A phenomenological theory of itinerant weak ferromagnetism.

A phenomenological theory is explored for itinerant weak ferromagnetism. Free energy including the higher order terms of m 4 and h m 3 is introduced and examined. This free energy is derived from the mean-field free energy expression with the use of the Bragg–Williams entropy. We apply the present theory to the typical itinerant weak ferromagnetic materials ZrZn 2 , MnSi 2 , and others. To determine the model parameters, we use the experimental value of the magnetization under the large magnetic field or the extension of the Arrott plot with a functional form of hyperbola. This extended Arrott plot would explain various experimentally observed M–H curves at finite temperatures. Finally, we discuss the Sommerfeld coefficient in magnetic fields based on the present theory. [ABSTRACT FROM AUTHOR]