L. N. Demianets

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.

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.

Triplet spin resonance of the Haldane magnetPbNi2V2O8with interchain coupling

Spin resonance absorption of the triplet excitations is studied experimentally in the Haldane magnet PbNi2V2O8. The spectrum has features of spin S=1 resonance in a crystal field, with all three components, corresponding to transitions between spin sublevels, being observable. The resonance field is temperature dependent, indicating the renormalization of excitation spectrum in interaction between the triplets. Magnetic resonance frequencies and critical fields of the magnetization curve are consistent with a boson version of the macroscopic field theory [Affleck 1992, Farutin & Marchenko 2007], implying the field induced ordering at the critical field, while contradict the previously used approach of noninteracting spin chains.

Coexistance of spiral and commensurate structures in a triangular antiferromagnet KFe(MoO4)2

Magnetization, specific heat, magnetic resonance and neutron diffraction measurements are used to study the magnetic structure of KFe(MoO4)2. This stacked triangular antiferromagnet (TN=2.5 K) demonstrates an unusual breaking of the spin system into two intercalated and almost independent 2D subsystems. One is a collinear antiferromagnet with a simple spin-flop behavior. The other is a spiral magnet. The spin structure may be explained assuming two types of inequivalent magnetic planes with distorted triangular lattices of Fe3+(S=5/2) ions.