Antiferromagnetic domain structure in tetragonal CuMnAs films: A picturebook of domains, domain walls and everything in between

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

Spectral functions of CVD grown MoS2_2 monolayers after chemical transfer onto Au surface

The recent rise of van der Waals (vdW) crystals has opened new prospects for studying versatile and exotic fundamental physics with future device applications such as twistronics. Even though the recent development on Angle-resolved photoemission spectroscopy (ARPES) with Nano-focusing optics, making clean surfaces and interfaces of chemically transferred crystals have been challenging to obtain high-resolution ARPES spectra. Here, we show that by employing nano-ARPES with submicron sized beam and polystyrene-assisted transfer followed by annealing process in ultra-high vacuum environment, remarkably clear ARPES spectral features such as spin-orbit splitting and band renormalization of CVD-grown, monolayered MoS2 can be measured. Our finding paves a way to exploit chemically transferred crystals for measuring high-resolution ARPES spectra to observe exotic quasi-particles in vdW heterostructures

Coupling of ferromagnetic and antiferromagnetic spin dynamics in Mn2_{2}Au/NiFe thin-film bilayers

We investigate magnetization dynamics of Mn$_{2}$Au/Py (Ni$_{80}$Fe$_{20}$) thin film bilayers using broadband ferromagnetic resonance (FMR) and Brillouin light scattering spectroscopy. Our bilayers exhibit two resonant modes with zero-field frequencies up to almost 40 GHz, far above the single-layer Py FMR. Our model calculations attribute these modes to the coupling of the Py FMR and the two antiferromagnetic resonance (AFMR) modes of Mn2Au. The coupling-strength is in the order of 1.6 T$\cdot$nm at room temperature for nm-thick Py. Our model reveals the dependence of the hybrid modes on the AFMR frequencies and interfacial coupling as well as the evanescent character of the spin waves that extend across the Mn$_{2}$Au/Py interfac

Direct observation of altermagnetic band splitting in CrSb thin films

Altermagnetism represents an emergent collinear magnetic phase with compensated order and an unconventional alternating even-parity wave spin order in the non-relativistic band structure. We investigate directly this unconventional band splitting near the Fermi energy through spinintegrated soft X-ray angular resolved photoemission spectroscopy. The experimentally obtained angle-dependent photoemission intensity, acquired from epitaxial thin films of the predicted altermagnet CrSb, demonstrates robust agreement with the corresponding band structure calculations. In particular, we observe the distinctive splitting of an electronic band on a low-symmetry path in the Brilliouin zone that connects two points featuring symmetry-induced degeneracy. The measured large magnitude of the spin splitting of approximately 0.6 eV and the position of the band just below the Fermi energy underscores the signifcance of altermagnets for spintronics based on robust broken time reversal symmetry responses arising from exchange energy scales, akin to ferromagnets, while remaining insensitive to external magnetic fields and possessing THz dynamics, akin to antiferromagnets.Comment: 10 pages, 7 figures (including supplementary information

Experimental electronic structure of the electrically switchable antiferromagnet CuMnAs

Tetragonal CuMnAs is a room temperature antiferromagnet with an electrically reorientable N\'eel vector and a Dirac semimetal candidate. Direct measurements of the electronic structure of single-crystalline thin films of tetragonal CuMnAs using angle-resolved photoemission spectroscopy (ARPES) are reported, including Fermi surfaces (FS) and energy-wavevector dispersions. After correcting for a chemical potential shift of $\approx-390$ meV (hole doping), there is excellent agreement of FS, orbital character of bands, and Fermi velocities between the experiment and density functional theory calculations. Additionally, 2x1 surface reconstructions are found in the low energy electron diffraction (LEED) and ARPES. This work underscores the need to control the chemical potential in tetragonal CuMnAs to enable the exploration and exploitation of the Dirac fermions with tunable masses, which are predicted to be above the chemical potential in the present samples.Comment: Submitted to Physical Review X. 20 pages. 9 figure

Atomically sharp domain walls in an antiferromagnet

The interest in understanding scaling limits of magnetic textures such as domain walls spans the entire field of magnetism from its relativistic quantum fundamentals to applications in information technologies. The traditional focus of the field on ferromagnets has recently started to shift towards antiferromagnets which offer a rich materials landscape and utility in ultra-fast and neuromorphic devices insensitive to magnetic field perturbations. Here we report the observation that domain walls in an epitaxial crystal of antiferromagnetic CuMnAs can be atomically sharp. We reveal this ultimate domain wall scaling limit using differential phase contrast imaging within aberrationcorrected scanning transmission electron microscopy, which we complement by X-ray magnetic dichroism microscopy and ab initio calculations. We highlight that the atomically sharp domain walls are outside the remits of established spin-Hamiltonian theories and can offer device functionalities unparalleled in ferromagnets.Comment: 8 pages, 4 figures, Supplementary informatio

Current polarity-dependent manipulation of antiferromagnetic domains

Antiferromagnets have several favourable properties as active elements in spintronic devices, including ultra-fast dynamics, zero stray fields and insensitivity to external magnetic fields. Tetragonal CuMnAs is a testbed system in which the antiferromagnetic order parameter can be switched reversibly at ambient conditions using electrical currents. In previous experiments, orthogonal in-plane current pulses were used to induce 90° rotations of antiferromagnetic domains and demonstrate the operation of all-electrical memory bits in a multi-terminal geometry. Here, we demonstrate that antiferromagnetic domain walls can be manipulated to realize stable and reproducible domain changes using only two electrical contacts. This is achieved by using the polarity of the current to switch the sign of the current-induced effective field acting on the antiferromagnetic sublattices. The resulting reversible domain and domain wall reconfigurations are imaged using X-ray magnetic linear dichroism microscopy, and can also be detected electrically. Switching by domain-wall motion can occur at much lower current densities than those needed for coherent domain switching

Defect-driven antiferromagnetic domain walls in CuMnAs films

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

Antiferromagnetic domain structure in tetragonal CuMnAs films: A picturebook of domains, domain walls and everything in between

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

Nonlinear terahertz N\'eel spin-orbit torques in antiferromagnetic Mn2_2Au

Antiferromagnets have large potential for ultrafast coherent switching of magnetic order with minimum heat dissipation. In novel materials such as Mn$_2$Au and CuMnAs, electric rather than magnetic fields may control antiferromagnetic order by N\'eel spin-orbit torques (NSOTs), which have, however, not been observed on ultrafast time scales yet. Here, we excite Mn$_2$Au thin films with phase-locked single-cycle terahertz electromagnetic pulses and monitor the spin response with femtosecond magneto-optic probes. We observe signals whose symmetry, dynamics, terahertz-field scaling and dependence on sample structure are fully consistent with a uniform in-plane antiferromagnetic magnon driven by field-like terahertz NSOTs with a torkance of (150$\pm$50) cm$^2$/A s. At incident terahertz electric fields above 500 kV/cm, we find pronounced nonlinear dynamics with massive N\'eel-vector deflections by as much as 30{\deg}. Our data are in excellent agreement with a micromagnetic model which indicates that fully coherent N\'eel-vector switching by 90{\deg} within 1 ps is within close reach.Comment: 16 pages, 4 figure