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Papers by Author: S.V. Demishev

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Authors: M.A. Anisimov, A.V. Bogach, V.V. Glushkov, S.V. Demishev, N.A. Samarin, N.Y. Shitsevalova, A.V. Levchenko, V.B. Filipov, A.V. Kuznetsov, N.E. Sluchanko

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.

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Suppression of Ferromagnetism in Eu_xCa_1-xB₆

Authors: V.V. Glushkov, A.V. Kuznetsov, I. Sannikov, A.V. Bogach, S.V. Demishev, V.Y. Ivanov, A.V. Dukhnenko, A.V. Levchenko, N.Y. Shitsevalova, N.E. Sluchanko

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)^-1at 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.

New Magnetoresistance Results in GdB₆

Authors: M.A. Anisimov, A.V. Bogach, V.V. Glushkov, S.V. Demishev, N.A. Samarin, Natalya Shitsevalova, A.V. Levchenko, Volodymyr Filipov, A.V. Kuznetsov, N.E. Sluchanko

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.

Spin-Dependent Variable Range Hopping Conductivity

Authors: A.D. Bozhko, S.V. Demishev, V.V. Glushkov, E.A. Kataeva, N.E. Sluchanko, A.G. Lyapin

Abstract: Basing on the study of the transport in disordered carbon nanomaterials we argue that the correct description of the magnetoresistance can be obtained in a spin polarization theory, which processes separately the spin and orbital degrees of freedom in hoping conductivity in magnetic field. It is found that the polarization of the spin part of the electron wave function controls the probabilities of the hops between the single and double occupied localized states and, moreover, the spin polarization process can be treated as in the paramagnet with spin S=1/2. The experimental checking of the universal magnetoresistance scaling expected in the spin polarization model is provided.

545

High Field Magnetoresistance Behavior in PrB₆ and NdB₆

Authors: M.A. Anisimov, V.V. Glushkov, S.V. Demishev, N.A. Samarin, N.Y. Shitsevalova, N.E. Sluchanko

Abstract: The comprehensive study of low temperature magnetoresistance (MR) has been carried out on high quality PrB6 and NdB6 single crystals at temperatures 2–20K and in magnetic fields up to 8T. The negative MR (Δρ/ρ<0) was found in the paramagnetic state at T>TN for both compounds. The analysis of the experimental data allows us to establish three contributions to MR. In addition to the main Brillouin type negative component –Δρ/ρ~Mloc2~H2 which may be interpreted in terms of Yosida theory [Phys. Rev. 107 (1957) 396] both the linear and nonlinear magnetic contributions were also deduced. According to detailed analysis in the framework of spin–polaron approach proposed by A.V. Bogach et al. [Physica B 378–380 (1957) 769] the last component should be ascribed to the ferromagnetic (FM) nanodomains embedded in the metallic RB6 matrix. The estimation of local magnetic susceptibility χloc=(–1/H•(d(Δρ/ρ)/dH))1/2 is deduced directly from the quadratic part of negative MR and reveals Curie–Weiss type behaviour in paramagnetic (PM) phase with paramagnetic Curie temperatures Θp~– 22K and –31K for PrB6 and NdB6 correspondingly. The results of undertaken analysis allow to conclude in favour of the concurrence between AF and FM iteractions as the main reason of incommensurate (IC) magnetic structure formation in RB6.

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Hall Effect in LaB₆ and NdB₆

Authors: M.A. Anisimov, A.V. Bogach, V.V. Glushkov, S.V. Demishev, N.A. Samarin, N.Y. Shitsevalova, N.E. Sluchanko

Abstract: To shed more light on the peculiarities of ground state formation in RB6 Hall effect in NdB6 and LaB6 has been investigated on the single crystals at temperatures 2K for LaB6 and <100> for NdB6). A very similar temperature behavior of RH(T) with the step–like anomaly near T ~ 25K has been established for both magnetic NdB6 and diamagnetic LaB6 at intermediate temperatures T>10K. The temperature independent behaviour of Hall coefficient (RH(LaB6) ~ –3.6•10-4 cm3/C, RH(NdB6) ~ –3.7•10-4 cm3/C) observed in the interval 10K

525

Magnetic Resonance and Nature of Magnetism in CeB₆

Authors: S.V. Demishev, A.V. Semeno, A.V. Bogach, N.A. Samarin, H. Ohta, S. Okubo, N.Y. Shitsevalova, N.E. Sluchanko

Abstract: We report results of the study of the recently discovered magnetic resonance in CeB6, which is characteristic to the phase II in the magnetic phase diagram of this strongly correlated heavy fermion metal. Experiments were carried out in a wide frequency range /2=44-360 GHz in magnetic field up to 30 T. It is found that the previously reported magnetic resonance exists at all frequencies studied and the g-factor for this resonance increases with frequency from g(/2=44 GHz)~1.55 to g(/2>250 GHz)~1.7. In addition to the aforementioned mode of magnetic oscillations an additional magnetic resonance with the g-factor 1.2-1.3 is detected for the frequencies exceeding 200 GHz. This effect may be a consequence of a complicated magnetic structure of the phase II, which is not foreseen in the standard models of the low temperature magnetic ordering in CeB6.

353

Electron Spin Resonance in EuB₆

Authors: A.V. Semeno, V.V. Glushkov, A.V. Bogach, N.E. Sluchanko, A.V. Dukhnenko, V.B. Fillippov, N.Y. Shitsevalova, S.V. Demishev

Abstract: Cavity measurements of a high frequency (60 GHz) electron spin resonance (ESR) have been carried our for a single crystal of EuB6 at temperatures 4.2-50K in magnetic field up to 7 T, which has been aligned along [001] crystallographic direction. The influence of the experimental geometry on the parameters of the resonance line has been studied. It is found that in conditions of a homogeneous magnetic field the ESR spectrum in EuB6 is formed by a single resonance line at all temperatures including the ferromagnetic ordering region. The presence of the gradient of the magnetic field in the sample causes strong broadening of the resonance line at low temperatures and even may induce a double peak ESR structure. The analysis of the linewidth temperature dependence in EuB6 shows the anomalous growth of the line width below Curie temperature TC ~15 K.

349

Anomalous Magnetism in Eu(Ca)B₆

Authors: V.V. Glushkov, M.A. Anisimov, A.V. Bogach, S.V. Demishev, N.A. Samarin, A.V. Kuznetsov, A.V. Dukhnenko, A.V. Levchenko, N.Y. Shitsevalova, K. Flachbart, N.E. Sluchanko

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.

307

Electron Spin Resonance in VO_x Nanotubes

Authors: A.L. Chernobrovkin, A.V. Semeno, V.V. Glushkov, N.E. Sluchanko, N.A. Samarin, S.V. Demishev, A.V. Grigorieva, E.A. Goodilin

Abstract: We report the first direct experimental evidence of the presence of antiferromagnetic dimers in VOx nanotubes based on the high frequency (60 GHz) electron spin resonance study carried out in the temperature range of 4.2-200 K. Arguments favouring a possible observation of the low temperature quantum critical phenomena in VOx nanotubes are provided.

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