Field-free spin-orbit switching of perpendicular magnetization enabled by dislocation-induced in-plane symmetry breaking

Abstract

Current induced spin-orbit torque (SOT) holds great promise for next generation magnetic-memory technology. Field-free SOT switching of perpendicular magnetization requires the breaking of in-plane symmetry, which can be artificially introduced by external magnetic field, exchange coupling or device asymmetry. Recently it has been shown that the exploitation of inherent crystal symmetry offers a simple and potentially efficient route towards field-free switching. However, applying this approach to the benchmark SOT materials such as ferromagnets and heavy metals is challenging. Here, we present a strategy to break the in-plane symmetry of Pt/Co heterostructures by designing the orientation of Burgers vectors of dislocations. We show that the lattice of Pt/Co is tilted by about 1.2° when the Burgers vector has an out-of-plane component. Consequently, a tilted magnetic easy axis is induced and can be tuned from nearly in-plane to out-of-plane, enabling the field-free SOT switching of perpendicular magnetization components at room temperature with a relatively low current density (~10¹¹ A/m²) and excellent stability (> 10⁴ cycles). This strategy is expected to be applicable to engineer a wide range of symmetry-related functionalities for future electronic and magnetic devices.

Fig. 1. The schematic of in-plane crystal symmetry breaking by dislocations.

a For the (100)-orientation, there are two $\frac{1}{2}<110>$ lattice vectors in the film plane. The gold balls indicate the atoms constructing FCC lattice. Representative Burgers vector B is shown in b, resulting in c, dislocation with in-plane Burgers vector and no tilting of crystal lattice. d For (110)-orientation, there is only one $\frac{1}{2}<110>$ lattice vector in the film plane. Therefore, Burgers vector B with out-of-plane component are required. Representative Burgers vector B is shown in e, results in f dislocation with an out-of-plane Burgers vector component and tilting of crystal lattice that is quantified by the angle θ_e. The gray arrows indicate $\frac{1}{2}<110>$ lattice vectors. Gray and blue balls indicate two distinct crystal lattices, and the Burgers circuits and Burgers vectors are indicated by green and red arrows, respectively. It is noted that FCC structure is not shown in Figs. c and f for better visualization.

Fig. 2. The observation of in-plane crystal symmetry breaking by dislocations.

a HAADF-STEM image of the (100)-orientation Pt(5)/Co(1.5)/NiO(20)/MgO heterostructure. Values in parentheses are thickness in nm. The statistical results of θ_e are shown in b. c HAADF-STEM image of the (110)-orientation Pt(5)/Co(1.5)/NiO(20)/MgO heterostructure. The statistical results of θ_e are shown in d. The white dash boxes show the Burgers circuits, and the red arrows show the projected Burgers vectors of dislocations. The symbol $\perp$ indicates the dislocations. The blue line is the gaussian fitting of the data. The error bars represent the standard deviation of θ_e.

Fig. 3. The controllable tilted magnetic easy axis induced by in-plane symmetry breaking.

a The schematic diagram for the tilted magnetic easy axis induced by dislocations in (110)-orientation Pt/Co/NiO/MgO heterostructures. b The magnetic hysteresis loops along the [001]-, [$\overline{1}$10]-, [110]-direction of (110)-orientation Pt(5)/Co(1.2)/NiO(20)/MgO. c Field dependence of polar MOKE signals by sweeping the magnetic field along in-plane [001] and [$\overline{1}10$] directions. Note that polar MOKE signals mainly correspond to the perpendicular magnetization. A data shift is induced for better visualization. d The polar angular (γ) dependent Hall resistance (R_xy) by rotating external magnetic field H in the ($\overline{1}10$) plane. The inset is the geometry set for the measurement. e The modulation of θ_M by increasing the thickness of inserting Pt layer (t_in-Pt), with blue line as guideline to the eye. The inset is the schematic diagram of the sample for the controllable tilted magnetic easy axis by inserting Pt layer between the Co and NiO layers.

Fig. 4. Field-free current-induced switching of magnetization with perpendicular components.

The schematic diagram of the set-up for SOT switching of perpendicular magnetization is shown in a, with small θ_M and b, with large θ_M. The yellow arrows indicate the ferromagnetic magnetization. The black arrows indicate the I_pulse. The results of field-free SOT switching with I_pulse (pulse width of 2 ms) applied along [$\overline{1}10$] and [001] are shown in c, for Pt(5)/Co(1.2)/NiO(20)/MgO heterostructure with θ_M = 16° and d, for Pt(5)/Co(0.6)/Pt(1.4)/NiO(20)/MgO heterostructure with θ_M = 84°, respectively. Field-free SOT switching is only observed as I_pulse is applied along [$\overline{1}$10]. e The schematic diagrams illustrate the switching of magnetization by SOT. The yellow arrow indicates the magnetization. The red arrow indicates the spin polarization $\mathit{\sigma }$ provided by Pt as I_pulse is applied along [$\overline{1}$10] direction. The black curve arrow indicates the direction of magnetization relaxation. f Reversible field-free SOT switching with ±J_pulse about 5.7 × 10¹¹A/m² and pulse width of 300 μs along $\left[\overline{1}10\right]$ in (110)-orientation Pt(5)/Co(0.6)/Pt(1.4)/NiO(20)/MgO heterostructure for more than 10⁴ cycles. Values in parentheses are thickness in nm.