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 low-temperature $(l60\phantom{\rule{0.3em}{0ex}}\mathrm{K})$ transport properties of as-grown ${\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}\mathrm{Mn}{\mathrm{O}}_{3}$ ultrathin films, deposited by the molecular beam epitaxy, have been investigated as a function of the sample thickness (from $40\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}3.5\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$) and in the presence of an external magnetic field. With decreasing thickness, a clear low-temperature resistivity minimum slightly affected by the application of the magnetic field has been observed, and its presence has been possibly interpreted in terms of quantum interference effects. As a function of the thickness, a crossover from a three-dimensional (3D) to a two-dimensional (2D) behavior of the system takes place below $20\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$. A re-entrant 3D behavior is induced in ultrathin films by the application of large $(g20\phantom{\rule{0.3em}{0ex}}\mathrm{kOe})$ magnetic fields. Negative values of the magnetoresistance have been observed in all of the investigated samples for all of the measured magnetic fields.

Low-temperature resistivity of La 0.7 Sr 0.3 Mn O 3 ultra thin films: Role of quantum interference effects

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

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

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

S Biswas

Papers

Electrical and magnetic properties of perovskite at low temperature

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