Bloch's law (the 3/2 law), the dependence of the spontaneous magnetisation Js of ferromagnets on temperature T, established theoretically by the American physicist F. Bloch in 1930 (for the range of temperatures well below the Curie point θ):
Js = Js0[1-α(T/θ)3/2],
where Js0 is the maximum value of Js at T= 0 K, α is a constant characteristic of the given substance. The formula given above represents the first terms of the expansion of Js(T) in powers of T. The following terms of this series are small (for T<θ), and the Bose law, as experience shows, holds well up to T≈θ/2. The decrease of Js as T rises is due to the disturbance of the ordered orientation of the atomic (spin) magnetic moments in a ferromagnet under the action of the thermal motion of the particles of the substance. At low temperatures the disturbance of the magnetic order has the character of elementary excitations - magnons (see Spin waves). Their number grows with rising temperature in proportion to T3/2, which is reflected in Bloch's formula.
Js = Js0[1-α(T/θ)3/2],
where Js0 is the maximum value of Js at T= 0 K, α is a constant characteristic of the given substance. The formula given above represents the first terms of the expansion of Js(T) in powers of T. The following terms of this series are small (for T<θ), and the Bose law, as experience shows, holds well up to T≈θ/2. The decrease of Js as T rises is due to the disturbance of the ordered orientation of the atomic (spin) magnetic moments in a ferromagnet under the action of the thermal motion of the particles of the substance. At low temperatures the disturbance of the magnetic order has the character of elementary excitations - magnons (see Spin waves). Their number grows with rising temperature in proportion to T3/2, which is reflected in Bloch's formula.