Isotropic and anisotropic magnetoelastic interactions in heavy and light RCo₂ Laves phase compounds

The thermal expansion and anisotropic magnetostriction of the RCo ₂ Laves phases were studied in the temperature range 4-500 K using the X-ray powder diffraction method. In the heavy RCo₂ the magnetic moment of the itinerant d electron subsystem derived from the magnetovolume effect was found to fit well with the magnetization curve of YCo₂. A pronounced paraprocess above the metamagnetic transition has been observed when increasing the f-d exchange field. The type and temperature variation of the distortion of the cubic unit cell of the three compounds PrCo₂, NdCo₂ and SmCo₂ have been studied in detail. The corresponding magnetostriction constants lambda ₁₁₁ or lambda ₁₀₀ were calculated. At 4 K the following values have been obtained: PrCo₂, tetragonal distortion, easy axes (100), lambda ₁₀₀=-3.4*10^-3: SmCo₂, rhombohedral distortion, easy axes (111), lambda ₁₁₁=-4.6*10^-3; NdCo₂, tetragonal distortion and easy axes (100) for 42 K<T<T_c, orthorhombic distortion and easy axes (110) for T<42 K with mod lambda ₁₁₁ mod =1.9*10^-3 and lambda ₁₀₀=-4.6*10^-3.