E. V. Eremin

Studies of Ferroelectric and Magnetic Phase Transitions in Multiferroic PbFe0.5Ta0.5O3

X-ray, magnetic, and Mossbauer studies of a PbFe0.5Ta0.5O3 powder have been carried out. For the first time temperature dependence of the unit cell parameters was obtained. Antiferromagnetic Neel temperature TN ≈180 K was determined using the results of Mossbauer studies. In the vicinity of TN an anomaly in the temperature dependence of the monoclinic angle β was revealed in PFT. This anomaly seems to be due to the magnetoelectric coupling.

Features of the Magnetic and Magnetoelectric Properties of HoAl 3 (BO 3 ) 4

The main features of the magnetic and record magnetoelectric properties of a HoAl3(BO3)4 aluminoborate single crystal have been studied experimentally and theoretically. It has been found that the electric polarization that was previously detected in HoAl3(BO3)4 and is record for multiferroics is significantly larger, ΔPba(Ba) ≈ −5240 μC/m2, with an increase in the magnetic field to 9 T at T = 5 K. The measured magnetic properties and revealed features have been interpreted within a united theoretical approach based on the molecular field approximation and on calculations in the crystal field model for a rare-earth ion. The experimental temperature (from 3 to 300 K) and field (up to 9 T) dependences of the magnetization have been described. The parameters of the crystal field of trigonal symmetry for a Ho3+ ion in HoAl3(BO3)4 have been determined from the interpretation of the experimental data.

Vibrational Spectra and Elastic, Piezoelectric, and Magnetoelectric Properties of HoFe 3 (BO 3 ) 4 and HoAl 3 (BO 3 ) 4 Crystals

Raman spectra of light are obtained for HoFe3(BO3)4 and HoAl3(BO3)4 crystals at various temperatures and are used for determining the frequencies of crystal lattice vibrations at the center of the Brillouin zone. It is also found that the HoFe3(BO3)4 crystal exhibits a phase transition at Tc ≈ 366 K. The magnetoelectric effect in the paramagnetic phase of these compounds is studied experimentally. The lattice vibration frequencies, elastic and piezoelectric moduli, Born dynamic charges, and the high-frequency permittivity are calculated using the density functional method. A peculiar behavior of the transverse acoustic vibration branch is observed in the Γ → Z direction of the Brillouin zone of the HoFe3(BO3)4 crystal. The electric polarization induced by an external field is estimated using the calculated values of piezoelectric moduli and experimental values of magnetostriction.

Reversible UV induced metal–semiconductor transition in In2O3 thin films prepared by autowave oxidation

We have prepared thin indium oxide films by the autowave oxidation reaction. Measurements of temperature dependence of resistivity, Hall carrier concentration and Hall mobility have been conducted in the temperature range 5–272 K. Before ultraviolet (UV) irradiation, the indium oxide film had a semiconductor-like temperature dependence of resistivity ρ and the ratio of ρ (5 K)/ρ(272 K) was very limited (∼1.2). It was found that after UV irradiation of the In2O3 film, the metal–semiconductor transition (MST) was observed at ∼100 K. To show that this MST is reversible and repeatable, two full cycles of ‘absence of MST–presence of MST’ have been done using UV irradiation (photoreduction) as the induced mechanism and exposure to an oxygen environment as the reversible mechanism, respectively. MST in transparent conducting oxide (TCO) is possibly associated with the undoped structure of metal oxide, which has some disorder of oxygen vacancies. It was suggested that reversible UV induced metal–semiconductor transition would occur in other TCOs.

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...

Direct and inverse magnetoelectric effects in HoAl3(BO3)4 single crystal

The direct (MEH-) and inverse (MEE-) magnetoelectric effects in the HoAl3(BO3)4 single crystal are studied. Temperature and magnetic field dependences of permittivity of the crystal are investigated. A relation between the investigated effects was established. It was found that the magnetoelectric effect can exist in crystals without magnetic order or spontaneous polarization. It was shown that the phenomena investigated are due to magnetostriction or magnetoelastic effect. The thermodynamic potential was considered for describing magnetoelectric effect at low magnetic fields. The results obtained are explained within a proposed qualitative microscopic model, based on interplay of configuration of 4f- electron subshell of the rare-earth element and applied magnetic or electric field.

Spin-glass magnetic ordering in CoMgGaO2BO3 ludwigite

Single crystal, needle shaped samples of diamagnetically diluted cobalt ludwigite CoMgGaO2BO3 have been grown by the flux method. X-ray diffraction and both dc and ac magnetic measurements are described. The unit cell volume changes significantly with dilution, from 328.31 A3 for the parent compound Co3O2BO3 to 345.46 A3 for CoMgGaO2BO3. The magnetic transition temperature is considerably lower for the latter compound (25 K against 43 K for Co2O2BO3). The dc magnetization temperature dependences are split between the field cooled and zero-field cooled regimes and the ac magnetic susceptibility temperature curves are frequency dependent, which indicates possible spin-glass freezing in the magnetic system.

Current-driven channel switching and colossal positive magnetoresistance in the manganite-based structure

The transport and magnetotransport properties of a newly fabricated tunnel structure manganite/depletion layer/manganese silicide have been studied in the current-in-plane (CIP) geometry. A manganite depletion layer in the structure forms a potential barrier sandwiched between two conducting layers, one of manganite and the other of manganese silicide. The voltage–current characteristics of the structure are nonlinear due to switching conducting channels from an upper manganite film to a bottom, more conductive MnSi layer with an increase in the current applied to the structure. Bias current assists tunnelling of a carrier across the depletion layer; thus, a low-resistance contact between the current-carrying electrodes and the bottom layer is established. Below 30 K, both conducting layers are in the ferromagnetic state (magnetic tunnel junction), which allows control of the resistance of the tunnel junction and, consequently, switching of the conducting channels by the magnetic field. This provides a fundamentally new mechanism of magnetoresistance (MR) implementation in the magnetic layered structure with CIP geometry. MR of the structure under study depends on the bias current and can reach values greater than 400% in a magnetic field lower than 1 kOe. A positive MR value is related to peculiarities of the spin-polarized electronic structures of manganites and manganese silicides.