Abstract
Heavy metal layers, exemplified by Pt, are known to play a significant role in the magnetization behaviour of thin-film ferromagnets by three distinct mechanisms that can each contribute to the reversal process. These include modifying the local magnetization state via an interfacial Dzyaloshinskii-Moriya interaction (IDMI), enhancement of the damping, via d-d hybridisation and spin-pumping across the interface, and the mediation of the magnetization switching, with the flow of current through a system, via the spin-Hall effect. Here we show for a system with weak interfacial DMI (NiFe/Pt) that the measurement of magnetic field-driven magnetization reversal, mediated by domain wall (DW) motion, is dominated by the enhanced intrinsic damping contribution as a function of the Pt capping layer thickness. But, we also show micromagnetically that the IDMI and damping also combine to modify the domain wall velocity behaviour when the damping is larger. It is also noted that Walker breakdown occurs at lower fields and peak DW velocity decreases in the presence of IDMI. These results highlight the significance of the relative contributions of the damping and the IDMI from the heavy metal layer on the magnetization reversal and provide a route to controlling the DW behaviour in nanoscale device structures.
Highlights
Heavy metal layers in contact with ferromagnetic materials have been shown to lead to several remarkable phenomena associated with large spin-orbit coupling that can modify the micromagnetic state of the magnetization, enhance the damping of magnetization precession and drive magnetization reversal via the propagation of pure spin-current generated by the spin-Hall effect in the heavy metal layer[1,2,3,4,5,6]
It requires a larger magnetic field to propagate the domain wall (DW) and produce magnetization reversal. These results suggest that while the damping plays a major role in increasing the DW propagation field, across the range of Pt thicknesses, the role of the interfacial Dzyaloshinskii-Moriya interaction (IDMI) becomes more significant as the damping increasing and the influence of both on the magnetization reversal become more significant for thicker Pt layers
Experimental results and micromagnetic analyses on domain wall magnetization behaviour show the influence of the different phenomenon occurring at the interface in NM/FM bilayers, where the ferromagnetic layer is magnetized in-plane and coupled to a heavy metal, in this case Pt
Summary
To understand the role of other mechanisms, and interfacial DMI, that can occur at the NM/FM interface and affect the reversal behaviour, detailed micromagnetic simulations were performed In these simulations nanowires were modelled with selected damping values and the strength of the IDMI was varied over a range consistent with previous experimental observations[36, 37] which suggest that the IDMI strength has some dependence on the ferromagnetic and heavy metal layer thickness. This indicates that both the Walker field and velocity are modified when interfacial effects such as damping and IDMI occur simultaneously This effect may be explained by the local re-orientation of the spin-structure at the interface due to the influence of the IDMI on the magnetization orientation at the interface, as indicated by the magnetization components extracted from micromagnetic simulations as a function of domain wall position in the nucleation process, see Fig. 7(a) and (b). Simulations indicate that the domain wall translates and rotates around the nanowire axis and it leads to back-and-forth motion slowing down the domain wall velocity for larger magnetic fields
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