Interlayer exchange coupling (Jex) between a hard magnetic Nd-Fe-B layer and a soft magnetic Ni80Fe20 layer is studied by means of time-resolved magneto-optical Kerr effect (TRMOKE) microscope. Whereas a single 16 nm thick Nd-Fe-B layer without Ni80Fe20 showed high coercivity of μ0Hc = 2 T and resonance frequency of fr = 161 GHz at external bias magnetic field of μ0Hb = 2 T due to the high anisotropy field, those of the bi-layer Nd-Fe-B (16 nm)/Ni80Fe20 (5 nm) are dramatically reduced to μ0Hc = 1.34 T and fr = 74.4 GHz. When the Nd-Fe-B and Ni80Fe20 are separated by a 1 nm thick non-magnetic Mo layer, by contrast, the coercivity recovered partially to μ0Hc = 1.9 T but the frequency further reduced to fr = 63.4 GHz. We derived Jex based on a simple macrospin model, whose value reduced from 3.9 ± 0.1 mJ/m2 for the bi-layer without the Mo layer to 0.1 ± 0.1 mJ/m2 with the Mo layer. The reduction in Jex suggested that the interlayer exchange decoupling between the Nd-Fe-B and the Ni80Fe20 layers was responsible to the recovery of μ0Hc and the reduction of fr by the insertion of the non-magnetic layer. We successfully estimated the interlayer exchange coupling constant in the hard/soft magnetic bilayer system by TRMOKE and macrospin-modeling, which had been previously difficult because of its high anisotropy and high coercivity. This method is applicable also to the quantitative estimation of the intergranular exchange coupling.
ASJC Scopus subject areas
- Electronic, Optical and Magnetic Materials
- Condensed Matter Physics