Measurement of the magnetic susceptibility of Cu(NH4)2Br4.2H2O near its curie temperature

Abstract

The zero-field susceptibility of Cu(NH4)2Br4.2H2O has been measured, using an a.c. induction method. In the critical temperature region the susceptibility can be described by the power law:χTC=A(1−TcT)−γ, with γ=1.31 ± 0.02 and A = = 1.22 ∓ 0.15. The transition temperature has been determined independently as Tc=1.773 ± 0.001 K, by heat-capacity measurements on the same sample. Since Cu(NH4)2Br4.2H2O and isomorphous copper salts have been found to be representative of the b.c.c. Heisenberg ferromagnet (S=12) - for which theory predicts γ = = 1.43 ± 0.01 - the obtained γ value is surprisingly low. This suggests that either the presence of a small anisotropy (0.7%),or scaling relations are checked tentatively. influence the exponent considerably. The scaling relations are checked tentatively. A relaxation effect has been observed that shows a narrow maximum just below Tc and vanishes upon application of a constant field of about 50 Oe. Such an effect has been predicted on the basis of the Landau theory of second-order phase transitions.