Spin-Orbit Torque Metrology
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Metrology is the study of measurements. It turns out that many of the common measurement techniques we use to quantify SOTs give results that disagree with one another, and depending on the practitioner, can give results that disagree with themselves. These discrepancies are not rarities. In fact they occur frequently and serve as the underpinning motivation for this whole thesis. As the field of SOTs has progressed, standard measurement techniques have persisted and have been used to measure various material systems, and discrepancies are often chalked up to material complications. However, similar measurement discrepancies exist even for simple, bread-and-butter SOT material systems (e.g. Pt/Py, where, in this work, Py = Ni81Fe19 -- a very-weakly-magnetoelastic alloy). To re-litigate these measurements and understand the cause of these measurement discrepancies, our studies will focus on standard, widely-studied heavy metal/ferromagnet (HM/FM) bilayers where the understanding and interpretation of measurement results ought to be simple. We will see that measurement results are complicated, even in these systems. In Chapter 1, we will see that long-known artifacts (spin pumping and resonant heating) affect the most common measurement technique used to quantify SOTs in in-plane-magnetized heterostructures in various HM/FM systems: spin-torque ferromagnetic resonance. These artifacts are particularly pronounced when the HM or FM thickness is large. In Chapter 2, we will see that improper data analysis and unintended experimenter bias can have a huge effect on the quantification of SOTs in another very common measurement technique used to quantify SOTs in in-plane-magnetized heterostructures in various HM/FM systems: dc-biased spin-torque ferromagnetic resonance. This bias and discrepancy is abated by following a systematic data analysis framework that we outline. In Chapter 3, we see the very surprising result that the planar Hall magnetoresistance seems to not contribute to the quantification of SOTs in the most common measurement technique used to quantify SOTs in out-of-plane-magnetized heterostructures in various HM/FM systems: the second-harmonic Hall method. Our suggestion is to ignore the planar Hall magnetoresistance when quantifying the SOT, suggesting that the planar Hall effect manifests differently when the magnetization is tilted with a magnetic field vs. a SOT. This challenges our conventional understanding of the planar Hall magnetoresistance, altogether.
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Fuchs, Gregory David