Ágnes Antal

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.

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.

Magnetic fluctuations above the Néel temperature in κ-(BEDT-TTF)2Cu[N(CN)2]Cl, a quasi-2D Heisenberg antiferromagnet with Dzyaloshinskii–Moriya interaction

Ágnes Antal, Titusz Fehér, Bálint Náfrádi, 2 László Forró, and András Jánossy ∗ Budapest University of Technology and Economics, Department of Physics and Condensed Matter Research Group of the Hungarian Academy of Sciences, P.O. Box 91, H-1521 Budapest, Hungary Institute of Physics of Complex Matter, FBS, Swiss Federal Institute of Technology (EPFL), CH-1015 Lausanne, Switzerland (ΩDated: May 3, 2014)

Two-dimensional Magnetism in κ-(BEDT-TTF)2Cu[N(CN)2]Cl, a Spin-1/2 Heisenberg Antiferromagnet with Dzyaloshinskii–Moriya Interaction

The finite phase transition temperature, T-N, of quasi two-dimensional (2D) Heisenberg antiferromagnetic (AF) crystals results from in-plane anisotropy or coupling between layers. It is usually not known which is the determining factor when both are weak. We show by an electron spin resonance experiment that in the quasi 2D antiferromagnetic crystal,kappa-(BEDT-TTF)(2)Cu[N(CN)(2)] Cl, TN is determined by in-plane anisotropy while interlayer coupling plays a minor role. The compound has a large isotropic Heisenberg exchange interaction between sites with S = 1/2 spins. The Dzyaloshinskii-Moriya (DM) interaction is the main source of anisotropy, while in-plane anisotropy and the interlayer coupling are very weak. The external-field field-induced static and fluctuating AF magnetizations are independent in adjacent layers above the (zero-field) ordering temperature.

Multiscale studies of complex magnetism of nanostructures based on first principles

We present a first principles formulation that allows the derivation of parameters of a classical spin-model. In particular, relativistic effects are included, inducing tensorial exchange interactions and on-site anisotropy. Three applications of the method are shown: (i) by using a mean field treatment of the spin-model we calculate the temperature dependence of the magnetic anisotropy energy of a Co3Cu(001) film; (ii) in terms of Monte Carlo simulations for an Fe2/W(110) film we demonstrate the sensitivity of the magnetic ground state to the layer relaxations; and (iii) we also investigate the magnetic ground state of an equilateral Cr trimer on top of a Au(111) surface. These examples clearly highlight the importance of the anisotropic exchange couplings and of the Dzyaloshinsky-Moriya interactions in studying and understanding the magnetism of nanostructures.