Hopkinson effect

For a ferromagnetic material, one speaks about effect Hopkison when the magnetic Susceptibilité or magnetization of material increases brutally just below the Température of Curie. This effect appears when the magnetic Anisotropie material decrease more quickly than its Aimantation when the temperature increases. It is characteristic of the transition between the ferromagnetic state and the paramagnetic state . It does not appear at the time of the reciprocal transition, which distinguishes it from a transition from phase magnetic.

This effect was observed for the first time by John Hopkinson in 1889, on an iron sample where a weak field was applied. This effect can also be observed for particles ferrimagnetic S close to their Température of Néel.

Demonstration in the case of a sample subjected to its field of demagnetization

Let us suppose for example that the system consists of ferromagnetic grains of small size. The magnetic grains are supposed sufficiently small to be monodomaines magnetically. This system initially is magnetized and supposed to be remanent, i.e. the particles present parallel magnetizations between them. The system is then heated. The only involved field is the field of demagnetization created by the magnetized system. Close to the temperature of Curie, the particles present for the majority a random orientation. A residual field of demagnetization exists always, created by the weak portion of particles having preserved a magnetization in a privileged direction.

However, on the assumptions of the model of Stoner-Wohlfarth, the magnetization of a magnetic system directed by chance in a weak magnetic field is expressed by:

$M (T) = \ frac {2} {3} \ rho M_s (T) \ frac {H} {H_a (T)}$

with $M_s (T)$ saturation magnetization, $\ rho$ the factor of compactness of the powder, $T$ the temperature, $H$ the field and $H_a$ the field of anisotropy.

For the weak fields, the field of anisotropy decrease more quickly than the saturation magnetization. The report/ratio of both thus can, according to the system, to lead to an addition of magnetization. Once the temperature of Néel exceeded, cooling does not lead to the observation of this peak of magnetization. Indeed, the initial system is then magnetically completely disordered. There is thus no field applied.

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