The specific heats of ferromagnetic substances

It is well known that a close relation must exist between the thermal and the magnetic properties of a ferromagnetic substance. On the basis of his theory of the internal molecular field, Weiss predicted a discontinuity in the specific heat of a ferromagnetic substance in the region of its critical point. His reasoning may be briefly summarised as follows. The mutual potential energy, E, of a number of elementary magnets, each of moment μ and making an angle θ with the applied field H, is given by E = —½ Ʃ μ H cos θ; so that when we consider a cubic centimetre of the given substance we may write E = — ½ H. I, where I is the intensity of magnetisation. Since the substance is ferromagnetic, we must suppose, according to Weiss, the existence of a molecular field of considerable magnitude, equal to N I, where N is a constant which is obtainable from a knowledge of the Curie constant and the critical point of the substance. Thus we may further write E = — ½NI2 , and, since E is negative, we must provide heat in order to demagnetise the substance. The amount of heat necessary to demagnetise 1 gm. of the substance is therefore 1/2J. N/ᵖ. I2 where ᵖ is the density of the substance. Now I varies with the temperature, so that the heat necessary to demagnetise a substance results in an apparent increase of its specific heat by an amount ∂/∂T (1/2J. N/ᵖ. I2 ) = 1/2J. N/ᵖ. ∂I2 /∂T.