Show Abstract ▼

Abstract: Antiferromagnetic materials hold potential for use in spintronic devices with fast operation frequencies and field robustness. Despite the rapid progress in proof-of-principle functionality in recent years, there has been a notable lack of understanding of antiferromagnetic domain formation and manipulation, which translates to either incomplete or nonscalable control of the magnetic order. Here, we demonstrate simple and functional ways of influencing the domain structure in Cu⁢Mn⁢As and Mn2Au, two key materials of antiferromagnetic spintronics research, using device patterning and strain engineering. Comparing x-ray microscopy data from two different materials, we reveal the key parameters dictating domain formation in antiferromagnetic devices and show how the nontrivial interaction of magnetostriction, substrate clamping, and edge anisotropy leads to specific equilibrium domain configurations. More specifically, we observe that patterned edges have a significant impact on the magnetic anisotropy and domain structure over long distances and we propose a theoretical model that relates short-range edge anisotropy and long-range magnetoelastic interactions. The principles invoked are of general applicability to the domain formation and engineering in antiferromagnetic thin films at large, which will hopefully pave the way toward realizing truly functional antiferromagnetic devices.

Open Access

Show Abstract ▼

Abstract: Topologically protected magnetic textures are promising candidates for information carriers in future memory devices, as they can be efficiently propelled at very high velocities using current-induced spin torques. These textures—nanoscale whirls in the magnetic order—include skyrmions, half-skyrmions (merons) and their antiparticles. Antiferromagnets have been shown to host versions of these textures that have high potential for terahertz dynamics, deflection-free motion and improved size scaling due to the absence of stray field. Here we show that topological spin textures, merons and antimerons, can be generated at room temperature and reversibly moved using electrical pulses in thin-film CuMnAs, a semimetallic antiferromagnet that is a testbed system for spintronic applications. The merons and antimerons are localized on 180° domain walls, and move in the direction of the current pulses. The electrical generation and manipulation of antiferromagnetic merons is a crucial step towards realizing the full potential of antiferromagnetic thin films as active components in high-density, high-speed magnetic memory devices.

Show Abstract ▼

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.

Open Access

Show Abstract ▼

Abstract: Efficient manipulation of antiferromagnetic (AF) domains and domain walls has opened up new avenues of research towards ultrafast, high-density spintronic devices. AF domain structures are known to be sensitive to magnetoelastic effects, but the microscopic interplay of crystalline defects, strain and magnetic ordering remains largely unknown. Here, we reveal, using photoemission electron microscopy combined with scanning X-ray diffraction imaging and micromagnetic simulations, that the AF domain structure in CuMnAs thin films is dominated by nanoscale structural twin defects. We demonstrate that microtwin defects, which develop across the entire thickness of the film and terminate on the surface as characteristic lines, determine the location and orientation of 180∘ and 90∘ domain walls. The results emphasize the crucial role of nanoscale crystalline defects in determining the AF domains and domain walls, and provide a route to optimizing device performance.

Show Abstract ▼

Abstract: Antiferromagnets offer spintronic device characteristics unparalleled in ferromagnets owing to their lack of stray fields, THz spin dynamics, and rich materials landscape. Microscopic imaging of antiferromagnetic domains is one of the key prerequisites for understanding physical principles of the device operation. However, adapting common magnetometry techniques to the dipolar-field-free antiferromagnets has been a major challenge. Here we demonstrate in a collinear antiferromagnet a thermoelectric detection method by combining the magneto-Seebeck effect with local heat gradients generated by scanning far-field or near-field techniques. In a 20-nm epilayer of uniaxial CuMnAs we observe reversible 180 ∘ switching of the Néel vector via domain wall displacement, controlled by the polarity of the current pulses. We also image polarity-dependent 90 ∘ switching of the Néel vector in a thicker biaxial film, and domain shattering induced at higher pulse amplitudes. The antiferromagnetic domain maps obtained by our laboratory technique are compared to measurements by the established synchrotron-based technique of x-ray photoemission electron microscopy using x-ray magnetic linear dichroism.