Papers by Author: A.V. Levchenko

Abstract: The comprehensive study of transverse magnetoresistance (MR) and magnetization has been carried out on the high quality single crystals of PrB₆ in the wide range of temperatures 2-40K and magnetic fields up to 80kOe. In order to estimate the role of boron vacancies in the formation of the new spin-glass (SG) phase detected by Alekseev et al. below 20K the experiments were carried out on the ordinary (initial state) and annealed single crystals of PrB₆. The data obtained demonstrate the appearance of spontaneous magnetization below T_SG21.3K with M~1.6 emu/mol for initial state and the absence of spontaneous magnetization for the annealed PrB₆ samples. On the contrary, quite similar behavior of MR was detected for various samples of PrB₆. Our results suggest the existence of the aggregated boron vacancies which provoke the new SG phase formation in PrB₆ at T_SG.

Abstract: We report the magnetic properties of Eu_xCa_1-xB₆ single crystals (0.756x1) studied in the wide range of temperatures (1.8-300 K) and magnetic fields (up to 50 kOe). It was found that low field magnetic susceptibility χ (T) follows the Curie-Weiss law χ~(T-Θ_p)^-1 at high temperatures for all the concentrations studied. The effective magnetic moment of the Eu²⁺ ion estimated from the data diminishes from the free ion value μ_eff7.93μ_B (μ_B - Bohr magneton) for x=1 to μ_eff7.3μ_B for x=0.756. A universal behavior of magnetic susceptibility χ~(T-Θ)^-α (α=1.5) is detected close to the Curie temperature T_C in the paramagnetic state at both metallic (x>x_C~0.8) and dielectric (xC.

Abstract: Detailed study of transverse magnetoresistance (MR) and magnetic susceptibility have been done on the high quality single crystals of antiferromagnet GdB₆ (T_N15.5K) in the wide range of temperatures 2-40K in magnetic fields up to 8T. The data obtained allow to establish the low temperature antiferromagnetic (AFMII) phase below T^*~4.7K with the complex behavior of MR including the field hysteresis of the magnetoresistance. It was shown that MR behavior depends on the cooling-warming prehistory in AFM state of GdB₆. The analysis of experimental data allowed us to deduce three contributions to MR in AFM(I), (T^*) and paramagnetic (PM) phases of GdB6. In addition to the main negative component Δρ˿ρ~ H² interpreted in terms of Yosida model both the linear and nonlinear magnetic contributions were also established. The anomalies of MR found in AFM(I) state seem to be associated with the local spin polarization of 5d-electron states of Gd³⁺ ion.

Abstract: The transport and magnetic properties of the Eu1-xCaxB6 (0x0.4) single crystals were studied in the wide ranges of temperatures (1.8-300 K) and magnetic fields (up to 8 T). The experimental data allow to identify a metal-insulator transition (MIT) at the critical Ca concentration xMIT≈0.3, which agrees well with the predictions of double exchange model (V.M. Pereira et al. Phys. Rev. Lett., 93 (2004) 147202). A significant enhancement of magnetoresistive effect is observed below 100K for Eu1-xCaxB6 compounds corresponding to the metallic side of the MIT (xxMIT). The drastic decrease of paramagnetic Curie temperature evaluated from the magnetic and magnetotransport data is discussed in terms of quantum MIT scenario recently proposed for this low carrier density system.

Abstract: High precision measurements of magnetoresistance =f(T,H) and magnetization M(T,H) have been carried out on single crystals of rare earth dodecaboride TmB12 at temperatures 1.8–30 K in magnetic fields up to 80 kOe. The high accuracy measurements and precise temperature stabilization allowed us to perform numerical differentiation and analyze quantitatively a behavior of derivative d()/dH=f(T,H) and magnetic susceptibility (T,H)=dM/dH in paramagnetic and magnetically ordered phases of thulium dodecaboride. It was shown that negative magnetoresistance anomalies observed in present study in paramagnetic state of TmB12 can be consistently interpreted in frameworks of a simple relation between resistivity and magnetization - /M2 proposed by K. Yosida (Phys. Rev., 107, 396 (1957)). A local magnetic susceptibility loc(T,H)=(1/H(d(/)/dH))1/2 was deduced directly from the magnetoresistance measurements and compared with bulk susceptibility (T,H)=dM/dH results of the present study. Moreover, the susceptibility dependences loc(T,H) and (T,H) have been applied to analyze in detail the H-T magnetic phase diagram of TmB12.