We studied spin excitations of the non-centrosymmetric Ba2CoGe2O7 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 multi-boson spin-wave theory describes these unconventional modes, including spin-stretching modes, characterized by oscillating magnetic dipole and quadrupole moment. The lack of the 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 non-centrosymmetric lattice structure. Magnons are collective spin excitations in crystals with long-range magnetic order, often investigated by electromagnetic absorption and neutron scattering experiments. Both classical and quantum spin-wave theory of S = 1/2 systems predict one magnon branch in the spin-excitation spectrum for each spin in the magnetic unit cell [1]. This rule about the number of magnon branches is generally accepted and experimentally verified for S > 1/2 spin systems as long as the conventional spin-wave theory applies, requiring that the lengths (i.e., the absolute values of the expectation values) of the spins are preserved in the excited states and only their orientations change relative to the ground-state configuration [2]. However, the picture of one magnon mode per spin in the magnetic unit cell needed to be surpassed in several f -electron compounds with complicated quadrupolar ordering, such as CeB 6 [3] and UO 2 [4].Recently, additional spin-wave modes have been observed by far infrared (FIR) spectroscopy [5] and inelastic neutron scattering (INS) [6] in Ba 2 CoGe 2 O 7 , a simple two-sublattice easy-plane antiferromagnet (AF) with S = 3/2 spins [7, 8]. This material has attracted much interest owing to its multiferroic ground state where delicate magnetic control of the ferroelectric polarization [8, 9] and chirality [10] were realized. Moreover, spin waves in Ba 2 CoGe 2 O 7 exhibits giant directional dichroism and natural optical activity at THz frequencies due to the large ac magnetoelectric effect [5, 10]. A recent numerical diagonalization study on finite spin clusters found, besides the two conventional AF modes, additional spin resonances with peculiar optical properties [10, 11]. Nevertheless, the understanding of the unconventional magnon modes and the coupled dynamics of spins and electronic polarization on a fundamental level remained an open issue.In this Letter, we investigate the spin-wave excitations in Ba 2 CoGe 2 O 7 over a broad photon energy range combining electron spin resonance (ESR) and high-resolution FIR spectroscopy. The largest magnetic field, 33 T, applied in this study drastically changes the antiferromagnetic spin configuration for any field direction, in contrast to former e...

We present calculations of the magnetic ground states of Cr trimers in different geometries on top of a Au(111) surface. By using a least square fit method based on a fully relativistic embedded-cluster Green's function method first we determined the parameters of a classical vectorspin model consisting of second and fourth order interactions. The newly developed method requires no symmetry constraints, therefore, it is throughout applicable for small nanoparticles of arbitrary geometry. The magnetic ground states were then found by solving the Landau-Lifshitz-Gilbert equations. In all considered cases the configurational energy of the Cr trimers is dominated by large antiferromagnetic nearest neighbor interactions, whilst biquadratic spin-interactions have the second largest contributions to the energy. We find that an equilateral Cr trimer exhibits a frustrated 120 • Néel type of ground state with a small out-of-plane component of the magnetization and we show that the Dzyaloshinsky-Moriya interactions determine the chirality of the magnetic ground state. In cases of a linear chain and an isosceles trimer collinear antiferromagnetic ground states are obtained with a magnetization lying parallel to the surface.

We acknowledge the support of the Hungarian National Research Fund OTKA NN76727, CNK80991, T72954, K107228, the New Hungary Development Plan TA ḾOP-4.2.1/B-09/1/KMR-2010-0002, and the Swiss NSF and its NCCR MaNEP. E .K. acknowledges support of the Bolyai Jańos Scholarship of the Hungarian Academy of Sciences.

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.