Comparative study of the third-order magnetic susceptibility and the specific heat of antiferromagnetic MnBr2 center dot 4H(2)O

We report a comparative study of the third-order magnetic susceptibility chi(3)(T) and of the specific heat C-p(T) of an antiferromagnetically ordering MnBr2 . 4H(2)O single crystal with T-N = 2.13 K. The dominant feature in chi(3)(T) is a positive singularity near T-N. The critical exponents of chi(3) are negative both above and below T-N On the contrary, the critical behaviour of the magnetic contribution C-m(T) to the specific heat C-p(T) is logarithmic below T-N and may be described by a large and positive critical exponent definitely exceeding the value of 0.3 above T-N. These results suggest that the critical behaviour of chi(3)(T) Of a, moderately anisotropic antiferromagnet is qualitatively different from that of C-m(T).