Solid State Phenomena Vols. 233-234

Abstract: The phase transitions in antiferromagnetic Ising model are studied on a body-centered cubic lattice by taking the interactions of next-nearest neighbors into account. The model is investigated on basis of the replica Monte Carlo algorithm and the histogrammic analysis of data. The diagram of the critical temperature dependence on an interaction value of next-nearest neighbors is plotted. The studied model reveals the phase transitions of second order. A static magnetic critical indices is calculated using the finite-dimensional scaling theory.

Abstract: Transport, magnetotransport and magnetic properties of structures GaAs/InAs/GaAs with a InAs quantum dot (QD) layer have been investigated in the temperature interval 4.2<T<300 K. The structures were delta-doped by Mn from a one side to provide magnetic properties and by carbon from the other side to enhance a p-type conductivity. The ferromagnetic phase up to 400 K was detected by SQUID magnetometer. Anomalous Hall-effect was observed at low T. The role of additional disorder in conducting channel due to the QD layer was investigated. It is shown a principal role of a fluctuation potential of Mn layer separated from conducting QD layer by a spacer in anomalous transport properties of structures. The negative magnetoresistance was observed at low T due to the reduction of the spin-flip scattering by aligning spins by magnetic field.

Abstract: The galvanomagnetic properties in weak magnetic fields (4.2≤T≤300 K, B≤0.07 T) as well as magnetic properties (2≤T≤300 K, B≤9 T) of the single-crystal Pb_1-ySc_yTe (y≤0.02) alloys have been investigated. We find that an increase of Sc impurity content leads to a monotonous growth of the free electron concentration (from 10¹⁶ cm^-3 to 10²⁰ cm^-3). In heavily doped alloys (y>0.01), it tends to saturation, indicating the pinning of the Fermi energy by the scandium resonant level located inside the conduction band. The energy of the level is estimated (E_Sc≈E_c+280 meV) and the model of electronic structure rearrangement of Pb_1-ySc_yTe alloys with doping is proposed. In the frame of this model, using experimental temperature and magnetic field dependences of magnetization, the concentrations of magnetically active scandium ions are determined and connection of the electronic structure with the magnetic properties of the alloys are discussed.

Abstract: GaMnAs layers fabricated on GaAs(001) substrates by laser ablation technique were studied using the magneto-optical transversal Kerr effect (TKE) (E = 0.5 - 4.0 eV) and spectral ellipsometry (E = 1.4 - 4.5 eV). Obtained spectral, temperature and magnetic field TKE dependences shown that the used technique allowed us to obtain ferromagnetic (Ga,Mn)As layers, whose composition was close to single-phase one (without MnAs inclusions). Spectral dependences of the off-diagonal components (e’ = e’₁ - ie’₂) of the permittivity tensor and also of the magnetic circular dichroism (MCD) were calculated for ferromagnetic (Ga,Mn)As sample. The calculated dependences were compared with the published magneto-optical spectra of (Ga,Mn)As layers grown by molecular beam epitaxy.

Abstract: The Bi₂(Sn_0.95Mn_0.05)₂O₇ compound existing simultaneously in two polymorphic modifications, namely, orthorhombic and cubic has been synthesized for the first time by solid-phase synthesis. The magnetic, dielectric and electrical properties of the compound have been studied. Anomalies in the temperature dependences of the electrical resistance and magnetic propoties have been found. These features are explained as martensitic phase transitions.

Abstract: Transport and magnetic properties of δ-Mn doped GaAs/InGaAs/GaAs quantum wells (QW) with various In content were studied at temperatures 4.2K≤T≤300K. Fluctuation potential (FP) appeared to be crucial for transport characteristics of structures under investigation. The magnetic percolation transition was observed at temperature T_p in the range 20 - 40K. The T_p dependence on the In content is nonmonotonic due to the peculiarities of free-carrier mediated exchange interaction mechanisms. The change of the anomalous Hall effect (AHE) sign with decreasing temperature was detected at temperatures close to the T_p. The main reason of the AHE sign change is the variation of contributions of different AHE mechanisms (intrinsic and side-jump) caused by the reduction of spin-dependent scattering intensity with temperature decrease. We believe that our results are the experimental observation of the AHE intrinsic mechanism in 2D.

Abstract: ²⁰⁹Bi NQR experiments, including analysis of zero-field line shapes, Zeeman-perturbed patterns and zero-field spin-echo envelopes were made to examine magnetic splitting of resonances revealed in the spectra of Main group element compounds of general composition Ba_kBi_lA_mO_n (A=Al, В, Ge, Br, Cl). The results were explained assuming the existence in the compounds of ordered internal magnetic fields from 5 to 250 G which notably exceed those of nuclear magnetic moments. A dramatic (8−10-fold) increase in the resonance intensities, instead of broadening and fading, was observed for such compounds upon applying weak (below 500 Oe) external magnetic fields. The effect was shown to relate to the spin dynamics, namely, to the influence of external magnetic field on the nuclear spin-spin relaxation of the compounds with anomalous magnetic properties. In α-Bi₂O₃, paramagnetism depending on the thermal prehistory of a sample was found using SQUID-technique; magnetoelectric effect linear in magnetic field was also observed for this oxide.

Abstract: The data on transport and magnetic properties (ac linear and nonlinear (second and third orders) susceptibilities) are presented for La_0.7Sr_0.3MnO₃ single crystal with metallic behavior in paramagnetic (PM) region and ferromagnetic (FM) metallic ground state. The FM clusters originate in the PM matrix of the compound below some temperature T* > 425 K, their nonlinear response being weakly T-dependent down to 367 K. This was attributed to clusters arising in the preferable sites formed by chemical inhomogeneities introduced by doping. On cooling below T^# ≈ 366 K > T_C ≈ 363.3 K, a fast growth of cluster response without the change of its parameters is observed that was attributed to the development of homogeneous nucleation of the FM clusters. The latter stage continues below T_C and is accompanied by a crossover to the steeper decreasing of resistivity with cooling that suggests metallic properties of the clusters. The cluster nonlinear response masks completely that of matrix at T = 360.3 K < T_C, where it is well described by the model of ensemble of magnetic single-domain nanoparticles in superparamagnetic regime based on the formalism involving Gilbert-Landau-Lifshits equation. Below T_D = 359.6 K at the stage of domain formation, a weak interaction of matrix and cluster subsystems leads to their mutual ordering, which is accompanied by a sharp decrease of the nonlinear response to a weak ac field in small steady field H. The latter suggests an “antiferromagnetic” type of arrangement of these subsystems that provides decreasing the magnetostatic energy of the sample.

Abstract: The temperature dependences of the magnetic susceptibility χ (T) of diluted magnetic semiconductor HgSe:Ni (1·10¹⁸ < N_Ni < 1·10¹⁹) cm^-3 were investigated. It was found that paramagnetic part of the susceptibility which is wholly due to the effect of the impurities in the range of temperature from 1.8 to 200 K is described by the Curie-Weiss law. The analysis of the concentration dependence of the Curie constant C(N_Ni) based on the model of the hybridized electronic states was carried out. The application of this model allowed us to determine the microscopic parameters of the electronic structure of these crystals (the effective spin of the hybridized electronic states at the nickel impurity S_i and the resonance concentration of donor electrons n_0d).

Abstract: The magnetic field dependence of ultrasonic attenuation α (B) of slow shear waves in the ZnSe:Cr²⁺ crystal at a number of fixed temperatures from T=1.4 K to 20 K in magnetic fields of up to B=14 T were investigated. For magnetic fields B above 5 T we found that the attenuation increases with B monotonically, and at a given temperature it is proportional to the magnitude of relaxation attenuation at B=0. We show that the magnetic field dependent attenuation is due to the change in populations of the lowest energy levels of the impurity centers Cr_Zn4Se, produced by the Jahn-Teller effect and split by the spin-orbital interaction and the magnetic field. The calculations carried out without fitting parameters are in good agreement with the experimental data.