Spin Pumping Study in Ion-Beam Sputtered β-W/Co₂FeAl Heterostructures and Effects of Different Interlayers (Al, Mg, Ta, Mo)

The study of spin pumping in a ferromagnetic/nonmagnetic (FM/NM) system has gained increasing interest in the last decade for the fundamental understanding of spin-orbit coupling (SOC) materials and their promising applications in spin-orbit torque based magnetic random-access memories. The angular momentum transfer through the FM/NM interface is susceptible to the quality of the interface between the particular materials involved and their properties. Several attempts have been made to increase the efficiency of SOC materials by alloying of the two elements, but this approach suffers from the drawback related to the presence of undesirable extrinsic scattering contributions [1].<br/>In this work, the spin pumping in ion-beam sputtered <i>β</i>-W/Co₂FeAl(CFA)/Al heterostructure grown on <i>Si(100)</i> substrate is systematically investigated. In the heterostructure, the thickness of the CFA and capping Al layer is fixed at 8 nm and 3.5 nm, respectively, but the thickness of the <i>β</i>-W layer is varied from 0.5 nm to 10 nm. While the <i>α_eff</i> for bare CFA(8 nm)/Al(3.5 nm) was 0.0061, after the introduction of 6 nm of W,<i> α_eff</i> becomes ~0.0087 <i>i.e.,</i> almost an enhancement of nearly ~47% in <i>α_eff</i>. From the variation of effective damping constant (<i>α_eff</i>) with the <i>β</i>-W thickness and considering ballistic transport model [2], the real part of spin mixing conductance (g_eff^↑↓) of the interface and spin diffusion length (<i>λ_S</i>) of the <i>β</i>-W are found to be 11.76±0.34 nm^-2 and 2.17±0.27 nm, respectively.<br/>Further to understand the impact of the different modified interface on the spin pumping, different interlayers (ILs) with different SOC strengths and different thicknesses have been introduced in between the <i>β</i>-W and CFA layers in <i>β</i>-W(6 nm)/CFA(8 nm)/Al(3.5 nm). Four different ILs were chosen, two of them (Al and Mg) are weak-SOC materials and the other two (Ta and Mo) are high SOC materials with different SOC strengths (SOC strength of Ta is reported to be higher than that in Mo).<br/>In the case of the two weak-SOC ILs, the spin pumping from the CFA layer to the <i>β</i>-W layer is blocked with the <i>α_eff</i> ~0.007. This is understandable since the low SOC strength of the ILs, it does not permit the transfer of spin angular momentum from CFA to <i>β</i>-W. On the other hand, in the case of the two high SOC ILs, although the variation in<i> α_eff</i> is similar, however, it is completely different from that observed in the case of weak-SOC ILs. For Ta (Mo), up to 2 nm (2 nm), <i>α_eff</i> increases to 0.0106 (0.0098) and then reduces up to 4 nm (4 nm) and finally saturates with an <i>α_eff</i> value of ~0.0085 (0.0079). This can be explained in terms of the <i>λ_S</i> of the IL and transparency of the IL -<i> β</i>-W interface. The initial increase in the <i>α_eff</i> up to 2nm of IL probably due to combined effect of the IL and the <i>β</i>-W layer as the spin angular momenta can pass through the IL and to <i>β</i>-W for IL thickness lesser than the <i>λ_S</i> of the ILs (which varies 2-4 nm depending upon the interface for both the ILs). However, for higher IL thickness (&gt; <i>λ_S</i>), the spin angular momentum will experience damping within the IL only and will not be able to transfer into the<i> β</i>-W layer. Hence, the only contribution of the IL will be observed, as if high SOC <i>β</i>-W layer is not there at all. The<i> α_eff</i> value will be saturated depending on the SOC strength of that particular IL. Here, as the SOC strength of Ta (Mo) is lesser than that of <i>β</i>-W, the<i> α_eff</i> saturates at a lower value than that of <i>β</i>-W(6 nm)/CFA(8 nm)/Al(3.5 nm). Now among the two high SOC ILs, Ta has higher SOC strength compared to Mo and hence <i>α_eff</i> saturates at a higher value.<br/>Thus, in this way, one can tune the transfer of spin angular momentum by choosing the specific interlayer of appropriate thickness without necessitating the alloying of the layers. Our study provides the fundamental understanding of how the different SOC layers altogether can increase spin pumping, which is important for future spintronics applications.<br/>References:<br/>[1] doi: 10.1103/PhysRevB.96.140405.<br/>[2] doi: 10.1063/1.1853131.