Magnetovolume effects

In this section some examples for spontaneous magnetovolume effects in cubic Gd based compounds will be presented. As will be discussed in section 5, in cubic systems also distortions of the crystal symmetry have been observed. In all cases of our knowledge these distortions are, however, very small compared to strains which conserve the crystal symmetry and which will be the main topic of the rest of this chapter. The symmetry breaking effects are so small that they probably can only be observed in the highly symmetric cubic systems, where the detection of such distortions is easier. 

Fig. 5. Temperature variation of the hexagonal lattice parameters and of the volume of pure gadolinium measured by x-ray powder diffraction (this work). The values have been normalized to 300 K in order to show the relative changes. (The values at 300 K are a = 3.632 0.002 A, c = 5.782 0.002 A.) The lines represent the extrapolation of the lattice contribution from temperatures above Tq assuming a Debye temperature of 184 K (Bodnakov et al. 1998). The lowest part of the figure shows the magnetovolume effect, obtained by subtracting the lattice contribution from the volume expansion.

Shimizu (1977, 1978, 1980, 1981) regarded the magnetovolume effects in the framework of a thermodynamic approach with the help of mean field and band models. He expanded the free energy in a power series of magnetization and relative volume change AV/F. The Curie temperature shift under compression was determined from the position of the spin susceptibility pole x T,P) = 0. From this condition the formula for calculating dJc/dP was derived (Inoue and Shimizu 1985) ... 

Shi] Shirae, K., Ueda, Y, Kachi, S., Kosuge, K., Phase Diagram and Magnetovolume Effect of Pseudobinary CrB-FeB System , Mater. Res. Bull., 22(4), 521-526 (1987) (Crys. Structure, Phase Diagram, Experimental,, 10)... 

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