Phase-transitions and magnetic properties of rare earth - transition metal glasses and multilayer films

Field-dependent ac susceptibility measurements as a function of temperature are presented for rare earth-transition metal amorphous (a-) alloys. The samples were a-TbFeGa and a-DyFeB, both of which possess strong random magnetic anisotropy. For both cases sharp susceptibility peaks were found, suggesting the presence of well defined phase transitions. As the applied external dc field increased the susceptibility peak height decreased and moved toward lower temperature. The similarities in the low temperature properties of our alloys and standard spin-glasses stimulated us to analyse the magnetic properties near the ordering temperature in terms of a spin-glass phase-transition model. Using the scaling equations derived by Suzuki for such phase transitions, critical exponents were determined for the systems. For a-DyFeB, dc magnetization measurements were also performed. Critical exponents determined from these measurements were similar to those obtained from ac susceptibility measurements. This work presents strong evidence for the validity of a phase-transition description of the spin freezing in the rare earth-transition metal glasses with strong random magnetic anisotropy. ac susceptibility measurements as a function of temperature and magnetization measurements as a function of field at room temperature are presented for Fe/Er and Fe/Nd multilayer films. Evidence for quasi-two-dimensional speromagnetic transitions at interfaces in Fe/Er multilayers is presented. In the Fe/Nd films with individual Fe layer thicknesses less than about 11 A, a coherent (uniaxial) anisotropy, perpendicular to the film plane, causes the magnetization direction to be flipped from in-plane to perpendicular to the film surface. The coherent, perpendicular anisotropy is produced by the Fe-Nd interfaces. Estimates of the interface and volume anisotropy are derived, but no microscopic models for calculating these quantities are available