Magnetoelastic Effects

1 12.1 The Main Magnetoelastic Effects

1.1 1.1 ANOMALOUS EXPANSION SPONTANEOUS AND FORCED EXCHANGE MAGNETOSTRICTION

Because exchange interactions are sensitive to interatomic distance, the order parameter 〈S _{i ·} S _j〉 couples to the dimensions of a magnetic sample. This magnetoelastic coupling generates exchange magnetostriction, the spontaneous distortion which is observed below the ordering temperature.

Figure 12.1 shows the thermal variation of the lattice parameter of nickel. Above T_c = 631 K, the behavior is normal, and extrapolation can yield the value a^* of the lattice parameter which nickel would have at 0 K if it were not magnetic. An anomaly in thermal expansion is observed around T_c, as well as a relative decrease in the lattice parameter, which at low temperature reaches (a₀ − a^7*)/a^* = −4 × 10⁻⁴, as was already noted for the compound GdAl₂ (fig. 3.19). This deformation does not lower the symmetryof the material. In isotropic and cubic materials, it can therefore be...