Weiss theory of ferromagnetism

To explain the phenomenon of ferromagnetism, Weiss proposed a hypothetical concept of ferromagnetic domains. He postulated that the neighboring atoms of the ferromagnetic materials, due to certain mutual exchange interactions, from several number of very small regions, called domains.

Weiss theory of ferromagnetism is also called domain theory of ferromagnetism. It has following points:

The domains which are aligned approximately along the direction of the applied magnetic field grow in size at the cost of unfavorably oriented domains, that is, those align opposite to the field direction get reduced. In other words, the domain boundaries move so as to expand the favorable domains.

Also domains rotate and orient themselves in the direction of the external magnetic field.

In the presence of the weak external field, the magnetisation in the material occur mostly by the process of domain growing, but in the strong magnetic field the material is magnetised mostly by the process of domain alignment. When the field is removed, the domain boundaries do not recover their original positions and thus the material is not completely demagnetised, but some residual magnetism remains in it.

Temperature dependence:It is shown that the thermal decrease of the spontaneous magnetization of iron differs characteristically depending on the preparation conditions. In all cases, the deviations from saturation at absolute zero can be accurately described by a single temperature power termTεwhich holds up to several hundreds of kelvins. The empirical exponents areε = 2for crystalline bulk iron,ε = 3/2for amorphous iron as well as for isotropic (crystalline) two-dimensional iron films, butε = 5/2for inhomogeneous (pseudomorphic) monatomic layers. In crystalline films which are sufficiently thick (~100–300 nm) to allow the excitation of standing spin waves, theT2bulk law is confirmed for the uniform precession mode, but for states with a periodically modulated magnetization due to spin wave resonance, aT3/2dependence holds. For ultrathin iron films with more than three atomic layers, a thermodynamic crossover fromT3/2toT2is observed at higher temperatures. This is indicative of a dimensionality change of the relevant interactions from 2 to 3. Crystalline and amorphous bulk iron also exhibit a crossover, but only for the amplitude and not for the exponent of theTεlaw. This is assumed to be caused by the thermal variation of the magnetic interaction strength. All observed empirical exponentsεfit into a recently proposed scheme of universality classes for the low-temperature behaviour of the order parameter in materials with half-integer spin quantum number and three-, two-and one-dimensional interactions. It can therefore be concluded that the thermodynamics of iron is like that of an insulating material withs= 1/2.