Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - Standard EF (The research of spin orbit torques in perpendicular magnetic anisotropy systems.)

Teaser

Fast, high density and low power microelectronic devices are crucial enablers of today’s IT technology. With semiconductor devices facing severe limitations for their performance in the future, spintronics technologies have been recently identified as the most likely...

Summary

Fast, high density and low power microelectronic devices are crucial enablers of today’s IT technology. With semiconductor devices facing severe limitations for their performance in the future, spintronics technologies have been recently identified as the most likely technology for the next generation of non-volatile random access memory. This is due to the fact that magnetic technologies are inherently non-volatile and thus retain their information without power. However, current spintronic approaches based on magnetic bits made of “single domain” spin structures or “domain walls” result in limited stability and an unacceptably high level of power consumption during operation due to the high currents and current densities required for manipulating the spins by spin transfer torque. Recently a radically new scientific and technological approach is necessary to tackle these key drawbacks, and obtain small and stable spin structures as well as new mechanisms to efficiently manipulate these. A key element in obtaining small and stable spin structures is known as the Dzyaloshinskii-Moriya interaction (DMI). The DMI which arises in the presence of spin orbit coupling and inversion asymmetry leads to a non-collinear interaction resulting in a spin texture such as chiral domain walls and skyrmions. Along with the DMI in the presence of spin orbit coupling a mechanism to exploit these spin structures arises which is called the spin orbit torque (SOT).
These effects are intriguing for its possible high efficient manipulation and stabilization of spin structures for application memory applications. However, in order to manipulate and maximize these effects first we must understand the underlying mechanisms. The project has focused on understanding the underlying physics of DMI and SOT by studying these effects in various systems. By fully understanding the effects and being able to manipulate the effects in a manner to fully maximize the efficiency. This would lead to possible technologies for designing an ultra-efficient memory devices.

Work performed

A key objective of the project is to study the Dzyaloshinskii-Moriya interaction (DMI) and spin-orbit torques (SOT) in various systems to uncover their underlying mechanisms. The DMI has been studied in various systems and conditions using different methods (Figure 1). The different methods have shown a discrepency in the results although the material systems were the same. Thus, finding a „gold standard“ to determine the DMI in systems was the first step of the project. We have access to different methods to measure the DMI using the Kerr microscope and transport measurements. We had measured and compared the DMI values in different systems using different methods and some have shown a good agreement and others have shown a difference. We have selected judiciously certain methods to measure the temperature dependence of DMI in Pt/Co/AlOx. These results show that the mechanism of DMI is rather complex and needs further research to really understand the underlying physics. During the project we have also found a missing component of the interlayer exchange interaction, namely the interlayer DMI. The results show that in a synthetic antiferromagnet in which two ferromagnetic layers with a spacing layer that is coupling by the interlayer exchange coupling have a hidden interlayer DMI. This leads to a non-collinear alignment between the two ferromagnetic layers. Interestingly, the interlayer DMI in combination with the intralayer DMI can possibly lead to novel three dimensional topological spin structures.
Along with the research on DMI we have studied the SOT in various systems to get a grasp on the underlying mechanism. Here we have also measured the SOT by different methods to validate whether there could be a standard for quantifying SOTs (Figure 2). Here, we have found out in determining SOT the existence of a domain wall makes it complex to compare the values with that measured in a mono-domain system. On the basis of these results, the SOT was evaluated in various material systems. We have measured the magnetic material thickness dependence and the temperature dependence. The temperature dependence revealed that the skew scattering mechanism of the spin Hall effect was dominant for the Pt based material stacks we observed. Also we have looked at possible correlations between the DMI and SOT in a material system where the DMI changes sign with the composition of the magnetic layer.
As a result of this research a number of papers were published and the results were disseminated at national and international conferences.

Final results

In spintronics, the means to manipulate magnetization in order to design efficient magnetic memory devices is a crucial task at hand. Especially the current method in manipulating magnetization has limitations in efficiency. A new concept of utilizing the spin orbit effects has arisen for designing a smaller and stable spin texture and a mechanism to manipulate it in a more efficient manner. Mainly, the Dzyaloshinskii-Moriya interaction (DMI) is responsible for the prior and spin orbit torques (SOT) are responsible for the latter. The project aims to better understand DMI and SOT and furthermore design material systems that will maximize the effects for high efficiency. The research results let us have a better understanding of SOT and DMI. By making the efficient magnetic random access memory possible it will assist in the IT industries making it possible to reduce the sizes of electronic devices. Higher efficiency will result in reduction of energy consumption leading to new possibilities of miniaturized gadgets for everyday life. Even accelerating the possibility of realizing the reality of higher virtual reality and ubiquitous applications based on the internet of things.

Website & more info

More info: https://lkjoon82.wixsite.com/standard-ef.