Spin currents can apply useful torques in spintronic devices. The spin Hall effect has been proposed as a source of spin current, but its modest strength has limited its usefulness. We report a giant spin Hall effect (SHE) in β-tantalum that generates spin currents intense enough to induce efficient spin-torque switching of ferromagnets at room temperature. We quantify this SHE by three independent methods and demonstrate spin-torque switching of both out-of-plane and in-plane magnetized layers. We furthermore implement a three-terminal device that uses current passing through a tantalum-ferromagnet bilayer to switch a nanomagnet, with a magnetic tunnel junction for read-out. This simple, reliable, and efficient design may eliminate the main obstacles to the development of magnetic memory and nonvolatile spin logic technologies.

Spin-torque switching with the giant spin hall effect of tantalum

In most ferromagnets the magnetization rotates from one domain to the next with no preferred handedness. However, broken inversion symmetry can lift the chiral degeneracy, leading to topologically rich spin textures such as spin spirals and skyrmions through the Dzyaloshinskii-Moriya interaction (DMI). Here we show that in ultrathin metallic ferromagnets sandwiched between a heavy metal and an oxide, the DMI stabilizes chiral domain walls (DWs) whose spin texture enables extremely efficient current-driven motion. We show that spin torque from the spin Hall effect drives DWs in opposite directions in Pt/CoFe/MgO and Ta/CoFe/MgO, which can be explained only if the DWs assume a Neel configuration with left-handed chirality. We directly confirm the DW chirality and rigidity by examining current-driven DW dynamics with magnetic fields applied perpendicular and parallel to the spin spiral. This work resolves the origin of controversial experimental results and highlights a new path towards interfacial design of spintronic devices.

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Current-driven dynamics of chiral ferromagnetic domain walls

Spin-polarized currents provide a powerful means of manipulating the magnetization of nanodevices, and give rise to spin transfer torques that can drive magnetic domain walls along nanowires. In ultrathin magnetic wires, domain walls are found to move in the opposite direction to that expected from bulk spin transfer torques, and also at much higher speeds. Here we show that this is due to two intertwined phenomena, both derived from spin–orbit interactions. By measuring the influence of magnetic fields on current-driven domain-wall motion in perpendicularly magnetized Co/Ni/Co trilayers, we find an internal effective magnetic field acting on each domain wall, the direction of which alternates between successive domain walls. This chiral effective field arises from a Dzyaloshinskii–Moriya interaction at the Co/Pt interfaces and, in concert with spin Hall currents, drives the domain walls in lock-step along the nanowire. Elucidating the mechanism for the manipulation of domain walls in ultrathin magnetic films will enable the development of new families of spintronic devices. The influence of magnetic fields on the current-driven motion of domain walls in nanowires with perpendicular anisotropy shows that two spin–orbit-derived mechanisms are responsible for their motion.

Chiral spin torque at magnetic domain walls

The magnetization of a magnetic material can be reversed by using electric currents that transport spin angular momentum. In the reciprocal process a changing magnetization orientation produces currents that transport spin angular momentum. Understanding how these processes occur reveals the intricate connection between magnetization and spin transport, and can transform technologies that generate, store or process information via the magnetization direction. Here we explain how currents can generate torques that affect the magnetic orientation and the reciprocal effect in a wide variety of magnetic materials and structures. We also discuss recent state-of-the-art demonstrations of current-induced torque devices that show great promise for enhancing the functionality of semiconductor devices.

https://physics.nyu.edu/kentlab/Papers/2012/NatureMaterials_11_372_2012.pdf

Current-induced torques in magnetic materials

Spin–orbit torques in heavy metal/ferromagnetic layers have a complex dependence on the magnetization direction. This dependence can be exploited to increase the efficiency of spin–orbit torques.

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Symmetry and magnitude of spin-orbit torques in ferromagnetic heterostructures

We show that in a perpendicularly magnetized Pt/Co bilayer the spin-Hall effect (SHE) in Pt can produce a spin torque strong enough to efficiently rotate and switch the Co magnetization. We calculate the phase diagram of switching driven by this torque, finding quantitative agreement with experiments. When optimized, the SHE torque can enable memory and logic devices with similar critical currents and improved reliability compared to conventional spin-torque switching. We suggest that the SHE torque also affects current-driven magnetic domain wall motion in Pt/ferromagnet bilayers.

/pdf/current-induced-switching-of-perpendicularly-magnetized-22yd09s0at.pdf

Current-induced switching of perpendicularly magnetized magnetic layers using spin torque from the spin Hall effect.

We study deterministic magnetic reversal of a perpendicularly magnetized Co layer in a Co/MgO/Ta nanosquare driven by spin Hall torque from an in-plane current flowing in an underlying Pt layer. The rate-limiting step of the switching process is domain wall (DW) depinning by spin Hall torque via a thermally assisted mechanism that eventually produces full reversal by domain expansion. An in-plane applied magnetic field collinear with the current is required, with the necessary field scale set by the need to overcome DW chirality imposed by the Dzyaloshinskii-Moriya interaction. Once Joule heating is taken into account the switching current density is quantitatively consistent with a spin Hall angle ${\ensuremath{\theta}}_{SH}\ensuremath{\approx}0.07$ for 4 nm of Pt.

Central role of domain wall depinning for perpendicular magnetization switching driven by spin torque from the spin Hall effect

We model, using the macrospin approximation, the magnetic reversal of a perpendicularly magnetized nanostructured free layer formed on a normal, heavy-metal nanostrip, subjected to spin-orbit torques (SOTs) generated by short (≤0.5 ns) current pulses applied to the nanostrip, to examine the potential for SOT-based fast, efficient cryogenic memory. Due to thermal fluctuations, if solely an anti-damping torque is applied, then, for a device with sufficiently low anisotropy ( Hanis0 ∼ 1 kOe) suitable for application in cryogenic memory, a high magnetic damping parameter ( α∼0.1−0.2) is required for reliable switching over a significant variation of pulse current. The additional presence of a substantial field-like torque improves switching reliability even for low damping ( α≤0.03).

Macrospin modeling of sub-ns pulse switching of perpendicularly magnetized free layer via spin-orbit torques for cryogenic memory applications

We report on spin valve devices that incorporate both an out-of-plane polarizer (OPP) to quickly excite spin torque (ST) switching and an in-plane polarizer/analyzer (IPP) For pulses <200 ps, we observe reliable precessional switching due largely to ST from the OPP Compared to a conventional spin valve, for a given current amplitude from ∼2 to 3 times the zero-thermal-fluctuation critical current (Ic0), the addition of the OPP can decrease the pulse width necessary for switching by a factor of 10 or more The effect of the IPP also has beneficial ST consequences for the short pulse switching behavior

http://ralphgroup.lassp.cornell.edu/papers/APL-95-012506-2009.pdf

Ultrafast switching of a nanomagnet by a combined out-of-plane and in-plane polarized spin current pulse

We present the first space- and time-resolved images of the spin-torque-induced steady-state oscillation of a magnetic vortex in a spin-valve nanostructure. We find that the vortex structure in a nanopillar is considerably more complicated than the 2D idealized structure often-assumed, which has important implications for the driving efficiency. The sense of the vortex gyration is uniquely determined by the vortex core polarity, confirming that the spin-torque acts as a source of negative damping even in such a strongly nonuniform magnetic system. The orbit radius is $\ensuremath{\sim}10\text{ }\text{ }\mathrm{nm}$, in agreement with micromagnetic simulations.

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O. J. Lee

Papers

Current-induced switching of perpendicularly magnetized magnetic layers using spin torque from the spin Hall effect.

Central role of domain wall depinning for perpendicular magnetization switching driven by spin torque from the spin Hall effect

Macrospin modeling of sub-ns pulse switching of perpendicularly magnetized free layer via spin-orbit torques for cryogenic memory applications

Ultrafast switching of a nanomagnet by a combined out-of-plane and in-plane polarized spin current pulse

Images of a spin-torque-driven magnetic nano-oscillator