Reimers, Sonka
(2022)
Antiferromagnetic domain structure in tetragonal CuMnAs films: A picturebook of domains, domain walls and everything in between.
PhD thesis, University of Nottingham.
Abstract
In antiferromagnetic (AF) materials, magnetic moments align in a regular pattern such that the moments cancel perfectly in each magnetic unit cell. Hence AF materials do not show a net magnetisation and are largely inert against magnetic fields. Thus, the hidden order of antiferromagnets has only been revealed in the last century. For spintronic applications, the use of antiferromagnets promises numerous advantages compared to conventional spintronics based primarily on ferromagnetic (FM) materials. Amongst the key materials for AF spintronics research are tetragonal, antiferromagnetic CuMnAs films, because in addition to being antiferromagnetically ordered at room-temperature, tetragonal CuMnAs is one of only two conductive AF materials, for which it has been shown that the AF order can be manipulated with electrical currents. This has raised hopes for antiferromagnetic memory devices where the AF order in CuMnAs is switched electrical between two different states.
The magnetic moments in CuMnAs films form ferromagnetic sheets (parallel alignment) which are stacked antiparallel along the crystallographic c-direction. The spin axis is confined within the ab-plane, but varies on a microscopic scale, which produces a variety of different AF domain structures. This thesis adresses the question: “what underlies the AF domain structures and how can they be manipulated efficiently?”
Visualising antiferromagnetic domain structures remains experimentally challenging, because the domains do not show a net magnetisation. Here, it is realised by combining photoemission electron microscopy (PEEM) with x-ray magnetic linear dichroism (XMLD), which yields sensitivity to the spin axis. These measurements require x-rays with precisely tunable energy. Therefore, this work has largely been performed at a synchrotron, namely Diamond Light Source.
Here, direct imaging of the response of the AF domain structure upon the application of electrical current pulses is used to study the microscopic mechanisms of electric switching in CuMnAs films. In the films studied here, the most efficient switching was found to occur via reversible AF domain wall motion induced by electrical current pulses of alternating polarity. The measurements also reveal the limiting factors of electrical switching in CuMnAs films, namely domain pinning which limits device efficiency and domain relaxation which hinders long-term memory. This illustrates that one needs to be able to precisely tune the material properties for a specific application in order to build efficient AF spintronic devices. Hence, the factors, which govern the AF spin textures in the CuMnAs films, need to be revealed.
This is done by combining direct imaging of the AF domain structure with complementary techniques including electrical measurements, scanning X-ray diffraction and low-energy electron microscopy and diffraction (LEEM, LEED). The measurements reveal that the AF domain patterns are highly sensitive to the crystallographic microstructure including patterned edges and crystallographic defects. In particular, crystallographic microtwin defects are found to largely define the AF domain structure in non-patterned films. The coupling between defects and AF domains can lead to magnetostructural kinetics, where defects and AF domains grow together over weeks at room temperature and over minutes at slightly elevated temperatures of 50°C -70°C. In devices, patterned edges are found to influence the AF domains over tens of micrometers. Combining the knowledge about the effects of microtwin defects and patterned edges on the AF structure helps to understand the microscopic effects of electric current pulses and can form the basis for targeted AF domain engineering. Although simple functionalities can be achieved even with devices fabricated from a single magnetic film, ferromagnetic spintronic research and technology has demonstrated that device performances can be significantly improved by using multilayer structures, which allows not only to tune particular material properties, but also to exploit a full range of other effects arising at the interface. These effects depend sensitively on the interface quality and the termination of the individual layers. The surfaces of the CuMnAs films studied here are found to be rough on a microscopic scale and micrometer-sized atomically flat areas are scarce if at all present. Nonetheless, the AF domain structure is found to be imprinted on the ferromagnetic domain structure in CuMnAs/Fe bilayer structures, albeit with each AF domain corresponding to several ferromagnetic domains with mutually antiparallel orientation.
In summary, this work provides a detailed investigation of the factors which govern microscopic AF domain structures in CuMnAs films. This is directly beneficial to current and future AF spintronics research on this particular material. In addition, it shows the level of detail at which the crystallographic microstructure and its effect on the AF order need to be known in order to understand, predict and tailor the equilibrium AF domain structure and AF domain kinetics in antiferromagnetic thin films.
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