Deterministic switching of perpendicularly magnetic layers by spin orbital torque through stray field engineering

📝 Abstract

We proposed a novel multilayer structure to realize the deterministic switching of perpendicularly magnetized layers by spin orbital torque from the spin Hall effect through stray field engineering. In our design, a pinned magnetic layer is introduced under the heave metal separated by an insulator, generating an in-plane stray field in the perpendicularly magnetized layer. We have confirmed the deterministic switching of perpendicularly magnetized layers through micromagnetic simulation and theoretical analysis. The in-plane stray field accounts for the deterministic switching exhibited in the structure and the reversal ultimate state of the magnetic layer is predictable when the applied spin current density is above the critical spin current density. Moreover, the stray field is easily tunable in a wide range by adjusting the saturation magnetization and dimensions of the pinned layer, and can accommodate different perpendicularly magnetized materials without any external magnetic field.

💡 Analysis

We proposed a novel multilayer structure to realize the deterministic switching of perpendicularly magnetized layers by spin orbital torque from the spin Hall effect through stray field engineering. In our design, a pinned magnetic layer is introduced under the heave metal separated by an insulator, generating an in-plane stray field in the perpendicularly magnetized layer. We have confirmed the deterministic switching of perpendicularly magnetized layers through micromagnetic simulation and theoretical analysis. The in-plane stray field accounts for the deterministic switching exhibited in the structure and the reversal ultimate state of the magnetic layer is predictable when the applied spin current density is above the critical spin current density. Moreover, the stray field is easily tunable in a wide range by adjusting the saturation magnetization and dimensions of the pinned layer, and can accommodate different perpendicularly magnetized materials without any external magnetic field.

📄 Content

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Deterministic switching of perpendicularly magnetic layers by spin orbital torque through stray field engineering

Sumei Wang1, Meiyin Yang2, and Chao Zhao1

ICAC, Institute of Microelectronics of CAS, University of CAS, No.3 BeiTuCheng West Rd., Beijing 100029, China 2. SKLSM, Institute of Semiconductors of CAS, P. O. Box 912, Beijing 100083, China

We proposed a novel multilayer structure to realize the deterministic switching of perpendicularly magnetized layers by spin orbital torque from the spin Hall effect through stray field engineering. In our design, a pinned magnetic layer is introduced under the heave metal separated by an insulator, generating an in-plane stray field in the perpendicularly magnetized layer. We have confirmed the deterministic switching of perpendicularly magnetized layers through micromagnetic simulation and theoretical analysis. The in-plane stray field accounts for the deterministic switching exhibited in the structure and the reversal ultimate state of the magnetic layer is predictable when the applied spin current density is above the critical spin current density. Moreover, the stray field is easily tunable in a wide range by adjusting the saturation magnetization and dimensions of the pinned layer, and can accommodate different perpendicularly magnetized materials without any external magnetic field.

Extensive experiments have been devoted to study the deterministic switching of perpendicularly magnetized layers in heavy metal/ferromagnet devices [1-10]. Particularly, the spin orbital torque (SOT) induced by the spin Hall effect (SHE) is one of the most promising candidates for next-generation memory devices due to many advantages such as low power consumption and absence of current leakage. The SOT may induce the rotation of magnetization in the magnetic layer, but the final state of the magnetic layer (pointing up or down) is uncertain [11, 12]. Several attempts have been made to eliminate this uncertainty. In Liu’s experiment[12], a fixed external field was applied along the charge current direction. The external field succeeded in breaking the symmetry of the rotation in response to the SOT and deterministic switching is achieved. Other alternatives are also put forward to circumvent the complexity of applying an external field, such as introducing antiferromagnetic interaction[10], establishing a lateral structure asymmetry[6], constructing a hybrid ferromagnetic/ferroelectric structure[7], etc. However, the range of assisted field is relatively limited or the design is not easily scalable[8]. Here, we proposed a novel structure to accomplish the deterministic switching of the perpendicular magnetized layer. In the design, a fixed or pinned layer is introduced at the bottom of the magnetic layer and its stray field on the magnetic layer servers as assisted field to break the symmetry. We have confirmed the deterministic switching behavior and feasibility of our design through micromagnetic simulation and theoretical analysis, and the detailed dynamics during switching are also analyzed. The stray field the pinned 2

layer produces on the magnetic layer is independent of the exchange interaction with the magnetic layer, which facilitates the design and application of such a structure. The magnitude of the stray field can be tuned by altering the magnetization and dimensions of the pinned layer. Besides, pinned magnetic layers have been extensively used in industries and are anticipated to be easily implemented.
Illustrations of our design of the multilayer structure can be seen in Fig.1 (a). The key lies in the introduction of the pinned magnetic layer at the bottom of the spintronic device. The magnetization of the fixed layer or the pinned layer is along -y direction (in-plane) and hence produces a stray field in the opposite direction (+y) on the magnetic layer. To be simple, we refer to the stray field produced by the pinned layer on the magnetic layer as the stray field. The pinned layer is separated by an insulator layer from the magnetic layer. The SOT is a short-range interaction since the torque is caused by spin-orbit interaction, and hence the magnetization of the pinned layer is hardly affected by the SOT due to the separation of the insulator layer. The dynamics of the reversal process are difficult to capture in experiments, but readily accessible by micromagnetic simulation. The dynamics of magnetization are described by the LLG equation with the SOT induced by the SHE, shown in Eq. (1).
d𝑚̂ d𝑡= −γ𝑚̂ × H⃗⃗ eff + 𝛼𝑚̂ × d𝑚̂ d𝑡+ ℏ𝐽𝑠 2𝑒𝑡𝑀s 𝑚̂ × 𝜎̂ × 𝑚̂ (1) The last term can be combined with the effective field Heff equivalently after a simple derivation and we define this field-like SOT term as H⃗⃗ SOT = ℏ𝐽𝑠 2𝑒𝑀s𝑡𝑚̂ × 𝜎̂. This field-like term is always perpendicular to the magnetization. Therefore, the total field becomes H⃗⃗ ′eff=H⃗⃗ eff + H⃗⃗ SOT = H⃗⃗ a + H⃗⃗ st + H⃗⃗ SOT where H⃗⃗

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