Isotropic and anisotropic magnetoelastic interactions in heavy and light RCo₂Laves 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 RCo 2 the magnetic moment of the itinerant d electron subsystem derived from the magnetovolume effect was found to fit well with the magnetization curve of YCo 2 . 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 PrCo 2 , NdCo 2 and SmCo 2 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: PrCo 2 , tetragonal distortion, easy axes (100), lambda 100 =-3.4*10 -3 : SmCo 2 , rhombohedral distortion, easy axes (111), lambda 111 =-4.6*10 -3 ; NdCo 2 , 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 .