skip to main content

This content will become publicly available on October 4, 2022

Title: Separation of Artifacts from Spin-Torque Ferromagnetic Resonance Measurements of Spin-Orbit Torque for the Low-Symmetry van der Waals Semi-Metal ZrTe
We measure spin-orbit torque generated by exfoliated layers of the low-symmetry semi-metal ZrTe3 using the spin-torque ferromagnetic resonance (ST-FMR) technique. When the ZrTe3 has a thickness greater than about 10 nm, artifacts due to spin pumping and/or resonant heating can cause the standard ST-FMR analysis to overestimate the true magnitude of the torque efficiency by as much as a factor of 30, and to indicate incorrectly that the spin-orbit torque depends strongly on the ZrTe3 layer thickness. Artifact-free measurements can still be achieved over a substantial thickness range by the method developed recently to detect ST-FMR signals in the Hall geometry as well as the longitudinal geometry. ZrTe3/Permalloy samples generate a conventional in-plane anti-damping spin torque efficiency ξDL|| = 0.014 ± 0.004, and an unconventional in-plane field-like torque efficiency |ξFL||| = 0.003 ± 0.001. The out-of-plane anti-damping torque is negligible. We suggest that artifacts similarly interfere with the standard ST-FMR analysis for other van der Waals samples thicker than about 10 nm.
Authors:
; ; ; ;
Award ID(s):
1708499
Publication Date:
NSF-PAR ID:
10301384
Journal Name:
ArXivorg
Page Range or eLocation-ID:
2110.01720
ISSN:
2331-8422
Sponsoring Org:
National Science Foundation
More Like this
  1. Efficient manipulation of antiferromagnetically coupled materials that are integration-friendly and have strong perpendicular magnetic anisotropy (PMA) is of great interest for low-power, fast, dense magnetic storage and computing. Here, we report a distinct, giant bulk damping-like spin–orbit torque in strong-PMA ferrimagnetic Fe 100− x Tb x single layers that are integration-friendly (composition-uniform, amorphous, and sputter-deposited). For sufficiently thick layers, this bulk torque is constant in the efficiency per unit layer thickness, [Formula: see text]/ t, with a record-high value of 0.036 ± 0.008 nm −1 , and the damping-like torque efficiency [Formula: see text] achieves very large values for thick layers, up tomore »300% for 90 nm layers. This giant bulk torque by itself switches tens of nm thick Fe 100− x Tb x layers that have very strong PMA and high coercivity at current densities as low as a few MA/cm 2 . Surprisingly, for a given layer thickness, [Formula: see text] shows strong composition dependence and becomes negative for composition where the total angular momentum is oriented parallel to the magnetization rather than antiparallel. Our findings of giant bulk spin torque efficiency and intriguing torque-compensation correlation will stimulate study of such unique spin–orbit phenomena in a variety of ferrimagnetic hosts. This work paves a promising avenue for developing ultralow-power, fast, dense ferrimagnetic storage and computing devices.« less
  2. It was recently demonstrated in bilayers of permalloy and platinum, that by combining spin torques arising from the spin Hall effect with Oersted field-like torques, magnetization dynamics can be induced with a directional preference.1 This “unidirectional” magnetization dynamic effect is made possible by exploiting the different even and odd symmetry that damping-like and field-like torques respectively have when magnetization is reversed. The experimental method used to demonstrate this effect was the spin-torque ferromagnetic (ST-FMR) resonance technique; a popular tool used in the phenomenological quantification of a myriad of damping-like and field-like torques. In this report, we review the phenomenology whichmore »is used to describe and analyze the unidirectional magnetization dynamic effect in ST-FMR measurements. We will focus on how the asymmetry in the dynamics also is present in the phase angle of the magnetization precession. We conclude by demonstrating a utility of this directional effect; we will outline an improved experimental method that can be used to distinguish a phase-shifted field-like torque in a ST-FMR experiment from a combination of field-like and damping-like torques.« less
  3. We adapt Sagnac interferometry for magneto-optic Kerr effect measurements of spin-orbit-torque-induced magnetic tilting in thin-film magnetic samples. The high sensitivity of Sagnac interferometry permits for the first time optical quantification of spin-orbit torque from small-angle magnetic tilting of samples with perpendicular magnetic anisotropy (PMA). We find significant disagreement between Sagnac measurements and simultaneously-performed harmonic Hall (HH) measurements of spin-orbit torque on Pt/Co/MgO and Pd/Co/MgO samples with PMA. The Sagnac results for PMA samples are consistent with both HH and Sagnac measurements for the in-plane geometry, so we conclude that the conventional analysis framework for PMA HH measurements is flawed. Wemore »suggest that the explanation for this discrepancy is that although magnetic-field induced magnetic tilting in PMA samples can produce a strong planar Hall effect, when tilting is instead generated by spin-orbit torque it produces negligible change in the planar Hall signal. This very surprising result demonstrates an error in the most-popular method for measuring spin-orbit torques in PMA samples, and represents an unsolved puzzle in understanding the planar Hall effect in magnetic thin films.« less
  4. Abstract

    Giant spin-orbit torque (SOT) from topological insulators (TIs) provides an energy efficient writing method for magnetic memory, which, however, is still premature for practical applications due to the challenge of the integration with magnetic tunnel junctions (MTJs). Here, we demonstrate a functional TI-MTJ device that could become the core element of the future energy-efficient spintronic devices, such as SOT-based magnetic random-access memory (SOT-MRAM). The state-of-the-art tunneling magnetoresistance (TMR) ratio of 102% and the ultralow switching current density of 1.2 × 105 A cm−2have been simultaneously achieved in the TI-MTJ device at room temperature, laying down the foundation for TI-driven SOT-MRAM. Themore »charge-spin conversion efficiencyθSHin TIs is quantified by both the SOT-induced shift of the magnetic switching field (θSH = 1.59) and the SOT-induced ferromagnetic resonance (ST-FMR) (θSH = 1.02), which is one order of magnitude larger than that in conventional heavy metals. These results inspire a revolution of SOT-MRAM from classical to quantum materials, with great potential to further reduce the energy consumption.

    « less
  5. Protecting intellectual property (IP) has become a serious challenge for chip designers. Most countermeasures are tailored for CMOS integration and tend to incur excessive overheads, resulting from additional circuitry or device-level modifications. On the other hand, power density is a critical concern for sub-50 nm nodes, necessitating alternate design concepts. Although initially tailored for error-tolerant applications, imprecise computing has gained traction as a general-purpose design technique. Emerging devices are currently being explored to implement ultra-low-power circuits for inexact computing applications. In this paper, we quantify the security threats of imprecise computing using emerging devices. More specifically, we leverage the innatemore »polymorphism and tunable stochastic behavior of spin-orbit torque (SOT) devices, particularly, the giant spin-Hall effect (GSHE) switch. We enable IP protection (by means of logic locking and camouflaging) simultaneously for deterministic and probabilistic computing, directly at the GSHE device level. We conduct a comprehensive security analysis using state-of-the-art Boolean satisfiability (SAT) attacks; this study demonstrates the superior resilience of our GSHE primitive when tailored for deterministic computing. We also demonstrate how probabilistic computing can thwart most, if not all, existing SAT attacks. Based on this finding, we propose an attack scheme called probabilistic SAT (PSAT) which can bypass the defense offered by logic locking and camouflaging for imprecise computing schemes. Further, we illustrate how careful application of our GSHE primitive can remain secure even on the application of the PSAT attack. Finally, we also discuss side-channel attacks and invasive monitoring, which are arguably even more concerning threats than SAT attacks.« less