GdCo films have been widely used in spintronic applications, owing largely to their tunable degree of ferrimagnetic compensation. However, all key properties likewise depend on the alloy composition, and a systematic study of the interdependent spintronic properties with composition has not been reported. Here, we report the compositional dependence of key spintronic properties, including anisotropy, symmetric, and antisymmetric (Dzyaloshinskii–Moriya, DMI) exchange interactions, effective spin Hall angle, and domain wall mobility in a 3 nm Pt/GdCo composition series. We measure the magnetic anisotropy and determine an interfacial Pt/Co and bulk GdCo pair-ordering contribution to total anisotropy. Additionally, we estimate the exchange stiffness of all three interactions in GdCo as a function of composition. We conduct two types of domain wall motion experiments on patterned racetracks to determine the effective spin Hall angle and current-driven domain wall mobility. We find a 5× increase in effective spin Hall angle with increasing Gd concentration, suggesting an improvement in spin transfer efficiency in rare earth materials. Finally, we observe a monotonic decrease in the DMI strength with increasing Gd content, suggesting that DMI arises from the Pt/Co interfacial interaction.
Rare-earth (RE) transition-metal (TM) amorphous alloys are a promising class of ferrimagnetic materials for spintronics. RE-TM alloys consist of a RE sublattice antiferromagnetically coupled to the TM sublattice. Altering the proportion of RE and TM atoms allows one to control the magnetic properties of RE-TM films, including the saturation magnetization, , net spin density, coercivity, and magnetic anisotropy. This tunability has been used to engineer RE-TM films with low or low net angular momentum, in order to exhibit spintronic phenomena such ultra-small room temperature stable skyrmions,1,2 high-speed current-driven domain wall motion,2–5 and voltage-controlled skyrmion generation.6 However, despite the large volume of work performed on RE-TM films, a comprehensive study of the spintronic properties of RE-TM alloys, particularly in technologically relevant thin films, has yet to be conducted.
In this Letter, we analyze the magnetic properties of 3 nm GdCo across a wide range of compositions with the ultimate aim of quantifying the strength of the Dzyaloshinskii–Moriya interaction (DMI) as a function of composition using 1D domain wall (DW) motion experiments in patterned magnetic racetracks. DMI is an antisymmetric exchange interaction that stabilizes chiral spin textures, a necessity for skyrmion stability and current-driven DW motion. However, extracting this parameter from DW dynamics measurements requires knowledge of other parameters, such as DW width, which depend on material parameters that vary significantly with composition. Here, we undertake a systematic study to quantify all key parameters, providing a roadmap for compositional engineering of skyrmion host materials including magnetic anisotropy and exchange stiffness.
Films of Ta(4)/Pt(4)/GdxCo1–x(3)/Ta(4)/Pt(2) were grown by D.C. magnetron sputter deposition (numbers in parenthesis indicate nominal thickness in nanometers) under a background pressure Torr on thermally oxidized Si wafers with . The GdCo layer was grown by co-sputtering from Gd and Co targets with deposition rates of ∼3.5 and ∼1 nm/min, respectively, and with target power densities ranging from 0 to 8.8 and 4.9 W/cm2, respectively. The lower Pt layer was selected to induce interfacial DMI and inject spin in the adjacent GdCo layer, enabling current-driven DW motion. A thin GdCo layer was chosen to maximize the interfacial DMI while minimizing potential bulk DMI contributions, as have been previously reported in thick RE-TM films.7 The Ta/Pt cap was used to prevent sample oxidation while preserving an asymmetric heterostructure for DMI maximization.
The 3 nm GdCo composition series saturation magnetizations, , are shown in Fig. 1(a). Magnetic compensation occurs at , in contrast to the commonly reported bulk compensation composition of .8,9 We have previously explained this discrepancy in terms of a combined environment and dead layer model, in which a RE dead layer results in a reduced average magnetic moment per RE atom (RE atomic magnetic moment) in RE-TM films nm relative to thick films, and the average atomic magnetic moments of both the RE and TM decrease with increasing RE concentration at room temperature.10 In this model, the thickness of the RE dead layer is independent of RE-TM film thickness and increases with RE concentration, leading to expected Gd dead layer thicknesses of 0.5–2 nm for [see Fig. 4(d) of Suzuki et al.10]. Additionally, at room temperature, the two dominant exchange interactions in RE-TM films are the RE-TM and TM-TM interactions. This results in Gd and Co atomic magnetic moments that depend on the number of Co nearest neighbors, which decreases with increasing . Thus, at room temperature, the Gd and Co atomic magnetic moments decrease monotonically with increasing , approaching zero for pure Gd. The combined environment and dead layer model allows for the accurate estimation of individual sublattice contributions, and (where ), in the grown series, shown as red and blue dashed lines in Fig. 1(a).
Figure 1(b) shows the effective magnetic anisotropy, , of the GdCo series, measured using hard-axis hysteresis loops via vibrating sample (VSM) and magneto-optical Kerr effect (MOKE) magnetometry. The films exhibit perpendicular magnetic anisotropy (PMA) for compositions . Subtracting the magnetostatic component gives the uniaxial anisotropy constant . decreases significantly with increasing Gd concentration at low and then increases and plateaus from before finally tending toward zero for . The variation of with suggests two primary contributions. The most significant contribution is from the Pt/Co interfacial interaction, which is known to promote PMA in ultrathin Pt/Co films, overcoming the large magnetostatic penalty of the Co layer.11–14 In GdCo, this interaction appears to drop rapidly with increasing Gd content. In addition, anisotropic pair–pair correlations have been found to introduce bulk PMA in RE-TM ferrimagnets.15–19 This bulk PMA term increases with the number of RE-TM pairs, with pair models indicating a maximum in the pair-ordering induced anisotropy from , consistent with the observed trend in Fig. 1(b).
Combining and yields the exchange stiffness of each interaction in GdCo, shown in Fig. 2(c). As expected, due to the low Curie temperature of Gd ( K), is insignificant compared to the other two interactions at room temperature. Additionally, the Co–Co interaction dominates except at relatively high . The value of is reduced by an order of magnitude from as the Co–Co exchange is reduced due to fewer Co–Co pairs. We have estimated using the sum of the inter- and intra-sublattice exchange stiffness, as shown in Fig. 2(d). Despite the large decrease in exchange stiffness over this range, the corresponding reduction in anisotropy results in a relatively weak variation of , ranging from ∼3 to 9 nm in the studied range.
The saturation velocity, , at is shown in Fig. 5(b). With no applied longitudinal field, , with the net spin density. The data exhibit constant m/s except near the angular momentum compensation point ( ), where m/s. This is consistent with previous investigations of ferrimagnetic materials near angular momentum compensation where the DW velocity has similarly increased with reduced spin density.2–4 The data suggest that except at , the ratio remains approximately constant.
In summary, we have analyzed the spintronic properties of 3 nm GdCo. A number of GdCo magnetic properties critical to spintronic devices and stabilization of chiral spin textures are found to strongly depend on RE concentration. The total effective anisotropy is found to strongly decrease with increased Gd concentration, consistent with a Pt/Co interfacial origin. We also observe a sub-dominant increase in uniaxial anisotropy that is maximized at , consistent with a pair-ordering bulk anisotropy origin that is often used to explain bulk PMA in RE-TM alloys. The combination of DW velocity and depinning measurements under longitudinal and polar applied fields allows for determination of a variety of dynamic magnetic properties in Pt/GdCo heterostructures, including DMI effective field, damping, spin density, spin transfer efficiency, and effective spin-Hall angle. These fundamental measurements are combined with knowledge of the ferromagnetic and antiferromagnetic exchange interactions in GdCo to determine the magnitude of antisymmetric exchange constant . is found to monotonically decrease with increasing Gd content, suggesting that the Pt/Co interface is the primary source of DMI in Pt/GdCo films. This thorough characterization of GdCo's spintronic properties should allow for improved design of materials for next-generation DW and skyrmionics devices.
This work was supported in part by the DARPA TEE Program and the Samsung Global Research Outreach (GRO) Program.
Conflict of Interest
The authors have no conflicts to disclose.
Daniel Hiroshi Suzuki: Conceptualization (equal); Data curation (equal); Formal analysis (equal); Investigation (equal); Methodology (equal); Software (equal); Visualization (equal); Writing – original draft (equal); Writing – review & editing (equal). Byung Hun Lee: Data curation (supporting); Formal analysis (supporting); Methodology (supporting); Writing – review & editing (supporting). Geoffrey S. D. Beach: Conceptualization (equal); Formal analysis (equal); Funding acquisition (equal); Methodology (equal); Project administration (equal); Resources (equal); Supervision (equal); Writing – review & editing (equal).
The data that support the findings of this study are available from the corresponding author upon reasonable request.