SPONTANEOUS MAGNETIZATION OF BCC HEISENBERG FERROMAGNET CU(NH4)2BR4.2H2O

Data are reported on the measurement of the spontaneous magnetization vs. temperature relation for the simple Heisenberg s = 1 2 ferromagnet Cu(NH4)2Br4.2H2O (Tc=1.74±0.01 K). In the aa plane an anisotropy field has been observed which is 150 Oe at 0.1 K and decreases only to 60% at Tc. This phenomenon is interpreted in terms of anisotropic exchange coupling between pairs of spins. On a relative temperature scale, Tc has been determined with an accuracy of 0.1%. The spontaneous magnetization is deduced from magnetization vs. temperature data obtained by a sample induction method. The spontaneous magnetization has been obtained for 0.05 < T/Tc < 0.997. The observed temperature dependence agrees quite well with the recent second order's Green's functions calculations by Cooke and Gersch (1967). The critical behaviour is described by M(T)/M(0) = B(1-T/Tc)β with B = 1.33 (±0.15) and β = 0.38 (±0.04). The critical isotherm is given by H ≈ Mδ with δ = 3.9 (±0.2). It is suggested that either γ' < γ, or α' + 2β + γ' > 2 with γ' ≡ γ. © 1969.