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.

/pdf/current-driven-dynamics-of-chiral-ferromagnetic-domain-walls-1g9i94jrpa.pdf

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.

/pdf/symmetry-and-magnitude-of-spin-orbit-torques-in-525ou2rmgx.pdf

Symmetry and magnitude of spin-orbit torques in ferromagnetic heterostructures

We demonstrate reliable spin-torque-driven ballistic precessional switching using 50 ps current impulses in a spin-valve device that includes both in-plane and out-of-plane spin polarizers. Different threshold currents as the function of switching direction and current polarity enable the final orientation of the magnetic free layer to be steered, in accord with a macrospin analysis, by the sign of the pulse, eliminating the need for read-before-write toggle operation. The pulse amplitude windows for this deterministic operation are wider and more symmetric as a function of current polarity for shorter impulses, while inhomogeneous fringe fields from the polarizers lead to asymmetries as a function of current direction.

/pdf/spin-torque-driven-ballistic-precessional-switching-with-50-pim0ujukqt.pdf

Spin-torque-driven ballistic precessional switching with 50 ps impulses

Techniques, systems, and devices are disclosed for implementing a quasi-linear spin-torque nano-oscillator based on exertion of a spin-transfer torque on the local magnetic moments in the magnetic layer and precession of the magnetic moments in the magnetic layer within a spin valve. Examples of spin-torque nano-oscillators (STNOs) are disclosed to use spin polarized currents to excite nano magnets that undergo persistent oscillations at RF or microwave frequencies. The spin currents are applied in a non-uniform manner to both excite the nano magnets into oscillations and generate dynamic damping at large amplitude as a feedback to reduce the nonlinearity associated with mixing amplitude and phase fluctuations.

Quasi-linear spin torque nano-oscillators

We report enhanced spin-torque oscillator results obtained in spin-valve nanopillars. When biased within the optimal range of a moderate, ≤600 Oe, hard axis field, the spin-torque-driven oscillations exhibit a sharp increase in power and a sharply narrowed linewidth, ≤10 MHz, which, based on micromagnetic simulations, we ascribe to a transition from incoherent to coherent dynamics. The simulations indicate that the coherent dynamics are enabled by the combination of strong coupling between the two oscillator end modes of the magnetic free layer and strong non-linear damping arising from a non-uniform magnetization that leads to a spatially varying anti-damping spin torque.

Coherent and incoherent spin torque oscillations in a nanopillar magnetic spin-valve

We report an approach to improving the performance of spin torque nano-oscillators (STNOs) that utilizes power-dependent negative feedback to achieve a significantly enhanced dynamic damping. In combination with a sufficiently slow variation of frequency with power this can result in a quasi-linear STNO, with very weak non-linear coupling of power and phase fluctuations over a range of bias current and field. An implementation of this approach that utilizes a non-uniform spin-torque demonstrates that highly coherent room temperature STNOs can be achieved while retaining a significant tunability.

Quasilinear spin-torque nano-oscillator via enhanced negative feedback of power fluctuations

Submitted for the MAR09 Meeting of The American Physical Society Ultrafast switching of a nanomagnet by a combined in-plane and out-of-plane polarized spin-current pulse OUKJAE LEE, V.S. PRIBIAG, P.M. BRAGANCA, P.G. GOWTHAM, E.M. RYAN, D.C. RALPH, R.A. BUHRMAN, Applied and Engineering Physics, Cornell University — For fast write operation of a spin-torque (ST) magnetic storage device, the exertion of a strong initial torque can switch the nanomagnet moment without the help of the thermal fluctuations. Use of an out-of-plane polarized reference layer can very quickly excite large free layer motion but reliable reversal requires precise ST pulse timing. The combination of strong in-plane and out-of-plane polarized spin currents can substantially relax this pulse-timing requirement. We have fabricated CPP spin-valve devices that incorporate both an out-of-plane polarizer, and an in-plane polarizer to quickly excite and reverse the moment of an in-plane polarized free layer. For pulse currents ranging between 100ps – 10ns, the reversal speeds are notably faster and much less thermally distributed than for a conventional spin-valve with the same pulse current amplitude. We will discuss the details of the short-pulse behavior of these device structures and the optimization of this approach for high-speed magnetic memory. Oukjae Lee Applied and Engineering Physics, Cornell University Date submitted: 28 Nov 2008 Electronic form version 1.4

http://absimage.aps.org/image/MAR09/MWS_MAR09-2008-006236.pdf

O. J. Lee

Papers

Spin-torque-driven ballistic precessional switching with 50 ps impulses

Quasi-linear spin torque nano-oscillators

Coherent and incoherent spin torque oscillations in a nanopillar magnetic spin-valve

Quasilinear spin-torque nano-oscillator via enhanced negative feedback of power fluctuations

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