Spin torque nano-oscillator driven by combined spin injection from tunneling and spin Hall current

Abstract

In this paper, a 3-terminal spin-transfer torque nano-oscillator (STNO) is studied using the concurrent spin injection of a spin-polarized tunneling current and a spin Hall current exciting the free layer into dynamic regimes beyond what is achieved by each individual mechanism. The pure spin injection is capable of inducing oscillations in the absence of charge currents effectively reducing the critical tunneling current to zero. This reduction of the critical charge currents can improve the endurance of both STNOs and non-volatile magnetic memories (MRAM) devices.It is shown that the system response can be described in terms of an injected spin current density Js which results from the contribution of both spin injection mechanisms, with the tunneling current polarization p and the spin Hall angle θ acting as key parameters determining the efficiency of each injection mechanism. The experimental data exhibits an excellent agreement with this model which can be used to quantitatively predict the critical points (Js = -2.26±0.09 × 10 9 ħ/e A/m 2 ) and the oscillation amplitude as a function of the input currents. In addition, the fitting of the data also allows an independent confirmation of the values estimated for the spin Hall angle and tunneling current polarization as well as the extraction of the damping α = 0.01 and non-linear damping Q = 3.8±0.3 parameters. Index Terms-Spin Hall Effect, Spin Torque Nano-oscillator, Magnetic Tunnel Junctions.Recent reports demonstrate that the Spin Hall Effect (SHE) can be used to generate pure spin currents, capable of exerting a spin transfer torque (STT) that induces oscillations in a ferromagnetic layer 1,2 . This pure spin current is created by a charge current in a nonmagnetic material with strong spin-orbit coupling where up and down spins are scattered in opposite directions resulting in a spin current orthogonal to the electrical current 2-6 . A central challenge is to quantify the efficiency of the charge current to spin current conversion, which results from the difficulty of measuring spin currents. The spin-orbit material is characterized by a material property called the spin Hall angle, which quantifies the ratio between the generated spin current density ( Hall spin s J ) at an applied charge current density ( Hall spin c J ). The spin Hall angle is expressed as   Hall spin c Hall spin s J J e    with the charge of the electron e and the reduced Plank constant ħ ensuring dimensional consistency. Several techniques have been used to quantify θ of transition metals such as Au, Pd, Pt, Ta, and W. A particularly interesting material is Ta since it is a typical cap and seed layer in magnetic tunnel junction (MTJ) devices and in direct contact with the ferromagnetic free layer. The reported θ values of Ta are in a wide range of 1.4% < θ < 15%, primarily due to dependences on the crystalline phase 6-9 . e J J J J J  stripe of Hall spin c J = -73 × 10 9 A/m 2 injects an equivalent Js into the free layer. At this value the spin Hall effect should excite oscill...

Spin–orbit torque nano-oscillator with giant magnetoresistance readout

“…This field is sufficiently high to rotate the free layer magnetization, while the pinned layer magnetization in the GMR SHO still has a large component parallel to the nanowire axis. For ϕ H near 90°, the antidamping action of SHT is maximized because spin Hall current polarization is nearly opposite to the free layer magnetization 20 .…”

“…An alternative route to achieve large output microwave power is to use a three-terminal spin torque oscillator that consists of an MTJ embedded on top of a heavy metal nanowire 15,20,67 . An example schematic can be seen in Fig.…”

“…Liu et al 15 first demonstrated microwave output powers of 0.25 nW using the three-terminal MTJ SHO design. Recently, Tarequzzaman et al 20 demonstrated microwave output powers of up to 48 nW in a the three-terminal MTJ SHO driven by combination of SHT and spin-transfer torque. In this case, the bias current applied to the MTJ was not only used as a probe current, but was actually large enough to drive spin-transfer torque.…”

“…The simplest example of such a SOT is spin Hall torque (SHT) arising from pure spin current in the NM layer that flows in the direction orthogonal to both the charge current and the FM/NM interface [9][10][11][12][13] . When injected into the FM layer, this pure spin current applies SHT that can act as negative magnetic damping and thereby excite autooscillations of the FM magnetization [14][15][16][17][18][19][20][21] . The current-driven auto-oscillations of magnetization result in a microwave voltage generation by the FM/NM bilayer due to anisotropic magnetoresistance (AMR) of the FM 15,22 .…”

Spin-Torque-Nano-Oscillator based neuromorphic computing assisted by laser

“…4 (d) and (e). This effect is also seen in spin Hall nano-oscillators (SHNOs) [33]. As a result, the minimum bias current needed to detect the STNO oscillation of the fired neuron by sensing circuit decreases.…”