Molecular realizations of 3D Heisenberg magnet: Critical scaling

The critical properties of two related bimetallic compounds {[MII(H2O)2]2[NbIV(CN)8]·4H2O}n (M = Mn, Fe) are reported. The compounds exhibit the transition to a magnetically ordered phase at Tc = 43 K in the case of the Fe-based sample (FeNb), and at Tc = 50 K for the Mn-based sample (MnNb). They differ in relative orientation of the M and Nb sublattices, which is parallel, i.e. ferromagnetic, in FeNb and antiparallel, i.e. ferrimagnetic, in MnNb. Critical exponents β and γ, governing the temperature dependence of the spontaneous magnetization below Tc and the temperature dependence of the initial susceptibility above Tc, respectively, have been determined using the Kouvel-Fisher method. Critical exponent δ has been obtained through the analysis of the critical magnetization isotherm. Thus determined values of the critical exponents were verified by employing two independent forms of the scaling equation of state. All the magnetization data are demonstrated to collapse either onto two universal branches corresponding to T〈Tc and T〉Tc, respectively, or onto a single universal curve. The critical exponents allow to ascribe FeNb as well as MnNb to the universality class of the three-dimensional (3D) Heisenberg model.