Magnetization switching by spin-orbit torque in an antiferromagnet/ferromagnet bilayer system (1507.00888v1)

Abstract: Spin-orbit torque (SOT)-induced magnetization switching shows promise for realizing ultrafast and reliable spintronics devices. Bipolar switching of perpendicular magnetization via SOT is achieved under an in-plane magnetic field collinear with an applied current. Typical structures studied so far comprise a nonmagnet/ferromagnet (NM/FM) bilayer, where the spin Hall effect in the NM is responsible for the switching. Here we show that an antiferromagnet/ferromagnet (AFM/FM) bilayer system also exhibits a SOT large enough to switch the magnetization of FM. In this material system, thanks to the exchange-bias effect of the AFM, we observe the switching under no applied field by using an antiferromagnetic PtMn and ferromagnetic Co/Ni multilayer with a perpendicular easy axis. Furthermore, tailoring the stack achieves a memristor-like behaviour where a portion of the reversed magnetization can by controlled in an analogue manner. The AFM/FM system is thus a promising building block for SOT devices as well as providing an attractive pathway towards neuromorphic computing.

• The paper demonstrates zero-field spin-orbit torque switching in AFM/FM bilayers via exchange bias effects.
• It employs PtMn/Co/Ni stacks with controlled thickness to reveal memristive behavior critical for neuromorphic applications.
• Measured SOT efficiency reaches ~78 mT per 10^12 A/m^2, rivaling traditional nonmagnet/ferromagnet systems.

Insights on Magnetization Switching by Spin-Orbit Torque in Antiferromagnet/Ferromagnet Bilayer Systems

The paper investigates spin-orbit torque (SOT)-induced magnetization switching in a bilayer structure composed of antiferromagnetic (AFM) PtMn and ferromagnetic (FM) Co/Ni multilayers. Historically, research focused on nonmagnet/ferromagnet (NM/FM) stacks due to their practical application in spintronics via the spin Hall effect (SHE). However, this work makes strides by showing that AFM/FM systems can also demonstrate significant SOT for magnetization switching, which is pivotal for future spintronic applications without the need for external magnetic fields.

Methodology and Results

The research employs PtMn/[Co/Ni] stacks with varying PtMn thicknesses and illustrates efficient SOT magnetization switching through careful sample preparation and experimental measurement. These samples show exchange bias effects, crucial for internal magnetic field generation, which aids in achieving magnetization switching without external fields. Notably, the stacks demonstrate memristive behavior, essential for neuromorphic computing applications.

Key experimental findings include:

• Zero Field SOT Switching: Evidence supports that the PtMn/Co/Ni stack can achieve SOT switching at zero external fields, indicating its practical advantage over NM/FM configurations requiring a collinear magnetic field.
• Memristive Behavior: The AFM/FM setup exhibits a behavior where the degree of magnetization reversal is continuously variable with the applied current, resembling a memristor. This property is especially relevant for neuromorphic applications since it mimics synaptic activity.
• Efficient SOT Magnitude: The effective SOT field, evaluated at approximately 78 mT per 10^12 A/m^2, is comparable in magnitude to traditional NM/FM systems like Ta/CoFeB/MgO, suggesting that AFM materials could rival or surpass them in terms of spin-torque efficiency.

Theoretical and Practical Implications

Theoretically, the paper explores the SHE in AFMs, affirmed by the positive spin Hall angle of PtMn, and proposes a direct SHE contribution to the SOT switching behavior. This challenges conventional focus on NM/FM systems, advocating for alternative pathways in spintronics. Practically, the demonstrated memristive behavior proposes potential for AFM/FM structures in memristive computing devices, promoting advances in energy-efficient, high-density memory and neuromorphic processors.

Future Directions

This paper opens multiple avenues for research in the field of AFM/FM spintronics. Future efforts may investigate:

• Material Optimization: Identifying optimal material combinations and configurations to further enhance SOT efficiency and memristive behavior.
• Device Integration: Developing prototypes of spintronic devices leveraging AFM/FM systems for both memory and logic applications to showcase industrial applicability.
• Expanded Applications: Exploring the integration of such systems in advanced neuromorphic computing architectures, driving innovation in artificial intelligence hardware.

In closing, the paper provides valuable insights into the field of magnetization switching using AFM/FM bilayers, offering a promising alternative to traditional NM/FM systems. These findings hold the potential to significantly impact the development of next-generation spintronic devices and contribute to advancements in computational architectures.