Two-Dimensional XY Behaviours in an Antiferromagnetic Mn(HCOO)2·2H2O Induced by the Magnetic Field

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Abstract
Two-dimensional (2d) XY behaviours have been revealed in a quasi-2d isotropic antiferromagnet Mn(HCOO) 2 ·2H 2 O ( T N =3.69 K) in the external magnetic field of appropriate strength. The anomalous increase of the observed Néel temperature T N ( H ) in the field has been explained to be attributed to a crossover of spin symmetry from the Heisenberg type to the XY type induced by the field. The absolute values of magnetic heat capacity of this 2d XY system induced by the field of H =20 kOe have been compared with the theoretical values expected for the 2d plane rotator model. A quantitative agreement has been found between them in the temperature region higher than T N ( H ). Discussion has been given of the heat capacity peak near the critical region.