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

A binary stochastic neuron (BSN) or a probabilistic bit (p-bit) randomly fluctuates between digitized “0” and “1” with a controllable functionality of time-averaged value. Such an unconventional bit is the most essential building block for the recently proposed stochastic neural networks and probabilistic computing. Here, we experimentally implement a magnetic tunnel junction (MTJ) for BSN, with relaxation times on the order of tens of milliseconds that can be modulated by a current-induced spin-transfer torque. The NIST Statistical Test Suite (800-22a) is used to verify true random number generation by the BSN-MTJ device. Our results suggest the possibility of using the artificial BSN MTJ device in neuromorphic applications as well as in a recently proposed probabilistic computing.

Demonstration of in-plane magnetized stochastic magnetic tunnel junction for binary stochastic neuron

Variation of spin-orbit torque and spin transport properties by V alloying in β-W-based magnetic heterostructures

We present detailed investigations of the magnetic properties of Pt/CoFeB/MgO layers as studied using the angle-resolved spin-torque ferromagnetic resonance method. Although the measurements provide reasonable magnetic parameters, we obtain an unusual value in the interfacial spin transparency if spin-pumping is assumed to be the dominant source for enhancing magnetic damping ( αeff) in the nanometer thickness regime. However, the thickness dependence of the Landé g-factor ( geff) for CoFeB indicates that the interfacial spin–orbit coupling plays a role in determining αeff. In addition, the azimuthal asymmetry in the magnetic system may not be related to the possibility of generating unconventional spin polarization. The results in this work are expected to aid in understanding various magnetic properties and current-induced spin-torques in a heavy-metal/ferromagnet bilayer.

Investigation of magnetic properties of Pt/CoFeB/MgO layers using angle-resolved spin-torque ferromagnetic resonance spectroscopy

Sputtered Bi2Se3 has strong potential for use as a topological insulator in spintronic devices because of its perfect spin polarization and ability to be grown on a large scale. In a Bi2Se3/Ni81Fe19 device, electric field control of spin–orbit torque is clearly observed using second-harmonic measurements. The gate voltage modulates the Fermi level as well as the channel types (i.e., p- or n-type). The strengths of damping-like and field-like torques induced by current are separately extracted for various gate voltages. We find that only damping-like torque is modulated by the gate electric field, showing its maximum value near the Dirac point. In addition, thermal effects mixed with spin–orbit torques are also resolved on the basis of the magnetic field dependence. This work not only evaluates the magnitudes of spin–orbit torques quantitatively but also demonstrates the gate-controlled damping-like torque in a sputtered topological insulator/ferromagnet bilayer. 

Gate Control of Spin–Orbit Torque in a Sputtered Bi2Se3/Ni81Fe19 Device


 Nonreciprocal charge transport is observed in a non-centrosymmetric system without a ferromagnetic layer. To observe the nonreciprocity of the Rashba system, an InAs-based two-dimensional electron gas channel is utilized and an angular dependent harmonic Hall measurement is performed. From the amplitude of the curve, a nonreciprocal coefficient of 1.36 A−1T−1 is extracted at 1.9 K. While the estimated value of the nonreciprocal coefficient decreases to 0.44 A−1T−1 at 300 K, we can clearly observe the nonreciprocal charge transport at room temperature. In addition, the independent transport measurements clarify that the amplitude of nonreciprocal coefficient is closely connected with the strength of the Rashba effect.

O. J. Lee

Papers

Demonstration of in-plane magnetized stochastic magnetic tunnel junction for binary stochastic neuron

Variation of spin-orbit torque and spin transport properties by V alloying in β-W-based magnetic heterostructures

Investigation of magnetic properties of Pt/CoFeB/MgO layers using angle-resolved spin-torque ferromagnetic resonance spectroscopy

Gate Control of Spin–Orbit Torque in a Sputtered Bi2Se3/Ni81Fe19 Device

Room-Temperature Nonreciprocal Charge Transport in an InAs-Based Rashba Channel