AntifeRromagnetic spin Transport and Switching

Magnetic materials and devices play a tremendous role in information technology and are a key tool to meet many current societal challenges. They might enable the exploration of the human brain with non-invasive sensors and IT devices with low environmental impact. Antiferromagnetic materials are magnetic materials with alternating orientation of the atomic magnetic moments, having thus a zero net stray magnetic field. This is different from ferromagnetic materials like fridge magnets. Antiferromagnetic spintronics is considered as a disruptive approach, enabling efficient spintronic devices, potentially replacing silicon-based microelectronics components in the future. Louis Néel received in 1970 the Nobel prize in Physics for his studies on antiferromagnetic materials, describing them as “interesting but useless”, which was believed at the time. Today we know that ultimate stability and speed indicate significant untapped potential of this class of materials, where information can be stored in the antiferromagnetic magnetic moment orientation. In antiferromagnetic insulators information can be transported by spin currents without Joule heating, thus being promising for applications where low power dissipation is important. We thus investigated spin switching and transport primarily in particularly low damping insulating antiferromagnetic materials. Our key goals have been (i) To develop and employ an all-electrical read-out and control of the antiferromagnetic magnetic moments, potentially paving the way to store magnetic information in this class of materials. (ii) To achieve and study long distance spin current transport in antiferromagnets, potentially enabling information transport with low dissipation.

During the funding period of this project, we demonstrated all-electrical control and reading-out of the insulating antiferromagnetic materials NiO and CoO, by means of devices patterned with metallic Pt in contact with the antiferromagnetic materials, allowing for the injection of current pulses. The application of a current pulse generates thermal gradients, which in turn lead to local deformation of the insulating materials, allowing for the control of the orientation of the magnetic moments via magnetoelastic effects. From a device perspective, one needs to apply current pulses in two orthogonal directions, as shown in the Figure below for the case of CoO. Note that the magnetic moments in the CoO (small black arrows) are alternating as present in antiferromagnetic materials. The direction of each individual magnetic moment points from the south to the north pole. In fact, a collinear antiferromagnetic material is nothing but a series of alternated atomic magnets. The direction of the antiferromagnetic moments can be read out electrically in a measurement configuration similar to the Hall effect measurement. We now describe the process leading to writing of information: the atomic magnetic moments of the Co (small black arrows) reorient orthogonally with respect to the applied current pulse (large red arrow). The large arrows indicate the current direction and should not be confused with the small arrows indicating the magnetic moments. A magnetic state like the one in panel (a) can correspond by convention to a bit of type “0”. If then conversely a pulse in the orthogonal direction is applied (large blue arrow), the magnetic moments also reorient orthogonally, the magnetic state corresponding to a bit of type “1”. We demonstrated this functionality experimentally in devices we fabricated, thus demonstrating the basic building block of a memory element in this class of materials, and we quantified the magnitude of the acting torques.
We additionally achieved long distance spin transport across several microns in antiferromagnetic hematite thin films. Here we found that the presence of magnetic domain walls can decrease the spin diffusion length, if the antiferromagnetic domains are smaller than the intrinsic spin diffusion length. This is important for applications, as one needs to prepare antiferromagnetic materials with large domains to optimize the spin transport properties.
We disseminated the project results to the scientific community via the publication of papers (14 papers and 1 preprint), conference contributions (10 oral contributions and 4 posters) and communicated to the general public via the via the project website (https://spintronicsartes.wordpress.com/tag/spintronics/) the Kläui group website (https://www.klaeui-lab.physik.uni-mainz.de/news/) and two press releases for the Physical Review Letters papers (https://www.uni-mainz.de/presse/aktuell/10211_ENG_HTML.php and https://www.uni-mainz.de/presse/aktuell/11958_ENG_HTML.php) both in English and in German. All papers have been published in open access repositories (arXiv respectively the Gutenberg Open Science repository provided by the JGU) and the accepted version can be accessed without subscriptions. More than 10 students have been involved in this project. In the host group, activities with a number of microelectronics companies are going on, where the results of the project are disseminated.

Our results convincingly and positively answered the questions we tackled, both on the feasibility of all-electrical control of insulating antiferromagnetic materials and on the long distance spin transport in antiferromagnets, thus fully achieving the key goals of the project. This is potentially relevant to new memory or logic devices. As we identified a new switching mechanism, previously not considered, new questions have been raised by our work, namely how fast, scalable and efficient can be the thermomagnetoelastic switching we identified. If these new questions receive a positive answer in the future, our result will pave the way for applications based on antiferromagnetic spintronics.

Periodic Reporting for period 1 - ARTES (AntifeRromagnetic spin Transport and Switching)

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