L. A. Prozorova

Magnetic phase diagram of the frustrated S = 1 chain magnet LiCu2O2

We present the results of the magnetization and dielectric constant measurements on untwinned single crystal samples of the frustrated S=1/2 chain cuprate LiCu_2O_2. Novel magnetic phase transitions were observed. A spin flop transition of the spiral spin plane was observed for the field orientations H||a,b. The second magnetic transition was observed at H~15 T for all three principal field directions. This high field magnetic phase is discussed as a collinear spin-modulated phase which is expected for an S=1/2 nearest-neighbor ferromagnetic and next-nearest-neighbor antiferromagnetic chain system.

Magnetic excitations in the geometrically frustrated pyrochlore antiferromagnet Gd2Sn2O7 studied by electron spin resonance

The spin dynamics in the geometrically frustrated pyrochlore antiferromagnet Gd2Sn2O7 is studied by means of the electron spin resonance. In the ordered phase (TN = 1K), we have detected three gapped resonance modes. Their values agree well with the developed spin-wave theory which takes into account the Heisenberg nearest-neighbor exchange, the single-ion anisotropy and the long-range dipolar interactions. The theory also predicts a fourth lowest-frequency gap, which lies beyond the experimental range of frequencies, but determines the exponential decrease of the specific heat at low temperature.

Magnetic phase diagram, critical behavior, and two-dimensional to three-dimensional crossover in the triangular lattice antiferromagnet RbFe"MoO4…2

We have studied the magnetic and thermodynamic properties as well as the NMR spectra of the Heisenberg antiferromagnet RbFe(MoO4)2. The observed temperature dependence of the order parameter, the critical indices and the overall magnetic H-T phase diagram are all in a good agreement with the theoretical predictions for a 2D XY model. The temperature dependence of the specific heat at low temperature demonstrates a crossover from a T^2 law characteristic of a two-dimensional antiferromagnet to a three-dimensional T^3 law.

Spin reorientation in the easy-plane hexagonal antiferromagnet under a canted magnetic field

Spin reorientation process in the quasi-one-dimensional easy-plane antiferromagnet on a stacked triangular lattice has been considered theoretically and studied in an experiment on CsMnBr3. The second-order phase transition associated with the flip of two pairs of sublattices has been shown to persist if the magnetic field is canted at an angle phi from the basal plane. The field and angular dependences of the magnetic torques measured at T=1.8 K, although in qualitative agreement with the classical theory, demonstrate a strong effect of quantum fluctuations.

Chiral and collinear ordering in a distorted triangular antiferromagnet

Magnetization, specific heat, and neutron diffraction measurements are used to map out the entire magnetic phase diagram of KFe(MoO4)2. This stacked triangular antiferromagnet is structurally similar to the famous multiferroic system RbFe(MoO4)2. Because of an additional small crystallographic distortion, it contains two sets of inequivalent distorted magnetic triangular lattices. As a result, the spin network breaks down into two intercalated yet almost independent magnetic subsystems. One is a collinear antiferromagnet that shows a simple spin-flop behavior in applied magnetic fields. The other is a helimagnet that instead goes through a series of exotic commensurate-incommensurate phase transformations. In the various phases one observes either true three-dimensional or unconventional quasi-two-dimensional ordering.

On the possible coexistence of spiral and collinear structures in antiferromagnetic KFe(MoO4)2

The static and resonance properties of a quasi-two-dimensional antiferromagnet phase on a distorted triangular lattice of KFe(MoO4)2 have been experimentally studied. Magnetization curves exhibit features corresponding to the spin-flop transition in a collinear biaxial antiferromagnet and simultaneously show a magnetization plateau characteristic of a triangular spin structure. The magnetic resonance spectra also display absorption lines corresponding to the spin structures of both types. The experimental data are described in terms of a model comprising alternating weakly bound magnetic layers, in which the main two exchange integrals have different values. Below the Néel temperature (TN=2.5 K), some of these layers possess a collinear antiferromagnetic structure, while the other layers have a triangular or spiral structure.

Relativistic AFMR modes in the hexagonal antiferromagnet CsNiCl3

The antiferromagnetic resonance spectra in hexagonal AFM CsNiCl3 has been investigated both experimentally and theoretically, with special attention to relativistic modes. The very-low-energy mode at frequencies around 1 GHz has been observed. This allowed the authors to specify the values of the magnetic constants.

Magnetic structure of the quasi-one-dimensional frustrated antiferromagnet LiCu2O2 with Spin S = 1/2

The magnetic properties of LiCu2O2 single-crystal samples without twinning are investigated using electron spin resonance and nuclear magnetic resonance spectroscopy. The experimental results obtained are described in terms of the model of a planar spiral antiferromagnet for the orientation of the magnetic field H ‖ b or H ‖ c and the model of a collinear spin-modulated antiferromagnet for the orientation of the static magnetic field H ‖ a.

New magnetic states in copper metaborate CuB2O4

The static and resonance properties of copper metaborate CuB2O4 were experimentally studied in a magnetic field applied in the crystal tetragonal plane. The field-induced second-order phase transition to a weakly ferromagnetic state was observed in the temperature range 10–20 K. The low-field state is characterized by the absence of spontaneous moment, and it represents, presumably, a long-period helicoid. At temperatures below 2 K, two sequential first-order phase transitions were observed. They were accompanied by jumps in resonance absorption with a hysteresis upon changing field-scan direction. These transitions can be caused by the transformation of the incommensurate spin structure into the helicoidal states with periods commensurate with the lattice translation period.

NMR study of the high-field magnetic phase of LiCuVO 4

We report on NMR studies of the quasi--1D antiferromagnetic