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Magnetic exchange and charge transfer in mixed-valence manganites and cuprates
Abstract
Metallic resistivity occurs at cryogenic temperatures in insulators with small carrier trap energies, e.g., superconducting cuprates. A similar metallic regime has been reported for the lanthanide (RE) manganites (RE<sup>3+</sup> 1-x A<sup>2+</sup> x )MnO 3 . To interpret the anomalous resistivity ρ as a function of temperature and magnetic field in these compounds, a model constructed from the relation for mobility activated semiconduction and the Brillouin–Weiss theory of ferromagnetism has been developed. The resistivity maximum occurs at the susceptibility peak slightly above the Curie temperature T C and its magnitude is related to the hopping electron trap energy E hop by exp(E hop /kT C ). Where T≪T C , ρ is metallic because E hop is small due to the collinear polarization of spins. For T≥T C , E hop increases to a value ∼0.1 eV equal to the decrease in stabilization energy of the transfer electrons caused by the transition from spin alignment to disorder. The magnetoresistance sensitivity dρ/dH at T=T C is controlled by T C through (1/T C )exp(E hop /kT C ). The relative sensitivity (1/ρ)dρ/dH, however, is proportional to 1/T<sup>2</sup> C . These results also reinforce the concept that metallic resistivity in the superconducting cuprates occurs because of the frustration of antiferromagnetism. © 1996 American Institute of Physics.
... It is noticeable that the extremely steep slope of the R T curve just below TC (® gure 46) cannot be accounted for by the above models. Consequently, several groups have developed other models to account for the temperature and magnetic ® eld dependences of the resistivity, and have proposed scaling relationships between q and M. [232,375,399,458,459]. Pierre et al. [460] proposed an exchange-ind uced band crossing model, like EuO (® gure 11). ...
... Nonetheless, quantitative agreement depends on the ® tting parameters ( Ms is one third of the experimental magnetizatio n) and their interpretation is unclear. Dionne [459] proposed a similar model where the thermally-activated hopping energy depends on magnetiza- tion [458]. A nother form of phenomenol ogy incorporates a two-current model [462,463], where the conductivity of one channel is purely dependent on magnetic order, and the other is due to excitation of carriers across a gap E0. ...
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.
... Fitting of resistivity above the transition is analyzed for LBCMO3 sample using SPH model and VRH model which are given by equations (6a) and (6b) [39]. ...
We report the doping effect of Bi on structural, magnetic and magneto-resistive properties of nanocrystalline La0.67-xBixCa0.33MnO3 (x = 0, 0.05, 0.1) (LBCMO) samples. Rietveld refinement of the X-ray diffraction (XRD) patterns revealed that all the three samples crystallize into orthorhombic crystal structure having Pbnm space group. The average crystallite size was calculated using Debye Scherer's formula and the crystallite size varies from 25-30 nm. Scanning electron microscopy (SEM) images shows that the particles are spherical in nature and distributed uniformly. The temperature dependent magnetization data infers that all the three samples undergo paramagnetic to ferromagnetic (PM-FM) phase transition with transition temperature (Tc) ∼245K, 250K and 230K upon cooling. La0.67Ca0.33MnO3 (LBCMO1), La0.62Bi0.05Ca0.33MnO3 (LBCMO2) samples exhibit second order magnetic phase transition, whereas La0.57Bi0.1Ca0.33MnO3 (LBCMO3) sample exhibit first order like transition. The maximum magnetic entropy changes |ΔSMmax| were calculated for nanocrystalline LBCMO samples using thermodynamic relations and the values were 3.73 Jkg⁻¹K⁻¹, 4.72 Jkg⁻¹K⁻¹ and 4.47 Jkg⁻¹K⁻¹ respectively, where field change was μoH = 5 T. As a consequence of temperature dependence of electrical resistivity, data shows metallic behavior before metal-semiconductor transition temperature (TMS) and semiconducting behavior after TMS. The magnetic field dependent electrical resistivity data reveals a huge change in resistivity near TMS and the maximum magnetoresistance (MR) value around ∼60% for an applied field of 5T around the TMS for LBCMO3 sample. The coexistence of magnetocaloric effect and moderately high value of magnetoresistance makes the studied samples promising for multifunctional device applications near its critical temperature.
... Generally, the exchange interaction Mn 3+ -O 2− -Mn 3+ is considered to be weak AFM. However, if localized Jahn-Teller effects come into the picture, a vibronic FM coupling may dominant over AFM [43,49,50]. The exchange Fe 3+ -O 2− -Mn 3+ is FiM in nature due to the partial B-site ordering of Fe 3+ and Mn 3+ ions, as discussed before. ...
The double perovskite La2FeMnO6 is ideally expected to be a ferrimagnet with a low saturation moment of 1 μB/f.u. Inhomogeneity in the Fe/Mn sites, along with other lattice disorders, can modify the exchange interactions in the material and result in a net saturation moment of more than 1 μB/f.u. Here, the origin of complicated magnetic behavior is examined of a pure phase La2FeMnO6 sample prepared by the sol-gel method. XRD analysis established that the material crystallizes in the orthorhombic Pbnm symmetry. The comprehensive analysis of x-ray photoelectron spectroscopy, x-ray near edge structure, and magnetic measurements acknowledge an antiferromagnetic coupling between Fe³⁺ and Mn³⁺ cations, thereby, resulting in the ferrimagnetic ground state for La2FeMnO6. The magnetic hysteresis loop obtained at 5 K shows a large coercivity of ∼820 Oe and a saturation moment of 1.6 μB/f.u., hinting at partial B-site ordering in La2FeMnO6. The ac susceptibility and dc magnetization measurements indicate the existence of magnetic glassy states (of cluster-glass type) with two distinct dynamical freezing points at ∼27 and 92 K, along with a Griffiths-like phase in the material. The experimental results are discussed taking several possible exchange interactions among Fe³⁺ and Mn³⁺ ions in the system into consideration. The magnetic complexity of this system makes it attractive for fundamental research and technological applications.
... The electrical behavior of our sample, in the semiconductor region can explained using two different models viz., SPH model and VRH model. 32 The high temperature region above q D /2 (where q D is Debye's temperature 33 ) is analyzed using the SPH model, whereas below q D /2 and above T M-SC , it is analyzed using VRH model. ...
... ii-Thermally activated conduction via polaron hopping is given by [27]. ...
The temperature dependence of magnetic susceptibility χ (Τ), 78<Τ(Κ)<295, and electrical resistivity ρ (T), 10 <T (K) <290, of La0.65Sr0.35Mn0.96 (Fe(1-x)Mgx)0.04O3 with (0.0≤x≤1.0) are investigated. All samples crystallize in rhombohedral system (R¯3 C). The unit cell dimensions increases to a maximum whereas grain size (67.254 < GS(nm) >59.634) decreases to a minimum for sample x= 0.6. Highest resistivity (ρ) and ac susceptibility (χ) are observed for x= 0.6. Metal- semiconductor transition temperature Tρ decreases gradually with Mg doping, in contrast to ferromagnetic-paramagnetic Curie temperature Tc which shows weak dependence on Mg doping level. Low temperature resistivity showed upturn in the temperature range 28 K- 37 K. At high temperatures, variable range hopping conduction predominates with density of states N(Ef) ~3x10¹⁹− 6x10¹⁹ (eV⁻¹cm⁻³). Low temperature resistivity follows ferrometallic nano-particle relation: ρ = ρo + ρ1T² + ρ2T4.5. AC susceptibility of x= 0.0 and 0.6 fit well to Curie law, samples x= 0.2, 0.4, 0.8 and 1.0 are better described in the framework of Neel ferrimagnetism. Transport and magnetic susceptibility results are interpreted in terms of core-shell model where ferromagnetic core interacts anti-ferro-magnetically with ferromagnetic shell.
... The electrical behavior of our sample, in the semiconductor region can explained using two different models viz., SPH model and VRH model. 32 The high temperature region above q D /2 (where q D is Debye's temperature 33 ) is analyzed using the SPH model, whereas below q D /2 and above T M-SC , it is analyzed using VRH model. ...
We investigated the magneto-electrical properties of a La0.8Ba0.1Ca0.1MnO3 polycrystalline sample, prepared by the polymerization-complex sol–gel method. Comparison of experimental data with the theoretical models showed that in the metal-ferromagnetic region, the electrical behavior of the sample is quite well described by a theory based on electron–electron, electron–phonon and electron–magnon scattering and Kondo-like spin dependent scattering. For the high temperature paramagnetic insulating regime, the adiabatic small polaron hopping model was found to fit well with the experimental curves. The estimated critical exponents, obtained from resistivity, were (β = 0.516 ± 0.013, γ = 1.181 ± 0.005 and α = 0.004). They were very close to those predicted by the mean-field model. These results were in good agreement with the analysis of the critical exponents from magnetization measurements.
... The electrical resistivity in the semiconductor region can be explained using two different models viz., SPH model and VRH model [46]. The high temperature region above θ D /2 (where θ D is Debye's temperature [47]) is analyzed using the SPH model, whereas below θ D /2 it is analyzed using VRH model. ...
The La0.8Ba0.1Ca0.1Mn0.85Co0.15O3 sample was prepared by polymerization complex sol-gel method. The structural, magnetocaloric and electrical properties were studied. The sample crystallizes in the orthorhombic structure with Pbnm space group. In addition to the experimental study, theoretical approaches were adopted to investigate the magnetocaloric and electrical behavior of this sample. The important parameters such as maximum entropy change, full width at half maximumimageand relative cooling power (RCP) were explained qualitatively. The analysis of electrical resistivity data shows the semiconductor behavior in all temperatures, which can be explained by small polaron hopping (SPH) and Mott's variable range hopping (VRH) models.
... The conductivity of the mixed valence semiconductors obeys the standard relation with activated mobility: s neeDakT exp ÀE hop akT, where eD/kT is the Einstein diffusion mobility, n the carrier density, D the diffusion constant and E hop the hopping-energy. The resistivity is [28]: r kTane 2 D exp E hop akTX For (T b T c ), we deduce, from the linear part of the curve log(r/T) = f(1/T) (Fig. 6), the values of the hopping energy E hop for all the samples (0 x 0.5). ...
We present the electrical properties of polycrystalline samples Pr1—xSrxMnO3 (0 ≤ x ≤ 0.5). These properties are correlated to the ratio of the Mn3+/Mn4+ cations. The transport properties of the Pr1—xSrxMnO3 samples could be explained on the basis of double exchange mechanism between pairs of Mn3+ and Mn4+ cations, with conductivity occurring by electron hopping between manganese ions along Mn–O–Mn bonds. A semiconductor-to-metal transition is observed as a function of temperature for the mixed valence samples Pr1—xSrxMnO3 (0.25 ≤ x ≤ 0.4) with a metallic-like behaviour above a critical temperature Tϱ. A semiconductor-like one is observed for all the range of temperature (50 to 300 K) for (0 ≤ x ≤ 0.2 and x= 0.5). Below the Curie temperature Tc, for the ferromagnetic regimes, the spin disorder scattering process has been observed to play an important role in resistivity behaviour. The evolution of activated energies with the carrier concentration has been investigated.
... The conductivity of the mixed valence semiconductors obeys the standard relation with activated mobility: s neeDakT exp ÀE hop akT, where eD/kT is the Einstein diffusion mobility, n the carrier density, D the diffusion constant and E hop the hopping-energy. The resistivity is [28]: r kTane 2 D exp E hop akTX For (T b T c ), we deduce, from the linear part of the curve log(r/T) = f(1/T) (Fig. 6), the values of the hopping energy E hop for all the samples (0 x 0.5). ...
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... Several conduction mechanisms are used to explain the electrical transport properties at high temperature above í µí± MI . The most prevalent models are used to describe the conduction mechanisms by Mott's variable range hopping model (í µí± MI < í µí± < í µí¼ í µí°· /2) [38] and another one small polaron hopping model (í µí± > í µí¼ í µí°· /2) where í µí¼ í µí°· is the Debye temperature [39]. Two models are linearly best fitted with experimental results. ...
The structural and transport properties of manganites with and 0.2 prepared by solid state reaction route are studied. These compounds are found to be crystallized in orthorhombic structural form. A shift in the metal-semiconductor/insulator transition temperature () towards room temperature (289 K) with the substitution of Nd by La, as the value of is varied in the sequence (0, 0.1, and 0.2), has been provided. The shift in the , from 239 K (for ) to near the room temperature 289 K (for ), is attributed to the fact that the average radius of site-A increases with the percentage of La. The maximum temperature coefficients of resistance (TCR) of ( and 0.2) are found to be higher compared to its parent compound which is almost independent of . The electrical resistivity of the experimental results is explored by various theoretical models below and above . An appropriate enlightenment for the observed behavior is discussed in detail.
... ii. Thermally activated conduction via polaron hopping given by [35] r ¼ ...
We have applied the pseudo-potential plane wave method to study the structural, elastic, electronic, lattice dynamic and optical properties of GaN and AlN in the wurtzite lattice and BN with zinc-blende structure. We have found that all elastic constants depend strongly on hydrostatic pressure, except for C 44 in wurtzite AlN and GaN that shows a weaker dependence. AlN and GaN present a direct band gap –, whereas BN has an indirect band gap –X . The indirect –K band gap in AlN occurs at about 35 GPa. The top of the valence bands reflects the p electronic character for all structures. There is a gap between optical and acoustic modes only for wurtzite phases AlN and GaN. All peaks in the imaginary part of the dielectric function for the wurtzite lattice GaN and AlN move towards lower energies, while those in the zinc-blende BN structure shift towards higher energies with increasing pressure. The decrease of the static dielectric constant and static refractive index in zinc-blende BN is weaker and it can be explained by its higher elastic constants.
... ii. Thermally activated conduction via polaron hopping given by [35] r ¼ ...
X-ray structure parameters of La0.7Ca0.3Mn0.96(InxAl(1−x))0.04O3 perovskite manganites determined from the Rietveld analysis of the orthorhombic unit cell, space group Pbnm, showed strong dependence on x. Resistivity (ρ) and ac magnetic susceptibility (χ) measurements showed decrease in the metallic-to-semiconductor transition temperature Tρ and the ferromagnetic-to-paramagnetic Curie temperature TC as x increases. The large drop in Tρ and TC for x=0.0 and 1.0 is ascribed to the difference between the electronic configuration (E-factor) and ionic radius (structure factor) of the dopants and the Mn3+ ions. Decrease in Tρ and TC for samples (0.2<x<0.8) is mainly due to the size mismatch between the A and B sites (structure factor).Research highlights► Tρ and TC decreased as In content increased in the (Al/In) doped LCMO manganites. ► Drop in Tρ and TC for x=0 and 1 is ascribed to electronic and structure factors of (Al/In) and Mn3+ ions. ► For x ranging from 0.2 to 0.8 the drop is mainly due to the size mismatch between the A and B sites.
The structural, electrical and magnetoresistance properties of Nd1-xPbxMnO3 with x = 0.1, 0.2, 0.3 & 0.4 have been investigated. All the samples were prepared through the modified sol-gel method. X-ray data were analyzed and found that the samples with x=0.3 & 0.4 show the single-phase nature while the samples with x=0.1 & 0.2 show the presence of minor extra NdMn2O5 phase with orthorhombic symmetry. Scanning electron microscopy (SEM) shows that the variation of grain growth was found with a change in the concentration of Pb. The temperature-dependent resistivity data shows the metal-insulator transition (TMI) with the higher values of Pb content (x=0.3 & 0.4) and TMI shifts to higher temperatures with an applied magnetic field (2T to 8T) excluding the x=0.1 & 0.2 samples. Resistivity data were explored by the different suitable theoretical models below and above TMI. A very high value of MR (nearly 99.47%) was observed for x=0.1 at 10 K under the applied magnetic field of 80 kOe. A sample with x=0.3 also shows a higher MR of about 96.93% at 200 K with the applied magnetic field of 80 kOe.
Magnetism in ionic compounds has been investigated from the earliest decades of the twentieth century, culminating during the middle years with a plethora of research results from a variety of academic, government, and industrial laboratories. Motivation behind these efforts came mainly from two directions: microwave spectroscopy of individual magnetic ions in paramagnetic crystals that served as the foundation of amplification devices, e.g., the maser, that were intended for advanced radar and communications applications, and the multidiscipline investigations of ferrimagnetic oxide materials (ferrites) for high frequency and microwave applications, as well as for the burgeoning field of information storage and the more conventional functions requiring high permeability combined with electrical insulating properties for electromagnet and inductor cores. The scientific endeavors over these years were staggering in scope as is evidenced by the volumes of literature produced during the period following World War II and extending into the 1970s. More recently three advances that involve the transport of charge carriers has re-energized the field of magnetic oxides. Among these initiatives is high-temperature superconductivity, magneto-resistance, and polarized-spin transport found in select transition-metal oxide compounds that offer the promise of integration for the enabling of future electronic technologies.
Rare earth ceramics, La 0.67Ba 0.33Mn 1-xFe xO 3(x=0, 0.05, 0.1), has been prepared. The saturation magnetization and Curie temperature decrease with increasing Fe content in the compounds La 0.67Ba 0.33Mn 1-xFe xO 3, since Fe does not participate in double exchange interaction and the amount of bond Mn 3+-O 2--Mn 4+ decreases. The ferromagnetic interaction decreases with Fe doping in the compounds La 0.67Ba 0.33Mn 1-xFe xO 3. Fe doping also dereases the transition temperature of the metal-semiconductor transition peak and increases the resistance of La 0.67Ba 0.33Mn 1-xFe xO 3. The colossal magneloresistance(CMR) effect at low temperatures has been improved for doped La 0.67Ba 0.33Mn 1-xFe xO 3. The magnetoresistance ratio [R(6T)-R(0)]/R(0) changes from-40% to -60% with increasing Fe content from x=0 to x=0.1. The results also show that the metal-semiconductor transition peak and the negative CMR effect are well described by the Dionne model.
In the present work, the rare earth oxides La0.65BaxMnO3 with Ba vacancies were prepared. With increasing Ba vacancies in La0.65BaxMnO3, the crystal structure transforms from the higher symmetry to the lower. The crystal structure of oxides for x>0.30 are cubic and becomes rhombohedral for x≤O.25. In La0.65BaxMnO3, with decreasing Ba content x from 0.35 to O.33, there is a peak for Curie temperature and a valley for room temperature resistivity. With further decreasing Ba content (x≤O.30), Curie temperature decreases and the room temperature increases. Analysis based on the polaron resistance model shows that the electron mobility is in the order of x=0.35>x=0.25>x=0.20. This is consistent with the content x dependence of the room temperature resistivity. Based on the Dionne model, we calculated the temperature dependence of the resistivity for La0.65BaxMnO3 (x=0.35, 0.25, 0.20). The calculated results agree with the experimental data quite well. Our results also show that effect of lattice, electron-phonon interaction and carriers-lattice (defect) coupling must be considered in the explanation of the electronic transport and colossal magnetoresistance effect.
In the previous chapters, the origins of spontaneous magnetism for parallel (ferromagnetism) and antiparallel spin alignments (antiferromagnetism) have been reviewed. In their pristine forms, the former occurs through direct exchange in metals and alloys, and the latter in nonmetallic ionic compounds comprising oxygen or other elements from the right-hand side of the Periodic table as the anion lattice. Utilitarian applications of ferromagnets are self-evident to even the most casual observer of physical phenomena, but the situation is much less so in the case of antiferromagnetism. For the most part, antiferromagnetism has been a portal to fundamental research in materials, particularly involving the diagnostic methods of neutron and more recently, muon diffraction and scattering.
There are, however, select groups of transition-metal oxides that combine the magnetic properties of ferromagnetic metals with the electrically insulating characteristics of the antiferromagnetic compounds described in the previous section. These magnetic insulators are termed ferrimagnets, and the phenomenon that characterizes their magnetic properties is called ferrimagnetism. Ferrimagnetic oxides have also served as rich sources of knowledge about the fundamental physics of materials, but unlike the antiferromagnetic oxides, they continue to add to their already widespread uses in modern electronics technology. For these reasons, the properties of ferrites, as they are commonly designated, will be treated generously for the remainder of this book.
Electrical conductivity in solids is traditionally associated with collective electron metals, intermetallic compounds, and metal alloys. With outer shell s and p electrons unbound in the sense of a Sommerfeld gas, the analysis of electrical properties usually takes the form of a density of states calculation from a theory that assumes a periodic lattice potential and applies Fermi statistics. The exercise leads to the creation of broadened energy states (bands) and the definition of a Fermi level to serve as the zero energy reference. For transition metals with unfilled “inner” d-shells, complications arise from the hybridization of “free” electron s states with states of unpaired spins in the d shell that forms the collective electron band structure. The first successful attempt to model ferromagnetism in collective electron metals was developed by Stoner employing a phenomenological theory that explained the net magnetization by introducing an exchange field that separated the d-spin populations into up (α) and down (β) bands [1], analogous to the Hund’s rule sorting of spins in an individual ion, illustrated by Figs. 2.3 and 2.19. In subsequent years, great strides were made in formalizing these concepts with the help of quantum theory and other aspects of modern physics [2].
In the previous chapters, the emphasis is placed on the electronic origins of local and collective molecular magnetism in transition-metal oxides and their behavior in alternating magnetic fields. Models of magnetic resonance based on precessing magnetic moments provide a classical analog to quantum mechanical transitions provided that the internal magnetic fields are large enough to produce the Zeeman energy splittings for the particular frequency of interest. In the energy range that can be easily reached by fields from laboratory electromagnets, electron paramagnetic resonance (EPR) and ferromagnetic resonance (FMR) occur in the microwave bands. However, resonances can also occur in magnetically ordered systems at the energies of magnetic exchange. Since the exchange effects occur in the submillimeter and far-infrared bands, but have the properties of a magnetic-dipole stabilization, this topic will serve as a transition to the subject of magneto-optics that is based on magnetically polarized electric-dipole interactions with optical waves.
The structure and magnetic behavior of Cu-substituted Sr2+Ti4+O3-δ (STCu) thin films with ∼22% Cu on the Ti sites, grown on CeO2/yttria stabilized zirconia buffered (001) Si substrates were investigated. STCu films grew with (110) orientation or with a mixed (001)+(110) orientation and were under in-plane compressive strain. They showed weak room temperature ferromagnetism with a saturation magnetization of 0.3 ∼ 1.7 emu/cm3. Cu ions in STCu grown in an oxygen atmosphere existed mainly as Cu3+ with a small proportion of Cu2+, while in STCu grown in vacuum, Cu1+ ions were dominant with small amounts of Cu2+ and Cu3+. The saturation magnetization was attributed to the Cu2+ (3d9, S = 1/2) concentration, and the majority of Cu3+ (3d8, S = 0) ions were stabilized in the low-spin diamagnetic electron configuration similar to that found in mixed-valence layered perovskite superconductor compounds.
The interpretation of the published electrical transport data of the
group IVB transition-metal pentatellurides, ZrTe5 and
HfTe5, have been reconsidered. These materials evince a
strong resistivity peak at a temperature where the Hall effect and the
thermopower manifest a change in sign. Inspired by recent investigations
of the ``metal-insulator transition'' in rare-earth manganates, the
anomalous behavior of the pentatellurides have been tentatively
interpreted as consequences of polaronic transport.
A systematic investigation of photoinduced properties is carried out in La0.67Ca0.33MnO3 film prepared on LaAlO3 (100) substrate by magnetron sputtering method. At T 270 K, the resistivity of film induced by laser increases because of the demagnetization effect of manganites. The photoinduced
relaxation character of film indicates that the time constant increases with increasing temperature, which is attributed to
the growing thermal fluctuation. After laser irradiation, the resistivity returns to the original value and the relaxation
time seems to be independent of temperature. In insulating state, laser irradiation induces the reduction in resistivity of
film due to the excitation of small polarons.
Mixed-valence manganese oxides (R1-xAx)MnO 3 (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 th 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, the the properties of related ferromagnetic oxides are also discussed.
The (Ba0.8K0.2)(Ti0.3Fe0.7)O3 ceramic was prepared by solid-state reaction, and post-annealed in oxygen ambient. By comparison with Ba(Ti0.3Fe0.7)O3 made under identical conditions, the effect of non-isovalent A-site substitution of K+ on microstructure and magnetism of as-prepared and annealed Ba(Ti0.3Fe0.7)O3 samples was investigated using X-ray diffraction, Mössbauer spectroscopy, vibrating sample magnetometer and iodometric titration. It is found that all samples have a single 6H-BaTiO3-type hexagonal perovskite structure without any impurities detected, regardless of A-site K+ substitution or annealing. In the as-prepared state, non-isovalent A-site substitution of K+ induces the variation in Fe occupational site, resulting in the disappearance of room-temperature ferromagnetism. The super-exchange interactions of Fe3+ at tetrahedral and octahedral Ti sites determine the paramagnetism of (Ba0.8K0.2)(Ti0.3Fe0.7)O3. During the O2 annealing process, the presence of Fe4+, an unusual valence for iron, besides Fe3+ is observed, both distributed over octahedral Ti site. By A-site substitution of K+ with a lower valence than Ba2+, the charge compensation mechanism is further enhanced, and thus more Fe3+ ions are oxidized to Fe4+ in annealed (Ba0.8K0.2)(Ti0.3Fe0.7)O3. Consequently, the ferromagnetic Fe4+–O2−–Fe4+ super-exchange interactions are strengthened, which leads not only to a paramagnetism–ferromagnetism transition but also to a higher saturation magnetization compared with annealed Ba(Ti0.3Fe0.7)O3.
The photo-induced resistance change has been observed in the La2/3(Ca1–xSrx)1/3MnO3 films with x = 0, 1/3 and 2/3 prepared by RF magnetron sputtering on the LaAlO3 substrate. The photo-induced relative change in resistance increases initially with increasing temperature, followed by a decrease. The maximum values of the relative change in resistance decrease from 43.5 to 27.2% with increasing x, and the corresponding temperatures increase from 252 to 317 K. The intriguing mechanism of the photo-induced character and the effect of the Sr content in the perovskite films are discussed.
SrTi1-xMxO3 films in which M = Fe, Co, or Cr and x=0.04-0.5 have been grown epitaxially on CeO2-buffered Si, SrTiO3, and LaAlO3 substrates. Films grown in vacuum containing Fe or Co typically showed a strong uniaxial magnetic anisotropy in the out-of-plane direction, a saturation moment on the order of 0.5muB/Fe or Co, and a magnetization that persists to temperatures of order 1000 K. In contrast, films containing Cr showed no evidence of a spontaneous magnetic moment. The films are typically in a high in-plane compressive strain state due to epitaxial growth. The results are discussed in terms of the magnetoelastic effects from specific ionic valence states, and the magnetization versus temperature curves are fitted to a model for magnetoelastic spin ordering.
BiFeO3 and Bi2FeMnO6 films were epitaxially grown on SrTiO3 (001) substrates by pulsed-laser deposition, and their structural, magnetic, magneto-optical and optical properties were measured. In Bi2FeMnO6 , Fe is mainly present in the 3+ valence state, while Mn shows multivalence states. Bi2FeMnO6 exhibits low magnetization at room temperature and at 5 K indicating there is no significant B -site ordering. The BiFeO3 film shows high optical transparency, while Bi2FeMnO6 shows high absorption loss in the infrared. Density-functional theory modeling of BiFeO3 , BiMnO3 and Bi2FeMnO6 was carried out by applying the generalized gradient approximation (GGA) and GGA+U methods. The formation enthalpy of ordered Bi2FeMnO6 is positive for several crystal symmetries and for ferromagnetic (FM) or antiferromagnetic (AFM) spin structures at 0 K temperature, indicating B -site ordering is not favored. The electronic structure calculations are consistent with the electronic and optical properties of these films.
The giant negative magnetoresistance in manganites has been investigated from the Curie temperature Tc up to 600 K (2.6 Tc) in magnetic fields up to 8 T. Nonadiabatic small polaron hopping can successfully describe the temperature dependence of the resistivity. The magnetic field influence on the activation energy is explained by the interaction of unclustered ions with small spin clusters of four ions.
Measurements of magnetization and magnetoresistance have been carried out for crystals of (La1 - yNdy)(1/2)Sr3/2MnO4 (y = 0, 0.5, 0.75 and 1) in pulsed magnetic fields up to 40 T. The enormous changes of resistivity, which are more than three orders of magnitude at T < 100 K, enable us to clarify the functional formula of resistivity vs. magnetization. We will compare these experimental results with various models which have been proposed to explain the colossal magnetoresistance in manganites up to the present.
The use of computer aided design (CAD) for microstrip circulators is discussed. Manganese (MnFe2O4) and manganese/magnesium (MgMn)Fe2O4) systems are related to the Ni ferrite spinels, but with octahedral-tetrahedral exchange coupling much reduced. The tapes of a single-component ferrite material are suitable for low-cost fabrication for some circulator configurations but others require multiple ferrite compositions. Highly anisotropic aligned uniaxial hexagonal ferrites provide the desired permanent magnetization and can be incorporated into the microwave circuit substrate to realize compact, internally biased configurations. Hexagonal ferrite material should have a large coercive field in order to maintain significant magnetization. Meshing for finite element processing is chosen by specifying an initial minimum extent grid which uniquely defines the regions of ferrite and air.
Ba(Ti0.65M0.05Fe0.3)O3 (M=Cr, Mn and Ni) ceramics were prepared by solid-state reaction, and post-annealed in vacuum and oxygen, respectively. Mössbauer spectroscopy was used to investigate the local environments of Fe atoms. Combined with the configuration of the other transition metal M, the mechanisms responsible for magnetism were discussed for different samples. The results show that all samples are single-phase with a 6H–BaTiO3-type hexagonal perovskite structure. The as-prepared ceramics are all paramagnetic, while the annealed ones are all ferromagnetic at room temperature. The origin of ferromagnetism changes with doping element and annealing atmosphere, including the double-exchange mechanism of transition metal M with mixed valences, the super-exchange mechanism of Fe3+ in different occupational sites, and the super-exchange mechanism of Fe4+–O2−–Fe4+.
Structural, magnetic and electrical properties of Pr deficient Pr0.8−x□xSr0.2MnO3 (0≤x≤0.2) samples have been investigated. Powder X-ray diffraction patterns are indexed in the orthorhombic perovskite system with Pnma space group. The unit cell volume decreases with increasing vacancy contents. The investigated samples exhibit a ferromagnetic to paramagnetic transition with increasing temperature. The highest obtained value of the Curie temperature is about 190 K. The presence of vacancies in structure increases the Curie temperature. At low vacancy concentrations a semiconductor like behaviour was observed. However, for x≥0.15, the samples exhibit a semiconducting to metallic transition with decreasing temperature.
A systematic study of the structural, magnetic and electrical properties of the manganites Pr1-xSrxMnO3(0≤ x ≤ 0.5) has been carried out. X-ray diffraction investigation shows a structural change with composition, from orthorhombic (0 ≤ x ≤ 0.2) to rhombohedral (0.25 ≤ x ≤ 0.5). The magnetic properties of Pr1-xSrxMnO3 samples could be explained on the basis of a double exchange mechanism between pairs of Mn and Mn ions. These properties are strongly dependent on the ratio of Mn/Mn. The maximum of the ferromagnetic transition temperature Tc is reached at x ≈ 0.35 corresponding to a value 1.85 of this ratio. The investigation of the electrical properties shows a semiconductor to metal transition as a function of temperature (0.25≤x≤0.4) with a metallic-like behaviour above a critical temperature Tp. A semiconducting-like one is observed for all the range of temperature (50–300 K) for (0 ≤ x ≤ 0.2 and x = 0.5). The evolution of activated energies with the carrier concentration has been investigated.
We present a detailed study of the activated resistivity of La0.67Ca0.33MnO3 films up to 600 K under the influence of high magnetic fields. Data in zero field can be explained by small polaron hopping as treated in the Friedman-Holstein theory. Based on the spin orientation of ferromagnetic clusters in a magnetic field, we develop a phenomenological model describing the temperature and field dependence of the resistivity with a minimum of free parameters. We find that the polarons have a magnetic contribution to their activation energy for hopping which depends on the variation of the spin order with increasing temperature and can be modified by applied magnetic fields. The average magnetic clusters contain 4–6 ions. Hall measurements show a temperature dependent electronlike linear low field slope predicted by the theory of the Hall mobility of small polarons.
The highly Mn+4-doped compound La0.35Ca0.65MnO3 has been studied up to high temperature (700 K) by using thermal-expansion, magnetostriction, magnetoresistance, and neutron-diffraction techniques. From 700 K down to room temperature the electrical conduction takes place through thermally activated hopping of polarons with Ehop=45 meV. At the charge-ordering (CO) transition temperature TCO=275 K, pronounced anomalies in the resistivity and the lattice are observed. The neutron thermodiffractogram clearly establishes that the CO state occurs in the paramagnetic (P) phase and is accompanied by a large anisotropic lattice distortion with a simultaneous large distortion of the MnO6 octahedra. The antiferromagnetic (AF) phase appears at TN=160±3 K. At this temperature no lattice effect is observed. The CO-P and the CO-AF ground states are stable under an applied magnetic field up to 12 T.
We report anisotropic resistivity measurements on a La1.2Sr1.8Mn2O7 single crystal over a temperature T range from 2 to 400 K and in magnetic-fields H up to 14 T. For T>~218K, the temperature dependence of the zero-field in-plane resistivity ρab(T) obeys the adiabatic small polaron hopping mechanism, while the out-of-plane resistivity ρc(T) can be ascribed by an Arrhenius law with the same activation energy. Considering the magnetic character of the polarons and the close correlation between resistivity and magnetization, we developed a model which allows the determination of ρab,c(H,T). The excellent agreement of the calculations with the measurements indicates that small polarons play an essential role in the electrical transport properties in the paramagnetic phase of bilayer manganites.
The effect of Cu-doping at Mn-site on structural, magnetic and transport properties in electron-doped manganites La0.85Te0.15Mn1−xCuxO3 (0⩽x⩽0.20) has been investigated. Based on the analysis of structural parameter variations, the valence state of the Cu ion in Cu-doped manganites is suggested to be +2. All samples undergo the paramagnetic-ferromagnetic (PM-FM) phase transition. The Curie temperature Tc decreases and the transition becomes broader with increasing Cu-doping level, in contrast, the magnetization magnitude of Cu-doping samples at low temperatures increase as x⩽0.15. The insulator-metal (I-M) transition moves to lower temperatures with increasing Cu-doping content and disappears as x>0.1. In addition, the higher temperature resistivity ρ peak in double-peak-like ρ(T) curves observed in no Cu-doping sample is completely suppressed as Cu-doping level x=0.1 and ρ(T) curve only shows single I-M transition at the low temperature well below TC. The results are discussed according to the change of magnetic exchange interaction caused by Cu-doping.
We fabricated ∼200 nm thin (001)-oriented films of the ferromagnetic metallic perovskite La2/3Ca1/3MnO3 on single crystal (001)-SrTiO3 substrates by dc-sputtering at high oxygen pressure. The samples feature a Curie temperature TC∼260 K and a magnetic moment μ(T→0 K)∼3 μB per Mn atom. The magnetization loops are nearly square-shaped with a coercive field Hc(5 K)=0.03 T that decreases linearly in temperature down to TC. At low temperature, the magnetization shows a decrease ΔM∼T2 and the resistivity an increase Δρ∼T2 as is expected for itinerant electron ferromagnets where single particle excitations dominate. As a further indication of the high quality of the samples, the resistivity without external magnetic field peaks right at TC with a maximum value ρ(H=0,T=TC) of only ∼2 mΩ cm. The magnetoresistance ratio Δρ/ρ0=[ρ(H,T)−ρ(H=0,T)]/ρ(H=0,T) also reaches its maximum value at TC, with Δρ/ρ0(H=6 T,T=TC)∼−50%. The complete absence of this colossal magnetoresistance effect in the low-temperature limit is a further indication of the high sample quality which may be due to a high degree of oxygenation as a result of our preparation condictions. The relation ρ(H,T)=ρmexp[−M(H,T)/M0] observed below TC as well as the temperature characteristic of the resistivity above TC can both be explained by a simple magnetic polaron hopping tunneling model where the tunneling barrier height depends in a simple way on the relative orientation of the local magnetization at both ends of the hopping path. However, here it must be considered that the polarons completely change their character at TC from large and delocalized to small and highly localized entities. © 2004 American Institute of Physics.
The behavior of temperature dependences of electrical resistance and magnetoresistance of polycrystalline substituted lanthanum manganite (La0.5Eu0.5)0.7Pb0.3MnO3 at low temperatures was thoroughly studied. A broad hysteresis was found in the field dependences of electrical resistance in the low-temperature region. Above 40 K, no hysteresis feature was observed. The temperature T = 40 K corresponds to the temperature of minimum electrical resistance and the temperature TN to the antiferromagnet–paramagnet phase transition of the material of the intergrain boundaries. In this work we propose a model which explains the observed features of the ρ(T) and ρ(H) curves at temperatures below TN by the formation of a network of ferromagnet-antiferromagnet-ferromagnet tunnel contacts.
Strain effect on La0.67Ca0.33MnO3 and La0.8Ba0.2MnO3 films is investigated by x-ray diffraction and x-ray absorption spectrums. Findings show that due to the different ionic sizes of doped Ca or Ba ions, the strain effect acts differently in the way it deforms. The interfacial strain effect produces opposite influences on the lattice symmetry, the average Mn–O bond lengths, the average oxygen disorders, the coupling symmetries inside and in the vicinity of the MnO6 octahedrons, as well as producing an opposing trend in metal-insulator and magnetic transition temperatures of the strained La0.67Ca0.33MnO3 and La0.8Ba0.2MnO3 films.
This letter is on the observation of anisotropic electrical transport and magnetoresistance (MR) in rf sputtered thin films of La0.62Bi0.05Ca0.33MnO3 on Si. Films sputtered on (111)Si are polycrystalline, whereas textured films with (110) orientation are obtained from (100) Si. Both films show a reduction in the lattice constant and relatively high Curie temperatures (262–266 K vs 230 K for bulk polycrystals). Although the films have identical magnetic parameters, resistivity ρ and MR data for the (110) textured films show the following features indicative of possible dependence of magneto transport on crystallographic orientation: (i) a two order of magnitude enhancement in the low temperature ρ value compared to polycrystalline films, (ii) a semiconductorlike electrical conduction and the absence of any metallic to semiconductor transition, and (iii) a relatively large MR, as high as 80% at 40 kOe. © 1997 American Institute of Physics.
The effect of elemental substitutions on the properties of the
ferromagnetic, conducting, highly magnetoresistive compound
La2/3Ca1/3MnO3 has been studied. The
results of Co doping and Ni doping on the Mn site and Gd doping on the
La site are reported. These compounds were investigated by x-ray
diffraction, magnetization measurements, resistivity measurements,
thermopower measurements, and by paramagnetic resonance. The result of
replacing La by Gd atoms is to lower the ferromagnetic (or
metal-insulator) transition temperature, an effect which is shown to be
due to bond bending caused by the lattice adjusting to the size
differential between the La and Gd ions. On the other hand, the
reduction of the magnetic transition temperature when Mn ions are
replaced with Co or Ni ions is not attributed to changes in the size of
the ions. Instead, we ascribe the lowering of the ferromagnetic
transition temperature to a weakening of the double-exchange interaction
between two unlike ions. The resistivity and Seebeck coefficient in
these materials have been investigated as a function of elemental
substitution. The magnetic polaron theory of Zhang is used,
phenomenologically, to provide a quantitative explanation of these
transport properties. In addition, the effect of these elemental
substitutions on the linewidths of the paramagnetic resonances is
studied and is discussed in terms of exchange narrowing and spin-lattice
relaxation.
Strontium substituted lanthanum manganite La0.5Sr0.5MnO3 powder has been synthesized by solid state reaction method at 1673 K. X-ray diffraction shows that the material exists in a single phase having a perovskite-like structure. Some physical properties such as magnetization versus temperature and electrical transport measurements have been investigated. The material is ferromagnetic with Curie temperature Tc = 370 K. The zero-field resistivity has a semiconducting behavior in all the range 4–300 K according to Mott's law. We have explored the magnetoresistance effect at 40 and 300 K under a field up to 54 kOe.
The non-stoichiometric solution Ln1−xSrxMnO3-δ□δ was prepared by the classic ceramic method. The physical properties as crystallographic, magnetic and electrical properties were studied. A structural phase transition from rhombohedral to orthorhombic was observed at a concentration of Mn between 10% and 15% per Mn atom in the unit formula. The magnetic properties are very sensitive to the presence of vacancies at the oxygen sites. The non-stoichiometric, samples change from metallic to insulating behaviour depending on their vacancy concentration. In the semiconductor phase, the activation energy value changes with the structural phase, increasing in the rhombohedral phase and decreasing in the orthorhombic phase.
Nonstoichiometric substituted lanthanum manganite powder has been synthesized by solid state reaction method. X-ray diffraction shows that the material exists in single phase having a perovskite structure. A structural phase transition separating a rhombohedral phase and an orthorhombic phase was observed at the rate δ ≈ 0.127 corresponding to a Mn concentration of about 85 %. The material is ferromagnetic for 0 ≤ δ ≤ 0.127 and antiferromagnetic for 0.15 ≤ δ ≤ 0.20 The Curie temperature is very sensitive to the presence of vacancies. The investigation of electrical properties shows metallic behavior at low temperature for δ ≤ 0.10 and as insulator for 0.127 ≤ δ ≤ 0.20. The variation of activation energies with the rate of vacancies has been investigated.
We report the electron transport studies in the rhombohedral LaMn1-xAlxO3+δ series (0≤x≤20%) of samples. Though all the samples are ferromagnetic below a transition temperature (TC), only the samples with x≤5% exhibit a metal-insulator transition (MIT) at TC while the samples with x≥10% are all semiconducting above and below TC. The sample with x = 7.5% is a borderline case where MIT at TC is immediately taken over by semiconducting behaviour at lower temperatures. Thus a progressive crossover from ferromagnetic-metallic state to ferromagnetic-insulating state is observed in this series within the same structure. This is accompanied by a decrease in Mn4+ content across the magnetic and conduction percolation thresholds. This systematic crossover from a double exchange dominated regime to an exclusively superexchange regime, preserving the rhombohedral symmetry, makes the present series an important and unique one for the study of electron transport in the colossal magnetoresistance manganites. An effective medium approach is employed to explain the resistivity behaviour in this series over the whole temperature range, which gives strong support for polaronic conduction in all the samples. This polaronic conduction justifies the presence of dynamic Jahn-Teller effects in the samples and the change in the character of polarons across TC illustrates the role of electron-lattice interaction as well as its coupling to magnetic states (core spins) of the samples. The dominance of double exchange is evident from metallic resistivity in the samples with x≤5% which have Mn4+ much above the percolation threshold for metallicity. However the observed non-trivial temperature dependence of metallic resistivity could not be explained by double exchange alone and it is shown in this study that one needs to take into account superexchange interactions even in the double exchange dominated regime to understand the electron transport, thus supporting the current understanding in these pervoskite manganites.
La0.67Sr0.33Mn0.95Zn0.05O3 film has been deposited on LaAlO3 (100) substrate by the magnetron sputtering method. The results concerning the photoinduced character of the film are reported. In the ferromagnetic metallic state, the increasing photoinduced resistance is caused by photoinduced demagnetization of the spin correlation system in manganites. The photoinduced relaxation character of the film indicates that the time constant increases with increasing temperature, which is attributed to the thermal fluctuation.
Magnetically doped SnO 2 is a promising dilute magnetic semiconductor and may also be applicable in a variety of magneto-optical applications. Epitaxial films of Sn 1−x Co x O 2 x 0.2 and Sn 1−x Fe x O 2 x 0.4 were grown by pulsed-laser deposition on R-plane Al 2 O 3 substrates. Structural, magnetic, and magnetotransport measurements consistently point to a source of magnetism within the host lattice rather than from an impurity phase. The films are strained and their magnetic anisotropy is consistent with the presence of substantial amounts of magnetoelastic high-spin Fe 2+ , or high-or low-spin Co 2+ . Sn 0.9 Co 0.1 O 2 films have a reasonably high Faraday rotation of 570°/cm, and the refractive index n and extinction coefficient k at 1550 nm wave-length are 1.957 and 0.0102, respectively.
In this article we show magnetotransport of two prototypical (nearly) half metallic perowskites La2/3Ca1/3MnO3 and Sr2FeMoO6. In a half metal the spin polarisation at the Fermi energy is complete and tunneling magnetoresistive devices of high sensitivity
can be realized with small external magnetic fields. In the vicinity of the metal-insulator phase transition (MIT) temperature
of the manganite an external magnetic field can induce ’colossal’ magnetoresistive effects. In the simple perovskites La1-x
Ca
x
MnO3 the charge transport above the MIT is of polaronic nature. Hall-effect measurements on the compound La0.67Ca0.33MnO3 below the MIT show a compensated Fermi-surface consisting of electron and hole contributions and an unusual quadratic temperature
dependence of the resistivity. In Sr2FeMoO6 we found a clear correlation between electrical conductivity and ordered occupation of the Fe, Mo atomic positions. While
the low temperature ferrimagnetic phase shows similar transport the high temperature phase stays metallic.
A study of the effect of Li substitution on the electrical properties of the oxides MnO, CoO, NiO, and CuO has been made. It has been found that (1) the activation energy for conduction increases abruptly near the antiferromagnetic Curie point; (2) as Li is added the activation energy and resistivity initially decrease rapidly and then level off in the neighborhood of 2% substituted Li; (3) this initial decrease becomes progressively greater in going from MnO to CuO in the sequence of the periodic table. A rather complete understanding of these materials can be had by treating the conduction process as a thermally activated diffusion of positive holes which are trapped by the local strain induced by their own polarization field. The magnitude of the activation energy will be related to the work done against elastic forces necessary to reduce the local strain at a positive hole to zero.
The resistivity-versus-temperature behavior of high-T(c) superconductors in the normal state is modeled according to the electron hopping theory for mixed-valence metal oxides. The study focuses on the interpretation of a resistivity minimum in the temperature regime near T(c), based on the decrease of activation energies E(hop) with increases in magnetic dilution reported in NiO and CuO systems. It is shown that at temperatures above T(c), the behavior follows a rising linear curve (metallic slope); for single-phase specimens, the slope and the intercept of the asymptote are related to E(hop) and the carrier concentration.
A new copper (+III) compound SrLaCuO4 has been prepared under oxygen pressure and investigated. It has the K2NiF4 structure (a = 3.765 ± 0.002 Å c = 13.27 ± 0.02 Å). A comparison with La2Li0.50−Cu0.50O4 suggests a low-spin d8 state for copper +III, but with associated Cu
A large magnetoresistance change (ΔR/RH) of −550% has been observed at 270 K in (La0.8Ca0.2)MnO3 thin films. The films were prepared in situ on LaAlO3 substrates by single-liquid-source metal-organic chemical vapor deposition. M(thd)n (M = La, Ca, and Mn, and n = 2, 3) were dissolved together in an organic solution and used as precursors for the deposition of (La0.8Ca0.2)MnO3 thin films. Deposition was conducted at an oxygen partial pressure of 1.2 Torr and a substrate temperature ranging from 600 °C to 700 °C. The mechanism for the large magnetoresistance change in this manganese oxide is briefly discussed.
The breakdown of antiferromagnetic order and its coincidence with the onset of the superconducting state (the T N , T c →0 condition) is explained by the itinerancy of polarons that are created by ‘‘impurity’’ cations or by oxygen vacancies. In both La 2-x Sr x CuO 4 and YBa 2 Cu 3 O y systems, the electrostatic balance is maintained in a traditional sense by the formation of Cu<sup>3+</sup>(d<sup>8</sup>) ions, here in low‐spin (S=0) electron configurations because of tetragonal crystal fields at the Cu sites. Since the spins of the host Cu<sup>2+</sup>(d<sup>9</sup>) ions (S=1/2) surrounding the diamagnetic Cu<sup>3+</sup> p‐type polaron ion will undergo canting that results from the magnetic dilution effects, a dynamic region of frustration will exist within the confines of the polaron cell boundaries. As cells begin to merge through increased polaron concentration, percolation of cells will destroy magnetic order at a threshold determined by the product of cell radius and carrier concentration. Analysis of the cell radii indicate that frustration should set in at a Cu<sup>3+</sup> concentration (x≊0.07) that is consistent with the T N , T c →0 condition observed in both of the above superconductor families. From these concepts, a rationale emerges for a superconductor phase diagram that links the magnetic, insulating, superconducting, and normal metallic regions as functions of temperature and composition.
Various manganites of the general formula La3+Mn3+O32−-Me2+Mn4+O32− have been prepared in the form of polycrystalline products. Perovskite structures were found, i.a. for all mixed crystals LaMnO3CaMnO3, for LaMnO3SrMnO3 containing up to 70% SrMnO3, and for LaMnO3BaMnO3 containing less than 50% BaMnO3. The mixed crystals with perovskite structure are ferromagnetic. Curves for the Curie temperature versus composition and saturation versus composition are given for LaMnO3CaMnO3, LaMnO3SrMnO3, and LaMnO3BaMnO3. Both types of curves show maxima between 25 and 40% Me2+Mn4+O32−; here all 3d electrons available contribute with their spins to the saturation magnetization. The ferromagnetic properties can be understood as the result of a strong positive Mn3+Mn4+ exchange interaction combined with a weak Mn3+Mn3+ interaction and a negative Mn4+Mn4+ interaction. The Mn3+Mn4+ interaction, presumably of the indirect exchange type, is thought to be the first clear example of positive exchange interaction in oxidic substances.
An investigation has been made of the relationship between crystallographic
symmetry and Mn3+ - O2- - Mn3+ 180 superexchange interactions in
several perovskite systems. In particular, crystallographic and magnetic
measurements have been made on a number of samples in the systems
La(Mn1-xMx)O3+ delta , where M=Ga, Co, Ni. In all three systems,
the Mn3+ - O2- - Mn3+ interactions are found to be ferromagnetic
for O-orthorhombic samples having a<c / sqrt[2]<b. For x<0.5 in the
system M=Ga, there is O'-orthorhombic symmetry (c / sqrt[2]<a<b)
and ferrimagnetism that is suggestive of anisotropic Mn3+ - O2- -
Mn3+ interactions, similar to those found in LaMnO3, and preferential
ordering of the Ga3+ into one magnetic sublattice. Measurements of
Curie temperature vs composition in this system support ordering
of the gallium in the compositional range x <= 0.4, partial ordering
in the range 0.4<x <= 0.6. These observations are consistent with
the magnetic measurements of various other workers on the systems
(La, M'2+)MnO3+ delta , La(Mn,Cr)O3 (La,Ba) (Mn,Ti)O3.
The ferromagnetic Mn3+ - anion - Mn3+ interactions that occur in the
perovskites with O-orthorhombic or rhombohedral symmetry and in the
NiAs-type compounds cannot be accounted for by present superexchange
theory if the electron configuration about a Mn3+ ion is assumed
fixed with one electron arithmetically averaged over the two eg orbitals,
or if static, local distortions are randomly distributed through
the structure. It is pointed out that Jahn-Teller electronic ordering
is fast relative to the atomic vibrations so that there is strong
coupling of the vibrational modes and the eg-electron configuration.
This means that the electron configuration that is used in the superexchange
calculation must be correlated with the vibrational modes. If this
is done, a ferromagnetic Mn3+ - anion - Mn3+ interaction follows
from the superexchange theory.
A negative isotropic magnetoresistance effect more than three orders of magnitude larger than the typical giant magnetoresistance
of some superlattice films has been observed in thin oxide films of perovskite-like La0.67Ca0.33MnOx. Epitaxial films that are grown on LaAIO3 substrates by laser ablation and suitably heat treated exhibit magnetoresistance values as high as 127,000 percent near 77
kelvin and ∼1300 percent near room temperature. Such a phenomenon could be useful for various magnetic and electric device
applications if the observed effects of material processing are optimized. Possible mechanisms for the observed effect are
discussed.