Magnetization profiles in Fe/SmCo Spring Magnets with Graded Interfaces

To understand the improved effectiveness of the exchange coupling in Fe/SmCo spring magnets with a graded interface, we have determined the magnetic structure with Polarized Neutron Reflectometry (PNR). PNR confirms that the Fe/SmCo interface is greatly intermixed. Magnetic hysteresis curves show well-separated coercive fields for the soft phase $H_C^S$ and the hard phase $H_C^H$. Large spin-flip (SF) scattering reveals a twisted magnetic structure at $H_C^S$. SF scattering initially decreases slightly when $H$ increases, then drops dramatically when $H> H_C^H$. This suggests that the twisted region first becomes more narrow and then suddenly diminishes once the SmCo layer switches. Combining 1D micro-magnetic simulations and PNR, we find that the SmCo layer has a much lower average in-plane anisotropy than the bulk. The scattering along the recoil loops strongly depends on the starting field, but in all cases a collinear magnetic structure is indicated at low fields. Overall, the magnetization reversal is mostly reversible rotation for $H < H_C^H$, but an abrupt irreversible switching happens when $H > H_C^H$.