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Magnon Excitations in Manganites
Abstract
A review of experimental studies of magnon excitations in manganites is presented. Two kinds of techniques: the inelastic neutron scattering (in bulk materials) and the microwave resonance (in thin films) are considered. Experimental studies of spin dynamics by inelastic neutron scattering in metallic ferromagnetic manganites have shown that at low temperature for small wave vectors κ → 0 the dispersion relation has a quadratic shape similar to that observed in Heisenberg ferromagnets. However, the above technique although very informative can be used only for sufficiently large samples of bulk materials. A complementary microwave resonance technique allows studying not only bulk properties, but also surface properties. There are two main theoretical approaches used to interpret the spin wave resonance spectra: the volume inhomogeneity and the surface inhomogeneity models. The last one introduced by Puszkarski has allowed for interpretation of the observed surface magnon excitations in thin films.
... There are a few reports concerning SWR in the thin films of manganites. Previously SWR was observed in La 0.7 Ba 0.3 MnO 3 [17,18] and La-(Sr,Ca)-Mn-O films [19] apart our previous publications [13,14,20]. The observation of the surface spin waves has a special interest in modern physics since resonance lines related to surface spin waves provide direct information concerning magnetic surfaces. ...
... It is seen that the splitting monotonously grows up to the Curie temperature T C = 205 K. This "doublet structure" of SWR spectrum can be comprehensively explained within the framework of the SI model and points to forming two magnetic sublattices (or stripes) on the surface [20]. The SWR does not answer the question: what makes these magnetic sublattices, but points that the surface anisotropy varies on one of the surfaces and this variation is periodical. ...
... In our films the Mn 3+ /Mn 4+ ratio is equal to 2:1, it is assumed that the stripe formation has a period of three lattice constant = 3a. SWR has no splitting at low temperature where exchange interaction length > 3a and two magnetic sublattices are effectively averaged [20]. The splitting of the SWR lines starts at that temperatures when exchange interaction length will decrease so much as it becomes comparable with or less than a stripe period. ...
A spin-wave resonance technique was used to collect information on elementary magnon excitations in high quality epitaxial La0.7Mn 1.3O2.84 films prepared by dc-magnetron sputtering. It was found that besides bulk spin-wave modes, the spin-wave resonance spectrum includes also the surface spin waves. The value of spin-wave stiffness constant D = 156 meV·Å2 was found. The values of the surface magnetic anisotropy on both surfaces and their angular dependence were determined. The experimentally found spin-wave resonance spectra were explained based on the surface-inhomogeneity model. The effect of surface anisotropy on the spin-wave excitation conditions in epitaxial La0.7Mn 1.3O2.84 films was investigated. Spin-wave resonance data give an indirect evidence of periodic charge-ordered stripes formation.
... The reduction of stiffness constants may arise from the suppression of magnetic coupling due to the tensile strain. Spin-wave theory [27] suggests that the dynamical interaction between the spin waves gives a T 5/2 behavior: ...
Tensile-strained epitaxial La0.67Ba0.33MnO3 (LBMO) film has been prepared by magnetron sputtering technique on (001) oriented spinel MgAl2O4 substrate. The transport and magnetic measurements give an insulator-metal transition and paramagnetic-ferro-magnetic transition occurring at ∼150 K and 250 K respectively, which implies the phase separation in such a tensile-strained film. By analyzing the angular and temperature dependences of the ferromagnetic resonance (FMR), we determine the magnetocrystalline anisotropy of the film. It is found that the tensile-strained film is dominated by an easy-axis corresponding to the compressive out-of-plane direction, though the magnitudes of anisotropy constants are relatively small and their temperature dependences are some complex. Furthermore, the FMR spectra show additional spin wave resonance (SWR), and the field positions can be indexed to follow a linear dependence on the square of index n. The scaling gives a spin-wave exchange stiffness D of 20.7 meV Å2 at low temperature, which is less than half of that in strain-free LBMO films, implying that the double exchange interaction is remarkably suppressed in the tensile-strained LBMO films.
We studied magnetic-field-dependent microwave absorption in epitaxial La0.7Sr0.3MnO3 films using an 9.5GHz Bruker electron-spin-resonance spectrometer. By analyzing the angular and temperature dependence of the ferromagnetic and spin-wave resonances we determine spin-wave stiffness and perpendicular anisotropy field. The spin-wave stiffness as found from the spectrum of the standing spin-wave resonances in thin films is in fair agreement with the results of inelastic neutron scattering studies on a single crystal of the same composition [L. Vasiliu-Doloc , J. Appl. Phys. 83, 7343 (1998)].
The magnetoresistive ρ(T,H) and thermoelectric (the Seebeck coefficient) S(T,H) properties of La0.7□0.3MnO3−δ thin films (□ is a cation vacancy) grown by the magnetron deposition technique are investigated. The magnetic polaron origin of the conductivity of such systems is established in the temperature range 77 K ⩽ T ⩽ 350 K in magnetic fields 0 ⩽ H ⩽ 10 kOe. The experimental dependences ρ(T,H) and S(T,H = 0) are approximated by a universal phenomenological expression. Thermopower measurements indicate a considerable change of the mobility as well as the density of states of charge carriers in the region of magnetic phase transition.
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.
FERROMAGNETIC perovskites of the form La1-XMexMnO3-Y (where Me is Ca or Sr) have been known1 since 1950, but there has been a recent resurgence of interest following the discovery of giant magnetoresistance in this class of compounds2,3. The compounds contain both Mn3+ and Mn4+ ions; as the electronic ground state of the Mn3+ ions is degenerate, their energy is lowered by a spontaneous distortion of the surrounding lattice-the Jahn-Teller effect4. The charge carriers in these materials are strongly coupled to (and mediate the ferromagnetic interaction between) the manganese ions5, suggesting that localized lattice distortions could also play an important role in determining the electronic and magnetic properties of these compounds. Here we investigate this possibility by examining the effect on the ferromagnetic transition temperature of varying the oxygen isotope mass (replacing 16O with 18O). For La0.8Ca0.2MnO3+y, we measure an isotope shift of >20 K, significantly larger than that found for any magnetic or electronic phase transition in other oxides. In contrast, we observe no significant isotope shift for the structurally related ferromagnet SrRuO3, in which the Jahn-Teller effect is negligible. These results imply that the large isotope shift arises from coupling of the charge carriers to Jahn-Teller lattice distortions, and we suggest that such Jahn-Teller 'polarons' may also be responsible for the magnetoresistive properties of these materials.
Neutron scattering has been used to study the structure and spin dynamics of La0.85Sr0.15MnO3. The magnetic structure of this system is ferromagnetic below TC≃235K. We see anomalies in the Bragg peak intensities and new superlattice peaks consistent with the onset of a spin-canted phase below TCA∼205K, which appears to be associated with a small gap at q=(0,0,0.5) in the spin-wave spectrum. Anomalies in the lattice parameters indicate a concomitant lattice distortion. The long-wavelength magnetic excitations are found to be conventional spin waves, with a gapless (<0.02meV) isotropic dispersion relation E=Dq2. The spin stiffness constant D has a T5/2 dependence at low T, and the damping at small q follows q4T2. An anomalously strong quasielastic component, however, develops at small wave vector above ∼200K and dominates the fluctuation spectrum as T⃗TC. At larger q, on the other hand, the magnetic excitations become heavily damped at low temperatures, indicating that spin waves in this regime are not eigenstates of the system, while raising the temperature dramatically increases the damping. The strength of the spin-wave damping also depends strongly on the symmetry direction in the crystal. These anomalous damping effects are likely due to the itinerant character of the eg electrons.
We have done inelastic neutron scattering investigations of the magnetic excitations in the ferromagnetic La0.7Ba0.3MnO3 which shows colossal magnetoresistance (CMR) behavior close to TC≈350 K. We have measured the dispersions of the spin waves along [100] and [110] and also their temperature dependence. We have fitted the dispersions with an effective localized spin model to get the nearest-neighbor exchange interaction. We have shown that the effective localized spin model is no longer valid for larger momentum transfer close to the zone boundary at which the spin-waves show softening. Also the spin-wave energy widths are found to be much larger than the instrumental resolution. We argue that the spin-wave softening and damping are generic to the double-exchange ferromagnet including those with large TC.
Neutron scattering studies on a single crystal of the highly correlated electron system, La1-xSrxMnO3 with x≈0.3, have been carried out elucidating both the spin and lattice dynamics of this metallic ferromagnet. We report a large measured value of the spin wave stiffness constant, which directly shows that the electron transfer energy of the d band is large. The spin dynamics, including magnetic critical scattering, demonstrate that this material behaves similar to other typical metallic ferromagnets such as Fe or Ni. The crystal structure is rhombohedral, as previously reported, for all temperatures studied (below ∼425 K). We have observed superlattice peaks which show that the primary rhombohedral lattice distortion arises from oxygen octahedra rotations resulting in an R3̅ c structure. The superlattice reflection intensities, which are very sensitive to structural changes, are independent of temperature demonstrating that there is no primary lattice distortion anomaly at the magnetic transition temperature TC=378.1 K; however, there is a lattice contraction.
Using inelastic neutron scattering we measured the microscopic magnetic coupling associated with the ferromagnetic clusters of the “colossal magnetoresistance” compound Pr0.70Ca0.30MnO3. When the insulating-to-metal (I-M) transition is induced by an external magnetic field there is a discontinuous change in the spin-wave stiffness constant. This result implies that the I-M transition is not achieved by the simple percolation of micron-sized metallic clusters as currently believed, but involves a first-order transformation.
We have performed diffraction and inelastic neutron scattering experiments on a La0.85Sr0.15MnO3 single crystal. In the ferromagnetic phase of this system (Tc = 235 K) the long wavelength dispersion relation along the (0,1,0) direction is well represented by E = E0+Dq2, but with the spin wave spectrum almost gapless (E0<0.1 meV), and with DT = 0 = 83.75±0.36 meV Å2. The spin-wave stiffness constant D(T) exhibits a power law behavior as a function of temperature, and appears to collapse as T→Tc. An orthorhombic–rhombohedral structural phase transition is observed at Ts = 360 K, and exhibits a hysteretic character suggesting a first-order transformation. Anomalies in magnetization measurements and Bragg peak intensities indicate the existence of a second phase transition at T = 200 K. © 1997 American Institute of Physics.
Cold neutron triple-axis measurements have been used to investigate the nature of the long-wavelength spin dynamics in strongly doped La1−xSrxMnO3 single crystals with x = 0.2 and 0.3. Both systems behave like isotropic ferromagnets at low T, with a gapless (E0<0.02 meV) quadratic dispersion relation E = E0+Dq2. The values of the spin-wave stiffness constant D are large (DT = 0 = 167 meV Å2 for x = 0.2 and DT = 0 = 176 meV Å2 for x = 0.3), which directly shows that the electron transfer energy for the d band is large. D exhibits a power law behavior as a function of temperature, and appears to collapse as T→TC. Nevertheless, an anomalously strong quasielastic central component develops and dominates the fluctuation spectrum as T→TC. Bragg scattering indicates that the magnetization near TC exhibits power law behavior, with β≃0.30 for both systems, as expected for a three-dimensional ferromagnet. © 1998 American Institute of Physics.
This chapter contains a brief review of the giant magnetoresistance (GMR) effect exhibited by magnetic multilayers, granular alloys, and related materials. Subjects covered include a description of the phenomenon, and the related oscillatory interlayer exchange coupling in magnetic multilayers; a simple model of giant magnetoresistance; the inverse GMR effect in spin-engineered magnetic multilayers; structures that display large changes in resistance in small magnetic fields, possibly for use in magnetic field sensors; and the dependence of GMR on various aspects of the magnetic structures.
MANGANESE oxides with the cubic perovskite structure (typified by LaMnO3) have stimulated considerable interest because of their magnetoresistive properties1–9; they exhibit extremely large changes in electrical resistance in response to applied magnetic fields, a property that is of technological relevance for the development of magnetic memory and switching devices. But for such applications to be viable, great improvements will be needed in both the sensitivity and temperature dependence of the magnetoresistive response. One approach under consideration for optimizing these properties is chemical substitution10. Here we demonstrate an alternative strategy, in which we synthesize layered variants of the cubic perovskite parent compounds that have a controlled number of MnO2 sheets per unit cell. This strategy is structurally analogous to that employed for the systematic exploration of the high-transition-temperature copper oxide superconductors11. We find that the magneto-resistive properties of these materials depend sensitively on the dimensionality of the manganese oxide lattice. Although the properties of our materials are still far from optimal, further exploration of this series of layered perovskites may prove fruitful.
In this work, we present the measurements of exchange-dominated nonpropagating surface and bulk spin-wave modes in the La-deficient epitaxial La0.7Mn1.3O3 films prepared by dc-magnetron sputtering. The angular and temperature dependences of the modes observed are discussed. The main result obtained is the observation of the spin-wave resonance (SWR) consisting of a series (17) of well resolved standing spin-wave modes in the perpendicular external magnetic field geometry. The surface spin-wave modes have been observed in manganites for the first time. As the magnetization is rotated out of perpendicular to the film surface, a `critical angle', cr, is fixed, at which the surface and first spin-wave modes have been transformed into the uniform mode. It is shown that only the uniform mode exists in the region 0<<cr. The surface mode data are consistent with the surface-inhomogeneity model in which the surface-anisotropy field acts on the surface spin. Possible origins of the surface anisotropy are discussed. Based on the temperature and angular dependences of SWR spectra, the main microscopic parameters (the spin-wave stiffness, exchange constant and g-factor value) are established.
The magnetic structure and dynamics in the colossal magnetoresistance (CMR) class perovskite La <sub> 0.53 </sub> Ca <sub> 0.47 </sub> MnO <sub> 3 </sub> have been studied by elastic and inelastic neutron scattering. This composition is near the 0.5 Ca transition from a metallic ferromagnet to an insulating antiferromagnet. Powder neutron diffraction on these samples showed coexisting ferromagnetic and antiferromagnetic phases at low temperature and a splitting of the lattice parameters of the antiferromagnetic phase near T<sub>N</sub>, reflecting the onset of the charge ordered state. Inelastic scattering measurements of the ferromagnetic excitations exhibited well-defined spin waves of resolution width below T<sub>c</sub>. The spin waves exhibited conventional Heisenberg behavior with dispersion of the form E=Dq<sup>2</sup>+Δ with spin stiffness D(T=0 K)≈135 meV Å <sup> 2 </sup> and an energy gap Δ≈0.1 meV. The value of the spin wave stiffness D is similar to that found for other ferromagnetic perovskite materials, and it renormalized with temperature in a manner consistent with the magnetization. © 1998 American Institute of Physics.
Following a concise review of present achievements in the study of magnetic surface states by spin wave resonance, a discussion of surface state theory and experimentally observed effects is given. The theoretical treatment, starting from the general model assumptions made when dealing with thin magnetic films, concentrates specifically on the Surface Inhomogeneity Model, for which a Hamiltonian is established and diagonalized, leading to the elementary magnetic excitation i.e. spin waves of the system. The appropriate spin wave resonance theory is developed in detail, and the resonance effects are expounded on the basis of a Pinning Diagram, especially constructed for this purpose. Particular attention is given to the existence conditions for surface states, the experimental identification of their respective resonance lines, as well as the use of these lines when determining the surface conditions of the thin film and measuring its magnetic surface anisotropy. The analysis of the surface state properties is based on the exact solution of the eigenvalue problem of a finite linear chain with arbitrary (asymmetric) boundary conditions.
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.
Inelastic neutron scattering was used to systematically investigate the spin-wave excitations (magnons) in ferromagnetic manganese perovskites. In spite of the large differences in the Curie temperatures ($T_C$s) of different manganites, their low-temperature spin waves were found to have very similar dispersions with the zone boundary magnon softening. From the wavevector dependence of the magnon lifetime effects and its correlation with the dispersions of the optical phonon modes, we argue that a strong magneto-elastic coupling is responsible for the observed low temperature anomalous spin dynamical behavior of the manganites. Comment: 11 pages, 4 figures
Elastic and inelastic neutron scattering was used to study two ferromagnetic manganites A$_{1-x}$B$_{x}$MnO$_3$ (x $\approx$ 0.3) with $T_c$=197.9 K and 300.9 K. The spin dynamical behavior of these is similar at low temperatures, but drastically different at temperatures around $T_c$. While the formation of spin clusters of size ($\sim20$ \AA) dominates the spin dynamics of the 197.9 K sample close to $T_c$, the paramagnetic to ferromagnetic transition for the 300.9 K sample is more conventional. These results, combined with seemingly inconsistent earlier reports, reveal clear systematics in the spin dynamics of the manganites. Comment: zipped RevTeX file (7 pages) and 4 postscript figures
Neutron scattering has been used to study the magnetic correlations and long wavelength spin dynamics of La1-xCaxMnO3 in the ferromagnetic regime (0<=x<1/2). For x=1/3 (Tc=250K) where the magnetoresistance effects are largest the system behaves as an ideal isotropic ferromagnet at low T, with a gapless (<0.04meV) dispersion relation E=Dq^2 and D(T=0)~170 meV-A-1. However, an anomalous strongly-field-dependent diffusive component develops above ~200K and dominates the fluctuation spectrum as T approaches Tc. This component is not present at lower x. Comment: Revtex, 4 pages, four ps figures available upon request. Physical Review Letters (in press)
In the Ln1-xAxMnO3 pseudoperovskites in which Ln is a lanthanide and A an alkaline-earth atom, an intrinsic colossal magnetoresistance (CMR) occurs in an O-orthorhombic phase near an O′-orthorhombic/O-orthorhombic phase boundary. For a fixed ratio Mn(IV)/Mn = 0·3, the transition through the O phase from localised-electron behaviour and orbital ordering in the O′ phase to itinerant-electron behaviour in an R-rhombohedral phase occurs with increasing geometric tolerance factor t ≡ ⟨A-O⟩/√2⟨Mn-O⟩, where ⟨A-O⟩ and ⟨Mn-O⟩ are mean equilibrium bond lengths. The CMR occurs in the temperature interval Tc ≤ T < Ts where there is a segregation, via cooperative oxygen displacements, into a Mn(IV)-rich ferromagnetic phase imbedded in a paramagnetic phase. The volume of the ferromagnetic, more conductive clusters increases from below to beyond a percolation threshold in response, above Tc, to an applied magnetic field and, below Tc, to a Weiss molecular field. In the O phase, the magnetic transition at Tc decreases on the exchange of 18O/16O and increases under hydrostatic pressure. Charge and orbital ordering below a Tco ≤ Tc is found in compositions with x ≈ ⅛ or x ≈ ½. With x ≈ ½, the charge-ordered phase CE is tetragonal and antiferromagnetic. An applied magnetic field stabilises the ferromagnetic, conductive phase relative to the insulator phase CE to give a second type of intrinsic CMR. For x ≈ 0·3, there is no static charge and orbital ordering; but for smaller t, strong electron-lattice coupling gives a ‘bad metal’ behaviour below Tc indicative of a dynamic phase segregation as in a traveling charge-density wave. In La1-xCaxMnO3 with ½ ≤ x ≤ ⅞, segregation of the CE x = 0·5 phase and the all-Mn(IV) x = 1 phase has been reported to take the form of a static charge-density wave. The origins of this complex behaviour are discussed.
We have studied the spin dynamics in Pr0.63Sr0.37MnO3 above and below the Curie temperature TC = 301 K. Three distinct new features have been observed: a softening of the magnon dispersion at the zone boundary for T<TC, significant broadening of the zone boundary magnons as T→TC, and no evidence for residual spin-wave-like excitations just above TC. The results are inconsistent with double exchange models that have been successfully applied to higher TC samples, indicating an evolution of the spin system with decreasing TC.
The general exchange boundary condition applicable to a line of spins placed in a static field of arbitrary orientation and whose end spins are subjected to different environments are first formulated. The result is then applied to the problem of spin wave resonance in thin magnetic films. Both the line and absorption spectra are calculated assuming different types of surface anisotropy. By comparing our theoretical results with various experiments, it is concluded that the shape of the energy surface of the surface anisotropy energy are uniaxial with the easy axis of magnetization in the direction of the surface normal and the hard plane of magnetization parallel to the film plane. The possible origin of this anisotropy, including the question of dynamic pinning, is discussed in some detail. Aside from the usual volume spin-wave resonance modes, we found also some surface spin wave modes whose presence may have an important bearing upon the magnetization reversal process in thin Permalloy films. An apparent departure from the k2 dispersion low for spin waves and its relevance upon the determination of the exchange constant is also discussed.
The spin dynamics of the La1−xSrxMnO3 system have been studied from the lightly (x < 0.1) to the heavily doped (x > 0.17) limit. In the low-doping limit we show that the spin waves exhibit a finite-energy gap and a strongly anisotropic dispersion relation, which can be described by a strong in-plane ferromagnetic coupling and a weak inter-plane antiferromagnetic coupling. As the dopant concentration increases, the spin-wave energy gap and the inter-plane coupling both decrease considerably with x. For x = 0.11 a second magnetic-phase transition occurs at T = 122 K evidenced by the appearance of new magnetic peaks at (0, 0, ) and of a new gap in the spin-wave spectrum at finite . In the metallic ferromagnetic state (x > 0.17), the system behaves like an isotropic ferromagnet at low T, with a quadratic long-wavelength dispersion relation E = E0 + Dq2 and no significant anisotropy gap (E0 < 0.04 meV).The spin-wave stiffness constant D increases strongly with x, conserving a large DT=0/kTc ratio, indicative of an itinerant soft ferromagnet. In this limit, the x-dependence of the stiffness constant is found to be in good agreement with the double-exchange model with an effective bandwidth of about 0.3 eV and a Hund's coupling constant 4JH ∼ 4.7 eV.
The spin-wave resonance (SWR) spectra have been observed in the (001) oriented La0.7Mn1.3O3−δ epitaxial films. The spectra consist of a series of well-resolved spin-wave modes clearly seen in the perpendicular external field configuration. It is shown that the spins are completely pinned on both sides of the film surface. Based on a study of the SWR spectra the value of the spin-wave exchange constant, D, is found to be 150 meV Å2. This value of D correlates well with this one determined for all known until now manganites.
It is possible to excite exchange and magnetostatic spin waves in a ferromagnet by a uniform rf field, provided that spins on the surface of the specimen experience anisotropy interactions different from those acting on spins in the interior. Modes with an odd number of half-wavelengths should be excited in a flat plate. The definition of what is meant by a different anisotropy interaction is worked out and is a rather lenient condition. Experiments which would determine the exchange energy constant should be possible using sufficiently thin platelets of single crystals having parallel faces. It is perhaps not unlikely that the theory may account for the observation by Waring and Jarrett of a large number of resonance peaks in NiMnO3.
As an effective model to describe the perovskite-type manganates (La,A)MnO3, the double-exchange model (the Kondo lattice modelwith ferromagnetic couplings) on a cubic lattice is investigated. The spin excitation spectrum of the model in theground state is studied using the spin wave approximation.The spin wave dispersion relation observed in the inelasticneutron scattering experiment for La0.7Pb0.3MnO3 is reproduced. Effective values for the electron bandwidth as well as Hund's coupling are estimated from the data.
A model for the doped rare-earth manganites such as La1-xSrxMnO3 incorporating the physics of dynamic Jahn-Teller and double-exchange effects is presented and solved via a dynamical mean field approximation. The interplay of these two effects as the electron phonon coupling is varied reproduces the observed behavior of the resistivity and magnetic transition temperature.
We propose the procedure of an analytical treatment of the boundary condition formulated on the surface of the periodically inhomogeneous material. By applying appropriate Fourier transformations the procedure converts the composite surface into an equivalent multiple homogeneous surface. In terms of effectivity, the method reduces the composite boundary condition to a specific eigenproblem condition, which constitutes the spectrum of eigenvalues of an effective surface parameter, a novel quantity we introduce to account for the nonhomogeneity of the composite surface.
Measuring the magnetization and the specific heat of La1-xSrxMnO3 single crystals with 0.11less than or equal toxless than or equal to0.15 we have investigated the charge order (CO) transition in lightly doped manganites. Though the properties of the ferromagnetic insulating state at low temperatures hardly depend on x, there is a very strong concentration dependence of the specific heat and the magnetization. While the specific-heat anomaly due to ferromagnetic order strongly increases with x, the jumps of both, entropy and magnetization at the CO transition decreases drastically. The analysis of our data shows that the entropy changes at the CO transition are mainly due to spin degrees of freedom, which strongly suggests that the melting of charge order is driven by magnetic energy.
The propensity of systems of charge and spin to form, under certain conditions, 'stripe' phases has recently attracted much attention, as it has been suggested that dynamically fluctuating stripe phases may be of central importance for an understanding of the physics of high-temperature superconductors(1-5), A related phenomenon-static charge stripes-characterizes(6) the insulating antiferromagnetic ground state of the manganese oxides, a class of materials which (like the copper oxide superconductors) have a perovskite structure, and are notable for their extraordinary electronic and magnetic properties, such as colossal magnetoresistance and charge ordering(7,8). Here we report a different pattern of charge localization in the charge-ordered phase of the manganese oxide La1-xCaxMnO3 (x greater than or equal to 0.5). This pattern takes the form of extremely stable pairs of Mn3+O6 stripes, with associated large lattice contractions (due to the Jahn-Teller effect), separated periodically by stripes of non-distorted Mn4+O6 octahedra. These periodicities, which adopt integer values between 2 and 5 times the lattice parameter of the orthorhombic unit cell, correspond to the commensurate carrier concentrations (x = 1/2, 2/3, 3/4 and 4/5): for other values of x, the pattern of charge ordering is a mixture of the two adjacent commensurate configurations. These paired Jahn-Teller stripes appear therefore to be the fundamental building blocks of the charge-ordered state in the manganese oxides, and so may be expected to have profound implications for the magnetic and transport properties of these materials.
Spin-wave resonance (SWR) in high quality epitaxial La0.7Mn1.3O3 films prepared by dc-magnetron sputtering was studied at a frequency of 9.25 GHz. SQUID magnetometry was used to study the film magnetization. A well-resolved structure of the SWR spectrum consists of surface and bulk spin waves modes. The experimentally found SWR spectra are in good agreement with the theory of SWR based on the surface-inhomogeneity model. Using the SWR and magnetization data, the spin-wave stiffness constant and the values of the bulk anisotropies were deduced.
A review is given that focuses on the spin dynamics in the ferromagnetic regime of the magnetoresistive oxides. At small wave vectors the quadratic dispersion relation is remarkably isotropic throughout the composition range, with a gap that is too small to measure with conventional neutron scattering techniques. At larger wave vectors the spin waves are strongly damped in the ground state, in contrast to expectations based on a simple half-metallic band structure, while the temperature dependence of the damping and renormalization of the spin waves is anomalous. An unusual spin-diffusion component to the fluctuation spectrum develops as the Curie temperature is approached, which appears to be related to the formation of polarons in the system and the consequent charge localization. Near and above T
C
the diffuse scattering from polarons is directly observed, as well as the correlations between the polarons.
Charge-ordered stripes in La <sub>1-x</sub> Ca <sub>x</sub> MnO <sub>3</sub> with x ≫0.5 were studied by electron diffraction and electron microscopy. Charge-ordered domains were observed in dark-field images and the stripes within the domain were revealed by high resolution lattice images. The modulation wave vector δ of the charge ordering follows closely the relationship of δ=1-x and shows a slight variation from area to area. The incommensurability of the charge ordering to the underlying fundamental lattice gives rise to the presence of discommensurations in the incommensurate phase. Configurations of discommensurations in this system do not follow the general rules observed in other incommensurate systems. © 1997 American Institute of Physics.
We report the observation of giant magnetoresistance near room temperature in ferromagnetic films of La 1-x Sr x MnO z for 0.16≤x≤0.33. For B=5 T, the maximum magnetoresistance ratio [R(0)-R(B)]/R(0) of an annealed film is 60% at 260 K for x=0.2, and 35% at 330 K for x=0.33. Annealed films have higher Curie temperature (T c ), a larger saturation moment and a larger magnetoresistance effect near T c than do as‐grown films. The temperature dependence of resistivity for all the samples investigated is unusual, activated above T c and metallic below T c . This and the giant magnetoresistance are possibly explained by scattering from magnetic polarons which dominate the transport near T c . © 1994 American Institute of Physics.
In the phase diagram of La1−xSrxMnO3 the compound with x=0.125, exhibits below TC=181 K, a ferromagnetic and metallic phase (FM). At TO′O″=159 K, a magneto-structural transition occurs towards a ferromagnetic and insulating (FI) phase with amazing properties. New superstructure Bragg peaks appear. At =(000.5), the spin wave energies split into two levels and, along [0 0 1], they lock successively on the values of phonon modes. This suggests a complex ground state for this (FI) phase with mixed magnetic and phononic excitations. These results are discussed in the frame of existing theories.
Epitaxial thin films of Ln0.7Mn1.3O3−δ have been grown on [0 0 1]-oriented LaSrAlO4 substrates by DC-magnetron deposition. Magnetization, resistivity, thermopower, and magnetic resonance spectra are investigated over a range of temperatures and magnetic fields. The magnetic polaron theory is used, phenomenologically, to provide a quantitative explanation of transport properties of the oxidized films both above and below Curie temperature TC. The main assumption is that metal–insulator transition near TC is due to a crossover in polaronic mobility. At low temperatures we observe an anisotropy of magnetic resonance spectra, both resonance field and linewidth. To qualitatively account for all the observations (resistivity, thermopower, and magnetic resonance spectra) we suggest an intrinsic electric and magnetic inhomogeneity of the samples.
As part of a general work on doped manganese perovskites, we have carried out detailed neutron-scattering experiments on powder and single crystals of the othorhombic phase of undoped LaMnO3. The temperature dependence of the sublattice magnetization has been determined in the antiferromagnetic phase (TN=139.5 K), and the critical exponent is beta=0.28, well below that corresponding to a pure three-dimensional Heisenberg antiferromagnet. We have measured the dispersion of the spin waves propagating in the highest symmetry directions solving the problems related to twinning. The whole spin wave spectrum is well accounted for with a Heisenberg Hamiltonian and a single ion anisotropy term responsible for the easy magnetization direction (b axis). This term induces a gap of 2.7 meV at low temperature in the spin wave dispersion curve. An important result is that the ferromagnetic exchange integral (J1~=0.83 meV), coupling the spins within the ferromagnetic basal plane (a,b), is larger by a factor 1.4 than the antiferromagnetic exchange integral (J2~=-0.58 meV) coupling spins belonging to adjacent MnO2 planes along c.
We use inelastic neutron scattering to measure the spin wave dispersion throughout the Brillouin zone of the double-exchange ferromagnet La0.7Pb0.3MnO3. Magnons with energies as high as 95 meV are directly observed and an unexpectedly simple Heisenberg Hamiltonian, with solely a nearest-neighbor coupling of 8.79±0.21meV, accounts for the entire dispersion relation. The calculated Curie temperature for this local moment Hamiltonian overestimates the measured Curie point (355 K) by only 15%. Raising temperature yields unusual broadening of the high frequency spin waves, even within the ferromagnetic phase.
We study the combined influence of spin double exchange and Jahn-Teller lattice coupling to holes in the La{sub 1{minus}{ital x}}{ital A}{sub {ital x}}MnO{sub 3} perovskites ({ital A}=Ca, Sr, Ba). Using a mean-field approximation for the double exchange and a variational Lang-Firsov approximation for the lattice degrees of freedom, we show that the lattice effects decrease the magnetic transition temperature, and also cause the maximal value of the transition temperature as a function of dopant concentration {ital x} to depend on the Jahn-Teller coupling strength. We find a continuous rapid crossover from a large polaronic state to a quasi-self-trapped small polaron state accompanying the magnetic transition. {copyright} {ital 1996 The American Physical Society.}
An electron in a solid, that is, bound to or nearly localized on the specific atomic site, has three attributes: charge, spin,
and orbital. The orbital represents the shape of the electron cloud in solid. In transition-metal oxides with anisotropic-shaped
d-orbital electrons, the Coulomb interaction between the electrons (strong electron correlation effect) is of importance for
understanding their metal-insulator transitions and properties such as high-temperature superconductivity and colossal magnetoresistance.
The orbital degree of freedom occasionally plays an important role in these phenomena, and its correlation and/or order-disorder
transition causes a variety of phenomena through strong coupling with charge, spin, and lattice dynamics. An overview is given
here on this “orbital physics,” which will be a key concept for the science and technology of correlated electrons.
Mixed-valent manganites are noted for their unusual magnetic, electronic and structural phase transitions. For example, the La(1-x)Ca(x)MnO(3) phase diagram shows that below transition temperatures in the range 100-260 K, compounds with 0.2 < x < 0.5 are ferromagnetic and metallic, whereas those with 0.5 < x < 0.9 are antiferromagnetic and charge ordered. In a narrow region around x = 0.5, these totally dissimilar ground states are thought to coexist. It has been shown that charge order and charge disorder can coexist in the related compound, La(0.25)Pr(0.375)Ca(0.375)MnO(3). Here we present electron microscopy data for La(0.5)Ca(0.5)MnO(3) that shed light on the distribution of these coexisting phases, and uncover an additional, unexpected phase. Using electron holography and Fresnel imaging, we find micrometre-sized ferromagnetic regions spanning several grains coexisting with similar-sized regions with no local magnetization. Holography shows that the ferromagnetic regions have a local magnetization of 3.4 +/- 0.2 Bohr magnetons per Mn atom (the spin-aligned value is 3.5 micro (B) per Mn). We use electron diffraction and dark-field imaging to show that charge order exists in regions with no net magnetization and, surprisingly, can also occur in ferromagnetic regions.
The interplay between spin and orbital degrees of freedom in the Mott-Hubbard insulator is studied by considering an orbitally degenerate superexchange model. We argue that orbital order and the orbital excitation gap in this model are generated through the order-from-disorder mechanism known previously from frustrated spin models. We propose that the orbital gap should show up indirectly in the dynamical spin structure factor; it can therefore be measured using the conventional inelastic neutron scattering method.
We report the first observation of spin wave resonances in 110 nm thick films of LBMO. The spin wave stiffness follows $D$ = 47 (1 - 3 \times 10$_{-7}$ $T^{5/2}$) meV${\AA_2}$. Comment: 5 pages LATEX, 3 figures available on request. Submitted to Nature. Please send all queries to sb27@umail.umd.edu
The spin dynamic of the metallic A-type antiferromagnetic manganites is studied. An effective nearest-neighbour Heisenberg spin wave dispersion is derived from the double exchange model taking into account the superexchange interaction between the core spins. The result of inelastic neutron scattering experiment on ${Nd}_{0.45}{Sr}_{0.55}{Mn} {O}_{3}$ is qualitatively reproduced. Comparing theory with experimental data two main parameters of the model: nearest-neighbour electron transfer amplitude and superexchange coupling between the core spins are estimated. Comment: to appear in Phys. Rev. B
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
Spin wave excitations were measured in the ferromagnetic phase of Nd$_{1/2}$Sr$_{1/2}$MnO$_{3}$ by neutron scattering. This compound is located in proximity to the A-type antiferromagnetic state, and it shows a clear anisotropy and anomalous softening of the spin wave excitations. The softening in the ferromagnetic phase is induced by the orbital ordering. Comment: 2 pages, 1 figure, proceeding of ICM2000. Corrected typos
- F Moussa
- M Hennion
- J Rodriguez-Carvajal
- H Moudden
- L Pinsard
- A Revcolevschi
F. Moussa, M. Hennion, J. Rodriguez-Carvajal, H. Moudden, L. Pinsard, A. Revcolevschi, Phys. Rev. B 54, 15149 (1996).
- S P P Parkin
S.P.P. Parkin, Ann. Rev. Mater. Sci. 25, 357 (1995).