Diamond Annual Review 2020/21

58 59 D I A M O N D L I G H T S O U R C E A N N U A L R E V I E W 2 0 2 0 / 2 1 D I A M O N D L I G H T S O U R C E A N N U A L R E V I E W 2 0 2 0 / 2 1 Nano-fragmentation of antiferromagnetic domains Related publication: Kašpar Z., SurýnekM., Zubáč J., Krizek F., NovákV., Campion R. P.,WörnleM. S., Gambardella P., Marti X., Němec P., Edmonds K.W., Reimers S., Amin O. J., Maccherozzi F., Dhesi S. S.,Wadley P.,Wunderlich J., Olejník K. & JungwirthT. Quenching of an antiferromagnet into high resistivity states using electrical or ultrashort optical pulses. Nat. Electron. 4 , 30–37 (2021). DOI: 10.1038/s41928-020-00506-4 Publication keywords : Electronic and spintronic devices; Magnetic properties andmaterials A ntiferromagnets are of potential use in spintronic devices due to their ultrafast dynamics, insensitivity to external magnetic fields and absence of magnetic stray fields. Researchers have observed large, transient changes in the electrical resistance in some antiferromagnetic thin filmdevices after applying electrical or optical pulses. Although the origin of these effects was unknown, it is now understood that they are related to the magnetic order. Researchers from the Czech Academy of Sciences and Charles University in Prague, the University of Nottingham, ETH Zurich, the University of Regensburg in Germany and Diamond Light Source investigated the micro-scale antiferromagnetic order in CuMnAs films before and after applying current pulses. They aimed to relate changes in the electrical resistance to changes in the magnetic microstructure. Using the X-PEEM End station on Diamond’s Nanoscience beamline (I06) enabled them to take images of the antiferromagnetic structures in microscale device structures. Electrical contacts on the sample allowed them to probe themagnetic state before and after applying current pulses. Theresultsshowthatafragmentationofmicrometre-scaleantiferromagneticdomainsaccompaniesthechangesintheelectricalresistance. The resulting textured structure has lengthscales comparable to, or even smaller than, the 30 nm spatial resolution of the X-PEEM End station. Their results establish a clear relationship of the electrical resistance changes with changes in the micromagnetic structure. The electrical switching and relaxation behaviour they observed can potentially mimic the characteristics of neural network components, offering the potential to develop efficient and high-speed neuromorphic computing applications that mimic neuro-biological architectures present in the human nervous system. In an antiferromagnet, the electron spins of adjacent atoms are ordered such that the total magnetic moment is zero. Information can be stored in the orientation of this collective magnetic order, but compared to their ferromagnetic counterparts, antiferromagnetism is challenging to harness for applications 1 . Current-induced spin-orbit fields have been shown to reorient the magnetic order vector, allowing antiferromagnetic devices to show a rewritable memory functionality. However, the magnetoresistive read-out signals in such devices are typically very small 2 . Thermally-induced quenching of the magnetic order offers prospects for realising large, tuneable, reversible resistive read-out in antiferromagnetic spintronic devices. Figure 1 shows the results of quench switching by unipolar electrical current pulses in an antiferromagnetic CuMnAs thin film resistor. A step increase in the electrical resistance is observed after applying a 100 μs current pulse of amplitude 1.2x10 7 A/cm 2 . Applying a current pulse of 8% smaller amplitude, in the same direction, then results in the resistance dropping back to a lower value. This pattern of changes in the resistance is MagneticMaterials Group Beamline I06 highly reproducible, as demonstrated by the behaviour shown in Fig. 1a,b for two different pulsing sequences. The rate of relaxation following each current pulse shows a simple exponential dependence on temperature. Furthermore, the quench switching can be induced by trains of sub-picosecond laser pulses, and can be probed optically using the sample reflectivity, opening prospects for ultrafast all-optical unconventional computing applications. Measurements were performed using X-ray PhotoEmission Electron Microscopy (XPEEM) on I06 in order to better understand the magnetic origin of the current-induced quench switching. Figure 2 shows antiferromagnetic domain images of a CuMnAs device before and after applying a current pulse at room temperature. The magnetic contrast is achieved using X-ray magnetic linear dichroism at the Mn L 3 absorption edge. The micron-sized magnetic domains in the as-grown sample are observed to fragment to lengthscales approaching the resolution of the XPEEM instrument ( ≈ 30 nm) after applying large-amplitude current pulses in situ . On the other hand, the X-ray absorption images displayed in the bottom-right of Fig. 2a,b showno significant difference after the current pulse, ruling out significant chemical or structural changes. The ability to directly image antiferromagnetic domains with XPEEM, combined with the chemical sensitivity given by the X-ray absorption images, provides invaluable insight into the microscopic origins of the resistive readout mechanism in CuMnAs devices. The fragmentation of the antiferromagnetic domain structure can be ascribed to local joule heating bringing the system close to the Néel temperature, where entropy gain favours local spin disorder. Further studies will be required to determine the origin of the high metastability of the nanoscale fragmented domain state, as well its precise relationship to the resistive signal. The quench switching observed in these antiferromagnetic materials has no counterpart in ferromagnetic systems. The observed tuneable multi- level output signals may allow antiferromagnetic spintronic devices to mimic components of spiking neural networks for neuromorphic computing applications 3 . Because electronic and magnetoelastic fluctuations may accompany the strong magnetic disorder, charge and spin-sensitive imaging with higher spatial and temporal resolution may shed new light on the underlying physics of this effect. References : 1. Železný J. et al. Spin transport and spin torque in antiferromagnetic devices. Nat. Phys. 14 , 220–228 (2018). DOI: 10.1038/s41567-018-0062-7 2. Wadley P. et al. Spintronics: Electrical switching of an antiferromagnet. Science. 351 , 587–590 (2016). DOI: 10.1126/science.aab1031 3. Kurenkov A. et al. Artificial Neuron and Synapse Realized in an Antiferromagnet/Ferromagnet Heterostructure Using Dynamics of Spin– Orbit Torque Switching. Adv. Mater. 31 , 1900636 (2019). DOI: 10.1002/ adma.201900636 Funding acknowledgement: The EU FET Open RIA grant no. 766566; the UK EPSRC (EP/P019749/1); the Ministry of Education of the Czech Republic (LM2018110, LNSM-LNSpin and LM2018096); the Czech Science Foundation (19-28375X). Corresponding author : Dr Kevin Edmonds, University of Nottingham, Kevin.Edmonds@nottingham.ac.uk Figure 1: (a) Electrical switching and relaxation in a CuMnAs antiferromagnet device, with the high and low resistance metastable states obtained using 100μs writing currents of 1.2x10 7 A/cm 2 and 1.0x10 7 A/cm 2 respectively. The device structure and measurement schematic are shown in the inset; (b) A sequence of repeated switching to high/low/low/high resistance states for the same device. Figure 2: (a) X-PEEM images for an as-processed CuMnAs device, showing the antiferromagnetic domain structure (bottom left and top zoomed-in) and the X-ray absorption signal (bottom right); (b) X-PEEM images for the same device after applying a 100 ms current pulse of amplitude ≈ 10 7 A/cm 2 along the direction indicated by the red arrow.