Magnetization-reversal mechanism of hard/soft exchange-coupled trilayers

Ni₈₀Fe₂₀/Sm₄₀Fe₆₀/Ni₈₀Fe₂₀ trilayers with in-plane uniaxial anisotropy induced by an applied magnetic field during deposition were prepared on (100)-Si substrates by dc magnetron sputtering. Magnetization hysteresis loops were measured by an alternating-gradient magnetometer for various angles between the external magnetic field and the easy axis. The experimental hysteresis loops were quantitatively described by a theoretical model that combines coherent rotation and domain-wall unpinning. When the external field is along the easy axis, the magnetization reversal is caused by domain-wall unpinning. When the external field is along or near the hard axis, the magnetization reversal is caused by coherent rotation. For other orientations, the magnetization first rotates gradually by coherent rotation and then sharply switches by domain-wall unpinning. The domain-wall angle and the pinning energy at the sharp switching strongly depend on the angle between the external field and the easy axis. Furthermore, the dependence of the coercivity measured in the easy direction on the thickness of Ni₈₀Fe₂₀ layers was theoretically reproduced by an extended model of domain-wall unpinning.