Modulation of interfacial magnetic relaxation timeframes by partially uncoupled exchange bias

Abstract

A set of partially uncoupled NiFe/Cu/IrMn exchange biased thin films with variable thickness of non-magnetic Cu spacer is characterized by ferromagnetic resonance (FMR) and Brillouin light scattering (BLS) techniques applied complementary to reveal time-scale dependent effects of uncoupling between ferromagnetic and antiferromagnetic layers on high-frequency magnetization dynamics. The results correlate with interfacial grain texture variations and static magnetization behavior. Two types of crystalline phases with correlated microwave response are revealed at the ferro–antiferromagnet interface in NiFe/Cu/IrMn thin films. The 1st phase forms well-textured NiFe/IrMn grains with NiFe (111)/IrMn (111) interface. The 2nd phase consists of amorphous NiFe/IrMn grains. Intercalation of NiFe/IrMn by Cu clusters results in relaxation of tensile strains at the NiFe/IrMn interface leading to larger size of grains in both the NiFe and IrMn layers. The contributions of well-textured and amorphous grains to the high-frequency magnetization reversal behavior are distinguished by FMR and BLS techniques. Generation of a spin-wave mode is revealed in the well-textured phase, whereas microwave response of the amorphous phase is found to originate from magnetization rotation dominated by a rotatable magnetic anisotropy term. Under fixed FMR frequency, the increase of Cu thickness results in higher magnetization rotation frequencies in the amorphous grains.