Continuous Tuning of the Magnitude and Direction of Spin‐Orbit Torque Using Bilayer Heavy Metals

Abstract

a Authors with equal contribution Spin-orbit torques (SOTs) have opened a new path to switch the magnetization in perpendicularly magnetized films and are of great interest due to their potential applications in novel data storage technology, such as the magnetic random access memory (MRAM). The effective manipulation of SOT has thus become an important step towards these applications.Here, current induced spin-orbit effective fields and magnetization switching are investigated in Pt/Ta/CoFeB/MgO structures with bilayer heavy metals. With a fixed thickness (1 nm) of the Ta layer, the magnitude and sign of current induced spin-orbit effective fields can be continuously tuned by changing the Pt layer thickness, consistent with the current induced magnetization switching data. The ratio of longitudinal to transverse spin-orbit effective fields is found to be determined by the Ta/CoFeB interface and can be continuously tuned by changing the Pt layer thickness. The Dzyaloshinskii-Moriya interaction (DMI) is found to be weak and shows an insignificant variation with the Pt thickness. The results demonstrate an effective method to tune SOTs utilizing bilayer heavy metals without affecting the DMI, a desirable feature which will be useful for the design of SOT-based devices.2

Characterization and Manipulation of Spin Orbit Torque in Magnetic Heterostructures

“…Since the magnetization oscillations (Δθ, Δφ) are induced by current‐induced effective fields, Equation (6) allows the determination of H L and H T by solving the force‐balance equations with known values of the anomalous Hall resistance ( R AHE ), the planar Hall resistance ( R PHE ), and the anisotropy field ( H an ) obtained from first‐harmonic voltage measurements. Moreover, the analytical expression of second‐harmonic analysis can be further simplified as…”

“…Since θ SH is strongly correlated with the material atomic number, various HMs, such as Pt, Ta, W, V, Ir, and Hf, have been explored for SOT study . Among these materials, Pt is the most well studied, and it is found of a large positive SHA.…”

Anomalous spin-orbit torque switching in synthetic antiferromagnets

“…In recent years, spin-orbit torques (SOTs), a new type of spin torque driven by in-plane currents flowing in heavy metals (HMs) [2][3][4][5][6], topological insulators [7][8][9], or antiferromagnets [10,11], have emerged as a more efficient way to manipulate magnetization. SOTs have been successfully employed to switch magnetization [2][3][4]8,10,[12][13][14][15][16][17][18], drive domain wall (DW) motion [19,20], and excite spin-torque nano-oscillators [21,22]. In many applications such as magnetic random access memory (MRAM), SOTs have advantages over STTs due to their higher efficiency and the ability to switch a MTJ without passing a large current through the tunnel barrier.…”

“…The final magnetization switches to the direction determined by H × σ . Here, H is the effective in-plane field that is provided by an external magnetic field [2][3][4]8,[12][13][14][15]18], exchange bias, or interlayer coupling [10,17], and σ is the spin polarization injected from adjacent materials. The spin Hall angle (SHA) of adjacent materials determines the direction of σ and thus the switching * cbi@email.arizona.edu † wgwang@physics.arizona.edu direction for a given H. Usually the adjacent materials can be classified into two basic types with a positive SHA, such as Pt [4], (Bi, Sb)Te [8], and PtMn [10], and a negative SHA, such as Ta [3] and W [26].…”

“…Therefore σ and the switching directions in Pt/ferromagnet (Pt/FM) and Ta/FM structures are opposite [2][3][4]8,10,[12][13][14][15][16][17]. So far all reported SOT switchings are the SOT switchings of a single ferromagnet [2][3][4]12,14,18,[23][24][25]27], in which reversing either current or in-plane field is necessary to switch magnetization [2][3][4]12,14,18,[23][24][25]27] and the switching rule shown in Figs. 1(a) and 1(b) is well obeyed.…”