Magnetization measurements along the main symmetry directions of the tetragonal structure, [100], [110] and [001], have been performed on several single crystals (R = Y, Tb and Ho), at applied high magnetic fields up to 12 Tesla and temperatures ranging from 4.2 to 300 K. From the measurements on the yttrium compound the second- and fourth-order anisotropy constants and the spontaneous magnetization of the Fe sublattice have been determined. The complex magnetic behaviour has been explained using a single-ion model for the crystal electric field (CEF) interaction and a mean-field model for the exchange interaction. A reliable set of CEF parameters and mean exchange field have been obtained for the and intermetallic compounds. All the experimental features observed, the first-order magnetization process under magnetic field in and spin reorientation transition (SRT) in , have been explained using such CEF and exchange parameters. A study of the character of the SRT observed in the intermetallic compound has been undertaken by measuring the parallel and perpendicular components of the magnetization to the applied magnetic field. The experimental results obtained have been explained considering the coexistence of two magnetic phases for temperatures close to the spin reorientation one.

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Single-crystal neutron diffraction was employed to study the magnetic transition in the pseudobinary Lavesphase compounds Er 1Ϫx Y x Co 2 (xϭ0, 0.4) under external magnetic fields up to 4 T. The magnetic scattering amplitudes measured for the reflections to which either the localized 4 f Er moment or the itinerant 3d Co moment solely contributes, give direct evidence that the onset of long-range magnetic order for both magnetic sublattices occurs at about the same temperature T C ϳ35.9 and 17.0 K for xϭ0 and 0.4, respectively. The magnetic-susceptibility and specific-heat data, obtained on the same specimens, were also measured. In agreement with the neutron-diffraction data the macroscopic measurements showed no multiple phase transitions as opposed to earlier measurements on powder samples, which are presented and discussed as well.