Magnetism and spin dynamics in MnPS₃ and pyridine intercalated MnPS₃: an electron paramagnetic resonance study

Abstract

A detailed investigation of the magnetic properties of single crystals of the two-dimensional layered Heisenberg antiferromagnet MnPS₃ and pyridine intercalated MnPS₃ by static magnetic susceptibility and electron paramagnetic resonance (EPR) is reported. A detailed analysis of the angular and temperature variation of the EPR linewidths and line shapes has been carried out. While MnPS₃ orders antiferromagnetically at T_N=78 K, the pyridine intercalated compound exhibits isotropic weak ferromagnetism below 70 K. It is known that in the intercalated compound there is a loss of center of inversion so that a canting of spins could arise from either Dzyaloshinsky-Moriya (DM) d_ijS_i×S_j interaction or single ion anisotropy, DS_z². The angular dependence of EPR linewidth and line shape of MnPS₃ even at 300 K shows none of the features expected for a low-dimensional magnetic system at high temperatures; a (3 cos² θ-1)² angular dependence of linewidth and Lorentzian line shape at θ=55°. Instead, over the entire temperature range studied, the linewidth showed a 1+cos² θ dependence and the line shapes predominantly Gaussian. The linewidth as well as the Gaussian character increased with decreasing temperature. The results have been interpreted to show that there is substantial intralayer spin-spin correlations and that critical fluctuations manifest at temperatures much above T_N. The pyridine intercalated compound, however, shows the expected W-shaped angular dependence of linewidth with a minimum at θ=55° and 135°. The angular variation of linewidth could be fitted to a A+B cos²θ+C(3 cos²θ-1)² dependence. However, the near-Lorentzian line shape at all angles implies that other relaxation mechanisms are operative which is also evidenced from the cos²θ contribution to the angular variation of linewidth. Evidence for both DM as well as single ion anisotropy in the intercalated compound is presented. The presence of both of the above terms would explain the isotropic weak ferromagnetism observed in the static susceptibility of pyridine intercalated MnPS₃.