Wiedemann効果の理論

抄録

As regards the classical problem of the Wiedemann effect in ferromagnetic polycrystalline substances, no satisfactory theory has yet been proposed except Fromy's theory for the case of thin-walled tube specimens. In a simple way, a general expression is derived for the Wiedemann effect or for the torsion angle per unit length, θ_r of a cylindrical layer of the radius _r in the case of a cylindrical rod of ferromagnetic substance, fixed at its one end and magnetized by a longitudinal magnetic field, H_l, parallel to, and by a circular magnetic field, H_cr, around, the rod axis. When the elastic energy is negligible as compared with the magnetic field energy as in normal ferromagnetics, the general expression is reduced to
θ_r=(2/r){λ_l(H_r) -λ_t(H_r)}(H_tH_cr/H_r²),
where Hr=(H_l²-H_cr²)^1/2 and λ_l(H_r) and λ_t(H_r) are, respectively, the longitudinal and transverse magnetostrictions accompanied with H_r. This expression is rewritten, for the surface of a rod of the radius a, as
θ_a=(2/a){λ_l(H_a)-λ_t (H_a)}(H_lH_ca/H_a²),
which holds also for a thin-walled tube and is the expression derived already by Fromy. This expression is further reduced to
θ_a=(3/a)•λ_l(H_a)•(H_lH_ca/H_a²),
if the volume magnetostriction is considered negligible as in normal ferromagnetics. It is shown that the above expressions can explain, completely qualitatively and to a considerable extent quantitatively, all of the available experimental facts concerning the Wiedemann effect.