Phenomenological theory of the phase transition in uniaxial ferromagnetic materials

The properties of ferromagnetic materials exhibiting uniaxial magnetocrystalline anisotropy in the vicinity of the Curie temperature T_c are treated in various ways. As a first step the equations of state are derived using a Landau theory. Then modified equations of state, which are applicable in the critical region, are obtained by inserting empirically critical exponents into the equations of the Landau theory. These modified equations of state describe the critical behaviour of ferromagnets with uniaxial anisotropy for an arbitrary angle θ₀ between the external field and the easy axis of the crystal. The validity of these equations is shown by comparing the theoretical results with experimental data obtained for Fe ₁₄Nd ₂B single crystals. From the modified equations of state the effective critical exponents γ( θ₀) and β( θ₀) depending on the angle between the applied field and the easy axis can be derived. Additionally we present experimental results of the critical behaviour of sintered ferromagnets. Small deviations of the critical exponents from those derived for single crystals are explained quantitatively by the misorientation of the magnetic grains of the sintered samples.