Switching of Perpendicularly Polarized Nanomagnets with Spin Orbit Torque without an External Magnetic Field by Engineering a Tilted Anisotropy (1409.0620v3)

Abstract: Spin orbit torque (SOT) provides an efficient way of generating spin current that promises to significantly reduce the current required for switching nanomagnets. However, an in-plane current generated SOT cannot deterministically switch a perpendicularly polarized magnet due to symmetry reasons. On the other hand, perpendicularly polarized magnets are preferred over in-plane magnets for high-density data storage applications due to their significantly larger thermal stability in ultra-scaled dimensions. Here we show that it is possible switch a perpendicularly polarized magnet by SOT without needing an external magnetic field. This is accomplished by engineering an anisotropy in the magnets such that the magnetic easy axis slightly tilts away from the film-normal. Such a tilted anisotropy breaks the symmetry of the problem and makes it possible to switch the magnet deterministically. Using a simple Ta/CoFeB/MgO/Ta heterostructure, we demonstrate reversible switching of the magnetization by reversing the polarity of the applied current. This demonstration presents a new approach for controlling nanomagnets with spin orbit torque.

• The paper introduces a tilted anisotropy approach that enables deterministic SOT switching in perpendicularly magnetized nanomagnets without requiring an external magnetic field.
• It employs a Ta/CoFeB/MgO/Ta heterostructure with precise ion milling and lithography to create a controlled tilt of approximately 2–5°, validated by AHE and AMR measurements.
• This breakthrough method paves the way for energy-efficient spintronic devices and advances in high-density data storage applications.

Overview of Magnetization Switching in Perpendicularly Polarized Nanomagnets Using Spin Orbit Torque

This paper addresses a significant challenge in the field of spintronics: the deterministic switching of perpendicularly polarized nanomagnets using spin orbit torque (SOT) without the necessity of an external magnetic field. Traditional methods necessitate an external in-plane magnetic field to break symmetry for magnetization switching. This paper introduces a pioneering technique that resolves this requirement by engineering a tilted anisotropy within the magnetic system, thus enabling deterministic switching solely with an in-plane current.

Spin orbit torque is a mechanism that leverages the interaction between an in-plane charge current and spin accumulation to apply a torque on a magnetic system. However, the inherent symmetry in perpendicularly magnetized systems typically prevents deterministic SOT-induced switching using only an in-plane current. The authors tackle this by introducing a slight tilt in the magnetic easy axis away from the perpendicular direction, effectively breaking the symmetry and facilitating the switching of magnetization without an external magnetic field.

Experimental Results and Characterization

The experimental setup involves a heterostructure composed of Ta/CoFeB/MgO/Ta. The crucial innovation lies in engineering a tilt in the magnetic anisotropy by creating a wedge in the CoFeB layer. This was accomplished by precise ion milling and lithographic techniques, resulting in nanodots with a smooth transition from a perpendicular magnetic anisotropy to a slight tilt.

The paper employs anomalous Hall effect (AHE) and anisotropic magnetoresistance (AMR) measurements to verify the occurrence and influence of this tilt. The AMR measurements reveal an asymmetry indicative of a tilted easy axis. The estimated tilt angle of approximately 2-5 degrees was confirmed through AHE data and micromagnetic simulations. Importantly, the experiments demonstrate deterministic switching of the nanomagnet's magnetization states in response to pulsed currents applied along specific in-plane directions, achieving full magnetization reversal without external fields.

Implications and Future Directions

The ability to control magnetization in perpendicularly polarized systems without an external magnetic field has far-reaching implications for the design of next-generation spintronic devices. Devices can be made more compact and energy-efficient by exploiting the low power requirements of SOT. This work could potentially integrate with existing technologies where perpendicular anisotropy is preferred due to superior thermal stability at small dimensions, such as in high-density data storage applications.

Future work could focus on refining the fabrication process for better control over the tilt angle and exploring the compatibility of this approach with various material systems. Furthermore, extending this technique to different geometries and devices could open new pathways for utilizing spintronic phenomena in practical applications. The integration of such systems with CMOS technology might also be a promising avenue, enhancing the scalability and manufacturability of spin-based electronic components.

In conclusion, this research presents a robust method for manipulating perpendicularly magnetized nanomagnets without external magnetic fields by leveraging the spin-orbit torque mechanism. The tilt-engineering approach marks a significant step forward in spintronic research and provides a foundation for advancing energy-efficient magnetic memory technologies.