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

Abstract

Ferrimagnetic thin films are attractive for low-power spintronic applications because of their low magnetization, small angular momentum, and fast spin dynamics. Spin orbit torques (SOT) can be applied with proximal heavy metals that also generate interfacial Dzyaloshinskii-Moriya interactions (DMI), which can stabilize ultrasmall skyrmions and enable fast domain wall motion. Here, the properties of a ferrimagnetic CoGd alloy between two heavy metals to increase the SOT efficiency, while maintaining a significant DMI is studied. SOT switching for various capping layers and alloy compositions shows that Pt/CoGd/(W or Ta) films enable more energy-efficient SOT magnetization switching than Pt/CoGd/Ir. Spin-torque ferromagnetic resonance confirms that Pt/CoGd/W has the highest spin-Hall angle of 16.5%, hence SOT efficiency, larger than Pt/CoGd/(Ta or Ir). Density functional theory calculations indicate that CoGd films capped by W or Ta have the largest DMI energy, 0.38 and 0.32 mJ m −2 , respectively. These results show that Pt/CoGd/W is a very promising ferrimagnetic structure to achieve small skyrmions and to move them efficiently with current.

Zero-Field Nucleation and Fast Motion of Skyrmions Induced by Nanosecond Current Pulses in a Ferrimagnetic Thin Film

“…The skyrmion velocity reported as higher in Pt/CoGd/W than in Pt/CoGd/Ta films is a result of a larger DMI and smaller angular momentum based on our previous studies. − For both CoGd films, the skyrmions move faster than DWs (Figures d and c). According to the Thiele model, the linearity of the skyrmion velocity with the current density ,, suggests that the SOT-driven skyrmion motion could surpass DWs that are described by the 1D model ,, as long as the skyrmion remains rigid.…”

“…In previous studies, we optimized the growth of CoGd thin films and carried out a systematic study of the Dzyaloshinksii-Moriya interactions (DMI) , and SOTs as a function of the capping layer. The CoGd structures were designed to have large perpendicular magnetic anisotropy and a fully saturated state at remanence , to allow the nucleation of isolated metastable skyrmions.…”

“…In previous studies, we optimized the growth of CoGd thin films and carried out a systematic study of the Dzyaloshinksii-Moriya interactions (DMI) , and SOTs as a function of the capping layer. The CoGd structures were designed to have large perpendicular magnetic anisotropy and a fully saturated state at remanence , to allow the nucleation of isolated metastable skyrmions. The alloy composition was chosen to yield magnetic and angular momentum compensation close to room temperature with T M = 250 K for both CoGd films and T A = 285 and 292 K for Pt/CoGd/Ta and Pt/CoGd/W, respectively, where T M and T A denotes the magnetic and angular momentum compensation temperature, respectively .…”

“…The CoGd structures were designed to have large perpendicular magnetic anisotropy and a fully saturated state at remanence , to allow the nucleation of isolated metastable skyrmions. The alloy composition was chosen to yield magnetic and angular momentum compensation close to room temperature with T M = 250 K for both CoGd films and T A = 285 and 292 K for Pt/CoGd/Ta and Pt/CoGd/W, respectively, where T M and T A denotes the magnetic and angular momentum compensation temperature, respectively . Minimization of the angular momentum at room temperature in our CoGd thin films indicates the potential of fast current-induced dynamics. ,, As seen in Figure a, the CoGd layer is inserted between two heavy metals which both contribute to the DMI and generation of spin currents.…”

“…Minimization of the angular momentum at room temperature in our CoGd thin films indicates the potential of fast current-induced dynamics. ,, As seen in Figure a, the CoGd layer is inserted between two heavy metals which both contribute to the DMI and generation of spin currents. W and Ta capping were chosen to optimize the SOTs due to their giant negative spin-Hall angle (SHA) − as opposed to commonly studied Pt/ferro- or ferri-magnet/oxide structures. ,, We measured a maximum effective SHA of 16.5% and 6.5% for Pt/CoGd/W and Pt/CoGd/Ta, respectively …”