Ionitronic manipulation of Racetrack memory devices

September 06, 2021

Racetrack memory devices are novel spintronic memory-storage devices that have been demonstrated to have highly attractive properties, including high speed and density, and non-volatility. A novel feature of these devices is that the digital data is encoded as nanoscopic magnetic domain walls that are shifted to and thro along magnetic “racetracks” using current pulses. Recently, a team of scientists at the Max-Planck Institute of Microstructure Physics in Halle (Saale) have successfully realized the ionitronic manipulation of such current-induced domain wall motion in synthetic antiferromagnetic racetrack devices. The results have been published in the journal “Nature Communications”.

Illustration of ionic liquid controlled domain wall switch funtion device

The current- or voltage- induced manipulation of magnetization is key to several spintronic technologies, including magnetic random-access memories (MRAMs) and magnetic racetrack memories. In the racetrack memory, a series of magnetic domain walls (DWs), which encode digital data, are manipulated in magnetic nanowires via current pulses. These nanowires have evolved from in-plane to out-of-plane magnetized materials, to heterostructures with strong interfacial spin–orbit coupling, and, most recently, to synthetic antiferromagnetic (SAF) structures. In the SAFs, where two magnetic sublayers are antiferromagnetically coupled through an ultra-thin metallic spacer layer, the DWs can be moved very efficiently with velocities that can exceed ~1 km/s by means of a giant exchange-coupling torque (ECT) that is generated by the current. Another means of manipulating magnetization are electric fields from gate voltages applied across insulating layers that have been shown, e.g., to tune the perpendicular magnetic anisotropy (PMA) of magnetic tunnel junctions and, thereby, allow for more energy-efficient MRAM.

Ionic liquid controlled transition between SAF and single ferromagentic domain walls

In a study published in Nature Communications, by applying gate voltages to SAF racetrack devices across an ionic liquid, the current-induced DW velocity is shown to be significantly modified, reversibly, and in a non-volatile manner. The efficient and effective manipulation of the DW velocity is shown to be due to gate-induced changes in the degree of oxidation of the upper magnetic layer that, thereby, influences the exchange-coupling torque. The gating effect is further shown to be able to transform SAF structures reversibly into a complete ferromagnetic structure. Utilizing such transformations, an ionic liquid gate-controlled switch function has been demonstrated: when a DW in an SAF structure is injected into a specially designed racetrack device with geometry, the DW can only pass through the device for certain gate voltages.

With the successful ionitronic manipulation of current-induced domain wall motion in synthetic antiferromagnetic racetrack systems, a new degree of freedom has been introduced into racetrack devices which can not only reversibly control the domain wall velocity over a wide range but also gives rise to the potential of realizing logic functions, thereby opening up novel routes to memory-in-logic applications.

The publication entitled “Ionitronic manipulation of current-induced domain wall motion in synthetic antiferromagnets” was published on Aug 18th in the journal Nature Communications.

DOI: 10.1038/s41467-021-25292-1

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