We have studied magnetic properties of two-dimensional (2D) triangular-lattice antiferromagnets (TAFMs) focusing on three compounds: NiGa2S4, CuCrO2, and Rb4Mn(MoO4)3 by means of electron spin resonance (ESR) and magnetization in high magnetic fields up to about 68 T.
In the case of NiGa2S4, from the temperature evolution of the ESR absorption linewidth, we find a distinct disturbing of the development of the 2D spin correlation by Z2-vortices, which are formed by vector spin chiralities, between 23 K and 8.5 K. This finding suggests the occurrence of a Z2 vortex-induced topological transition at 8.5 K.
The frequency dependence of the ESR resonance fields and the magnetization process of CuCrO2 are well explained by a mean-field analysis considering the out-of-plane 118°spiral spin structure on a distorted triangular-lattice model with two kinds of antiferromagnetic in-plane exchange interactions and rhombic anisotropy. Then, we have quantitatively clarified the ground state spin structure associated with its ferroelectricity and the first-order magnetic phase transition accompanied by the flop of the electric polarization.
For Rb4Mn(MoO4)3, we have successfully explained the observed frequency dependence of the ESR resonance fields and the reported magnetization curves for both parallel and perpendicular to the easy-axis normal to the triangular plane, by means of a mean-field approximation and a classical Monte Carlo simulation assuming a spin Hamiltonian of the 2D Heisenberg TAFM with Ising anisotropy. Then, we evaluate the anisotropy and the exchange constants accurately. Consequently, we have confirmed that Rb4Mn(MoO4)3 is a good example of a quasi-2D Heisenberg TAFM with Ising anisotropy.
As for the second topic, to advance the frontiers of physics in quantum spin and frustrated systems, it is sometimes necessary to combine extremely low temperatures and high magnetic fields. Thus, we have developed a compact 3He cryostat and a wide bore pulse magnet. This combination enables us to perform ESR measurements at extremely low temperatures down to 0.6 K in magnetic fields up to 60 T. It is a unique experimental apparatus in the world to measure ESR in such extreme conditions.