E. Rozenberg

Low-temperature resistivity minimum in ceramic manganites

Measurements of magnetoresistance and magnetization were carried out on ceramic samples of La0.5Pb0.5MnO3 and La0.5Pb0.5MnO3, containing 10 at. % Ag in a dispersed form. The results obtained for the resistivity at zero applied magnetic field exhibit a shallow minimum at the temperature T∼25–30 K which shifts towards lower temperatures upon applying a magnetic field and disappears at a certain field Hcr. Also the resistivity at helium temperature decreases upon applying magnetic fields. It is shown that the model of charge carriers tunneling between antiferromagnetically coupled grains may account for the results observed.

Low-temperature resistivity minima in single-crystalline and ceramic La0.8Sr0.2MnO3: Mesoscopic transport and intergranular tunneling

A slight minimum in the zero-field resistivity of a single crystalline La0.8Sr0.2MnO3 and shallow one for ceramic La0.8Sr0.2MnO3 samples were observed at T∼4 K and at T∼25–30 K, respectively. The minimum for the ceramic shifts towards lower temperatures, flattens with increasing magnetic fields (H), and vanishes at some critical H. The above effects are accompanied by an appreciable negative magnetoresistance (MR). On the other hand, the minimum for the single-crystalline sample is almost field independent and the MR in the relevant temperature range is very small. Two different mechanisms were found to account for the results observed in the single crystal and the polycrystalline samples: (i) mesoscopic corrections to the bulk resistivity that include Coulomb interaction and weak localization and (ii) intergranular tunneling. The resistivity of the large-grain ceramic sample comprises both types of behavior. The minimum of the bulk contribution becomes clearly seen under H, which suppresses the ceramic-t...

Correlation between electroresistance and magnetoresistance in La0.82Ca0.18MnO3 single crystal

The resistivity of La0.82Ca0.18MnO3 single crystal has been investigated as a function of external magnetic field and separately under an applied current flow. The measurements were carried out at various temperatures below and above the ferromagnetic transition temperature TC. It has been found that the dynamic electroresistance exhibits stunning similarities to the colossal magnetoresistance at the corresponding temperatures. The correlation observed between the electric- and magnetic-field effects is attributed to electrically induced magnetoresistance.

Adiabatic and non-adiabatic small-polaron hopping conduction in La1−xPbxMnO3+δ (0.0 ≤ x ≤ 0.5)-type oxides above the metal–semiconductor transition

Above the semiconductor-to-metallic transition (SMT) temperature (Tp), transport properties of the La1−xPbxMnO3+δ (0 ≤ x ≤ 0.5)-type mixed valence oxides with Tp between 230 and 275 K (depending on x) have been thoroughly examined for a small-polaron hopping conduction mechanism of the carriers. Although the variable range hopping (VRH) model was used earlier to fit the entire conductivity data above SMT, we noticed two distinct regions (above and below θD/2; θD is the Debye temperature) where different types of conduction mechanisms are followed. The high temperature (T > θD/2) conductivity data of all the Pb-doped samples follow the adiabatic hopping conduction mechanism, while those of LaMnO3 (x = 0) showing no SMT follow the non-adiabatic hopping conduction mechanism of Mott or Emin with reasonable values of polaron radius, hopping distance, polaron binding energy, activation energy, etc being different for different systems. The VRH model, however, fits the corresponding low temperature (T < θD/2) data of all the samples. Both resistivity ρ(T) and thermoelectric power S(T) follow a similar microscopic theory above Tp supporting the small-polaron hopping mechanism. Thermoelectric power also showed appreciable magnetic field dependence around SMT.

The nature of the low-temperature minimum of resistivity in ceramic manganites

Abstract Electrical transport measurements were carried out on a ceramic samples of La 0.5 Pb 0.5 MnO 3 containing 10 at.% of Ag in a dispersed form and La 0.8 Sr 0.2 MnO 3 . Their resistivity at zero applied magnetic field exhibits a shallow minimum at the interval of T ∼25–30 K. This minimum shifts towards lower T upon applying a magnetic field ( H ) and disappears at a certain field H cr , while residual resistivity decreases notably with increasing of H . It is found that the bulk-scattering model is not adequate for describing of such behavior of ρ in the presence of H . It seems that the proposed model of charge-carrier tunneling between antiferromagnetically coupled grains accounts for the results obtained in spite of significant difference in the crystallinity (grain size) of the samples.

Magnetic properties of NiMnSb(001) films grown on InGaAs/InP(001)

NiMnSb half Heusler alloy films were prepared by molecular beam epitaxy (MBE) on InP(001). The dc and rf magnetic properties were investigated by ferromagnetic resonance (FMR). The effective uniaxial anisotropy fields increased with increasing film thickness and reached nonzero asymptotic values. FMR linewidths rapidly increased with the film thickness due to the presence of two magnon scattering. Bulklike uniaxial anisotropies and two magnon scattering were caused by a self-assembled network of lattice defects. Gilbert damping parameter and spectroscopic g factor were found to be 3.1×107 and 2.03, respectively, indicating a weak role of spin orbit interaction.

Inherent inhomogeneity in the crystals of low-doped lanthanum manganites

The X-band electron paramagnetic resonance (EPR) technique and the model analysis of the EPR susceptibility versus temperature were employed to characterize the dopant distribution in a number of La-manganite crystals, low doped with Ca, Sr, and Ba in a La site. Such distribution appears to be inhomogeneous as a result of technological-driven effect. It is emphasized that the above chemical disorder influences strongly both magnetic state and transport of the low-doped manganite crystals.

Magnetic, electric and electron magnetic resonance properties of orthorhombic self-doped La1−xMnO3 single crystals

The effect of lanthanum deficiency on structural, magnetic, transport, and electron magnetic resonance (EMR) properties has been studied in a series of La1−xMnO3 (x = 0.01, 0.05, 0.11, 0.13) single crystals. The x-ray diffraction study results for the crystals were found to be compatible with a single phase of orthorhombic symmetry. The magnetization curves exhibit weak ferromagnetism for all samples below 138 K. It was found that both the spontaneous magnetization and the coercive field increase linearly with x. The pressure coefficient dTN/dP decreases linearly with self-doping, from a value of 0.68 K kbar−1 for La0.99MnO3 to 0.33 K kbar−1 for La0.87MnO3. The resistivity of low-doped La0.99MnO3 crystal is of semiconducting character, while that of La0.87MnO3 depends weakly on temperature between 180 and 210 K. It was found that the magnetic and transport properties of the self-doped compounds may be attributed to a phase separation involving an antiferromagnetic matrix and ferromagnetic clusters. The latter phases as well as their paramagnetic precursors have been directly observed by means of EMR.

Chemical disorder influence on magnetic state of optimally-doped La0.7Ca0.3MnO3

X-band electron magnetic resonance and dc/ac magnetic measurements have been employed to study the effects of chemical disorder on magnetic ordering in bulk and nanometer-sized single crystals and bulk ceramics of optimally-doped La0.7Ca0.3MnO3 manganite. The magnetic ground state of bulk samples appeared to be ferromagnetic with the lower Curie temperature and higher magnetic homogeneity in the vicinity of the ferromagnetic-paramagnetic phase transition in the crystal, as compared with those characteristics in the ceramics. The influence of technological driven “macroscopic” fluctuations of Ca-dopant level in crystal and “mesoscopic” disorder within grain boundary regions in ceramics was proposed to be responsible for these effects. Surface spin disorder together with pronounced inter-particle interactions within agglomerated nano-sample results in well defined core/shell spin configuration in La0.7Ca0.3MnO3 nano-crystals. The analysis of the electron paramagnetic resonance data enlightened the reasons f...

Magnetic Correlations and Spin Dynamics in Crystalline La

Electron paramagnetic resonance data of La1-xCaxMnO3 compounds with x=0,0.1,0.2,0.3 were analyzed in order to explore the changes in effective spin-spin interactions and spin dynamics parameters. The presence of two magnetic subsystems - host of superexchange coupled Mn3+ ions and Mn4+ associated clusters is characteristic for x=0 and 0.1 samples, while ferromagnetically correlated host is observed in x=0.2 and 0.3 ones. The contributions of both ionic host and doped carriers to spin relaxation parameters were determined using general spin-dynamics theory.