N. V. Volkov

Adiabatic calorimetric study of the intense magnetocaloric effect and the heat capacity of (La0.4Eu0.6)0.7Pb0.3MnO3

The temperature dependences of the intense magnetocaloric effect ΔTAD(T, H) and the heat capacity Cp(T) of the (La0.4Eu0.6)0.7Pb0.3MnO3 manganite are directly measured using adiabatic calorimetry. The experimental dependences ΔTAD(T) are in satisfactory agreement with those calculated from the data on the behavior of the magnetization. The factors responsible for the absence of an anomaly in the experimental temperature dependence of the heat capacity Cp(T) in the range of the magnetic phase transition are discussed.

Antiferromagnetic resonance in CuB2O4 single crystal

The frequency-field, temperature, and angular dependences of the antiferromagnetic resonance parameters for the tetragonal CuB2O4 single crystal are studied in the frequency range 2.6–80 GHz and at temperatures of 4.2–30 K. The results obtained confirm the fact that, in the high-temperature state in the range 10–21 K, this compound is an easy-plane weak ferromagnet. The temperature dependence of the Dzyaloshinski field is determined. An abrupt change observed in the frequency-field dependence of the magnetic resonance at T=4.2 K and H ⊥ C4 indicates the transition to the weak ferromagnetic state induced by the external field H⊥. The phase diagram for CuB2O4 is constructed on the H⊥-T coordinates. It is demonstrated that, in the low-temperature state, the magnetic moments of copper ions remain in the basal plane, but the weak ferromagnetism is absent.

Thermal properties, magneto- and baro-caloric effects in La0.7Pb0.3MnO3 single crystal

The results of heat capacity, thermal dilatation and T-p phase diagram studies on the La0.7Pb0.3MnO3 single crystal are reported. Direct measurements of intensive magnetocaloric effect are performed by means of adiabatic calorimeter. Barocaloric effect is determined using data of heat capacity and susceptibility to hydrostatic pressure. Caloric efficiency of manganite in the vicinity of ferromagnetic phase transition is discussed and compared with that of other magnetic materials.The results of heat capacity, thermal dilatation and T-p phase diagram studies on the La0.7Pb0.3MnO3 single crystal are reported. Direct measurements of intensive magnetocaloric effect are performed by means of adiabatic calorimeter. Barocaloric effect is determined using data of heat capacity and susceptibility to hydrostatic pressure. Caloric efficiency of manganite in the vicinity of ferromagnetic phase transition is discussed and compared with that of other magnetic materials.

Synthesis and magnetic properties of copper metaborate single crystals, CuB2O4

Large high-quality single crystals of copper metaborate are grown on the basis of the phase diagram of the ternary Li2O-CuO-B2O3 system. Bright blue crystals with a volume of about 1 cm3 were grown by the method of spontaneous crystallization while slowly cooling the melt. The magnetic susceptibility and electron spin resonance were measured. It is shown that the effective magnetic moment of a Cu2+ ion is equal to 1.6 μB and the g-factor, to 2.170 and 2.133 for the magnetic field oriented parallel and perpendicular to the fourfold axis, respectively. At 21 and 10 K, sharp anomalies of magnetic susceptibility are observed.

Extremely large magnetoresistance induced by optical irradiation in the Fe/SiO2/p-Si hybrid structure with Schottky barrier

We report giant magnetoresistance (MR) effect that appears under the influence of optical radiation in common planar device built on Fe/SiO2/p-Si hybrid structure. Our device is made of two Schottky diodes connected to each other by the silicon substrate. Photo-induced MR is positive and the MR ratio reaches the values in excess of 104%. The main peculiarity of the MR behavior is its strong dependence on the magnitude and the sign of the bias current across the device and, most surprisingly, upon polarity of the magnetic field. To explain such unexpected behavior of the MR, one needs to take into account contribution of several physical mechanisms. The main contribution comes from the existence of localized interface states at the SiO2/p-Si interface, which provide the spots for the photo-current conduction by virtue of the sequential tunneling through them or thermal generation and optical excitation of mobile charges. External magnetic field changes the probability of these processes due to its effect o...

Magnetization, Magnetoelectric Polarization, and Specific Heat of HoGa 3 (BO 3 ) 4

A comprehensive experimental and theoretical study of magnetic, magnetoelectric, thermal, and spectroscopic characteristics of HoGa3(BO3)4 gallium borate single crystals has been performed. A large magnetoelectric effect exceeding its values found in all iron and aluminum borates except HoAl3(BO3)4 has been observed. The magnetoelectric polarization of HoGa3(BO3)4 equals ΔPba(Ba) ≈ −1020 μC/m2 at T = 5 K in a magnetic field of 9 T. The theoretical treatment based on the crystal field model for rare-earth ions provides a unified approach for the consistent interpretation of all measured characteristics. The crystal-field parameters are determined. The temperature (in the 3–300 K range) and magnetic field (up to 9 T) dependences of the magnetization, the Schottky anomaly in the temperature dependence of the specific heat, and its shift in the field B ‖ c are described. To compare the thermal properties of HoGa3(BO3)4 with those of HoAl3(BO3)4 exhibiting record values of the polarization, the specific heat of HoAl3(BO3)4 at various B values and the temperature dependence of the polarization ΔPb(T) in the applied magnetic field of 9 T have been measured.

Frequency-dependent magnetotransport phenomena in a hybrid Fe/SiO2/p-Si structure

We report the large magnetoimpedance effect in a hybrid Fe/SiO2/p-Si structure with the Schottky barrier. The pronounced effect of magnetic field on the real and imaginary parts of the impedance has been found at temperatures 25–100 K in two relatively narrow frequency ranges around 1 kHz and 100 MHz. The observed frequency-dependent magnetotransport effect is related to the presence of localized “magnetic” states near the SiO2/p-Si interface. In these states, two different recharging processes with different relaxation times are implemented. One process is capture-emission of carriers that involves the interface levels and the valence band; the other is the electron tunneling between the ferromagnetic electrode and the interface states through SiO2 potential barrier. In the first case, the applied magnetic field shifts energy levels of the surface states relative to the valence band, which changes recharging characteristic times. In the second case, the magnetic field governs the spin-dependent tunneling...

Dielectric properties of a mixed-valence Pb3Mn7O15 manganese oxide.

We investigated the low-frequency dielectric properties of a Pb(3)Mn(7)O(15) single crystal with manganese ions in the mixed-valence state (Mn(3+)/Mn(4+)). Dielectric relaxation was found in the frequency window from 20 to 100 kHz in the temperature range 110-180 K. The dielectric spectra of the crystal were analyzed using a Debye model. Estimations made within the model and analysis of resistivity data suggest that the relaxation behavior of the dielectric constant is related to polaronic charge carrier hopping. Around 250 K, charge ordering occurs in the crystal when the Mn(3+) and Mn(4+) ions are arranged in a specific order among the crystal sites. With a decrease in temperature, an ac electric field can induce a charge hop between the equivalent lattice sites available, related to crystal symmetry. This hopping is equivalent to the reorientation of an electric dipole that yields Debye-type behavior of the complex dielectric constant. The observed anisotropy in the behavior of the dielectric properties and resistivity can be attributed to a pronounced two-dimensional character of the crystal structure.

Optically driven conductivity and magnetoresistance in a manganite-based tunnel structure

In the multilayer structure, La0.7Sr0.3MnO3/depleted manganite layer/MnSi, the photovoltaic effect has been discovered. The depleted manganite layer in the structure is dielectric and serves as a potential barrier between the ferromagnetic conducting La0.7Sr0.3MnO3 and MnSi layers by the formation of a magnetic tunnel junction. The photoinduced changes in the transport properties of the magnetic tunnel structure have been observed in the current-in-plane geometry. The changes are reversible and saturate at radiation power densities over 30?mW?cm?2. The photovoltaic effect has a threshold character: it reveals only at h? > 1.17?eV. Most likely, the effect of optical radiation is related to the formation of electron?hole pairs due to interband absorption of light in the dielectric layer. A photocurrent through the tunnel junctions separating the conducting layers causes a redistribution of the current channels between the conducting layers, which influences the conductivity and the magnetoresistance of the structure.

Magnetic resonance spectrum of a two-phase state in single crystals of La0.7Pb0.3MnO3 lanthanum manganite

Two absorption lines are observed over a wide temperature range below Tc in the magnetic resonance spectrum of an La0.7Pb0.3MnO3 single crystal. These lines correspond to two magnetic phases in the sample. The frequency-field dependence of spectra obtained in the range of microwave radiation frequencies 10–77 GHz allows these phases to be interpreted as ferromagnetic and paramagnetic phases. The phase volume ratio depends on the temperature and the magnitude of the external magnetic field. Features in the temperature behavior of parameters of the magnetic absorption line are observed in the region of the highest magnetic resistance of the sample. The results are interpreted within the mechanism of electronic phase separation.