We demonstrate that the spin Hall effect in a thin film with strong spin-orbit scattering can excite magnetic precession in an adjacent ferromagnetic film. The flow of alternating current through a Pt/NiFe bilayer generates an oscillating transverse spin current in the Pt, and the resultant transfer of spin angular momentum to the NiFe induces ferromagnetic resonance (FMR) dynamics. The Oersted field from the current also generates an FMR signal but with a different symmetry. The ratio of these two signals allows a quantitative determination of the spin current and the spin Hall angle. We study Pt/Permalloy bilayer films with a microwave-frequency (RF) charge current applied in the film plane (Permalloy = Py = Ni 81 Fe 19 ). An oscillating transverse spin current is generated in the Pt by the SHE and injected into the adjacent Py ( Fig. 1(a)), thereby exerting an oscillating spin torque (ST) on the Py that induces magnetization precession. This leads to an oscillation of the bilayer resistance due to the anisotropic magnetoresistance (AMR) of Py. A DC voltage signal is generated across the sample from the mixing of the RF current and the oscillating resistance, similar to the signal that arises from ST induced FMR in spin valves and magnetic tunnel junctions [12][13][14][15]. The resonance properties enable a quantitative measure of the spin current absorbed by the Py.Our measurement setup is shown in Fig. 1(c). Pt/Py bilayers were grown by DC magnetron sputter deposition. The starting material for the Pt was 99.95% pure. Highly resistive 3 Ta (1 nm) was employed as the capping layer to prevent oxidation of the Py. The bilayers were subsequently patterned into microstrips of 1 to 20 μm wide and 3 to 250 μm long. By using a bias tee, we were able to apply a microwave current and at the same time measure the DC voltage. A sweeping magnetic field H ext was applied in the film plane, with the angle θ between H ext and microstrip kept at 45° unless otherwise indicated. The output power of the microwave signal generator was varied from 0 to 20 dBm and the measured DC voltage was proportional to the applied power, indicating that the induced precession was in the small angle regime. All the measurements we present were performed at room temperature with a power of 10 dBm.We model the motion of the Py magnetic moment m by the Landau-Lifshitz-GilbertHere γ is the gyromagnetic ratio, α is the Gilbert damping coefficient, μ 0 is the permeability in vacuum, M s is the saturation magnetization of Py, t is the thickness of the Py layer, , / 2 S RF J erepresents the oscillating spin current density injected into Py, H RF is the Oersted field generated by the RF current, H eff is the sum of H ext and the demagnetization field 4π M eff , and σ is the direction of the injected spin moment. The third and fourth terms on the right hand side of Eq. (1) are the result of in-plane spin torque and the out-of-plane torque due to the Oersted field, respectively ( Fig. 1(a)). The mixing signal in response to a combination of in...

Antiferromagnetic materials could represent the future of spintronic applications thanks to the numerous interesting features they combine: they are robust against perturbation due to magnetic fields, produce no stray fields, display ultrafast dynamics, and are capable of generating large magnetotransport effects. Intense research efforts over the past decade have been invested in unraveling spin transport properties in antiferromagnetic materials. Whether spin transport can be used to drive the antiferromagnetic order and how subsequent variations can be detected are some of the thrilling challenges currently being addressed. Antiferromagnetic spintronics started out with studies on spin transfer and has undergone a definite revival in the last few years with the publication of pioneering articles on the use of spin-orbit interactions in antiferromagnets. This paradigm shift offers possibilities for radically new concepts for spin manipulation in electronics. Central to these endeavors are the need for predictive models, relevant disruptive materials, and new experimental designs. This paper reviews the most prominent spintronic effects described based on theoretical and experimental analysis of antiferromagnetic materials. It also details some of the remaining bottlenecks and suggests possible avenues for future research. This review covers both spin-transfer-related effects, such as spin-transfer torque, spin penetration length, domain-wall motion, and "magnetization" dynamics, and spin-orbit related phenomena, such as (tunnel) anisotropic magnetoresistance, spin Hall, and inverse spin galvanic effects. Effects related to spin caloritronics, such as the spin Seebeck effect, are linked to the transport of magnons in antiferromagnets. The propagation of spin waves and spin superfluids in antiferromagnets is also covered.

Antiferromagnetic spintronics is an emerging research field which aims to utilize antiferromagnets as core elements in spintronic devices 1,2 . A central motivation toward this direction is that antiferromagnetic spin dynamics is expected to be much faster than ferromagnetic counterpart because antiferromagnets have higher resonance frequencies than ferromagnets 3 . Recenttheories indeed predicted faster dynamics of antiferromagnetic domain walls (DWs) than ferromagnetic DWs 4-6 . However, experimental investigations of antiferromagnetic spin dynamics have remained unexplored mainly because of the immunity of antiferromagnets to magnetic fields. Furthermore, this immunity makes field-driven antiferromagnetic DW motion impossible despite rich physics of field-driven DW dynamics as proven in ferromagnetic DW studies. Here we show that fast field-driven antiferromagnetic spin dynamics is realized in ferrimagnets at the angular momentum compensation point TA. Using rare-earth-3d-transition metal ferrimagnetic compounds where net magnetic moment is nonzero at TA, the field-driven DW mobility remarkably enhances up to 20 km s −1 T −1 . The collective coordinate approach generalized for ferrimagnets 7 and atomistic spin model simulations 6,8 show that this remarkable enhancement is a consequence of antiferromagnetic spin dynamics at TA. Our finding allows us to investigate the physics of antiferromagnetic spin dynamics and highlights the importance of tuning of the angular momentum compensation point of ferrimagnets, which could be a key towards ferrimagnetic spintronics.Encoding information using magnetic DW motion is essential for future magnetic memory devices, such as racetrack memories 9,10 . High-speed DW motion is a key prerequisite for making the racetrack feasible. However, velocity breakdown due to the angular precession of DW, referred to as the Walker breakdown 11 , generally limits the functional performance in ferromagnet-based DW devices.Recently, it was reported that the DW speed boosts up significantly in antiferromagnets due to the suppression of the angular precession 4-6 . However, the immunity of antiferromagnets to magnetic fields yields notorious difficulties in creating, manipulating, and detecting antiferromagnetic DWs, compared to ferromagnetic ones. One possibility to avoid these difficulties is offered by the synthetic

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For a long time, there were no efficient ways of controlling antiferromagnets. Quite a strong magnetic field was required to manipulate the magnetic moments because of a high molecular field and a small magnetic susceptibility. It was also difficult to detect the orientation of the magnetic moments since the net magnetic moment is effectively zero. For these reasons, research on antiferromagnets has not been progressed as drastically as that on ferromagnets which are the main materials in modern spintronic devices. Here we show that the magnetic moments in NiO, a typical natural antiferromagnet, can indeed be controlled by the spin torque with a relatively small electric current density (~4 × 107 A/cm2) and their orientation is detected by the transverse resistance resulting from the spin Hall magnetoresistance. The demonstrated techniques of controlling and detecting antiferromagnets would outstandingly promote the methodologies in the recently emerged “antiferromagnetic spintronics”. Furthermore, our results essentially lead to a spin torque antiferromagnetic memory.