Tuning interfacial Dzyaloshinskii-Moriya interactions in thin amorphous ferrimagnetic alloys

Abstract

Skyrmions can be stabilized in magnetic systems with broken inversion symmetry and chiral interactions, such as Dzyaloshinskii-Moriya interactions (DMi). further, compensation of magnetic moments in ferrimagnetic materials can significantly reduce magnetic dipolar interactions, which tend to favor large skyrmions. tuning DMi is essential to control skyrmion properties, with symmetry breaking at interfaces offering the greatest flexibility. However, in contrast to the ferromagnet case, few studies have investigated interfacial DMi in ferrimagnets. Here we present a systematic study of DMI in ferrimagnetic CoGd films by Brillouin light scattering. We demonstrate the ability to control DMI by the coGd cap layer composition, the stack symmetry and the ferrimagnetic layer thickness. the DMi thickness dependence confirms its interfacial nature. In addition, magnetic force microscopy reveals the ability to tune DMI in a range that stabilizes sub-100 nm skyrmions at room temperature in zero field. our work opens new paths for controlling interfacial DMi in ferrimagnets to nucleate and manipulate skyrmions. Magnetic skyrmions due to their non-trivial topology have interesting properties 1-3 that make them attractive for spintronic applications, such as racetrack memory and logic devices 4-6. A magnetic skyrmion designates a chiral spin texture with a whirling spin configuration 7. Skyrmions can be stabilized by broken inversion symmetry and chiral interactions, such as the Dzyaloshinskii-Moriya interactions (DMI) 8,9 , which is an antisymmetric exchange interaction that favors non-collinear neighboring spins. Ultrathin magnetic materials with interfaces to heavy non-magnetic metals with large spin-orbit coupling exhibit interfacial DMI that stabilizes skyrmions and chiral domain walls 10-13. The interfacial DMI and the nucleation of skyrmions have been extensively investigated in ferromagnetic materials 10,14-18. Very recently, magnetic skyrmions and chiral domains were reported in ferrimagnetic systems 19-21. Nearly compensated thin ferrimagnetic films with interfacial DMI are interesting materials due to their low stray fields, reduced sensitivity to external magnetic fields, and fast spin dynamics, which are predicted to lead to ultrasmall and ultrafast skyrmions 19,22. Unlike in ferromagnets where fast current-induced motion of chiral textures is impeded by the Walker breakdown and domain wall pinning 13,23-25 , high domain wall velocities-reaching 1000 m s-1-have been observed in ferrimagnetic CoGd films near the angular momentum compensation temperature 19. In addition, the large dipolar fields in ferromagnets are obstacles to the formation of ultrasmall skyrmion 22. Hence, ferrimagnetic thin films are promising candidates for ultrafast skyrmion-based spintronics. Recently, bulk DMI was reported in an amorphous ferrimagnetic GdFeCo alloy 26. However, the significant advantages of interfacial DMI are that it can be controlled by the nature of the interfaces and widely tuned to stabilize skyrmions. Yet, interfacial DM...

Interplay between Spin‐Orbit Torques and Dzyaloshinskii‐Moriya Interactions in Ferrimagnetic Amorphous Alloys

“…Additionally, we previously experimentally investigated the interfacial DMI in Pt/CoGd/W films by Brillouin light scattering and we found DMI energies of the same order of magnitude as our DFT calculations. [ 23 ] Furthermore, in the SOT switching experiments, the metastable state, which was associated with chiral domain wall motion, was observed only in CoGd films capped by W and Ta and the DFT calculations confirm that these films have the largest DMI which is crucial for the formation of small skyrmions (see Figure 2b,d ). Therefore, our theoretical results suggest that, indeed, the DMI may be the origin of the nonuniform magnetization configuration in the current‐induced SOT switching as discussed in ref.…”

“…We have previously carried out a systematic study of the DMI in Pt/CoGd/W films as a function of the thickness and stack symmetry and we demonstrated the control of the DMI in a range that allowed the formation of sub‐100 nm skyrmions. [ 23 ] Typically, the ferrimagnetic stacks studied for skyrmion nucleation and motion are made with a Pt seed layer and capped by an oxide, such as Pt/CoGd/TaO x [ 15 ] or Pt/GdFeCo/MgO. [ 8 , 18 , 24 ] In this case, the Pt seed layer is the only source of DMI and spin currents via the spin‐Hall effect (SHE).…”

Tunable Magnetic Skyrmions in Ferrimagnetic Mn$_4$N

“…Recent studies have reported Bloch skyrmions in bulk B20 crystals with intrinsic DMI at low temperature 10,11 . On the other hand, Néel skyrmions have been reported in heterostructures with heavy metal interfaces, such as Pt/CoFe/MgO 12 , Ir/Fe/Co/Pt 13,14 , Pt/CoGd/Ta 15 , Pt/CoGd/Pt 1−x W x 16 , and (Pt/Co/Ru) x 17 . For reliable high-density data storage, small (near 10 nm) and stable skyrmions at room temperature are necessary.…”

“…One promising candidate with superior thermal stability compared to the amorphous rare-earth transition metals is rare-earth-free Mn 4 N. There are some key similarities and differences between the rare-earth transition metals and Mn 4 N. Both are ferrimagnetic metals, and there have been successful experimental demonstrations of thin film growth in both systems. Both materials show perpendicular magnetic anisotropy (PMA) in the thin film geometry 16,[20][21][22][23][24][25][26][27] . However, unlike the amorphous structure in the rare-earth transition metals, Mn 4 N is a crystalline compound that forms in the anti-perovskite crystal structure 27 .…”

“…In this paper, we report the experimental observation of skyrmions in Mn 4 N thin films. In a recent study, tuning of interfacial DMI (iDMI) was reported in Pt/CoGd/Pt 1−x W x through the dilution of Pt with W in the capping layer 16,30 . Based on the simulated skyrmion phase diagram, such tuning of iDMI was predicted to allow the control of skyrmion size for desirable applications 31 .…”