Spin Reflection-Induced Field-Free Magnetization Switching in Perpendicularly Magnetized MgO/Pt/Co Heterostructures

Abstract

Field-free magnetization switching is critical towards practical, integrated spin-orbit torque (SOT)-driven magnetic random-access memory with perpendicular magnetic anisotropy. Our work proposes a technique to modulate the spin reflection and spin density of states within a heavy-metal Pt through interfacing with a dielectric MgO layer. We demonstrate tunability of the effective out-of-plane spin torque acting on the ferromagnetic Co layer, enabling current-induced SOT magnetization switching without the assistance of an external magnetic field. The influence of the MgO layer thickness on effective SOT efficiency shows saturation at 4 nm, while up to 80% of field-free magnetization switching ratio is achieved with the MgO between 5 and 8 nm. We analyze and attribute the complex interaction to spin reflection at the dielectric/heavy metal interface and spin scattering within the dielectric medium due to interfacial electric fields. Further, through substituting the dielectric with Ti or Pt, we confirm that the MgO layer is indeed responsible for the observed field-free magnetization switching mechanism.

Current-induced perpendicular effective magnetic field in magnetic heterostructures

“…Development of high-density magnetic memory and computing technologies requires energy-efficient and scalable electrical switching of perpendicular magnetization. Microscopically, perpendicular magnetization can be switched by a current of transverse spins (σy) under the assist of an in-plane effective magnetic field along current (𝐻 𝑥 eff , to overcome the Dzyaloshinskii-Moriya interaction or to break the switching symmetry) 1 , or by an antidamping spin torque exerted by high-density current of perpendicular spins (σz) [2][3][4][5][6][7][8][9][10] , or by a strong perpendicular effective magnetic field (𝐻 𝑧 eff ) [11][12][13][14][15] . Since the first method (σy+𝐻 𝑥 eff ) is hardly energy-efficient or scalable, searching of σz or 𝐻 𝑧 eff in magnetic heterostructures becomes a very hot topic.…”

“…Experimentally, it is widely assumed that the presence of σz could be concluded from a small but sizable sin2φ dependent contribution in spin torque ferromagnetic resonance (ST-FMR) 2,7,8,[16][17][18] or a φ independent second harmonic Hall voltage response (HHVR) 5,19,20 of an in-plane magnetization (φ is the angle of external magnetic field with respect to the current). Presence of σz is also claimed from the occurrence of external-field-free current switching of a uniform perpendicular magnetization 4,[9][10][11][12][13][14][15] because the polarization and fieldlike spin-orbit torque (SOT) field of σz, if any, are along the film normal.…”

Wedge-shaped HfO2 buffer layer-induced field-free spin-orbit torque switching of HfO2/Pt/Co structure

“…Additionally, recent reports suggest that insulator buffer layers can promote field-free switching in MgO/Pt/Co structures, thus attracting considerable attention. [21] The current understanding attributes this phenomenon to the spin reflection-induced out-of-plane polarized spin current. [21] However, the generation of novel out-of-plane spin currents by reflection of the conventional in-plane spin Hall effect remains an open question.…”

Controlling In‐Plane Magnetic Anisotropy of Co Films on MgO Substrates using Glancing Angle Deposition

“…[23,24] While the combination of a heavy metal such as Pt and a ferromagnetic layer such as Co exhibits an efficient charge-to-spin conversion efficiency [25] and enables current-induced spin-orbit torque switching of an adjacent Co layer, [26] the proximity to an MgO layer induces perpendicular magnetic anisotropy in the Co layer. [27] This material combination also allows for a tuning of its energy terms, enabling the nucleation of magnetic skyrmion bubbles. [28] Here, we report the investigation on the ability of GLAD to control the anisotropy of Co films on Pt-coated MgO(001) and lower symmetry MgO(110) substrates.…”