Isothermal magnetic entropy change

The above physical quantity, denoted by AS and expressed in J K , is called the field-induced isothermal magnetic entropy change or simply isothermal magnetic entropy change. 

This variation, called the adiabatic temperature change calculation of the isothermal magnetic entropy change, is based on the following Maxwell relation ... 

For most paramagnetic and ferromagnetic materials, the isothermal magnetic entropy change per unit of magnetic induction usually ranges from 1 to 4 J.kg .K .T . 

Fig. 21. a) Isothermal M versus H plots of a simulated mixed-phase system, from 295 to 350 K (0.5 K step) and b) magnetic entropy change values resulting from the direct use of the Maxwell relation. 

Figure 4.1 Schematic diagram showing the thermodynamics of the demagnetization process. By changing the magnetic field from the initial value Hl to the final one Hf, with Hf < Hj a gain in magnetic entropy, ASm, occurs during the isothermal process from...

The choice of magnetic field variation as the process that distinguishes the two curves in Figs, la and lb is of practical, but not of theoretical, significance. If we could find any reversible isothermal process whose entropy change remained finite as 0 K was approached, it would be described by a diagram similar to Fig. la and theoretically permit the attainment of 0 K in a finite number of steps. The third law therefore requires the following ... 

Fig. 15. Schematic diagrams of a a) second-order and b) first-order M vs. H plots, showing the area between magnetic isotherms. From Eq. 33 these areas directly relate to the entropy change.

Table 7.3. Selected materials exhibiting a magnetocaloric effect Material Magnetocaloric Relative Isothermal specific entropy peak temperature cooling power change per unit (K) (K ) of magnetic induction (As /R)inJm T Adiabatic temperature drop per unit of magnetic induction (Ar/B)inKr 7 Magnetic Materials...

We can find the sign of the entropy change during the isothermal magnetization process shown as path A in Fig. 6.3 on page 158. In order to use T, p, and B as the independent... 

Since mmag at constant B decreases with increasing T, as explained above, we see that the entropy change during isothermal magnetization is negative. 

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