Physics of Magnetism

(Sean Pound) #1

SECTION 10.2. THE MAGNETOCALORIC EFFECT 93


be estimated from measurements on nickel alloys that show no magnetic ordering. The
total of these contributions has a temperature dependence as shown by the broken line in
Fig. 10.1.2. After subtraction of this contribution, one finds the magnetic contribution shown
in the lower part of the figure. Comparison with the molecular field result in Fig. 10.1.1 shows
that the general behavior is the same, the main difference being substantial contributions
also above in the experimental curve. This behavior is commonly attributed to so-called
short-range magnetic order. Above the long-range magnetic order that extends over
many interatomic distances disappears. Some short-range order in terms of correlations
between the directions of moments of nearest-neighbor atoms may persist, however, also
at temperatures above the magnetic-ordering temperature.

10.2. THE MAGNETOCALORIC EFFECT

The magnetocaloric effect is based on the fact that at a fixed temperature the entropy
of a system of magnetic moments can be lowered by the application of a magnetic field.
The entropy is a measure of the disorder of a system, the larger its disorder, the higher its
entropy. In the magnetic field, the moments will become partly aligned which means that
the magnetic field lowers the entropy. The entropy also becomes lower if the temperature
is lowered because the moments become more aligned.
Let us consider the isothermal magnetization of a paramagnetic material at a tempera­
ture The heat released by the spin system when it is magnetized is given by its change
in entropy

If the magnetization measurement is performed under adiabatic conditions, the temperature
of the magnetic material will increase. By the same token, if a magnetic material is adi­
abatically demagnetized, its temperature will decrease. The magnitude of the heat effects
involved can be calculated as follows.
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