase of W doping content. For the sample of x = 0.04, FM phase and AFM/CO phases coexist at low temperature, and FM is separated front PM both before and after the formation of CO phase; when x >= 0.12, CO state melts, and the paramagnetism-ferromagnetism (PM-FM) transition exists at extremely low temperature.
Using density functional calculations within the generalized gradient approximation and Hubbard U framework, the electronic and the magnetic properties of SrCoO 3 are investigated. The result shows that the spin-up t 2g and e g states of Co shift toward the lower energy with the increase of the U value, whereas the spin-down t 2g and e g states of Co shift toward the higher energy. The O 2p state almost does not shift with the increase of U value. The electronic structure of SrCoO 3 changes from metal state into half-metal state around U of 7-8 eV. The magnetic moment of Co ions increases linearly with U increasing for U < 7.0 eV, and almost does not change for U > 8.0 eV. Compared with the experimental results, U = 8.0 eV is thought to be suited for the study of SrCoO 3. The result shows that with U = 8.0 eV, the magnetic moment on Co site is 3.19 μB and SrCoO 3 has the half-metallic nature.
A series of Nd1−xCaxMnO3 (x=0.2, 0.33, 0.4, and 0.5) manganites was prepared by sol–gel route by sintering at 1300 °C, mainly to understand the correlation between electron, spin, and phonon couplings. The internal friction and longitudinal modulus along with electrical and magnetic properties have been measured. All the samples are found to exhibit anomalies at TC, TN, and TCO transition temperatures. The anomalies in longitudinal modulus and the internal friction peak at TCO are attributed to Jahn–Teller effect. A strong correlation between the temperature dependent elastic, anelastic, resistivity, and ac susceptibility properties has been observed and an effort has been made to explain the observed anomalous behavior by a qualitative model.
The microstructure and the magnetic and transport properties of as-deposited La1−yCeyMnO3(0.1⩽y⩽0.3) films prepared by pulsed laser deposition are investigated in a wide region of temperature and magnetic field. The microstructure analysis reveals that all films have a high c-oriented texture, an orthorhombic crystal lattice, and a negligible quantity of CeO2 inclusions. The observed strip-domain phase with a periodic spacing of about 3c, the crystal lattice of which is the same as for the basic film phase, exhibits magnetic behavior typical for the Griffiths phase. Regions of the double-period modulated phase are found at room temperature in the y=0.1 film, which is interpreted as Mn3+∕Mn2+ ordering with a partial ferromagnetic↠antiferromagnetic transition at TN⩽80 K. At the same time, the investigation reveals that the magnetic and transport properties of the electron–doped La1−yCeyMnO3 films, driven by cation doping, are similar to those for the hole-doped La∕Ca manganites. Therefore, one can conclude that there is no fundamental difference between the mechanisms of spin ordering and charge transport in the hole-doped and electron–doped manganites.
The magnetic and transport properties of La0.5Ca0.5MnO3 and La0.4Ca0.6MnO3 films with different thickness, prepared by rf-magnetron sputtering with the use of a so-called “soft” (or powder) target on a LaAlO3 substrate, are investigated. Electron-diffraction and high-resolution electron microscopy (HREM) studies show that the charge-ordered phase is observed at room temperature for all films. Both the paramagnetic-to-ferromagnetic transition at TC ≈ 250 K upon cooling and the appearance of an antiferromagnetic (AFM) phase at TN≲140 K are observed in the La0.5Ca0.5MnO3 films, while the La0.4Ca0.6MnO3 films exhibit the AFM transition only, at the same temperature, except for a small ferromagnetic (FM) response from a “dead” layer. It is shown that the volume fraction of the FM phase in the La0.5Ca0.5MnO3 film does not exceed 30% and that the FM phase coexists with the AFM phase at low temperature. All films manifest an exponential temperature dependence of the resistance, with no evidence of the metal-insulator transition. This is explained by the scarcity of the FM phase for the formation of an infinite percolating cluster and by the existence of a charge-ordered phase. The field-dependent magnetoresistance at low temperature is described in terms of the spin-assisted polaron-hopping model.
The structural and the magnetic properties of Nd0.5Sr0.5MnO3 films with column-like nano-clustered microstructure have been investigated. The high-resolution electron microscopy, X-ray diffraction and electron diffraction reveal that the average cluster diameter was about 10 nm, and that the charge-ordered state, typical for this composition, was not observed down to 87 K. The films manifest a superparamagnetic behavior with increasing temperature, and an additional antiferromagnetic transition at TN
Using density-functional calculations, we investigate the special properties of SrFe1−xCoxO3. The results show that the ground states have A-type antiferromagnetic order for x=0.1 and ferromagnetic order for x≥0.2 with Co ions distributed averagely. SrFe1−xCoxO3 exhibits half-metallic nature for 0.2≤x≤0.7 and full-metallic nature for other values of x, and the half-metallic gap decreases with increasing x. The tunneling between the half-metallic ferromagnetic phases drives the large magnetoresistance. In addition, the Co cations are in the intermediate-spin state, while the Fe cations are in the intermediate-spin state for x≤0.5 and the high-spin state for x≥0.6.
The Jahn-Teller distortion dependence of tetrahedral LaMnO3 and KCuF3 elastic tensor is derived using both direct uniform deformation differentiation of Jahn-Teller effect contribution into crystal energy and symmetry adapted energy Taylor series decomposition of WEELat and WRel matrices. The tetrahedral LaMnO3 and KCuF3 are considered in D54h and D184h phases. It is shown that Jahn-Teller and direct relaxation terms are identical in both phases, whereas relaxation terms are not. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
We investigate the influence of a well-defined reversible biaxial strain <= 0.12% on the magnetization (M) of epitaxial ferromagnetic manganite films. M has been recorded depending on temperature, strain and magnetic field in 20-50 nm thick films. This is accomplished by reversibly compressing the isotropic in-plane lattice parameter of the rhombohedral piezoelectric 0.72PMN-0.28PT (001) substrates by application of an electric field E <= 12 kV cm(-1). The magnitude of the total variable in-plane strain has been derived. Strain-induced shifts of the ferromagnetic Curie temperature (T-C) of up to 19 K were found in La0.7Sr0.3MnO3 (LSMO) and La0.7Ca0.3MnO3 films and are quantitatively analyzed for LSMO within a cubic model. The effective magnetoelectric coupling coefficient alpha=mu(0)dM/dE <= 6x10(-8) s m(-1) at ambient temperature in the magnetic-film-ferroelectric-substrate system results from the strain-induced M change. It corresponds to an enhancement of mu(0)Delta M <= 19 mT upon biaxial compression of 0.1%. The extraordinary large alpha originates from the combination of three crucial properties: (i) the strong strain dependence of M in the ferromagnetic manganites, (ii) large piezo-strain of the PMN-PT substrates, and (iii) effective elastic coupling at the film-substrate interface.
We report the effect of intrinsic strain on the coexistence of charge-ordered (CO) and charge-disordered (CD) phases in Bi0.4Ca0.6MnO3. The electronic transport measurements show that a CO phase forms below TCO∼320 K, accompanied by a distinct maximum in magnetization. The softening of shear modulus G in the vicinity of TCO is observed, strongly suggesting the presence of a strong coupling of electron phonon in this compound. Furthermore, two internal friction (Q−1) peaks at temperatures 320 and 260 K are observed in the mechanical energy dissipation spectra. The 320 K Q−1 peak originates from the CO phase transition with typical signatures of a martensitic transformation. The 260 K Q−1 peak related to a relaxation mechanism is ascribed to the motion of CO and CD domains. The CO and CD phases coexistence is caused by the presence of an intrinsic inhomogenous martensitic accommodation strain. This strain is thought to stabilize the large-scale phase separation in manganites.
We study the phase competition in charge ordering La0.33Ca0.67MnO3 across the length scale from 1 to 100 nm by in-situ electron imaging and diffraction. The charge ordering (CO) phase transition is characterized by electron diffractions and CO stripes are recorded by dark field electron imaging [1]. At the early stage of the CO formation, we observed inhomogeneity across different CO domains and inhomogeneity within a single CO domain. Within a single domain, CO follows three stages: disorder, cluster forming and percolation. We found that the charge ordering percolation temperature, which is different at different CO domains, depends critically on intrinsic strain. To explain the CO incommensuration observed in electron diffraction data, statistical measurements of CO stripes are done and show that the majority of the incommensurability results from the change of the spacing of CO strips during the transition. [1] Jing Tao and Jian-Min (Jim) Zuo, Phys. Rev. Lett., submitted
We have investigated the thermodynamic properties of perovskite manganite LaMnO3, the parent compound of colossal magnetoresistive manganites, with the Ca2+ doping at the A-site. As strong electron-phonon interactions are present in these compounds, the lattice part of the specific heat deserves proper attention. We have described the temperature dependence of the lattice contribution to the specific heat at constant volume (Cv(lattice)) of La1−xCaxMnO3 (x=0.125, 0.175, 0.25, 0.35, 0.50, 0.67, 0.75) as a function of temperature (1K–20K) by means of a rigid ion model (RIM).The trends of specific heat variations with temperature are almost similar at all the composition. The Debye temperatures obtained from the lattice contributions are found to be in somewhat closer agreement with the experimental data. The specific heat values revealed by using RIM are in closer agreement with the available experimental data, particularly at low temperatures for some concentrations (x) of La1−xCaxMnO3. The theoretical results at higher temperatures can be improved by including the effects of the charge ordering, van der Waals attraction and anharmonicity in the framework of RIM.
The magnetic phase transition and insulator–metal phase transition in charge ordering Nd0.50Sr0.45Ca0.05MnO3 system have been studied. As the temperature decreases below 100 K, the antiferromagnetic coupling becomes stronger but the ferromagnetic phase basically remains unchanged. At low temperature, the ferromagnetic and antiferromagnetic phases coexist in the system. From the measurement of magnetization and resistance with thermal cycling, a considerable instability is observed at 70⩽T⩽160 K. The phase competition is responsible for the presence of instability.
We studied the crystal and magnetic structure of the La1-xCaxMnO3 compound for (0.11 <= x <= 0.175) using stoichiometric samples. For x < 0.13 the system's ground state is insulating canted antiferromagnetic. For 0.13 <= x <= 0.175 below the Jahn-Teller transition temperature (T-JT) the crystal structure undergoes a monoclinic distortion. The crystal structure can be described with P2(1)/c space group which permits two Mn sites. The unit-cell strain parameter s=2(a-c)/(a+c) increases for T < T-JT, taking the maximum value at the Curie point, and then decreases. Below T-M'M''approximate to 60 K s abruptly changes slope and finally approaches T=0 K with nearly zero slope. The change of s at T-M'M'' is connected to a characteristic feature in the magnetic measurements. As x increases towards the ferromagnetic metallic boundary, although s is reduced appreciably, the monoclinic structure is preserved. The monoclinic structure is discussed with relation to the orbital ordering, which can produce the ferromagnetic insulating ground state. We also studied samples that were prepared in air atmosphere. This category of samples shows ferromagnetic insulating behavior without following the particular variation of the s parameter. The crystal structure of these samples is related to the so-called O-* (c > a > b/root 2) structure.
Nd0.52Sr0.48MnO3 films have been fabricated by dc magnetron sputtering on single-crystal LaAlO3 (001) and SrTiO3 (011) substrates with additional annealing to relax the lattice strain. Although the Nd0.52Sr0.48MnO3 films were deposited simultaneously on different substrates at the same deposition rate, they differ in thickness by a factor of ~=2. The observed difference in thickness is explained by the two-dimensional (layer-by-layer) film growth, rather than by a difference in growth rate controlled by the crystalline orientation of the substrate. An analysis of optical and transport properties reveals that the observed anisotropy in the polaron motion is governed by a strong anisotropy in the trapping energy, rather than in polaron formation. It is shown that the deposited Nd0.52Sr0.48MnO3 films exhibit magnetic behavior typical of two-phase magnetic systems and should be regarded as an assembly of interacting magnetic clusters.
We show that applying strain on half-doped manganites makes it possible to tune the system to the proximity of a metal-insulator transition and thereby generate a colossal magnetoresistance (CMR) response. This phase competition not only allows control of CMR in ferromagnetic metallic manganites but can be used to generate CMR response in otherwise robust insulators at half-doping. Further, from our realistic microscopic model of strain and magnetotransport calculations within the Kubo formalism, we demonstrate a striking result of strain engineering that, under tensile strain, a ferromagnetic charge-ordered insulator, previously inaccessible to experiments, becomes stable.
Using neutron and x-ray diffraction, we report the discovery of competing ground states near a multicritical point in A-site layer-ordered La1−xBa1+xMn2O6 materials. We demonstrate the dual effects of deliberate disorder on the system’s stability, the freezing of the competing states, and the drastic reduction in magnetic fields required for the suppression of charge- and orbital-ordered phases. Our work suggests that quenched disorder is not the primary reason for phase separation and magnetoresistance and that increased doping leads to electronic phase separation.
Energy-band-structure calculations of the ground-state orbital and charge orders for various magnetic structures have been performed in the tight-binding approximation for manganites of the La0.5Ca0.5MnO3 type with 16 Mn ions in the unit cell. The arising inhomogeneous charge distribution was accounted for self-consistently by assuming a linear dependence of Jahn-Teller (JT) splitting of the eg level on its occupation for each of the ions. This dependence promotes the phases with charge separation and the charge degree of freedom becomes an essential factor responsible for stabilization of such phases as a ground state. Among them are the CE structure and also a peculiar ferrimagnetic structure with magnetization twice as low as that for the saturated ferromagnet. The existence and the width of the gap in the electron spectrum of the CE phase depend on the relation between Hund exchange and JT splitting, so that the JT effect stabilizes the insulating state. A finite character of Hund coupling also favors stabilization of the CE phase in the region of reasonable values of the interatomic exchange and hopping integrals.
Structural evolution as a function of temperature through the Pnma↔incommensurate (IC) phase transition in Pr0.48Ca0.52MnO3 perovskite has been analyzed from the perspectives of symmetry and strain. The structure and stability of both phases are shown to depend on combinations of order parameters which have symmetries associated with irreducible representations M3+, R4+, M2+, Γ3+ and Σ2 of space group Pm3̅ m. The physical origin of these can be understood in terms of octahedral tilting, cooperative Jahn-Teller distortions and charge order/Zener polaron ordering. The M2+ order parameter describes the Jahn-Teller ordering scheme which develops in LaMnO3 while the Γ3+ order parameter relates to an ordering scheme in which the unique axes of the distorted octahedra are all aligned in the same direction. Irrep Σ2 contains two components with gradient coupling and provides the symmetry-breaking mechanism by which the IC transition can occur. Each order parameter couples with macroscopic spontaneous strains in a manner that depends strictly on symmetry and this leads to specific interactions between the order parameters through their coupling with common strains. In order to establish the extent and importance of this coupling, symmetry-adapted strains have been extracted from a new set of lattice parameters obtained by high-resolution powder neutron diffraction in the temperature interval 10–1373 K. It is found that the predominant strain of the incommensurate structure (up to ∼2.5%) is a tetragonal shear strain which arises by bilinear coupling with the Γ3+ order parameter. This combination is probably responsible for most of the energy reduction accompanying the Pnma↔IC transition and also gives it some characteristics typical of a pseudoproper ferroelastic transition. Strain coupling promotes mean-field behavior and the evolution of the symmetry-breaking order parameter can be described by a standard Landau tricritical solution, q4∝(Tc−T) with Tc=237±2 K. Octahedral tilting at high temperatures is closely similar to tilting in the Pnma structure of other perovskites, such as SrZrO3. This is accompanied by a degree of Jahn-Teller ordering on the basis of the M2+ scheme below ∼775 K but is replaced by the Γ3+ scheme below Tc. In contrast with the tilting and Jahn-Teller effects, magnetic ordering at the Néel temperature (∼180 K) is accompanied by only the slightest volume strain and is not likely to influence the evolution of the other order parameters to any significant extent, therefore. An additional change in the volume strain below ∼85 K is perhaps related to changes in magnetic structure at lower temperatures. Line broadening in powder diffraction patterns collected in the temperature interval ∼150–260 K appears to be related to the presence of ferroelastic twins arising from octahedral tilting and draws attention to the fact that the Pnma↔IC transition takes place in a material which already contains heterogeneities. Finally, correlation of the repeat distance of the IC structure with Γ3+ distortions of MnO6 octahedra shows that the nature of the IC structure itself is also determined essentially by geometrical factors and strain.
The elastic and anelastic properties of a polycrystalline sample of Pr0.48Ca0.52MnO3 have been investigated by resonant ultrasound spectroscopy, as a function of temperature (10–1130 K) and magnetic field strength (0–15 T). Marked softening of the shear modulus as the Pnma↔incommensurate phase transition at ∼235 K in zero field is approached from either side is consistent with pseudoproper ferroelastic character, driven by an order parameter with Γ3+ symmetry associated with Jahn-Teller ordering. This is accompanied by an increase in attenuation just below the transition point. The attenuation remains relatively high down to ∼80 K, where there is a distinct Debye peak. It is attributed to coupling of shear strain with the Γ3+ order parameter which, in turn, controls the repeat distance of the incommensurate structure. Kinetic data extracted from the Debye peak suggest that the rate-controlling process could be related to migration of polarons. Elastic softening and stiffening as a function of magnetic field at constant temperatures between 177 and ∼225 K closely resembles the behavior as a function of temperature at 0, 5, and 10 T and is consistent with thermodynamically continuous behavior for the phase transition in both cases. This overall pattern can be rationalized in terms of linear/quadratic coupling between the Γ3+ order parameter and an order parameter with Σ1 or Σ2 symmetry. It is also consistent with a dominant role for spontaneous strains in determining the strength of coupling, evolution of the incommensurate microstructure, and equilibrium evolution of the Jahn-Teller ordered structure through multicomponent order-parameter space.
The half-doped manganite, (Eu0.4La0.1)(Sr0.4Ca0.1)MnO3, has been found to exhibit sharp steplike metamagnetic transitions below 5 K. The number of magnetic steps increases with decreasing temperature and this number suddenly rises from 3 at 2 K to ∼50 at 1.7 K. The self-similar character of the multiple magnetic steps at reduced temperature identifies these steps as avalanchelike transitions. The occurrence of a multitude of magnetic steps at low temperatures, the decreasing effect of magnetic field sweep rate on the steplike metamagnetism, and the decreasing irreversibility of transition in the specific heat suggest that reduced spin-lattice coupling facilitates the transformation of antiferromagnetic to ferromagnetic state via an avalanchelike behavior.
The essential role of strain in determining the electronic states of perovskite manganites is clearly demonstrated in the form of anisotropic and substrate-dependent crossover of a first-order phase transition in charge-orbital ordered epitaxial thin films of (Nd1−xPrx)0.5Sr0.5MnO3. An application of (110)-oriented epitaxial growth technique allows the structural flexibility indispensable for the large anisotropic lattice deformation at the first-order transition. The technique enables us to fine-tune the Jahn-Teller distortion for manipulating the metal-insulator transition in thin films, a first step toward the strongly correlated electron devices.
The nature of orbital order-disorder transition has been studied in the La1−xNdxMnO3 (x=0.0–1.0) series which covers the entire range between two end points — LaMnO3 and NdMnO3 — as well as in La0.85Nd0.1Sr0.05MnO3 and La0.8Nd0.1Sr0.1MnO3. It has been observed that the first-order nature of the transition gives way to higher order with the increase in “x” in the case of pure manganites. The latent heat (L) associated with the transition, first, drops with a steeper slope within x=0.0–0.3 and, then, gradually over a range 0.3⩽x⩽0.9. This drop could, possibly, be due to evolution of finer orbital domain structure with “x.” In the case of Sr-doped samples, the transition appears to be of higher-order nature even for a doping level 5 at. %. In both cases, of course, the transition temperature TJT rises systematically with the drop in average A-site radius ⟨rA⟩ or rise in average Mn-O-Mn bond bending angle ⟨cos2 φ⟩ while no apparent correlation could be observed with doping induced disorder σ2. The cooperative nature of the orbital order, therefore, appears to be robust.
We report on the imaging of the coexistence of the CE-type charge-ordered insulator (COI) phase and the ferromagnetic metal (FMM) phase of Nd1∕2Sr1∕2MnO3 by means of polarized optical microscopy over a range in temperature of several degrees. Consistent with the different Mn-O bond lengths along the c axis and in the ab plane, optical anisotropy is observed in the COI, FMM, and paramagnetic insulator phases. Upon cooling, accommodation strain leads to the rapid formation of martensiticlike COI variants within the FMM phase. Upon warming, the FMM phase is observed to nucleate slowly on COI variant boundaries where the magnitude of the strain is locally maximized. Consistent with the presence of strain, COI twin boundaries sometimes demonstrate optical anisotropy and can act as sites for FMM phase nucleation.
The microstructure and the magnetic properties of La0.4Ca0.6MnO3 film, prepared by rf magnetron sputtering on a LaAlO3 substrate, have been investigated. The electron microscopy study reveals the presence of strip-domain phase with a periodic spacing of about 3c for the orthorhombic symmetry. The magnetic measurements show that in addition to the expected antiferromagnetic transition at TN ≃ 120 K with decreasing temperature, the film manifests the Griffiths phase behavior in a wide temperature range.
The electronic transport and magnetism in half-doped Nd0.50Ca0.25Sr0.25MnO3 manganites have been investigated. Contrary to general half-doped system, it only displays a paramagnetic-ferromagnetic phase transition associated with an insulator-metal transition instead of with any features of charge ordering. With the decrease of temperature, an electronic phase separation and spin glass state occur in low temperature. We suggest that the A-site cation disorder induced by the size mismatch between Sr2+ ion and Ca2+ ion is mainly responsible for this phenomenon.
We report the effects of disorder induced by a small amount of substitution of the smaller cations (Nd, Sm, Gd and Yb) for La in the La 0.50 Ca 0.50 MnO 3+d system. With decreasing size of the dopant, the ferromagnetic and metallic state is stabilized while the AFM and insulating behaviour is completely eliminating. The magnetic moment below T c increases in general, with decreasing dopant size. The behaviour is interpreted in terms of the destabilization of the charge ordering (CO) due to the disorder induced by the size mismatch of the cations. Our data support the view that close to the COI–FM phase boundary, the effect of disorder is to weaken the CO that is more sensitive to disorder, whereas it leaves the more robust double exchange relatively unaffected, thereby extending the region in phase space where the FM phase is stable. r 2005 Elsevier B.V. All rights reserved. PACS: 71.30.+h; 75.30.Vn; 75.30.Kz; 75.60.Lr; 72.60.+g
Using density-functional calculations within the generalized gradient approximation (GGA)+U framework, we investigate the structural, electronic, and magnetic properties of the ground states of SrFeOn (n = 2 and 2.5). The magnetism calculations show that the ground states of both SrFeO2 and SrFeO2.5 have G type antiferromagnetic ordering, with indirect band gaps of 0.89 and 0.79 eV, respectively. The electronic structure calculations demonstrate that Fe cations are in the high-spin state of (dz2)2 (dxz, dyz)2 (dxy)1(dx2–y2)1 (S = 2), unlike the previous prediction of (dxz, dyz)3 (dxy)1 (dz2)1 (dx2–y2)1 (S = 2) for SrFeO2, and in the high-spin state of (dxy, dxz, dyz, dx2–y2, z2)5 (S = 5/2) for SrFeO2.5.
In this paper, we report unusual thermal hysteresis behaviour in half-doped manganites Nd0.5Sr0.5−xCaxMnO3 (0.0 ≤ x ≤ 0.50). For 0.05 ≤ x < 0.25 samples, the magnetization curves show a counterclockwise hysteresis loop. For 0.25 < x ≤ 0.40 samples, however, they exhibit a clockwise loop. The inverse hysteretic behaviour stems from the competition between two kinds of strain, namely, the compressive strain and the tensile strain. The former produces a counterclockwise thermal hysteresis while the latter leads to a clockwise one. In the meantime, a magnetic instability is observed and the strain relaxation should be the main reason for it.
Within the framework of strong electron-lattice interaction of Mn3+ ions and orbital dependent exchange interaction model, the crystal and magnetic structures of charge ordered phase La1/3Ca2/3MnO3 are studied. The calculation results are in agreement with the “Wigner crystal” model. The estimated superexchange parameters confirm the qualitative conclusions of experimental works about frustrated magnetic structure with magnetic supercell (3a × b × 2c). The values of hyperfine magnetic fields on 55Mn nuclei in different charge states in charge ordering phase are calculated. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
Based on the double-exchange model in the tight-binding approximation, self-consistent energy-band-structure calculations are carried out for the first time for the orbital and charge ordering in La0.5Ca0.5MnO3-type manganites with 16 Mn ions in the unit cell under the assumption of a linear dependence of the Jahn-Teller splitting of the e
g
level on the occupancy of each ion. The equilibrium magnetic and orbital configurations are determined by minimizing the total energy with respect to the direction of the local magnetic moments and the orbital states of the Mn ions. The dependence of the splitting on the occupancy favors the stabilization of phases with charge separation. This separation is an important factor determining the magnetic structure of the ground state. The ground state can be an insulating antiferromagnetic structure of the CE or G type or a new ferrimagnetic structure with a magnetization lower than that of a saturated ferromagnet by a factor of 2. The existence and the width of the bandgap in the electronic energy spectrum of the CE phase depend on the ratio between the values of the Hund exchange and the splitting. When the splitting is sufficiently large, the Jahn-Teller effect stabilizes the insulating state. The finite value of the Hund coupling also favors the stabilization of the CE phase for the realistic values of the interionic exchange and the hopping integral of itinerant electrons.
The structure of manganites R
1−x
A
x
MnO3 (R = La, Sm, or a mixture of rare earth cations; A = Ca, Sr; x = 0.3, 0.5) has been studied by neutron diffraction to find out the reasons for the giant oxygen isotope effect—the transition
from a low-temperature metallic state to an insulating state as a result of replacement of 16O with 18O. It is shown that this effect is observed in compositions in which the two phases P1 and P2 coexist at low temperatures. They have the same crystal symmetry (sp. gr. Pnma) but different types of Jahn-Teller distortions of oxygen octahedra and different magnetic structure. Phase P1 has a conductivity of the metallic type, weakly distorted MnO6 octahedra, and a ferromagnetic structure. Phase P2 is insulating, with MnO6 octahedra extended or compressed in the apical direction and the moments of Mn ions forming an antiferromagnetic structure.
The relative volume of phases P1 and P2 in samples depends on the average radius of the A cation and changes occurring upon replacement of 16O with 18O. The percolation transition from the metallic to insulating state upon substitution of 16O with 18O is caused by the sharp decrease in the volume of the ferromagnetic metallic phase in favor of the insulating antiferromagnetic
phase. An effect of the sample microstructure on the formation of the two-phase state is found.
The influence of A -site size and disorder on metamagnetic transformation in polycrystalline Pr <sub>0.5-x</sub> La <sub>x</sub> Ca <sub>0.5</sub> MnO <sub>3</sub> and Pr <sub>0.5</sub> Ca <sub>0.5-x</sub> Ba <sub>x</sub> MnO <sub>3</sub> at low temperature has been systematically investigated. The introduction of larger A -site cations such as La <sup>3+</sup> or Ba <sup>2+</sup> will locally suppress the lattice distortion. This “counterdistortion” effect becomes more pronounced with the increase in the A -site average ionic radius <r<sub>A</sub>> , which is favorable to ferromagnetism. Thus the critical field H<sub>C1</sub> corresponding to the first sharp metamagnetic step will decrease, and the field-induced magnetization M<sub>5 T </sub> will increase. However, the A -site ionic radii variance σ<sup>2</sup> will dramatically increase when more barium is introduced and a spin glasslike state is induced, which is unfavorable to the stability of ferromagnetism state. Therefore, H<sub>C1</sub> will increase and M<sub>5 T </sub> will decrease. Moreover, the evolution of resistivity at low temperature with σ<sup>2</sup> exhibits strong correlation with that of magnetic properties.
Systematic studies of structural, magnetic, electronic, and elastic properties have been performed for the electron-doped manganite Sr0.95Ce0.05MnO3. The results show that light doping with Ce in place of Sr in SrMnO3 could stabilize the perovskite-type structure. The electronic transport and magnetism measurements show that the sample exhibits a charge ordering (CO) state below , accompanied by softening of Young’s modulus due to a strong electron–phonon coupling. Cluster-glass behavior and the magnetoresistance (MR) effect are observed at low temperatures, resulting from the induced double-exchange (DE) ferromagnetic (FM) clusters embedded in the CO antiferromagnetic (AFM) matrix. Above , the high temperature range appears to be dominated by local FM fluctuations, which is further supported by internal friction measurements. Our results indicate the existence of intrinsic magnetic inhomogeneity in electron-doped Sr0.95Ce0.05MnO3.
The Pnma incommensurate phase transition in the perovskite Pr(0.48)Ca(0.52)MnO(3) at ∼ 235 K is accompanied by shear strains of up to ∼ 2.5% (from neutron diffraction) and changes in the shear modulus of up to ∼ 40% (from resonant ultrasound spectroscopy, RUS), indicating strong coupling between the structural order parameter and strain. In contrast, the antiferromagnetic (AFM) ordering transition at ∼ 180 K displays no detectable static strain, implying that there is either no coupling or only very weak coupling between the magnetic order parameter and strain. Conventional analysis of RUS data, based on measurements of resonance peak frequencies and peak widths, also failed to detect any anomaly in elastic or anelastic properties due to the AFM ordering. A new approach to the analysis the RUS data, based on autocorrelation and convolution of the entire spectra, however, has revealed that the AFM order does indeed affect the elastic behaviour in an unexpected manner. The new analysis shows, firstly, that dynamical fluctuations of the charge density ordering at T > T(c) = 237 K lead to an increase of the RUS amplitude and of the spectral convolution function. Secondly, fluctuations and convolution effects peak at the transition point and decrease with decreasing temperatures. Below 180 K the stripe structure is essentially static. Finally, AFM ordering leads to an increase in the damping of the elastic resonances.
Space groups, order-parameter and strain/order-parameter coupling relationships in ABX3 perovskite structures which combine cooperative Jahn-Teller distortions and octahedral tilting have been investigated from the perspective of group theory using the computer program ISOTROPY. Two common Jahn-Teller ordering schemes are associated with the irreducible representations M2+ and R3+ of the space group Pm3m. A third, less-common ordering scheme is associated with Gamma3+. These combine with tilting instabilities associated with M3+ and R4+ to generate a predicted suite of Jahn-Teller structure types that includes many of the known structures of manganites, vanadates, Cu and Cr halides. Order-parameter coupling and possible phase transitions are described using Landau free-energy expansions, and general expressions for the relationships between symmetry-adapted spontaneous strains and particular order-parameter components are presented. These provide a general formal framework for determining structural evolution across multi-component order-parameter space and for characterizing the influence of tilting instabilities on Jahn-Teller instabilities or of Jahn-Teller ordering on octahedral tilting.
We present a novel ground state that explains the continuous charge modulated diagonal order recently observed in manganese oxides, at hole concentrations x larger than one-half. In this diagonal phase the charge is modulated with a predominant Fourier component inversely proportional to 1-x. Magnetically this state consists of antiferromagnetically coupled zigzag chains. For a wide range of physical parameters such as electron-phonon coupling, antiferromagnetic interaction between Mn ions, and on-site Coulomb repulsion, the diagonal phase is the ground state of the system. Also we find that the diagonal modulation of the electron density is only a small fraction of the average charge, a much smaller modulation than the one obtained by distributing Mn+3 and Mn+4 ions. We discuss also the spin and orbital structure properties of this new diagonal phase.
Transmission of information using the spin of the electron as well as its charge requires a high degree of spin polarization at surfaces. However, at surfaces this degree of polarization can be quenched by competing interactions. Using a combination of surface-sensitive X-ray and tunnelling probes, we show for the quasi-two-dimensional bilayer manganites that only the outermost Mn-O bilayer is affected: it is a 1-nm-thick insulator that exhibits no long-range ferromagnetic order, whereas the next bilayer displays the full spin polarization of the bulk. Such an abrupt localization of the surface effects is due to the two-dimensional nature of the layered manganite, and the loss of ferromagnetism is attributed to weakened double exchange in the reconstructed surface bilayer and a resultant antiferromagnetic phase. The creation of a well-defined surface insulator atop a fully spin-polarized bulk demonstrates the ability of two of the most demanding components of an ideal magnetic tunnel junction to self-assemble naturally.
Polycrystalline samples of La0.3Ca0.7Mn(1-x)W(x)O3 (0.00 < or = x < or = 0.15) were prepared by conventional solid-state reaction method. The influence of W doping at Mn site on the magnetic transition of La0.3Ca0.7Mn(1-x)W(x)O3 system was studied by the measurements of magnetization-temperature (MT) curves, magnetization-magnetic intensity (MH) curves, and ESR spectra. The results show that, with the increase in W doping amount, the magnetic transition of the system exhibits a complicated charge process, when the doping amount is 0.00 < or = x < or = 0.08, the charge-ordering (CO) phase exists in the system, AFM/CO states coexist below the transition temperature, and the charge-ordering temperature (Tco) goes up with the increase in W doping content; when x > or = 0.12, the charge ordering (CO) state of the system weakens and melts, and paramagnetism-ferromagnetism (PM-FM) transition exists at extremely low temperature.
The structural, electronic, and magnetic properties of mixed-valence compounds are believed to be governed by strong electron correlations. Here we report benchmark density-functional calculations in the spin-polarized generalized-gradient approximation (GGA) for the ground-state properties of doped CaMnO(3). We find excellent agreement with all available data, while inclusion of strong correlations in the GGA+U scheme impairs this agreement. We demonstrate that formal oxidation states reflect only orbital occupancies, not charge transfer, and resolve outstanding controversies about charge ordering.
The physics of manganites appears to be dominated by phase competition among ferromagnetic metallic and charge-ordered antiferromagnetic insulating states. Previous investigations (Burgy {\it et al.}, Phys. Rev. Lett. {\bf 87}, 277202 (2001)) have shown that quenched disorder is important to smear the first-order transition between those competing states, and induce nanoscale inhomogeneities that produce the colossal magnetoresistance effect. Recent studies (Motome {\it et al.} Phys. Rev. Lett. {\bf 91}, 167204 (2003)) have provided further evidence that disorder is important in the manganite context, unveiling an unexpected insulator-to-metal transition triggered by disorder in a one-orbital model with cooperative phonons. In this paper, a qualitative explanation for this effect is presented. It is argued that the transition occurs for disorder in the form of local random energies. Acting over an insulating states made out of a checkerboard arrangement of charge, with ``effective'' site energies positive and negative, this form of disorder can produce lattice sites with an effective energy near zero, favorable for the transport of charge. This explanation is based on Monte Carlo simulations and the study of simplified toy models, measuring the density-of-states, cluster conductances using the Landauer formalism, and other observables. The applicability of these ideas to real manganites is discussed.
Mixed-valence manganese oxides (R1-x Ax)MnO3 (R=rare-earth cation, A=alkali or alkaline earth cation), with a structure similar to that of perovskite CaTiO3, exhibit a rich variety of crystallographic, electronic and magnetic phases. Historically they led to the formulation of new physical concepts such as double exchange and the Jahn-Teller polaron. More recent work on thin films has revealed new phenomena, including colossal magnetoresistance near the Curie temperature, dense granular magnetoresistance and optically-induced magnetic phase transitions. This review gives an account of the literature on mixed-valence manganites, placing new results in the context of established knowledge of these materials, and other magnetic semiconductors. Issues addressed include the nature of the electronic ground states, the metal-insulator transition as a function of temperature, pressure and applied magnetic field, the electronic transport mechanisms, dielectric and magnetic polaron formation, magnetic localization, the role of cation disorder and the Jahn-Teller effect. Sample preparation, and the properties of related ferromagnetic oxides are also discussed.
A systematic study of phase separation effects in polycrystalline La0.5Ca0.5MnO3 obtained under different thermal treatments is reported. Samples with average grain size ranging from 200 to 1300 nm were studied. Magnetic and electrical measurements show quantitative differences among samples in their low-temperature behavior, indicating that the fraction of the ferromagnetic (FM) phase gradually decreases as the grain size increases. Percolation of the FM phase in samples with a small fraction of this phase suggests that grain boundaries play a distinctive role in the spatial distribution of coexisting phases. The defective structure at the grain surface could explain the local inhibition of the antiferromagnetic charge ordered phase, an effect that is gradually removed with increasing grain size. Qualitative agreement of the data with this description is found. This effect is also found to be highly dependent on the oxygen content of the samples and its spatial distribution.
The fundamental physical properties of doped LaMnO3, generically termed ``manganites,'' and much of the underlying physics, were known more than 40 years ago. This article first reviews progress made at that time, the concept of double exchange in particular, and points out the missing elements that have led to a massive resurgence of interest in these and related materials. More recent research is then described, treating first the ground states that emerge as divalent atoms are substituted for trivalent La. A wide range of ground states appear, including ferromagnetic metals, orbital- and charge-ordered antiferromagnets, and more complex stripe and spin-glass states. Because of the interest in so-called colossal magnetoresistance that occurs in the ferromagnetic/metallic composition range, a section is devoted to reviewing the atypical properties of that phase. Next the high-temperature phase is examined, in particular, evidence for the formation of self-trapped small polarons and the importance of Jahn-Teller coupling in this process. The transitions between the high-temperature polaronic phase and the ferromagnetic and charge-ordered states are treated in a fourth section. In each section, the authors stress the competition among charge, spin, and lattice coupling and review the current state of theoretical understanding. They conclude with some comments on the impact that research on these materials has on our understanding of doped oxides and other strongly correlated electronic materials.
Exchange interaction in magnetic substances containing ions with orbital degeneracy is considered. It is shown that, among with spin ordering, superexchange also results in cooperative ordering of Jahn-Teller ion orbitals, which, generally speaking, occurs at a higher temperature and is accompanied by distortion of the lattice (which is a secondary effect here). Concrete studies are performed for substances with a perovskite structure (KCuF3, LaMn03, MnF3). The effective spin Hamiltonian is obtained for these substances and the properties of the ground state are investigated. The orbital and magnetic struc-tures obtained in this way without taking into account interaction with the lattice are in accord with the structures observed experimentally. The approach employed also permits one to explain the strong anisotropy of the magnetic properties of these compounds and to obtain a reasonable estimate for the critical temperatures.
Insulator-metal phenomena depending on band filling (doping degree), temperature, and external magnetic field have been investigated for prototypical double-exchange ferromagnets, namely, crystals of La1-xSrxMnO3 (0≤x≤0.6). The electronic phase diagram in the plane of the temperature vs nominal hole concentration (x) has been deduced from the magnetic and electrical measurements on the melt-grown crystals. Around the ferromagnetic transition temperature TC, large negative magnetoresistance was observed. Irrespective of temperature, reduction of the resistivity is scaled with the field-induced magnetization (M) as -Δρ/ρ=C(M/Ms)2 for M/Ms≲0.3, where Ms is the saturated magnetization. The coefficient C strongly depends on x, i.e., C≊4 near the compositional insulator-metal phase boundary (xc∼0.17), but decreases down to ≊1 for x>=0.4, indicating the critical change of the electronic state.
The coupled magnetic and charge-order transition observed in the manganites of the type R1-xMxMnO3 near half filling (x≃1/2) is shown to be the result of the interplay between the double-exchange, superexchange, and the Coulomb interaction terms in an electronic Hamiltonian. At half filling and temperature T=0 we find, as we increase the strength of the extended-Hubbard repulsion, a first-order transition from a charge-nonordered ferromagnetic metal (FN) to a charge-ordered antiferromagnetic and insulating (AFO) ground state. The AFO-FN transition is also obtained by increasing T; however, a small degree of charge order remains in the ferromagnetic phase. The charge-ordered state also “melts,” as observed, on the application of a magnetic field, which causes a rapid drop in the transition temperature. Qualitative differences in behavior between members of the manganite series can be understood in terms of small variations in the interaction parameters.
We measured the pressure dependence of the room-temperature infrared
absorption spectrum of the pseudocubic
La0.75Ca0.25MnO3 manganite. On
increasing pressure up to 10 Gpa we observe the increase of a broad band
associated with polaronic absorption, which partially screens the
high-frequency phonon at high pressures. We also measured absorption
spectra of the same sample as a function of temperature, and of the
layered Sr1.5La0.5MnO4 manganite as a
function of pressure. The comparison between different sets of spectra
clearly shows that the increase of the broad-band absorption observed in
the La0.75Ca0.25MnO3 manganite is due
to a pressure-induced charge delocalization process. However, the
metallic state is not achieved at room temperature, at least in the
explored pressure range.
We report the characterization of the crystal structure, low-temperature charge and orbital ordering, transport and magnetization of Pr0.6Ca0.4MnO3 films grown on LaAlO3, NdGaO3 and SrTiO3 substrates, which provide compressive (LaAlO3) and tensile (NdGaO3 and SrTiO3) strain. The films are observed to exhibit different crystallographic symmetries from the bulk material and the low-temperature ordering is found to be more robust under compressive—as opposed to tensile—strain. In fact, bulk-like charge and orbital ordering is not observed in the film grown on NdGaO3, which is the substrate that provides the least amount of measured, but tensile, strain. This result suggests the importance of the role played by the Mn–O–Mn bond angles in the formation of charge and orbital ordering at low temperatures. Finally, in the film grown on LaAlO3, a connection between the lattice distortion associated with orbital ordering and the magnetization is reported.
The two-orbital model for manganites with both noncooperative and cooperative Jahn-Teller phonons is studied at hole density x = 0.5 using Monte Carlo techniques. The phase diagram is obtained by varying the electron-phonon coupling and the t(2g)-spins exchange. The insulating CE-type charge- and orbital-ordered state with the z-axis charge stacking observed in narrow-bandwidth manganites is stabilized in the simulations. Its charge gap Delta(CO) is much larger than the critical temperature k(B)T(CO). Metalliclike A-type and ferromagnetic states are also obtained in the same framework, and the phase boundaries among them have first-order characteristics.
We numerically examine the large-q asymptotics of the q-state random bond Potts model. Special attention is paid to the parametrization of the critical line, which is determined by combining the loop representation of the transfer matrix with Zamolodchikov's c-theorem. Asymptotically the central charge seems to behave like c(q)=1 / 2 log(2)(q)+O(1). Very accurate values of the bulk magnetic exponent x(1) are then extracted by performing Monte Carlo simulations directly at the critical point. As q-->infinity, these seem to tend to a nontrivial limit, x(1)-->0.192+/-0.002.
We study theoretically the phase diagram of perovskite manganites taking into account the double degeneracy of the $e_g$ orbitals in a $Mn^{3+}$ ion. A rich phase diagram is obtained in the mean field theory at zero temperature as functions of $x$ (hole concentration) and $J_S$ (antiferromagnetic interaction between $t_{2g}$ spins). The global features of the phase diagram is understood in terms of the superexchange and double exchange interactions, which are strongly depends on types of the occupied $e_g$ orbitals. The strong electron correlation induces the orbital polarization, which controls the dimension of the conduction band. A sequential change of the spin and orbital structures with doping holes is consistent with the recent experiments. In particular, metallic A-type (layered) antiferromagnetic state is found for $x\sim0.5$ with the uniform $d_{x^2-y^2}$ orbital ordering. Effects of the Jahn-Teller distortion are also studied. A short version of this paper has been already published (to appear in PRB-rapid June), but this paper contains additional and more detailed results.
In this paper, we report polarized optical microscopy and electrical transport studies of manganese oxides that reveal that the charge ordering transition in these compounds exhibits typical signatures of a martensitic transformation. We demonstrate that specific electronic properties of charge-ordered manganites stem from a combination of martensitic accommodation strain and effects of strong electron correlations. This intrinsic strain is strongly affected by the grain boundaries in ceramic samples. Consistently, our studies show a remarkable enhancement of low field magnetoresistance and the grain size effect on the resistivity in polycrystalline samples and suggest that the transport properties of this class of manganites are governed by the charge-disordered insulating phase stabilized at low temperature by virtue of martensitic accommodation strain. High sensitivity of this phase to strains and magnetic field leads to a variety of striking phenomena, such as unusually high magnetoresistance (10^10 %) in low magnetic fields. Comment: Short paper, 4 figures, to appear in Rapid Communication
The phase diagram of half-doped manganite systems of formula A
0.5
A
′
0.5MnO3 is investigated within a single-orbital model incorporating magnetic double-exchange and superexchange, together with intersite Coulomb and electron-lattice interactions. Strong Jahn-Teller and breathing mode deformations compete together and result in shear lattice deformations. The latter stabilize the charge-ordered CE-type phase, which undergo first-order transitions with temperature or magnetic field to either Ferromagnetic metallic or Paramagnetic insulating phases. An essential feature is the self-consistent screening of Coulomb and electron-phonon interactions in the ferromagnetic phase.
The effects of uniaxial strain on the structural, orbital, optical, and
magnetic properties of LaMnO_3 are calculated using a general elastic energy
expression, along with a tight-binding parameterization of the band theory.
Tensile uniaxial strain of the order of 2 % (i.e., of the order of magnitude of
those induced in thin films by lattice mismatch with substrates) is found to
lead to changes in the magnetic ground state, leading to dramatic changes in
the band structure and optical conductivity spectrum. The magnetostriction
effect associated with the Neel transition of bulk(unstrained) LaMnO_3 is also
determined. Due to the Jahn-Teller coupling, the uniform tetragonal distortion
mode is softer in LaMnO_3 than in doped cubic manganates. Reasons why the
observed (\pi \pi 0) orbital ordering is favored over a (\pi \pi \pi)
periodicity are discussed.
A study has been made of the magnetic properties of the series of perovskite-type compounds [(1-x)La, xCa]MnO3. The investigations have been made primarily by neutron diffraction methods, but x-ray diffraction measurements of lattice distortions and ferromagnetic saturation data are also included. This series of compounds exhibits ferromagnetic and antiferromagnetic properties which depend upon the relative trivalent and tetravalent manganese ion content. The samples are purely ferromagnetic over a relatively narrow range of composition (x∼0.35) and show simultaneous occurrence of ferromagnetic and antiferromagnetic phases in the ranges (0<x<0.25) and (0.40<x<0.5). Several types of antiferromagnetic structures at x=0 and x>0.5 have also been determined. The growth and mixing of the various phases have been followed over the whole composition range, the ferromagnetic and antiferromagnetic moment contributions to the coherent reflections have been determined, and Curie and Néel temperatures have been measured. The results have been organized into a scheme of structures and structure transitions which is in remarkable accord with Goodenough's predictions based on a theory of semicovalent exchange.
Phase diagram of half-doped perovskite manganites is studied within the
extended double-exchange model. To demonstrate the role of orbital degrees of
freedom both one- and two-orbital models are examined. A rich phase diagram is
obtained in the mean-field theory at zero temperature as a function of J
(antiferromagnetic (AFM) superexchange interaction) and V (intersite Coulomb
repulsion). For the one-orbital model a charge-ordered (CO) state appears at
any value of V with different types of magnetic order which changes with
increasing J from ferromagnetic (F) to AFM ones of the types A, C and G. The
orbital degeneracy results in appearance of a new CE-type spin order that is
favorable due to opening of the "dimerization" gap at the Fermi surface. In
addition, the CO state appears only for V>V_c for F and CE states while C-type
AFM state disappears and A-type AFM state is observed only at small values of V
as a charge disordered one. The relevance of our results to the experimental
data are dicussed.
The magnetic couplings in insulating ${\mathrm{LaMnO}}_{3}$, ${\mathrm{CaMnO}}_{3}$, and the planar analogs ${\mathrm{La}}_{\mathrm{n}+1}$${\mathrm{Mn}}_{\mathrm{n}}$${\mathrm{O}}_{3\mathrm{n}+1}$ are estimated using standard superexchange arguments. The orbital ordering observed in ${\mathrm{LaMnO}}_{3}$ is found to lead to the observed magnetic exchange constants if the effective Mn on-site interaction ${\mathrm{U}}_{\mathrm{Mn}}$ is larger than the charge transfer energy. Differences between the pseudocubic and the planar materials are accounted for. The effect of doping is discussed.
A tight-binding parametrization of the band structure, along with a mean-field treatment of the Hund, electron-electron, and electron-lattice couplings, is used to obtain the full optical conductivity tensor of LaMnO3 as a function of temperature. We predict striking changes with temperature in the functional form and magnitude of the optical absorption. Comparison of our results with existing data makes it possible to determine the electron-lattice and electron-electron couplings. The effective “Hubbard U” is found to be ≈1.6 eV, rather less than the full bandwidth ≈3.6 eV, putting the material in the weak-intermediate coupling regime.
The crystal distortion which arises from the Jahn-Teller effect is discussed in several examples. In the case of compounds containing Cu <sup>2+</sup> or Mn <sup>3+</sup> at octahedral sites, the lowest orbital level of these ions is doubly degenerate in the undistorted structure, and there is no spin-orbit coupling in this level. It is shown that, introducing a fictitious spin to specify the degenerate orbital states, we can discuss the problem by analogy with the magnetic problems. The “ferromagnetic” and “antiferromagnetic” distortions are discussed in detail. The transition from the distorted to the undistorted structure is of the first kind for the former and of the second kind for the latter. Higher approximations are discussed briefly. In compounds like FeO, CoO, and CuCr <sub>2</sub> O <sub>4</sub> , the lowest orbital level is triply degenerate, and the spin-orbit coupling is present in this level. In this case the distortion is dependent on the magnitude of the spin-orbit coupling relative to the strength of the Jahn-Teller effect term. The distortion at absolute zero temperature and its temperature dependence are discussed.
We have used a first-principles ultra-soft-pseudopotential method in conjunction with an efficient preconditioned conjugate-gradient scheme to investigate the properties of a series of eight cubic perovskite compounds. The materials considered in this study are BaTiO3, SrTiO3, CaTiO3, KNbO3, NaNbO3 PbTiO3 , PbZrO3, and BaZrO3. We computed the total-energy surface for zone-center distortions correct to fourth order in the soft-mode displacement, including renormalizations due to strain coupling. Quantities calculated for each material include lattice constants, elastic constants, zone-center phonon frequencies, Grüneisen parameters, and band structures. Our calculations correctly predict the symmetry of the ground-state structures of all compounds whose observed low-temperature structure retains a primitive five-atom unit cell. The database of results we have generated shows a number of trends which can be understood using simple chemical ideas based on the sizes of ions, and the frustration inherent in the cubic perovskite structure.
The theory of semicovalent exchange is reviewed and applied to the
perovskite-type manganites [La, M(II)]MnO3. With the hypothesis of
covalent and semicovalent bonding between the oxygen and manganese
ions plus the mechanism of double exchange, detailed qualitative
predictions are made about the magnetic lattice, the crystallographic
lattice, the electrical resistivity, and the Curie temperature as
functions of the fraction of Mn4+ present. These predictions are
found to be in accord with recent findings from neutron-diffraction
and x-ray data as well as with the earlier experiments on this system
by Jonker and van Santen.
A classical model for the lattice distortions of $\lax$ is derived and, in a
mean field approximation, solved. The model is based on previous work by
Kanamori and involves localized Mn d-electrons (which induce tetragonal
distortions of the oxygen octahedra surrounding the Mn) and localized holes
(which induce breathing distortions). Parameters are determined by fitting to
the room temperature structure of $LaMnO_3$. The energy gained by formation of
a local lattice distortion is found to be large, most likely $\approx 0.6$ eV
per site, implying a strong electorn-phonon coupling and supporting polaronic
models of transport in the doped materials. The structural transition is shown
to be of the order-disorder type; the rapid x-dependence of the transition
temperature is argued to occur because added holes produce a "random" field
which misaligns the nearby sites.
A model of localized classical electrons coupled to lattice degrees of freedom and, via the Coulomb interaction, to each other, has been studied to gain insight into the charge and orbital ordering observed in lightly doped manganese perovskites. Expressions are obtained for the minimum energy and ionic displacements caused by given hole and electron orbital configurations. The expressions are analyzed for several hole configurations, including that experimentally observed by Yamada et al. in La_{7/8}Sr_{1/8}MnO_3. We find that, although the preferred charge and orbital ordering depend sensitively on parameters, there are ranges of the parameters in which the experimentally observed hole configuration has the lowest energy. For these parameter values we also find that the energy differences between different hole configurations are on the order of the observed charge ordering transition temperature. The effects of additional strains are also studied. Some results for La_{1/2}Ca_{1/2}MnO_3 are presented, although our model may not adequately describe this material because the high temperature phase is metallic. Comment: 12 pages in RevTex, 5 figures in PS files, to appear in Phys. Rev. B (New paragraphs and references added, typos corrected)
Orbital degree of freedom of electrons and its interplay with spin, charge and lattice degrees of freedom are one of the central issues in colossal magnetoresistive manganites. The orbital degree of freedom has until recently remained hidden, since it does not couple directly to most of experimental probes. Development of synchrotron light sources has changed the situation; by the resonant x-ray scattering (RXS) technique the orbital ordering has successfully been observed . In this article, we review progress in the recent studies of RXS in manganites. We start with a detailed review of the RXS experiments applied to the orbital ordered manganites and other correlated electron systems. We derive the scattering cross section of RXS where the tensor character of the atomic scattering factor (ASF) with respect to the x-ray polarization is stressed. Microscopic mechanisms of the anisotropic tensor character of ASF is introduced and numerical results of ASF and the scattering intensity are presented. The azimuthal angle scan is a unique experimental method to identify RXS from the orbital degree of freedom. A theory of the azimuthal angle and polarization dependence of the RXS intensity is presented. The theoretical results show good agreement with the experiments in manganites. Apart from the microscopic description of ASF, a theoretical framework of RXS to relate directly to the 3d orbital is presented. The scattering cross section is represented by the correlation function of the pseudo-spin operator for the orbital degree of freedom. A theory is extended to the resonant inelastic x-ray scattering and methods to observe excitations of the orbital degree of freedom are proposed. Comment: 47 pages, 24 figures, submitted to Rep. Prog. Phys
We argue that elastic interactions between ions in different valence states can play an essential role in stabilization of stripes(or 2D "sheets")in doped oxides. These interactions are in general long-range and anisotropic (attractive in certain directions and repulsive in others). This can naturally give rise to stripe-like structures in insulating materials. We illustrate this general idea with certain specific examples and show that the situation can be described by the Ising model with anisotropic interactions. The case of anisotropic impurities, relevant e.g. for manganites, is also briefly discussed. Comment: 4 pages, RevTex, accepted for publication in Europhysics Letters
The study of the manganese oxides, widely known as manganites, that exhibit the ``Colossal'' Magnetoresistance (CMR) effect is among the main areas of research within the area of Strongly Correlated Electrons. After considerable theoretical effort in recent years, mainly guided by computational and mean-field studies of realistic models, considerable progress has been achieved in understanding the curious properties of these compounds. These recent studies suggest that the ground states of manganite models tend to be intrinsically inhomogeneous due to the presence of strong tendencies toward phase separation, typically involving ferromagnetic metallic and antiferromagnetic charge and orbital ordered insulating domains. Calculations of the resistivity versus temperature using mixed states lead to a good agreement with experiments. The mixed-phase tendencies have two origins: (i) electronic phase separation between phases with different densities that lead to nanometer scale coexisting clusters, and (ii) disorder-induced phase separation with percolative characteristics between equal-density phases, driven by disorder near first-order metal-insulator transitions. The coexisting clusters in the latter can be as large as a micrometer in size. It is argued that a large variety of experiments reviewed in detail here contain results compatible with the theoretical predictions. It is concluded that manganites reveal such a wide variety of interesting physical phenomena that their detailed study is quite important for progress in the field of Correlated Electrons. Comment: 76 pages, 21 PNG files with figures. To appear in Physics Reports
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