Manganite Compounds Research Articles

ESR study of Sr‐doped LaCoO3 cobaltites

AbstractThe electron spin resonance (ESR) method was used to investigate the internal spin dynamics of La1–xSrxCoO3 (x = 0.1, 0.2, 0.3) compounds in the temperature range 170 to ∼500 K. ESR spectra of the samples over the whole temperature range studied could be divided into three regions corresponding to different linewidths. This is due to the presence of an anisotropic field arising from ferromagnetic clusters. This is similar to manganite perovskites. The peak‐to‐peak ESR linewidth vs. temperature, ΔHpp(T), for the samples has a minimum value at a temperature Tmin (in this case Tmin ≈ 1.7TC, whereas Tmin ≈ 1.1TC for manganites). Asymmetric Lorentzian ESR spectra at temperatures T < Tmin become Gaussian at T > Tmin. ESR signals of La1–xSrxCoO3 cobaltites are thermally damped out at ∼460 K, this value being much lower than that for manganite compounds. The explanation of the experimental ΔHpp(T) data in the T > Tmin regime is based on the one‐phonon process and adiabatic hopping motion of small polarons. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Metamagnetic steps in Eu-based manganite compounds

Magnetization and specific heat measurements have been carried out on the ABO3 type half-doped (La0.3Eu0.2)(Ca0.3Sr0.2)MnO3 (LE-50) and electron-doped (La0.2Eu0.25)(Ca0.3Sr0.25)MnO3 (LE-55) compounds. These compounds exhibit a metamagnetic transition at low temperatures. In LE-50, the dominant feature of the metamagnetic transition manifests itself at T≲5K. The transition is sharp and steplike at 2K with a critical field (HC) of 3.3kOe and a transition width of 250Oe becoming broad with a width of ∼10kOe at 3K. Similarly, in the LE-55 sample, the steplike behavior occurs at T≲3K with HC=72kOe, whereas at T>3K, the transition is much broader. A feature consistent with the metamagnetic transition appears in the specific heat data in these compounds. Such metamagnetic steps have been observed prior to now only in Pr-based manganites. Observation of the same in Eu-based manganites suggests that these steps have no influence of any property intrinsic to Pr ions.

Sharp step-like metamagnetic transition in the charge-ordered manganite compound(La0.3Eu0.2)(Ca0.3Sr0.2)MnO3

We have performed transport, magnetization and specific heat measurements on theABO3-type half-doped(La0.3Eu0.2)(Ca0.3Sr0.2)MnO3 (LECSMO)manganite. At T = 2 K, a sharp step-like metamagnetic transition in the magnetization with a critical field of3.3 T is observed. The transition is much sharper than that seen at higher temperatures ( K). Such step-like transitions have previously been reported only in Pr-based manganites,but the present data show that the step-like metamagnetism can also be found in othermanganite compounds.

Low-field magnetoresistance in La0.7Ca0.3MnO3 manganite compounds prepared by the spray drying technique

Calcium-substituted lanthanum manganite compounds were synthesized by the spray drying technique. This method—whose main advantages are versatility, high reproducibility and scalability—yields small grain materials of high homogeneity and displaying low-field magnetoresistance effects. We report about the physical and chemical characterizations of these samples in order to investigate the potential interest of spray drying for the production of materials for low-field magnetoresistance applications. We have studied the dependence of the low-field magnetoresistance on the temperature and duration of the thermal treatment applied to the pelletized powders. The issue of the shape anisotropy (demagnetisation effects) influence on the magnetoresistance properties has also been dealt with.

Transport and Magnetic Properties of Eu and Sr Doped Manganite Compound La0.7Ca0.3MnO3

The effect of simultaneous increase in carrier density and size-disorder on the transport and magnetic properties of La0.7Ca0.3MnO3 has been investigated by studying the (La0.7 − 2x Eu x )(Ca0.3Sr x )MnO3 (0.05 ≤ x ≤ 0.2) (LECSMO) compounds. These compounds have been compared with standard La1 − x Ca x MnO3 (0.3 ≤ x ≤ 0.5) (LCMO) in which carrier density alone varies. In LECSMO, the insulator-metal transition temperature (T p) decreases from 180 K for x = 0.05 to 80 K for x = 0.15 sample vis-a-vis ∼240 K for x = 0.35 to ∼225 K for x = 0.45 in LCMO system. Similarly, the Curie temperature (T C) in LECSMO, decreases from 205 K for x = 0.05 to 75 K for x = 0.2 sample against 240 K for x = 0.35 to ∼220 K for x = 0.45 in LCMO system. Also, in LCMO the T C and T p are coincident whereas, in LECSMO system, with increasing x there is an increasing disparity between the two. At 5 K, in the metallic region, a large MR (>90%) is observed in x = 0.15 sample and is discussed in terms of phase segregation and inter-grain magnetoresistance.

Pressure dependence of the MI-transition temperature under competing double exchange–superexchange interactions close to CDO afm insulating states

The pressure derivative of the order–order transition T 1 and the order–disorder transition at small T c is calculated using a magnetostriction and polaron extended deGennes model in the low x-doped region. According to this model, in the case where there is a semiconducting gap there is no direct pressure dependence of the transition lines but only an indirect one which is connected with the pressure dependence of the transfer integral, even if we have polaronic states. In the case where T 1 and the metal–insulator transition T mi split, there is a direct (quadratic) dependence which is based on a static (polaron) susceptibility of the insulating band edge electronic–phononic states. Pressure experiments on manganite compounds are shown which are consistent with the predictions of the model in detail, e.g. a correlation between d T mi/d p shifts and resistivity changes.

Field-induced avalanche to the ferromagnetic state in the phase-separated ground state of manganites

Perovskite manganite compounds such as Pr_(1-x)(Ca_(1-y)Sr_y)_xMn3 can be tuned to exhibit a metastable ground state where two magnetic/crystallographic phases coexist in zero magnetic field. Field-dependent neutron diffraction measurements on both poly- and single-crystal samples with a range of Pr, Ca, and Sr dopings(0.3<x<0.35 and y<0.30) reveal that the charge-ordered, antiferromagnetic phase of the ground state suddenly and irreversibly jumps to the ferromagnetic state. The transition occurs spontaneously at some time after the field is set above a threshold field, indicating that once the transition is initiated an avalanche occurs that drives it to completion.

Polymer light-emitting diodes with spin-polarised charge injection

This study is concerned with the characterisation of the injection properties of the manganite compound La0.7Sr0.3MnO3 (LSMO) and its use as a spin-polarised anode in polymer light-emitting diodes. Charge carrier injection and electroluminescence in a polymer LED with an LSMO anode is achieved. A dependence of the current–voltage characteristics of LEDs with LSMO anodes on the temperature is observed and compared to the behaviour of LEDs with standard ITO anodes. The temperature dependence is shown to be due to improved hole injection at low temperatures, possibly originating from magnetic ordering upon cooling.

Magnetization steps in the phase-separated Cr-doped Nd0.5Ca0.5MnO3 compounds

The manganite Nd0.5Ca0.5MnO3 has a CE-type antiferromagnetic-insulating (AFI) ground state at low temperatures. This compound undergoes a charge-ordering (CO) transition at TCO∼250 K followed by a paramagnetic to AFI (orbital-ordered) OO phase transition at TN∼170 K. The ferromagnetic (FM) double-exchange interaction between Mn ions is quenched by such a CO effect. Upon Cr doping, as in single crystals of Nd0.5Ca0.5Mn0.96Cr0.04O3, suppression of the quenched CO-OO state occurs at TC∼140 K and is accompanied by a FM-metal-insulator transition. These Cr-doped materials have a low temperature ground state comprised of a mixture of 20–30 nm domains of the FM phase embedded in the CO-OO matrix, as inferred from Lorentz microscopy. Our results show that these Cr-doped materials undergo a field-dependent magnetic transition in a large range of temperature T&amp;lt;50 K, signed by a pronounced magnetization step in M(H) virgin curves. Moreover, from several isothermal hysteresis M(H) curves, we have observed that the FM state remains at H→0 and does not collapse back to the AFI state in M(H) cycles performed up to 18 T and T&amp;lt;50 K. These results are different from those found in La0.5Ca0.5MnO3 and Nd0.5Sr0.5MnO3 compounds, where the induced FM state by H collapses back to AFI at low H. At 1.4 K, the M(H) virgin curves exhibit a sharp ΔH∼0.1 T magnetization step at Hj∼4.3 T, which is followed by saturation of M(H)∼110 emu/g in magnetic fields up to H=18 T. The temperature dependence of Hj was found to obey an exponential decay up to T∼50 K. Such a temperature dependence of Hj indicates that the magnetization steps may not be related to a classical metamagnetic transition, as has been proposed for these manganite compounds.

Synchrotron studies of lattice distortions and lattice modulations in complex oxides

Abstract We review some recent studies of lattice modulations and lattice distortions in single crystals of the layered manganite compound La1.2Sr1.8Mn2O7 and the YBa2Cu3O6+ x cuprate superconductors obtained from X-ray diffuse scattering using synchrotron radiation. These results confirm the existence of a polaronic liquid (with short-range order) in the paramagnetic insulator phase of the manganite compound, and of lattice modulations (with short-range order) in the cuprate compounds, which co exist with superconductivity down to the lowest temperatures.

Study of the magnetic properties of La 0.53Tb 0.14Sr 0.33MnO 3± δ sintered in different conditions

We report the study of the magnetic properties of the manganite compounds of La 0.53Tb 0.14Sr 0.33MnO 3± δ sintered in Ar, N 2 and O 2 atmospheres. The crystal structure and ferromagnetic–paramagnetic transition region are considerably altered for the prepared samples, indicating the expected variation in the concentration of Mn 3+ and Mn 4+ ions, which was attributed to the oxygen stoichiometric effects. The AC susceptibility and DC magnetization measurements for the prepared samples indicate a cluster-glass state in low magnetic field at low temperatures, besides their respective spin freezing temperatures are similar in spite of the fact that the double-exchange and superexchange interactions are considerably altered. This result induces us to believe that this behavior does not depend only on the competition between those interactions, but also on the interaction between the Mn and Tb moments, since it was found to be a sort of antiferromagnetic ordering below 66 K at 5 kOe applied field. This ordering was ascribed to the Tb moments, suggesting that those are aligned opposite to Mn moments. The obtained magnitude of the saturation magnetization can be explained considering a ferromagnetic alignment of Mn and Tb moments under high fields.

Spin dependent distribution and Fermi surface of the perovskite manganite compound La0.7Sr0.3MnO3 via 2D‐ACAR measurements

AbstractUsing 2D angular correlation of positron annihilation radiation (ACAR) experiment, we have performed a systematic study of the spin dependent and Fermi surface of the colossal magneto‐resistance CMR La0.7Sr0.3MnO3. The measurements have been carried out using a re‐versal magnetic field direction (parallel and anti‐parallel to the direction of motion of the polarized posi‐trons). The measured spectra have been investigated in the momentum space as well as in the wave vector space. They revealed information about the hybridization effect of Mn(3d eg1) and O(2p) like states. Further, the results showed that the majority spin electrons predominated at the Fermi level. From another perspective, the Fermi surface of La0.7Sr0.3MnO3 revealed a cuboids hole surface centered on R point and a spheroid electron surface centered on Γ point. A comparison with the earlier results showed qualitative agreement. However, the current results could reveal the dimension of the electron surface centered on Γ point that was predicted in the earlier 2D‐ACAR measurements. (© 2004 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Study of magnetransport properties in manganites with fixed structural parameters

This work reports a detailed study of magnetic and transport properties of the manganite compounds ( R1, R2) 0.67( M1, M2) 0.33MnO 3 ( R1, R2=La, Gd, Pr, Tb, Ho; M1, M2=Ca, Sr, Ba). Taking into account that the ferromagnetic-to-paramagnetic and the metallic-to-semiconductor transition temperatures, as well as the magnetoresistance peak temperature, depend on the ionic radius of the A-site cations, 〈 r A 〉, as well as on the A-site ( A= R1, R2, M1 and M2) cationic size mismatch, σ 2( r A ), and also on the ratio Mn 3+/Mn 4+, compositions of the samples employed here were chosen to keep these parameters approximately constant. X-ray diffraction patterns indicate that all samples have similar structural characteristics. Experimental results show that they all present similar ferromagnetic–paramagnetic and metallic–semiconductor transition temperatures, as well as similar behavior of the magnetoresistance. We also found evidences of magnetic coupling between rare-earth and Mn moments at low temperatures, while at high temperatures the rare-earth ions have paramagnetic behavior. These results indicate that by controlling the structural parameters and preparation conditions, one can obtain samples with similar magnetic and electrical properties.

Calorimetric and electrical transport properties of stoichiometricLa1−xSrxMnO3 compounds

Heat capacity and electrical resistivity investigations are carried out for stoichiometricLa1−xSrxMnO3 manganite compounds. The temperature of the peak of the heat capacitycp associated with ferromagnetic transition is in agreement with theresistivity measurements. The electrical resistivity shows a roughlyAT2 dependence over a wide temperature range, where the coefficientA decreases as the content of Sr increases. The results show that the Kadowaki–Woodsempirical relation is not valid. It is attempted to explain the dependence ofA onthe x content of Sr within the framework of electron–electron Umklapp scattering.

Lattice Disorder, Electric Properties, and Magnetic Behavior of La1-xNaxMnO3+δ Manganites

La 1 - x Na x MnO 3 + Φ samples with well-defined cation molecularity and oxygen contents (x = 0.05,x = 0. 13, and -0.04 ≤ δ ≤ 0.078) are analyzed with XRPD. electrical conductivity, static magnetization, and EPR measurements. The majority defects induced by oxygen over- and under-stoichiometry are cation and oxygen vacancies; the cation vacancies affect the magnetic properties by interrupting the interaction paths between Mn (III) and Mn(IV) ions, whereas the oxygen vacancies have a stronger influence on the structural and electrical properties and act on the magnetic properties by the overall decrease of the Mn( IV amount and by the creation of sample regions with different magnetic features. We can state that the basic magnetic and electrical behaviors of the Na-doped lanthanum manganite compounds are driven from the Mn(IV)/Mn(III) ratio and all the peculiar behaviors are indeed caused by defects, concentration gradient and, more generally, by lattice disorder.

Magnetic relaxation inLa0.250Pr0.375Ca0.375MnO3with varying phase separation

We have studied the magnetic relaxation properties of the phase-separated manganite compound ${\mathrm{La}}_{0.250}{\mathrm{Pr}}_{0.375}{\mathrm{Ca}}_{0.375}{\mathrm{MnO}}_{3}.$ A series of polycrystalline samples was prepared with different sintering temperatures, resulting in a continuous variation of phase fraction between metallic (ferromagnetic) and charge-ordered phases at low temperatures. Measurements of the magnetic viscosity show a temperature and field dependence which can be correlated to the static properties. Common to all the samples, there appears to be two types of relaxation processes: at low fields associated with the reorientation of ferromagnetic domains and at higher fields associated with the transformation between ferromagnetic and nonferromagnetic phases.

Phase transitions of MnO3 compounds revealed by nonlinear magnetooptics

Magnetic second-harmonic generation (SHG) and three-photon difference-frequency generation (DFG) are used to investigate the magnetic phase diagrams of MnO3 3- compounds with five- and sixfold coordination of the Mn3+ (3d4) ion. In the hexagonal manganites, the six basic antiferromagnetic structures following from a one-dimensional representational analysis of the hexagonal unit cell are clearly distinguished on the basis of their different selection rules for SHG. All structures break the geometric frustration of the unit cell by triangular antiferromagnetic ordering of the Mn spins. The stability and the mutual interactions of in-plane and inter-plane exchange and the in-plane anisotropy are investigated. The three quantities act as almost decoupled degrees of freedom, which leads to independent in-plane and inter-plane reorientations of the Mn spin lattice. DFG was observed in the orthorhombic manganites Pr1-xCaxMnO3 and Nd1-xSrxMnO3. The DFG reflects the fourfold anisotropy of the quasicubic crystal, and the independent tensor elements for DFG are determined quantitatively. A metal–insulator transition with a two-order-parameter coupling to antiferromagnetism and charge ordering leads to additional difference-frequency contributions, which allow us to observe the formation of ∼100 μm magnetic domains. The variety of results gained from the two vastly different groups of manganite compounds, with spectral and spatial resolution used as additional degrees of freedom, demonstrates the versatility of nonlinear magnetooptical experiments for the investigation of magnetic structures and symmetries.

Evolution of the magnetic and magnetotransport properties of the Ga-substituted manganite compoundsLa2−xSrxMnGaO6

We report a series of magnetic and magnetotransport studies performed on Ga-substituted manganite compounds ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{MnGaO}}_{6}(x&lt;~0.7).$ We replace half of the Mn ions in ${\mathrm{LaMnO}}_{3}$ with diamagnetic ions ${\mathrm{Ga}}^{3+}$ and find that the system transforms into a ferromagnet as the result of local coherent and incoherent distortions. Following additional doping with Sr ions we find that the ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{MnGaO}}_{6}$ compounds remain ferromagnetic up to $x\ensuremath{\sim}0.3$ but for higher concentrations the competition between the ferromagnetic and antiferromagnetic superexchange interactions, together with the randomness in distribution of the magnetic ions, lead to magnetic frustration, cluster-glass and spin-glass behavior. All compounds are insulating and the activation energy decreases almost linearly with x. $\ensuremath{\mu}\mathrm{SR}$ measurements suggest the existence of short-range magnetic correlations in the high-temperature region. We measure a significant increase in the muon-spin relaxation rate at low temperatures due to fluctuations of random internal magnetic fields. The observed magnetoresistance in the high-temperature region is mainly a consequence of the reduction of the spin disorder on a local scale, and is as high as 50% in 14 T and $T&gt;100\mathrm{K}$ for samples with $x=0.3$ and $x=0.5.$

Transition temperature and magnetoresistance in double-exchange compounds with moderate disorder

We develop a variational mean-field theory of the ferromagnetic transition in compounds like lanthanum-manganite within the framework of the double-exchange model supplemented by modest disorder. We obtain analytical expressions for the transition temperature, its variation with the valence electron density, and its decrease with disorder. We derive an expression for the conductivity for both the paramagnetic and the ferromagnetic metallic phases, and study its dependence on the temperature and magnetic field. A simple relation between the resistivity in the ferromagnetic phase and the spontaneous magnetization is found. Our results are in a good agreement with the experimental data on transition temperatures and resistivity in the manganite compounds with relatively small disorder. We comment on the effects of increased disorder.

Photoacoustic effect at second-order phase transition in La 1− xCa xMnO 3

The gas-microphone photoacoustic (PA) technique has been applied to determine the second-order phase transition in manganite compounds La 1− x Ca x MnO 3 ( x=0.1 and 0.2). A comparative analysis of the PA result with magnetization and electrical resistivity measurements has been performed. The doping of Ca causes the occurrence of ferromagnetic (FM) ordering. The FM transition temperature T c, determined from magnetization measurement, is 145 and 180 K for x=0.1 and 0.2, respectively. The temperature at which the anomaly of PA signal occurs is consistent with T c determined from the magnetization measurement. The change of PA signal arises at the same temperature in both cases with decreasing and increasing temperatures. This implies that the paramagnetic to FM phase transition can be regarded as a second-order phase transition with no latent heat at T c.