Solid State Phenomena Vols. 233-234

Abstract: In this work we have researched the features of dynamic magnetoelectric and magnetoelastic interactions in multiferroic crystal, influenced by various external fields, with the group of symmetry . Based on the integrated approach, which combines N. N. Bogolyubov’s quantum-statistic methods, Green’s temperature functions, diagram technique and symmetry, energy spectrum’s and static spin susceptibility’s dependences on the temperature and external fields were calculated. The effective parameters of magnetoelectric and magnetoelastic interactions dependences on the external fields’ intesities were analysed. It is shown that these parameters have a distinct maximum in the resonance value. As was shown, it is possible to enhance the interactions between spins, ferroelectric and fonons subsystems in multiferroics by applicating the external fields in different crystallographic directions.

Abstract: Multiferroic films of BiFeO₃, (BiLa)FeO₃ and (BiNd)FeO₃ with various concentration of ions of Bi, La and Nd in dodecahedral sublattice utilising were fabricated on monocrystalline substrates of (001) SrTiO₃, (100) MgO and (100) Al₂O₃ by a number of technological methods: rf sputtering, vacuum laser ablation and metal-organic chemical vapor deposition (MOCVD). The film thickness varied in the range of 30-300 nm. The magnetic and magnetoelectric properties of the obtained films were investigated. The saturation magnetization of BiFeO₃ was about 9 emu/cm³ which is typical of strained films of this composition. Doping BiFeO₃ films by rear earth ions La (Nd) increases both the magnetisation saturation and Neel temperature, as well as magnetoelectric effects, which is explained by increase in magnetic crystal anisotropy and suppression of spatially modulated magnetic structure. It was demonstrated that the corona discharge treatment resulted in a substantial growth of the magnetisation saturation up to 35% whereas the changes in the Neel temperature were not noticible. This is explained by the induced electret state and giant magnetoelectric effect.

Abstract: We investigate Anomalous Hall effect in nonmagnetic metal/ferromagnetic insulator bilayer with rotating magnetization of the magnetic insulator. Spin-orbit interaction of Rashba type takes place near metal/insulator interface. Magnetization of the ferromagnetic insulator rotates with some frequency w by microwave radiation under ferromagnetic resonance condition. This rotation together with spin-orbit interaction in non-magnetic metal layer induced Hall current along the interface. The Hall current appears under zero bias in the system. The dependence of Hall current on the exchange splitting, the magnetization rotation frequency and the barrier height is calculated. We analyze various contributions in Hall current and discuss the limit of small frequencies.

Abstract: We report on apparently the first observation of terahertz stimulated radiation in magnetic junctions due to nonequilibrium spins injected by current at room temperatures. The contact between ferromagnetic layers is investigated and so the contact between ferromagnetic and antiferromagnetic layers. The current flowing in the structure creates an inversed population of spin energy levels. After a placing of such a structure in a resonator positive feedback and stimulated emission appears. When current is growing, we observe the relatively narrow (in frequencies) peak - analogous to laser generation but at terahertz frequencies. For the higher currents we observe the picture of electromagnetic turbulence.

Abstract: We present our last results on anomalous Hall effect (AHE) in (Co₄₁Fe₃₉B₂₀)_x(Al–O)_100-x nanocomposites focusing on the possible correlation between temperature dependence of AHE and resistivity. It is shown that the temperature dependence of conductivity G=1/R_xx, where R_xx is resistivity, for compositions with x=49-56% at 10K<T<T_k follows the relation, where the parameters A, , T_k depend on x. For x=47% this relation changes to the exponential law “1/2” R_xx ∝ exp (Т₀/T)^1/2. The correlation between AHE resistivity R_H (T) and resistivity R_xx (T) can be described as R_H ∝ (R_xx)^m , where m increases from 0.38 to 0.58 with an increase of x from 49 to 56 %.

Abstract: Spin transport and distribution of spin accumulation in CuPt/Fe heterostructure are numerically investigated, using linearized Levy-Fert model. It was shown that Spin Hall Effect in CuPt layer produces non-equilibrium spin accumulation in adjacent ferromagnetic layer. Spin accumulation vector is not collinear with the direction of magnetization in ferromagnetic layer which leads to the appearance of spin transfer torque. The absolute values and angular dependence of this torque were calculated and it was demonstrated that for the current the values of torques are sufficient for manipulation of magnetization of ferromagnetic layer.

Abstract: The authors have been investigated structural, magnetic and electrical transport properties for CFAS/n-GaAs junctions. From cross sectional TEM image, RHEED and XRD patternz of thin CFAS films, CFAS films found to be grown epitaxially on GaAs, and shown L2₁-ordered structure for the films with substrate temperature (T_CFAS) of 300°C and 400°C. It is hard to find some additional phase around the interface between CFAS and GaAs. Magnetic moment (and magnetic anisotropy energy) of CFAS increased (and decreased) with increasing T_CFAS up to 300°C and decreased (and increased) at T_CFAS of 400°C, respectively. The asymmetry of current (J)-voltage (V) curve for the junction with T_CFAS =300°C was found to be larger than those for other junctions. It was found there is the relation between T_CFAS dependence of spin signal obtained by three terminal Hanle or four terminal non-local measurement and that of magnetic moment, magnetic anisotropy field or asymmetry of J-V curve.

Abstract: In the present work the possibility of simultaneous localization of two electrons in Δ¹⁰⁰ and Δ⁰⁰¹ valleys in ordered structures with Ge/Si quantum dots (QD) was verified experimentally by electron spin resonance (ESR) method. ESR spectra obtained for the ordered ten-layered QD structure in the dark shows the signal corresponding to electron localization in Si at the Ge QD base edges, in Δ¹⁰⁰, Δ⁰¹⁰ valleys (g_zz=1.9985, g_in-plane=1.999). Light illumination causes the appearance of a new ESR line (g_zz=1.999) attributed to the electrons in Δ⁰⁰¹ valley localized at the QD apexes. Observed effect is explained by enhancement of electron confienment near QD apex by Coloumb attraction to the photogenerated hole trapped in Ge QD.

Abstract: The transport and magnetic properties of cation-substituted manganese sulphides CexMn_1-ХS in the 4K - 450K temperature range in magnetic fields up to 90 kOe are studied. The hysteresis of curve magnetization for X=0.01 and nonlinear field behavior of the magnetization at X = 0.05, the sharp Curie temperature drop were found. The sharp maximum in the temperature dependence of resistivity was observed. The shift of the maximum temperature to low temperatures at cerium ion concentration increasing and in magnetic field was established. Model of orbital polaron for explanation of experimental datа was used.

Abstract: The magnetic and electrical properties of (Co/Ge)_nfilms are experimentally studied. It is established that at the Co/Ge interfacean intermediate magnetic layer forms. Twophases of cobalt, one is a face-centered cubic phase and the other ispresumably a Co–Ge alloy with a weakly ferromagnetic order, have been found toexist. A “dead” layer no more than 2 nm in thickness is formed at the interface.This layer affects the magnetic behavior andmagnetoresistive effect in the investigated structures.