Second-order phase transitions in ferromagnetic materials in weak fields near the Curie point

A review is given of the present state of theoretical and experimental research on second-order phase transitions in anisotropic and nonunifonnly magnetized ferromagnetic and ferrimagnetic materials and on phenomena observed near the Curie point in weak magnetic fields ($H<H_A$, $H_d$, where $H_A$ and $H_d$ are the anisotropy field and demagnetizing field, respectively). The nature of these transitions is examined, and it is shown that for anisotropic and nonunifonnly magnetized ferromagnetic materials the Curie point is not an isolated point on the $H$–$T$ plane. The experimental and theoretical data indicate the existence of a line of second-order phase transitions in a magnetic field applied in certain definite directions with respect to the anisotropy axis. This line of transitions is described by the law $T_C(H)=T_C(0)(1– AH^\omega)$; theoretical estimates in the molecular-field approximation yield values $\omega=2$ for ferromagnetic materials of the easy-axis and easy-plane type and $\omega=2/3$ for cubic ferromagnetic materials. The experimental results on the equilibrium properties (the magnetization, susceptibility, specific heat, magnetostriction, Faraday effect, etc.) and dynamic properties (the speed and attenuation rate of ultrasonic waves, the dynamic susceptibility) not only confirm the existence of a line of phase transitions but also indicate that spin fluctuations play a decisive role in the formation of the transition between ferromagnetic and paramagnetic phases in a weak magnetic field.