Self-Bias Ferromagnetic Resonance and Quasi-Magnetic Isotropy of (FeCoB/MgO)6 Multilayers Prepared by Composition Gradient Sputtering

A 25 nm Fe_0.7Co_0.3-B film was deposited by a composition gradient sputtering (CGS) method with a gradient B composition (named CGS-FeCoB). The CGS-FeCoB film showed a well-defined uniaxial magnetic anisotropy with anisotropy field HK of 400 Oe and ferromagnetic resonance frequency f_FMR of 6.45 GHz. A (CGS-FeCoB/MgO)₆ multilayer was laminated by six CGS-FeCoB sublayers separated by 2 Å MgO spacers with the easy-axis direction of the neighboring CGS-FeCoB sublayers clockwise rotating 30° in turn. It is exciting that a quasi-magnetic isotropy with a quasi-isotropic hysteresis loop and omnidirectional ferromagnetic resonance around 4.77 GHz was achieved in the designed multilayer. This quasi-isotropic multilayer is promising for the application in inductors, since 100% hard-axis excitation can be achieved for inductors of any shape.