D. A. Shulyatev

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.

Colossal electroresistance and current induced switching in ferromagnetic insulating state of La0.82Ca0.18MnO3

Colossal electroresistance and current induced resistivity switching have been measured in the ferromagnetic insulating (FMI) state of single crystal manganite La0.82Ca0.18MnO3. The sample has a Curie transition temperature TC = 165 K and the FMI state is realized for temperatures T<100 K. The electroresistance (ER), arising from a strong nonlinear resistivity, attains a large value ( ≈ 100%) in the FMI state. However, this is accompanied by a collapse of the magnetoresistance (MR) to a small value even in magnetic field (H) of 10 T. This demonstrates that the mechanisms that give rise to ER and MR are effectively decoupled.

Kinetic effects in an La0.8Ba0.2MnO3 single crystal

Results are presented of a complex study of the magnetic and resistive properties, the Hall effect, the thermal emf, and the longitudinal Nernst-Ettingshausen effect of an La0.8Ba0.2MnO3 single crystal at temperatures between 77 and 400 K. A maximum was observed near the Curie temperature Tc on the temperature dependences of the resistivity, the thermal emf, and the normal Hall coefficient. It was established that the Hall mobility remains constant near Tc. It is shown that these anomalies in the kinetic properties are attributable to a change in the position of the mobility edge relative to the Fermi level. A semiphenomenological theory is put forward to quantitatively describe the temperature and magnetic-field dependences of the resistivity and thermal emf of lanthanum manganites near the phase-transition temperature.

Magnetoresistance and Hall effect in La0.8Sr0.2MnO3

A comparative study of the longitudinal ρxx and transverse ρxy resistivities and magnetic susceptibility χac of La0.8Sr0.2MnO3 single crystals and ceramic samples has been conducted in a wide range of temperatures T=1.7–370 K and magnetic fields, H=0–13.6 T. It turned out that the relation ρxy∼ρxx, which is expected to hold in the case of carrier scattering by magnetic fluctuations, applies to the single crystals. In polycrystals, an additional H-dependent contribution to the resistivity tentatively attributed to plane (near grain boundaries) and bulk “defects” of the magnetic sublattice has been detected. The scattering of carriers by these defects does not make a notable contribution to the anomalous Hall effect and magnetic susceptibility χac. As a result, the curve of ρxy versus ρxx seems to be steeper than a linear dependence. Under the assumption that the materials under investigation are metals with constant carrier concentrations, the conductivity σ=1/ρxx due to the critical magnetic scattering calculated in the molecular field approximation reproduces the main features of experimental data, namely, the drop in the amplitude and shift of the resistivity peak near the Curie point with increasing magnetic field H and also a relatively slow change in the derivative dσ/dH with increasing temperature in the region T⩽TC. The large hole concentration of about two per unit cell derived from Hall measurements indicates that carriers of opposite signs can coexist in these materials.

Conductivity oscillations in current-induced metastable states in low-doped manganite single crystals

Deterministic oscillations of current-induced metastable resistivity in changing voltage have been detected in La 0 . 8 2 Ca 0 . 1 8 MnO 3 single crystals. At low temperatures, below the Curie point, application of specificbias procedures switches the crystal into a metastable resistivity state characterized by the appearance of pronounced reproducible and random structures in the voltage dependence of the differential conductivity. In a certain bias range equally spaced broad conductivity peaks have been observed. The oscillating conductivity has been tentatively ascribed to resonances in a quantum well within the double-tunnel barrier of intrinsic weak links associated with twinlike defect boundaries.

CURRENT INDUCED TELEGRAPH NOISE IN CMR MANGANITES

Random telegraph fluctuations of the resistivity exceeding 10% have been observed in current induced low resistive metastable state of low-dopped La1-xCaxMnO3 single crystal at temperatures below the Curie point. Telegraph amplitudes and average life-times depend on dc bias. Near the characteristic minimum in the resistance vs current curve the bias dependent duty cycle of the telegraph signal crosses 0.5. The pronounced telegraph fluctuations are tentatively associated with fluctuating conductivity of intrinsic tunnel barriers between ferromagnetic domains joined by antiferromagnetic insulating matrix in percolating paths of the phase separated manganite.