Titusz Fehér

Anisotropy of superconducting MgB2 as seen in electron spin resonance and magnetization data.

We observed the conduction electron spin resonance (CESR) in fine powders of MgB2 both in the superconducting and normal states. The Pauli susceptibility is chi(s) = 2.0 x 10(-5) emu/mole in the temperature range of 450 to 600 K. The spin relaxation rate has an anomalous temperature dependence. The CESR measured below T(c) at several frequencies suggests that MgB2 is a strongly anisotropic superconductor with the upper critical field, H(c2), ranging between 2 and 16 T. The high-field reversible magnetization data of a randomly oriented powder sample are well described assuming that MgB2 is an anisotropic superconductor with H(ab)(c2)/H(c)(c2) approximately 6-9.

Spin-stretching modes in anisotropic magnets: Spin-wave excitations in the multiferroic ba2coge2o7

We studied spin excitations in the magnetically ordered phase of the noncentrosymmetric Ba(2)CoGe(2)O(7) in high magnetic fields up to 33 T. In the electron spin resonance and far infrared absorption spectra we found several spin excitations beyond the two conventional magnon modes expected for such a two-sublattice antiferromagnet. We show that a multiboson spin-wave theory describes these unconventional modes, including spin-stretching modes, characterized by an oscillating magnetic dipole and quadrupole moment. The lack of inversion symmetry allows each mode to become electric dipole active. We expect that the spin-stretching modes can be generally observed in inelastic neutron scattering and light absorption experiments in a broad class of ordered S > 1/2 spin systems with strong single-ion anisotropy and/or noncentrosymmetric lattice structure.

Magnetic memory based on La0.7Ca0.3MnO3/YBa2Cu3O7/La0.7Ca0.3MnO3 ferromagnet/superconductor hybrid structures

We report a memory concept utilizing ferromagnet/superconductor/ferromagnet La0.7Ca0.3MnO3/YBa2Cu3O7/La0.7Ca0.3MnO3 thin film hybrid structures. The orientation of the magnetic field with respect to the ferromagnetic easy axis has a strong effect on superconductivity as indicated by a strong variation in the magnetoresistance (MR). MR can be controlled by rotating a small magnetic field applied in the plane of the film in a way that is determined by the in-plane biaxial magnetic anisotropy. The proposed memory device has the advantages of superconducting detection elements (fast response and low dissipation), small (100–150 Oe) writing fields, and resistance read-out without need for applied field.

Spin diffusion and magnetic eigenoscillations confined to single molecular layers in the organic conductors κ-(BEDT-TTF)2Cu[N(CN)2]X (X=Cl,Br)

The layered organic compounds, kappa-(BEDT-TTF)2Cu[N(CN)2]X X=Cl, Br) are metals at ambient temperatures. At low temperatures, the Cl compound is a weakly ferromagnetic Mott insulator while the isostructural Br compound is a superconductor. We find by conduction electron spin resonance and antiferromagnetic resonance (AFMR) an extreme anisotropy of spin transport and magnetic interactions in these materials. In the metallic state spin diffusion is confined to single molecular layers within the spin lifetime of 10(-9) s. Electrons diffuse several hundreds of nm without interlayer hopping. In the magnetically ordered insulating phase of the Cl compound we observe and calculate the four AFMR modes of the weakly coupled single molecular layers. The interplane exchange field is comparable or less than the typically 1 mT dipolar field and almost 10(6) times less than the intralayer exchange field.

Pressure and temperature dependence of interlayer spin diffusion and electrical conductivity in the layered organic conductors kappa-(BEDT-TTF)(2)Cu[N(CN)(2)]X (X = Cl, Br)

A high frequency (111.2-420 GHz) electron spin resonance study of the inter-layer (perpendicular) spin diffusion as a function of pressure and temperature is presented in the conducting phases of the layered organic compounds, {\kappa}-(BEDT-TTF)2-Cu[N(CN)2]X ({\kappa}-ET2-X), X=Cl or Br. The resolved ESR lines of adjacent layers at high temperatures and high frequencies allows for the determination of the inter-layer cross spin relaxation time, Tx and the intrinsic spin relaxation time, T2 of single layers. In the bad metal phase spin diffusion is two-dimensional, i.e. spins are not hopping to adjacent layers within T2. Tx is proportional to the perpendicular resistivity at least approximately, as predicted in models where spin and charge excitations are tied together. In {\kappa}-ET2-Cl, at zero pressure Tx increases as the bad metal-insulator transition is approached. On the other hand, Tx decreases as the normal metal and superconducting phases are approached with increasing pressure and/or decreasing temperature.

Diagonal antiferromagnetic easy axis in lightly hole doped Y1-xCaxBa2Cu3O6.

Hole induced changes in the antiferromagnetic structure of a lightly Ca doped Gd:Y(1-x)CaxBa2Cu3O6 copper oxide single crystal with x approximately 0.008 is investigated by Gd3+ electron spin resonance. Holes do not localize to Ca2+ ions above 2.5 K since the charge distribution and spin susceptibility next to the Ca2+ are independent of temperature. Both hole doped and pristine crystals are magnetically twinned with an external magnetic field dependent antiferromagnetic domain structure. Unlike the undoped crystal, where the easy magnetic axis is along [100] at all temperatures, the easy direction in the hole doped crystal is along the [110] diagonal at low temperatures and changes gradually to the [100] direction between 10 and 100 K. The transition is tentatively attributed to a magnetic anisotropy introduced by hole ordering.

Skyrmion dynamics under uniaxial anisotropy

D. Ehlers, I. Stasinopoulos, V. Tsurkan, 3 H.-A. Krug von Nidda, T. Fehér, A. Leonov, I. Kézsmárki, D. Grundler, 6 and A. Loidl Experimentalphysik V, Elektronische Korrelationen und Magnetismus, Universität Augsburg, 86135 Augsburg, Germany Lehrstuhl für Physik funktionaler Schichtsysteme, Technische Universität München, Physik Department, 85748 München, Germany Institute of Applied Physics, Academy of Sciences of Moldova, MD 2028, Chisinau, Republica Moldova Department of Physics, Budapest University of Technology and Economics and MTA-BME Lendület Magneto-optical Spectroscopy Research Group, 1111 Budapest, Hungary IFW Dresden, Postfach 270016, 01171 Dresden, Germany EPFL STI IMX LMGN, MXC 241, Station 12, 1015 Lausanne, Switzerland (Dated: December 9, 2015)

A Fourier transform Raman spectrometer with visible laser excitation

We present the development and performance of a Fourier transformation (FT)-based Raman spectrometer working with visible laser (532 nm) excitation. It is generally thought that FT-Raman spectrometers are not viable in the visible range where shot noise limits the detector performance and therein they are outperformed by grating based, dispersive ones. We show that contrary to this common belief, the recent advances of high-performance interference filters makes the FT-Raman design a valid alternative to dispersive Raman spectrometers for samples which do not luminesce. We critically compare the performance of our spectrometer to two dispersive ones: a home-built single channel and a state-of-the-art charge coupled device-based instruments. We demonstrate a similar or even better sensitivity than the charge coupled device-based dispersive spectrometer particularly when the laser power density is considered. The instrument possesses all the known advantages of the FT principle of spectral accuracy, high throughput, and economic design. We also discuss the general considerations, which helps the community reassess the utility of the different Raman spectrometer designs. Copyright

Magnetic-field-induced density of states in MgB2: Spin susceptibility measured by conduction-electron spin resonance

The magnetic field dependence of the spin-susceptibility,

Comment on "Magnetoresistance anomalies in antiferromagnetic YBa2Cu3O6+x: fingerprints of charged stripes"

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