Effect of Fe layer thickness and Fe/Co intermixing on the magnetic properties of Sm–Co/Fe bilayer exchange-spring magnets

Sm–Co(20 nm)/Fe(tFe) bilayers are fabricated with different Fe layer thicknesses (tFe = 0, 2, 4, 6 and 8 nm) in order to systematically investigate the intermixing effect between Fe and Sm–Co layers and its influence on the magnetic properties. X-ray diffraction analysis indicates that the deposition of soft layer (Fe) promotes crystallization of hard layer (Sm–Co) at a low annealing temperature (400 °C). The annealed films consist of hexagonal Sm2Co7, Sm2Co17 and SmCo5 phases in the hard layer and bcc-structured Fe(Co) in the soft layer. Rutherford backscattering is employed to investigate the atomic composition of the individual layers and thereby the extent of diffusion of Co atoms into the Fe layer. The estimated Co content is higher for lower tFe. The magnetic properties of the bilayers very close to the interface are analysed by the magneto-optical Kerr effect. The calculated Kerr intensities (which represent the magnetization process) are significantly affected by the extent of diffused Co in the Fe layer. Superconducting quantum interference device magnetic measurements demonstrate smooth and single-phase magnetic behaviour for tFe up to 6 nm and a good combination of high coercivity (6.5 kOe) and high magnetic remanence (834 emu cm−3) is obtained for tFe = 4 nm. (paper)