Search

Spin-1/2 Heisenberg antiferromagnets Cs2CuCl4 and Cs2CuBr4 with distorted triangular-lattice structures are studied by means of electron spin resonance spectroscopy in magnetic fields up to the saturation field and above. In the magnetically saturated phase, quantum fluctuations are fully suppressed, and the spin dynamics is defined by ordinary magnons. This allows us to accurately describe the magnetic excitation spectra in both materials and, using the harmonic spin-wave theory, to determine their Exchange parameters. The viability of the proposed method was proven by applying it to Cs2CuCl4, yielding J/kB = 4.7(2) K, J'/kB = 1.42(7) K, [J'/J ≃ 0.30] and revealing good agreement with inelastic neutronscattering results. For the isostructural Cs2CuBr4, we obtain J/kB = 14.9(7) K, J'/kB = 6.1(3) K, [J'/J ≃ 0.41], providing exact and conclusive information on the exchange couplings in this frustrated spin system.

es hat kein Abstract vorgelegen

We report on an investigation of optical properties of multiferroic CoCr2O4 at terahertz frequencies in magnetic fields up to 30 T. Below the ferrimagnetic transition (94 K), the terahertz response of CoCr2O4 is dominated by a magnon mode, which shows a steep magnetic-field dependence. We ascribe this mode to an exchange resonance between two magnetic sublattices with different g factors. In the framework of a simple two-sublattice model (the sublattices are formed by Co2+ and Cr3+ ions), we find the inter-sublattice coupling constant, λ = −(18 ± 1) K, and trace the magnetization for each sublattice as a function of field. We show that the Curie temperature of the Cr3+ sublattice, θ2 = (49 ± 2) K, coincides with the temperature range, where anomalies of the dielectric and magnetic properties of CoCr2O4 have been reported in literature.

We report low-temperature electron spin resonance (ESR) studies of single-crystalline samples of Cs2CuBr4, a spin-1/2 antiferromagnet with a triangular spin-lattice structure. A remarkable angular dependence of the resonance field, including the splitting of the ESR line for some orientations of the magnetic field, and the presence of a gap in the ESR excitation spectrum at temperatures above TN ~1.3 K have been revealed. Our observations suggest that uniform Dzyaloshinskii-Moriya interaction affects the low-energy excitation spectrum in this frustrated compound. The results are compared with that obtained recently for the isostructural material Cs2CuCl4 [Povarov et al., Phys. Rev. Lett. 107, 037204 (2011)]

We report systematic tunable-frequency electron spin resonance (ESR) studies of the natural mineral azurite Cu3(CO3)2(OH)2. This material has a diamond spin-chain structure and exhibits unusual magnetic properties (including a magnetization plateau). ESR experiments have been done at frequencies from 50 to 1000 GHz in magnetic fields up to 51 T. The observed frequency-field dependence of the magnetic excitations clearly indicates the important role of magnetic frustrations in this material, which can be suppressed by high magnetic fields. Our findings are discussed in connection with results of inelastic neutron-scattering experiments.

Ba3Cr2O8 is a system of three-dimensionally coupled dimers, exhibiting a field-induced phase transition from a quantum-disordered to a long-range antiferromagnetically ordered state at Hc1 = 12.5 T. We report tunable-frequency electron spin resonance (ESR) studies of Ba3Cr2O8 single crystals performed in a frequency range between 50 and 700 GHz in fields up to 25 T (applied along the c axis). Two gaps in the magnetic excitation spectrum, Δ1 = 564 GHz and Δ2 = 400 GHz, were observed directly, confirming the spin-singlet ground state of Ba3Cr2O8 below Hc1. The observation of singlet-triplet ESR transitions, which are normally forbidden in spin-1/2 dimer systems by selection rules, indicates the break-down of the axial symmetry in this compound. Details of the ESR excitation spectrum of Ba3Cr2O8 across different regions of its phase diagram will be discussed.

The successful use of picosecond-pulse free-electron-laser (FEL) radiation for the continuous-wave THz-range electron spin resonance (ESR) spectroscopy has been demonstrated. The unique combination of two linac-based FELs (covering the wavelength range of 4 - 250 μm) with high magnetic fields at the Research Center Dresden-Rossendorf (FZD) allows for tunable-frequency ESR spectroscopy in an extraordinary broad frequency range of 1.2 - 75 THz in magnetic fields up to ~70 T. The new approach is of particular importance for studying magnetic excitations in materials exhibiting field-induced phenomena (including magnetic phase transitions) and in spin systems with a large zero-field splitting. The performance of the spectrometer is illustrated with ESR spectra obtained in the low-dimensional organic material (C6H9N2)CuCl3 and the multiferroic compound YMnO3 This work was made in collaboration with R. Wünsch, W. Seidel, H.D. Zhou, C. Wiebe.

The successful use of picosecond-pulse free-electron-laser (FEL) radiation for the continuous-wave THz-range electron spin resonance (ESR) spectroscopy has been demonstrated. The unique combination of two linac-based FELs [1] (covering the wavelength range of 4 - 250 μm) with high magnetic fields at the Research Center Dresden-Rossendorf (FZD) allows for tunable-frequency ESR spectroscopy in a frequency range of 1.2 - 75 THz in magnetic fields up to ~ 70 T with a spectral resolution better than 1% [2]. The new approach is of particular importance for studying magnetic excitations in spin systems with a large zero-field splitting and materials exhibiting field-induced phenomena. The performance of the spectrometer is illustrated with ESR spectra obtained in the low-dimensional organic material (C6H9N2)CuCl3 and the multiferroic compound YMnO3††.

We report on X-band electron spin resonance studies (ESR) of the quasi-one-dimensional anisotropic S = 1 Heisenberg antiferromagnet compound Ni(tn)2NO2BF4 (or NTNB for short). The presence of interacting S = 1/2 chain-end states is clearly observed in the ESR spectra even in nominally pure samples, visible as a g ~ 2 main peak with several satellites. The data are analyzed in the frame of the theory proposed by Batista et al. [Phys. Rev. B 60, R12533 (1999)]. The ESR angular dependence revealed a pronounced anisotropy, whose origin can be explained assuming the presence of the field-induced staggered magnetization.

Multiferroic rare-earth manganites have attracted much attention because of the coexistence of ferroelectric and magnetic orders. Among hexagonal manganites, the largest magnetoelectric effect was found in HoMnO3, which is ferroelectric below Tc = 875 K and antiferromagnetically ordered below TNMn = 75 K (Mn subsystem). In addition, at TNHo ~ 5 K the Ho subsystem undergoes a transition into an antiferromagnetically ordered state. The coupling between the ferroelectric and antiferromagnetic order parameters in HoMnO3 has not been yet unambiguously explained, revealing the importance of geometrical frustrations and a complex interplay between Ho and Mn magnetic subsystems [1, 2]. Here, the magnetic excitation spectrum in high-quality single-crystalline samples of HoMnO3 was probed by means of X-band (9.3 GHz) electron spin resonance spectroscopy. A relatively strong absorption was found below 5 K, which corresponds to the temperature of the magnetic ordering of the Ho subsystem. The observed mode exhibits a very pronounced anisotropic behavior and can be interpreted as excitations within the antiferromagnetically ordered Ho subsystem.