Study of dynamic properties and magnetic anisotropies of NiFe/MnPt in the critical thickness range

Abstract

The interfacial ferromagnetic/antiferromagnetic exchange coupling and ferromagnetic and antiferromagnetic anisotropies of polycrystalline ferromagnetic/antiferromagnetic exchange biased bilayers of NiFe/MnPt grown by RF sputtering have been investigated with quasi-static and dynamic measurements for different MnPt and NiFe thicknesses. The dynamic properties have been investigated through complex permeability frequency spectra measurements in the 100 MHz–3 GHz range. An effective field is deduced from dynamic measurements. A sharp enhancement of this effective field appears above a critical MnPt thickness (4.7 nm) lower than the critical thickness for the appearance of the easy axis hysteresis loop shift (7.8 nm). We observe also a strong enhancement of the damping parameter for the former thickness. This may be attributed to the superparamagnetic to antiferromagnetic transition in the antiferromagnetic layer. The anisotropy constant of the antiferromagnetic layer determined with dynamic measurements is in agreement with the one deduced from static measurements. The interface exchange energy JE deduced from dynamic measurement is twice higher than the one obtained with hysteresis loop measurements. The dispersion relation of the resonance frequency of imaginary part of the permeability shows hysteretic behaviour and discontinuities when measured in presence of a static magnetic field applied along the easy axis of the ferromagnetic layer. This is attributed to partial magnetization switching by rotation mechanism in the ferromagnetic layer.