Searcharxiv⌕ Search

arXiv subjects

K. Mikhalev

Publications and source records attributed to K. Mikhalev.

7 recordsLinked to original sources
arXiv2013

Metal-insulator transition in antiferromagnetic $Ba_{1-x}K_xMn_2As_2$ $(0\le x\le0.4)$ single crystals studied by $^{55}$Mn and $^{75}$As NMR

The magnetic structure and metal-insulator transition in antiferromagnetic (AFM) BaMn2As2 and Ba1-xKxMn2As2 single crystals have been investigated by 55Mn and 75As nuclear magnetic resonance (NMR) measurements. In the parent AFM insulator BaMn2As2 with a Neel temperature TN = 625 K, we observed a 55Mn zero-field NMR (ZFNMR) spectrum and confirmed the G-type AFM structure from the field dependence of the 55Mn spectra and 75As-NMR spectra below TN. In hole-doped crystals with x > 0.01, similar 55Mn ZFNMR spectra were observed and the AFM state was revealed to be robust up to x = 0.4 with the ordered moment nearly independent of x. The nuclear spin-lattice relaxation rates (1/T1) for both nuclei in the doped samples follow the Korringa relation T1T = const., indicating a metallic state. This confirms the coexistence of AFM ordered localized Mn spins and conduction carriers from a microscopic point of view. From the x-dependence of (T1T)^(-1/2) for both nuclei, we conclude that this transition is caused by vanishing of the hole concentration as the transition is approached from the metallic side.

cond-mat.str-el↗
arXiv2013

Melting of the orbital order in LaMnO3 probed by NMR

The Mn spin correlations were studied near the O'-O phase transition at T{JT} = 750 K, up to 950 K with 17O and 139La NMR in a stoichiometric LaMnO3 crystalline sample. The measured local hyperfine fields originate from the electron density transfered from the eg- and t2g-orbitals to the 2s(O) and 6s(La) orbits, respectively. By probing the oxygen nuclei, we show that the correlations of the Mn spins are ferromagnetic in the ab-plane and robust up to T{JT} whereas along the c-axis, they are antiferromagnetic and start to melt below T{JT}, at about 550 K. Above T{JT} the ferromagnetic Mn-Mn exchange interaction is found isotropic. The room temperature orbital mixing angle, ϕnmr = 109+-1.5°, of the eg ground state is close to the reported value which was deduced from structural data on Jahn-Teller distorted MnO6 octahedra. For T > T{JT} LaMnO3 can be described in terms of non-polarized eg-orbitals since both eg-orbitals are equally occupied.

cond-mat.str-el↗
arXiv2005

The charge ordered state in half-doped Bi-based manganites studied by $^{17}$O and $^{209}$Bi NMR

We present a $^{209}$Bi and $^{17}$O NMR study of the Mn electron spin correlations developed in the charge ordered state of Bi$_{0.5}$Sr$_{0.5}$MnO$_{3}$ and Bi$_{0.5}$Ca$_{0.5}$MnO$_{3}$. The unusually large local magnetic field $^{209}H_{loc}$ indicates the dominant $6s^{2}$ character of the lone electron pair of Bi$^{3+}$-ions in both compounds. The mechanism connecting the $s$ character of the lone pairs to the high temperature of charge ordering $T_{CO}$ is still not clarified. The observed difference in $^{209}H_{loc}$ for Bi$_{0.5}$Sr$_{0.5}$MnO$_{3}$ to Bi$_{0.5}$Ca$_{0.5}$MnO$_{3}$ is probably due to a decrease in the canting of the staggered magnetic moments of Mn$^{3+}$-ions from. The modification of the $^{17}$O spectra below $T_{CO}$ demonstrates that the line due to the apical oxygens is a unique local tool to study the development of the Mn spin correlations. In the AF state the analysis of the $^{17}$O spectrum of Pr$_{0.5}$Ca$_{0.5}$MnO$_{3}$ and Bi$_{0.5}$Sr$_{0.5}$MnO$_{3}$ prompts us to try two different theoretical descriptions of the charge-ordered state, a site-centered model for the first manganite and a bond-centered model for the second one.

cond-mat.str-el↗
arXiv2004

Spin Susceptibility of Ga-Stabilized delta-Pu Probed by {69}^Ga NMR

Spin susceptibility of stabilized δphase in the Pu-Ga alloy is studied by measuring {69,71}^Ga NMR spectra and nuclear spin-lattice relaxation rate {69}T_{1}^{-1} in the temperature range 5 - 350 K. The shift ({69}^K) of the {69,71}^Ga NMR line and {69}^T_{1}^{-1} are controlled correspondingly by the static and the fluctuating in time parts of local magnetic field arisen at nonmagnetic gallium due to transferred hyperfine coupling with the nearest f electron environment of the more magnetic Pu. The nonmonotonic with a maximum around 150 K behavior of {69}^K(T) χ_{s,5f}(T) is attributed to the peculiarities in temperature dependence of the f electron spin susceptibility χ_{s,5f}(T) in δphase of plutonium. The temperature reversibility being observed in {69}^K(T) data provides strong evidence for an electronic instability developed with T in f electron bands near the Fermi energy and accompanied with a pseudogap-like decrease of χ_{s,5f}(T) at T<150 K. The NMR data at high temperature are in favor of the mainly localized character of 5f electrons in δphase of the alloy with characteristic spin-fluctuation energy Γ(T) T^{0.35(5)}, which is close to $Γ(T) T^{0.5} predicted by Cox et al. [J. Appl. Phys. 57, 3166 (1985)] for 3D Kondo-system above T_Kondo}. The dynamic spin correlations of 5f electrons become essential to consider for {69}^T_{1}^{-1}(T) only at T<100 K. However, no NMR evidences favoring formation of the static magnetic order in δ-Pu were revealed down to 5K .

cond-mat.str-el↗
arXiv2002

55Mn NMR Study of the Field-Controlled Magnetic Phase Separation in (La0.25Pr0.75)0.7Ca0.3MnO3 with Different Oxygen Isotope Content

An influence of the 16O-18O isotope substitutions on magnetic state of perovskite-type manganite (La0.25Pr0.75)0.7Ca0.3MnO3 is studied by 55Mn NMR. Successive cycling with an isochronal exposure at different magnetic fields up to H=8T is used to study the field-induced transition from antiferromagnetic insulating (AFI) state to the ferromagnetic metal (FMM) one in the 18O-enriched sample. After exposure at H>H_{cr} \sim 5.3T the NMR spectrum of the 18O-sample evidences for magnetic phase separation (PS) resulted in the coexisting AFI and FMM domains. Further increase of exposing field leads to a progressive growth of the FMM phase at the expense of AFI domains. Its relative fraction can be controlled by external magnetic field and the resulting magnetic structure in the PS region is discussed. Anomalous T-dependence of the 55Mn nuclear spin-lattice relaxation rate is revealed in the FMM state of both 16O-and 18O-enriched samples. The manifestation of the Pr magnetic ordering at T \sim 40K is considered.

cond-mat.str-el↗
arXiv2001

Electronic States of Boron in Superconducting MgB$_2$ Studied by $^{11}$B NMR

NMR spectra and nuclear spin-lattice relaxation rate $T_1^{-1}$ of $^{11}$B have been measured in superconducting polycrystalline MgB$_{2}$ with $T_C^{ons}\cong 39.5K$. It is shown that $(T_{1}T)^{-1}$ and the Knight shift $K_{s}$ are independent of temperature and nearly isotropic above $T_C$. Both of these quantities are decreased gradually in going to superconducting state. According NMR data the density of states (DOS) near the Fermi level is flat at the scale $\approx$ 500K. Some conclusions on the orbital content of the DOS at the Fermi level was done and compared with the results of the band structure calculations.

cond-mat.supr-con↗