Omnidirectional zero-field ferromagnetic resonance driven by rotatable anisotropy in FeNi/FeMn bilayers without exchange bias

In this work, the zero- field GHz frequency permeability spectra and the rotatable anisotropy of FeMn/FeNi bilayers are investigated. Omnidirectional zero-field ferromagnetic resonance (FMR) driven by rotatable anisotropy can be achieved in the FeMn/FeNi bilayers without exchange bias. Instead of uniaxial anisotropy and exchange bias field, it is the rotatable anisotropy field that dominates in the unbiased bilayer, due to the absence of the exchange bias. The quantitative analysis of exchange bias field, uniaxial anisotropy and rotatable anisotropy is discussed by comparing the dynamical and static magnetic properties of FeNi single layer, biased and unbiased FeMn/FeNi bilayers.