Time domain magnetization dynamics study to estimate interlayer exchange coupling constant in Nd-Fe-B/Ni_(80)Fe_(20) films

摘要

Interlayer exchange coupling (J(ex)) 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 mu H-0(c)= 2 T and resonance frequency of f(r) = 161 GHz at external bias magnetic field of mu H-0(b) = 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 mu H-0(c) = 1.34 T and f(r) = 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 mu H-0(c) = 1.9 T but the frequency further reduced to f(r) = 63.4 GHz. We derived J(ex) based on a simple macrospin model, whose value reduced from 3.9 +/- 0.1 mJ/m(2) for the bi-layer without the Mo layer to 0.1 +/- 0.1 mJ/m(2) with the Mo layer. The reduction in J(ex) suggested that the interlayer exchange decoupling between the Nd-Fe-B and the Ni80Fe20 layers was responsible to the recovery of mu H-0(c) and the reduction of f(r) 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.