Magnetically Induced Enhanced Exchange Spring Effect in CoFe2O4/CoFe2/CoFe2O4 Films

Earlier, we reported exchange coupling between bilayers of CoFe2O4/CoFe2, where CoFe2O4 and CoFe2 are the hard and soft magnetic layers, respectively. These results suggested trilayers structures of CoFe2O4/CoFe2/CoFe2O4 are strong candidates for low hysteresis loss "exchange spring materials." To study this possibility, we extended our investigation to the synthesis of CoFe2O4/CoFe2/CoFe2O4 trilayers under different sputtering deposition conditions, which included oxygen pressure in the case of the CoFe2O4 component and the application of a magnetic field during the deposition of the soft CoFe2. The most significant finding was an increase in the exchange spring figure of merit (FoM) of 36% in-plane and 60% out-of-plane by the application of an applied field during the deposition of the soft CoFe2, resulting in a decrease in hysteretic losses by a similar percentage. We will discuss these spin spring results in terms of an increase in uniaxial anisotropy energy in the CoFe2 layers due to short-range ordering of Fe-Fe and Co-Co pairs during deposition and an enhancement in the coercivity and remnant magnetization of CoFe2O4. We also discuss a metric (FoM) for describing enhancements due to multiple optimized deposition conditions.