Quantum oscillations of resistance and magnetization in the degenerate semiconductor n-HgCr2Se4

In the magnetic field range ΔH=8–60 kOe we observed and studied the anomalous oscillations in the magnetic field dependence of the resistance and magnetization of single crystals of n-HgCr₂Se₄. The absence of periodicity in 1/H in the ΔH=8–20 kOe range can be explained by the non-Fermi-liquid behavior of the electron subsystem and agrees with the theory of the de Haas-van Alphen in systems with intermediate valence. In stronger fields, ΔH=20–60 kOe, the amplitude of the fundamental harmonic decreases, with the number and amplitude of the higher-order harmonics increasing. As a result, noise is superimposed on the signal as magnetic field strength grows. The temperature dependence of the magnetization is the sum of the monotonic spin-wave contribution and the oscillating part. Zh. éksp. Teor. Fiz. 113, 1877–1882 (May 1998)     

A new concept for a current switch based on a high-temperature superconductor

A new concept for a device for protecting an alternating-current network is offered. It is based on a combination of a superconducting limiter of a short-circuit current and a circuit breaker. A high-temperature superconductor in the form of a pile of plane rings is used as the active element of the limiter. The test results of a model of such a limiter are obtained in the steady-state short-circuit regime. The characteristics of composite silverless materials used for design of the breaker are given.     

Low-temperature magnetic behavior of the rare-earth cobaltites GdCoO<Subscript>3</Subscript> and SmCoO<Subscript>3</Subscript>

The temperature and magnetic field dependences of the static magnetization of the polycrystalline rare-earth cobaltites GdCoO₃ and SmCoO₃ have been measured. It is shown that, below room temperature, the magnetization of both compounds derives primarily from the rare-earth ion paramagnetism. The GdCoO₃ and SmCoO₃ compounds have been found to differ substantially in magnetic behavior, which can be traced to differences in their electronic shell structures. The magnetic behavior of GdCoO₃ is close to that of an array of free Gd³⁺ ions, whereas in SmCoO₃ the deviation from the free-ion properties is very large because of the Sm³⁺ ground state being crystal-field split. Van Vleck magnetic susceptibility measurements of SmCoO₃ suggest that the splitting is ～10 K.     

Fe nanowires in carbon nanotubes as an example of a one-dimensional system of exchange-coupled ferromagnetic nanoparticles

The cooperative phenomena revealed in the field and temperature dependences of the magnetization in a system of iron nanoparticles in carbon nanotubes were studied experimentally. The character of the temperature dependences of the magnetization indicates that the ferromagnetic Fe particles in carbon nanotubes are exchange-coupled. In the region where the magnetization approaches saturation, the magnetization curves reveal the power dependence ΔM～ H^?3/2 typical for a one-dimensional system of exchange-coupled ferromagnetic nanoparticles.     

Giant magnetoresistance of MexMn1−x S (Me=Fe, Cr) sulfides

The structural, electrical, and magnetic properties, as well as the magnetoresistance of polycrystalline Me_xMn_1?x S (Me=Fe and Cr) sulfides were investigated in longitudinal magnetic fields of up to 50 kOe over the temperature range 4.2–300 K. The ferromagnetic compound Fe_xMn_1?x S (x=0.29) exhibits the giant magnetoresistance (GMR) effect with magnitude δ_H=?450% in a field of 30 kOe at 50 K. Antiferromagnetic Cr_xMn_1?x S (x=0.5) sulfide undergoes a transition to the GMR state δ_H～?25% in a field of 30 kOe at 4.2 K) in the region of antiferromagnet-ferromagnet transition (T _c ～66 K). A mechanism of the GMR in these compounds is discussed.     

Specific features of magnetic properties of ferrihydrite nanoparticles of bacterial origin: A shift of the hysteresis loop

The results of the experimental investigation into the magnetic hysteresis of systems of superparamagnetic ferrihydrite nanoparticles of bacterial origin have been presented. The hysteresis properties of these objects are determined by the presence of an uncompensated magnetic moment in antiferromagnetic nanoparticles. It has been revealed that, under the conditions of cooling in an external magnetic field, there is a shift of the hysteresis loop with respect to the origin of the coordinates. These features are associated with the exchange coupling of the uncompensated magnetic moment and the antiferromagnetic “core” of the particles, as well as with processes similar to those responsible for the behavior of minor hysteresis loops due to strong local anisotropy fields of the ferrihydrite nanoparticles.     

Implementation of the Astrov method for measuring the ME E effect with the use of a vibrating-coil magnetometer

A setup is developed for measuring the ME _E effect by fixing the amplitude of magnetization oscillations upon the repolarization of a sample, caused by the application of ac voltage to the sample plates. The temperature dependence of the ME _E effect in a Ga_{2 ? x} Fe _x O₃ single-crystal sample is measured in external fields from 0.25 to 1 kOe at temperatures from 77.4 to 280 K. It is established that the effect disappears when the Curie temperature is attained. The hysteresis of the magnetoelectric effect related to the magnetization hysteresis is measured.     

Increase in the magnetization loop width in the Ba0.6K0.4BiO3 superconductor: Possible manifestation of phase separation

The magnetization of Ba_0.6K_0.4BiO₃ samples in fields up to 90 kOe in the temperature range from 2 to 30 K is investigated. It is shown that the observed increase in the width of the magnetization loop can be explained by a decrease in the phase nonuniformity upon an increase in the magnetic field. The asymmetric hysteretic dependence of magnetization with the secondary peak was successfully described by the extended critical state model taking into account the phase separation in the superconductor.     

Magnetoresistance of porous polycrystalline HTSC: Effect of the transport current on magnetic flux compression in intergranular medium

The hysteretic dependences of the magnetoresistance of porous (38% of the theoretical density) granular high-temperature superconductor (HTSC) Bi_1.8Pb_0.3Sr_1.9Ca₂Cu₃O _x have been analyzed in the model of the effective intergranular field. This effective field has been defined by the superposition of the external field and the field induced by magnetic moments of superconducting grains. The magnetic flux compression in an intergranular medium, characterized by the effective field, controls the hysteretic behavior of the magnetoresistance. It has been found that the magnetoresistance hysteresis width for the studied porous HTSC depends on the transport current, in contrast to the superconductor of the same composition with high physical density (more than 90% of the theoretical value). For a porous superconductor, a significant current concentration occurs in the region of the grain boundaries, which is caused by features of its microstructure. A current-induced increase in the effective boundary length results in a decrease in the flux compression, a decrease in the effective field in the intergranular medium, and a magnetoresistance hysteresis narrowing with increasing current.