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Register a new userDoping-induced spin Hall ratio enhancement in A15-phase, Ta-doped Beta-W thin films
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Added by
Mohsin Zamir Minhas
Publication date
2020
fields
Physics
and research's language is
English
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As spintronic devices become more and more prevalent, the desire to find Pt free materials with large spin Hall effects is increasing. Previously it was shown that Beta W, the metastable A15 structured variant of pure W, has charge-spin conversion efficiencies on par with Pt, and it was predicted that Beta W(Ta) alloys should be even more efficient. Here we demonstrate the enhancement of the spin Hall ratio (SHR) in A15-phase Beta W films doped with Ta (W(4-x)Tax where x is between 0.28 and 0.4, deposited at room temperature using DC magnetron co-sputtering. In close agreement with theoretical predictions, we find that the SHR of the doped films was approx. 9 percent larger than pure Beta W films. We also found that the SHRs in devices with Co2Fe6B2 were nearly twice as large as the SHRs in devices with Co4Fe4B2. This work shows that by optimizing deposition parameters and substrates, the fabrication of the optimum W3Ta alloy should be feasible, opening the door to commercially viable, Pt free, spintronic devices.
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Generating pure spin currents via the spin Hall effect in heavy metals has been an active topic of research in the last decade. In order to reduce the energy required to efficiently switch neighbouring ferromagnetic layers for applications, one should not only increase the charge- to-spin conversion efficiency but also decrease the longitudinal resistivity of the heavy metal. In this work, we investigate the spin Hall conductivity in W_{1-x}Ta_{x} / CoFeB / MgO (x = 0 - 0.2) using spin torque ferromagnetic resonance measurements. Alloying W with Ta leads to a factor of two change in both the damping-like effective spin Hall angle (from - 0.15 to - 0.3) and longitudinal resistivity (60 - 120 {mu}W cm). At 11% Ta concentration, a remarkably high spin Hall angle value of - 0.3 is achieved with a low longitudinal resistivity 100 {mu}W cm, which could lead to a very low power consumption for this W-based alloy. This work demonstrates sputter-deposited W-Ta alloys could be a promising material for power-efficient spin current generation.
We report on Cr doping effect in Mn3Sn polycrystalline films with both uniform and modulation doping. It is found that Cr doping with low concentration does not cause notable changes to the structural and magnetic properties of Mn3Sn, but it significantly enhances the anomalous Hall conductivity, particularly for modulation-doped samples at low temperature. A Hall conductivity as high as 184.8 {Omega}-1 cm-1 is obtained for modulation-doped samples at 50 K, in a sharp contrast to vanishingly small values for undoped samples at the same temperature. We attribute the enhancement to the change of Fermi level induced by Cr doping
The spin Hall effect (SHE) is the conversion of charge current to spin current, and non-magnetic metals with large SHEs are extremely sought after for spintronic applications, but their rarity has stifled widespread use. Here we predict and explain the large intrinsic SHE in $beta$-W and the A15 family of superconductors: W$_3$Ta, Ta$_3$Sb, and Cr$_3$Ir having spin hall conductivities (SHC) of -2250, -1400, and 1210 $frac{hbar}{e}(Omega cm)^{-1}$, respectively. Combining concepts from topological physics with the dependence of the SHE on the spin Berry curvature (SBC) of the electronic bands, we propose a simple strategy to rapidly search for materials with large intrinsic SHEs based on the following ideas: high symmetry combined with heavy atoms gives rise to multiple Dirac-like crossings in the electronic structure, without sufficient symmetry protection these crossings gap due to spin orbit coupling (SOC), and gapped Dirac crossings create large spin Berry curvature.
A key challenge in manipulating the magnetization in heavy-metal/ferromagnetic bilayers via the spin-orbit torque is to identify materials that exhibit an efficient charge-to-spin current conversion. Ab initio electronic structure calculations reveal that the intrinsic spin Hall conductivity (SHC) for pristine $beta$-W is about sixty percent larger than that of $alpha$-W. More importantly, we demonstrate that the SHC of $beta$-W can be enhanced via Ta alloying. This is corroborated by spin Berry curvature calculations of W$_{1-x}$Ta$_x$ ($x$ $sim$ 12.5%) alloys which show a giant enhancement of spin Hall angle of up to $approx$ $-0.5$. The underlying mechanism is the synergistic behavior of the SHC and longitudinal conductivity with Fermi level position. These findings, not only pave the way for enhancing the intrinsic spin Hall effect in $beta$-W, but also provide new guidelines to exploit substitutional alloying to tailor the spin Hall effect in various materials.
Heavy metal-ferromagnet bilayer structures have attracted great research interest for charge-to-spin interconversion. In this work, we have investigated the effect of the permalloy seed layer on the Ta polycrystalline phase and its spin Hall angle. Interestingly, for the same deposition rates the crystalline phase of Ta deposited on Py seed layer strongly depends on the thickness of the seed layer. We have observed a phase transition from $alpha$-Ta to ($alpha$+$beta$)-Ta while increasing the Py seed layer thickness. The observed phase transition is attributed to the strain at interface between Py and Ta layers. Ferromagnetic resonance-based spin pumping studies reveal that the spin-mixing conductance in the to ($alpha$+$beta$)-Ta is relatively higher as compared to the to $alpha$-Ta. Spin Hall angles of to $alpha$-Ta and to ($alpha$+$beta$)-Ta are extracted from inverse spin Hall effect (ISHE) measurements. Spin Hall angle of the to ($alpha$+$beta$)-Ta is estimated to be $theta$_SH=-0.15 which is relatively higher than that of to $alpha$-Ta. Our systematic results connecting the phase of the Ta with seed layer and its effect on the efficiency of spin to charge conversion might resolve ambiguities across various literature and open up new functionalities based on the growth process for the emerging spintronic devices.
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