Magnitic Phase Boundary for a Quasi-Two-Dimensional Isotropic Antiferromagnet Mn(HCOO)2·2H2O

Abstract
The fully mapped T - H (temperature-magnetic field) phase diagram of a quasi-two-dimensional (2d) antiferromagnet Mn(HCOO) 2 ·2H 2 O ( T N =3.69 K) has been determined by the measurements of magnetic heat capacity and susceptibility. The critical field has been estimated to be 105±5 kOe at T =0 K, which gives the intra-layer exchange constant | J |/ k B =0.35±0.02 K. By applying external field, an anomalous increase of the Néel temperature T N ( H ) has been found ( T N ( H )≈1.10 T N (0) at H =35 kOe) for the first time in this 2d compound as in the case of quasi-1d compounds.