MAGNETISM AND SPIN DYNAMICS IN MNPS3 AND PYRIDINE INTERCALATED MNPS3 - AN ELECTRON-PARAMAGNETIC-RESONANCE STUDY

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(ij) . S(i) x S(j) interaction or single ion anisotropy, DS(z)2. 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 theta-1)2 angular dependence of linewidth and Lorentzian line shape at theta=55-degrees. Instead, over the entire temperature range studied, the linewidth showed a 1+cos2 theta 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 theta=55-degrees and 135-degrees. The angular variation of linewidth could be fitted to a A+B cos2 theta+C(3 cos2 theta-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 cos2 theta 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.