Tunable charge to spin conversion in strontium iridate thin films

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Tunable charge to spin conversion in strontium iridate thin films

Arnoud S. Everhardt, Mahendra DC, Xiaoxi Huang, Shehrin Sayed, Tanay A. Gosavi, Yunlong Tang, Chia-Ching Lin, Sasikanth Manipatruni, Ian A. Young, Supriyo Datta, Jian-Ping Wang, and Ramamoorthy Ramesh

Phys. Rev. Materials 3, 051201(R) – Published 6 May 2019

Abstract

Efficient charge to spin conversion is important for low-power spin logic devices. Spin and charge interconversion is commonly performed using heavy metals and topological insulators, while the field of oxides is not yet fully explored. Strontium iridate thin films were grown, where the different crystal structures form a perfect playground to understand the key factors in obtaining high charge to spin conversion efficiency (i.e., large spin Hall angle). It was found that the semiconducting ${Sr}_{2} {IrO}_{4}$ has a spin Hall angle of $\sim 0.1$ (depending on measurement technique), which is promising for a spin-orbit coupled electronic system and comparable to Pt. In contrast, the perovskite ${SrIrO}_{3}$ , reported to have a Dirac cone near the Fermi level, has a larger spin Hall angle of 0.3–0.4 degrees. The largest difference between the two materials is a large degree of spin-momentum locking in ${SrIrO}_{3}$ , comparable to known topological insulators. A simple semiclassical relationship is found where the spin Hall angle increases for higher degrees of spin-momentum locking and it also increases for lower Fermi wave vectors. This relationship is then able to explain the decreased spin Hall angle below 10 nm film thickness in ${SrIrO}_{3}$ , by relating it to the correspondingly higher carrier concentration (related to the higher Fermi wave vector). Breaking the commonly believed anticorrelation between resistivity and carrier concentration paves a pathway to lower power losses due to resistance while keeping large spin Hall angles.

Received 24 January 2019

DOI:https://doi.org/10.1103/PhysRevMaterials.3.051201

Physics Subject Headings (PhySH)

Spin torque Spintronics

Laser ablation Transport techniques

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Arnoud S. Everhardt^1,2,*,†, Mahendra DC^3,*, Xiaoxi Huang², Shehrin Sayed^4,5, Tanay A. Gosavi⁶, Yunlong Tang^1,2, Chia-Ching Lin⁶, Sasikanth Manipatruni⁶, Ian A. Young⁶, Supriyo Datta⁵, Jian-Ping Wang^3,7, and Ramamoorthy Ramesh^1,2,‡

¹Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
²Department of Materials Science and Engineering and Department of Physics, University of California, Berkeley, California 94720, USA
³School of Physics and Astronomy, University of Minnesota, Minneapolis, Minnesota 55455, USA
⁴Department of Electrical Engineering and Computer Science, University of California, Berkeley, California 94720, USA
⁵School of Electrical and Computer Engineering, Purdue University, West Lafayette, Indiana 47907, USA
⁶Components Research, Intel Corporation, Hillsboro, Oregon 97124, USA
⁷Electrical and Computer Enginering Department, University of Minnesota, Minneapolis, Minnesota 55455, USA

^*These authors contributed equally to this work.
^†Corresponding author: arnoudeverhardt@lbl.gov
^‡Corresponding author: rramesh@berkeley.edu

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Vol. 3, Iss. 5 — May 2019

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Images

Figure 1
(a), (b) X-ray diffraction scan of ${SrIrO}_{3}$ and ${Sr}_{2} {IrO}_{4}$ , along with a diffraction pattern calculated from their simulated crystal structures. The agreement between the theoretical fits and the data shows that the right crystal structures have been formed. Lattice parameters are determined from the Bragg peak (00l) indices to be $({La}_{0.18} {Sr}_{0.82}) ({Al}_{0.59} {Ta}_{0.41}) O_{3} (substrate) = 3.87 Å$ , $SrIr O_{3} = 3.98 Å$ , and $S r_{2} Ir O_{4} = 25.8 Å$ . (c), (d) Transmission electron microscopy images of ${SrIrO}_{3}$ and ${Sr}_{2} {IrO}_{4}$ , showing the perovskite and layered structures, respectively, of the two materials. A $NiFe / Al O_{x}$ top layer is grown on the ${SrIrO}_{3}$ . (e) Resistivity $ρ$ and electron carrier concentration $n$ , mobility of 5- and 10-nm-thick films of ${SrIrO}_{3}$ . (f) Resistivity ρ, data for 10 nm of ${Sr}_{2} {IrO}_{4}$ .
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Figure 2
(a) The different types of torques present in a ferromagnet/iridate bilayer of materials. A charge current $J_{c}$ , driven through the bilayer, couples to the magnetization $M$ in the ferromagnet in three ways. The anti-damping-like torque $τ_{D L}$ acts parallel to the surface and couples to out-of-plane magnetization. The fieldlike torque $τ_{F L}$ acts perpendicular to the surface and couples to transverse magnetization, while the Oersted torque $τ_{O e}$ acts along the same axis as the fieldlike torque (combined in $τ_{⊥}$ ). The external magnetic field ( $H_{ext}$ ) can be applied under an in-plane angle $β$ with respect to the current direction. (b) ST-FMR resonance line shape for ${Sr}_{2} {IrO}_{4}$ (10 nm)/NiFe (6 nm) bilayer structure under 5 mA and a frequency of 9 GHz oscillating rf current. Lorentzian trial functions extracted symmetric and antisymmetric components from the mixing voltage $V_{mix}$ (the obtained quantity from the measurement), represented by the black and green solid lines, respectively. (c) Spin Hall angles extracted from the ST-FMR method for different frequencies $f$ , in 5 and 10 nm of ${Sr}_{2} {IrO}_{4}$ . (d) First-harmonic $R_{1 ω}$ and (e) second-harmonic $R_{2 ω}$ signals of the SHH measurement in a ${Sr}_{2} {IrO}_{4}$ (10 nm)/NiFe (6 nm) bilayer structure under a driving field of 10 mA at 5000 Oe. $R_{1 ω}$ gives the planar Hall effect, while $R_{2 ω}$ gives a combination of different angular contributions, which can be separated into the different contributions. Here only a $cos (β)$ contribution is measured, which gives a $τ_{D L}$ . (f) $H_{O O P}$ $H_{T}$ fields as a function of applied current, from which the spin Hall angle is calculated.
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Figure 3
(a) ST-FMR in ${SrIrO}_{3}$ (5 nm)/NiFe (6 nm) under an rf current of 10 mA and a frequency of 9 GHz. (b) Spin Hall angle calculated from ST-FMR. (c) $R_{1 ω}$ and (d) $R_{2 ω}$ signals of the SHH measurement in a ${Sr}_{2} {IrO}_{4}$ (10 nm)/NiFe (6 nm) bilayer structure under a driving field of 10 mA at 5000 Oe magnetic field, separated into their components $τ_{D L}$ by $cos (β)$ and $τ_{⊥}$ by $cos (3 β)$ . (e) $H_{O O P}$ and (f) $H_{T}$ fields as a function of applied current. (g) Spin Hall angle calculated from SHH measurement from the $H_{O O P}$ .
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Figure 4
(a) Resistivity and carrier concentration calculated from Hall measurements of ${SrIrO}_{3}$ films of different thickness without capping layer. (b) Calculated spin Hall angles with a fixed fitted $x$ factor of 5 ( $10^{18} m^{- 2}$ ) from Eq. (1)) with an exponential fit, given together with the measured spin Hall angles. (c) Electron mean free path $λ$ as determined from Eq. (2)) with the same fixed $x$ factor as in (b).
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