We report the observation of a frequency shift and splitting of the electron spin resonance (ESR) mode of the low-dimensional S=1/2 frustrated antiferromagnet Cs2CuCl4 in the spin-correlated state above the ordering temperature 0.62 K. The shift and splitting exhibit strong anisotropy with respect to the direction of the applied magnetic field and do not vanish in a zero field. The low-temperature evolution of the ESR is a result of the modification of the one-dimensional spinon continuum by the uniform Dzyaloshinskii-Moriya interaction within the spin chains.

The properties of two quantum spin chain materials, K 2 CuSO 4 Cl 2 and K 2 CuSO 4 Br 2 , are studied by a variety of experimental techniques, including bulk measurements, neutron spectroscopy, and electron spin resonance. The hierarchy of relevant terms in the magnetic Hamiltonian is established. It is shown that these two compounds feature substantial Dzyaloshinskii-Moriya interactions that are uniform within each chain, but antiparallel in adjacent chains. The result is a peculiar type of frustration of interchain interactions, which leads to an unusual field-temperature phase diagram.

Neutron scattering is used to study NiCl2−2xBr2x·4SC(NH2)2, x = 0.06, a bond-disordered modification of the well-known gapped S = 1 antiferromagnetic quantum spin system NiCl2·4SC(NH2)2. The magnetic excitation spectrum throughout the Brillouin zone is mapped out at T = 60 mK using high-resolution time-of-flight spectroscopy. It is found that the dispersion of spin excitation is renormalized, as compared to that in the parent compound. The lifetime of excitations near the bottom of the band is substantially decreased. No localized states are found below the gap energy ∆ 0.2 meV. At the same time, localized zero wave vector states are detected above the top of the band. The results are consistent with a more or less continuous random distribution of bond strengths, and a discrete, possibly bimodal, distribution of single-ion anisotropies in the disordered material.

Experiments in the antiferromagnetic phase of a quasi 2D S = 1/2 quasi-square lattice antiferromagnet Cu(pz)2(ClO4)2 reveal a biaxial type of the anisotropy, instead of the easy-plane one, considered before. The weak in-plane anisotropy, found by means of electron spin resonance spectroscopy and magnetization measurements, is about an order of magnitude weaker, than the off-plane anisotropy. The weak in-plane anisotropy results in a spin-flop phase transition for the magnetic field aligned along easy axis, and, thereby, in a bicritical point on the phase diagram. A remarkable feature of the weak in-plane anisotropy is the abrupt change of its sign at the spin-flop point. This anisotropy switching disappears at the tilting of magnetic field to the easy axis by the angle of 10• within the plane. The nature of the abrupt anisotropy reversal remains unclear. The phase diagram is characterized by the increase of the ordering temperature TN in the magnetic field used, except for a dip near the bicritical point.