The electronic structure of the perovskite LaCoO$_3$ for different spin states of Co ions was calculated in the LDA+U approach. The ground state was found to be a nonmagnetic insulator with Co ions in a low-spin state. Somewhat higher in energy we found two intermediate-spin states followed by a high-spin state at significantly higher energy. The calculation results show that Co 3$d$ states of $t_{2g}$ symmetry form narrow bands which could easily localize whilst $e_g$ orbitals, due to their strong hybridization with the oxygen 2$p$ states, form a broad $\sigma^*$ band. With the increase of temperature which is simulated by the corresponding increase of the lattice parameter, the transition from the low- to intermediate-spin states occurs. This intermediate-spin (occupation $t_{2g}^5e_g^1$) can develop an orbital ordering which can account for the nonmetallic nature of LaCoO$_3$ at 90 K$<$T$<$500 K. Possible explanations of the magnetic behavior and gradual insulating-metal transition are suggested.

https://arxiv.org/pdf/cond-mat/9709060.pdf

Intermediate-spin state and properties of LaCoO_3

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 minimum in ceramic manganites

Measurements of magnetoresistance and magnetization were carried out on ceramic samples of La_{0.5}Pb_{0.5}MnO_{3} and La_{0.5}Pb_{0.5}MnO_{3}, containing 10 at. % Ag in a dispersed form. The results obtained for the resistivity at zero applied magnetic field exhibit a shallow minimum at temperatures T about 25 K - 30 K, which shifts towards lower temperatures upon applying magnetic field and disappears at a certain (rather low) field. Also the resistivity at helium temperature decreases upon applying magnetic fields. It is shown that the model of spin-dependent carrier tunneling between antiferromagnetically coupled grains may account for the results observed.

A nature of low-temperature resistivity minimum in ceramic manganites

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-type minimum.

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

The compounds La 0.67 Ca 0.33 Mn 1− x Cr x O 3 ( x =0.00, 0.05, 0.10, 0.15, 0.20, 0.35, 0.50 and 0.60) have been studied by X-ray diffraction (XRD), AC and DC magnetization, and DC electrical resistivity measurement techniques. Ferromagnetism is observed up to as high as 60% of Cr substitution. Electrical resistivity behavior of the compounds as a function of temperature could be explained on the basis of adiabatic small polaron model.

Magnetic and electrical properties of La0.67Ca0.33MnO3 as influenced by substitution of Cr

Electrical and magnetic properties of have been extensively investigated at low temperature down to 1.8 K. The irreversibility of magnetic susceptibility between zero field cooling and field cooling increases with increasing Y content. The metal-insulator transition temperature shifts towards lower temperature and the peak resistivity and magnetoresistance increase with the increase of x. Electrical conduction above is dominated by the variable range hopping mechanism. The increase in resistivity at low temperature has been interpreted in terms of electron-electron interaction. The enhancement of magnetoresistance with the increase of Y content is due to the reduction in Mn-O-Mn bond angle and the mobility of charge carriers.

https://iopscience.iop.org/article/10.1088/0953-8984/10/43/024/pdf

Electrical and magnetic properties of perovskite at low temperature

Electrical properties of La0.6Re0. 1Ca0.3MnO3 (Re = Pr, Sm, Gd, Dy) at low temperature

The electrical resistance and magnetoresistance of La08Sr02FexCo1−xO3 (0025⩽X⩽03) have been studied at low temperatures down to 15 K and magnetic fields up to 75 T The large magnetoresistance at high and low temperature has been found The magnetoresistance exhibits a broad minima in the temperature interval between 150 and 50 K for all the compositions The large magnetoresistance and its broad minima may be interpreted as the interplay of spin state transition, Jahn–Teller distortion, and orbital ordering of Co ions

Giant magnetoresistance in La0.8Sr0.2FexCo1−xO3 (0.025⩽X⩽0.3)

The partial substitution of Pr by the rare earth elements (Y, Dy, Gd, Sm, Nd) in induces charge ordering at 120 K. The insulator to metal transition associated with a large negative magnetoresistance has been observed in the presence of magnetic field. The metallic state is destroyed under application of magnetic field and returns to an insulating state at low temperature. The magnetoresistance reveals a broad maximum upon cooling from 150 to 100 K. These results are interpreted by double-exchange interaction and Jahn - Teller distortion.

https://iopscience.iop.org/article/10.1088/0953-8984/10/13/001/pdf

M. Ghosh

Papers

Electrical and magnetic properties of perovskite at low temperature

Electrical properties of La0.6Re0. 1Ca0.3MnO3 (Re = Pr, Sm, Gd, Dy) at low temperature

Giant magnetoresistance in La0.8Sr0.2FexCo1−xO3 (0.025⩽X⩽0.3)

Charge ordered state and giant magnetoresistance in (R = Y, Dy, Gd, Sm, Nd)