Temperature dependence of the spontaneous magnetisation of ferromagnetic insulators: Does it obey the 3/2-5/2-β law?

Temperature dependence of the spontaneous magnetisation is analysed for three archetypal ferromagnetic insulators, EuS, EuO and CrBr3. The main point is the shape, rather than scale, of the curve Ms(T). As far as the available data enable us to judge, this shape is adequately described by the simple “3/2-5/2-β” expression (m=[1−sτ3/2−(1−s)τ5/2]β), proposed previously for metallic ferromagnets, and depends on a single dimensionless parameter s. The lack of direct experimental data makes this conclusion a preliminary one. In order to resolve the question definitively, accurate magnetic measurements on single crystals are required as a matter of priority.