We have had a research paper “Spintronic terahertz emitters exploiting uniaxial magnetic anisotropy for field-free emission and polarization control” by S. M. Hewett, C. Bull, A. M. Shorrock, C-H. Lin, R. Ji, M. T. Hibberd, T. Thomson, P. W. Nutter, and D. G. Graham, published in Applied Physics Letters. You can view the paper online here.

Abstract: We explore the terahertz (THz) emission from CoFeB/Pt spintronic structures in the below-magnetic-saturation regime and reveal an orientation dependence in the emission, arising from in-plane uniaxial magnetic anisotropy (UMA) in the ferromagnetic layer. Maximizing the UMA during the film deposition process and aligning the applied magnetic field with the easy axis of the structure, allows the THz emission to reach saturation under weaker applied fields. In addition, the THz emission amplitude remains at saturation levels when the applied field is removed. The development of CoFeB/Pt spintronic structures that can emit broadband THz pulses without the need for an applied magnetic field is beneficial to THz magneto-optical spectroscopy and facilitates the production of large-area spintronic emitters. Furthermore, by aligning the applied field along the hard axis of the structure, the linear polarization plane of the emitted THz radiation can be manipulated by changing the magnitude of the applied field. We therefore demonstrate THz polarization control without the need for mechanical rotation of external magnets.

We have had a Research Update “Polarized neutron reflectometry characterization of interfacial magnetism in an FePt/FeRh exchange spring” by W. Griggs, C. Bull, C. W. Barton, R. A. Griffiths, A. J. Caruana, C. J. Kinane, P. W. Nutter, and T. Thomson, published in Phys. Rev. Materials. You can view the paper online here.

Abstract: We report on the depth-sensitive, temperature-dependent exchange coupling in an FePt/FeRh thin-film exchange-spring structure. The depth-dependent in-plane magnetization is measured as a function of applied magnetic field and sample temperature using polarized neutron reflectometry (PNR). The magnetization profiles are interpreted in terms of the competition between anisotropy, exchange coupling, and dipolar coupling as the FeRh undergoes the magnetic phase transition from antiferromagnetic to ferromagnetic ordering. The PNR data are combined with bulk magnetometry and x-ray characterization, allowing us to determine characteristic length scales over which the exchange-spring mechanism is effective at ambient and elevated temperatures.

We have had a Research Update “Spintronic terahertz emitters: Status and prospects from a materials perspective” by C. Bull, S. M. Hewett, R. Ji, C-H. Lin, T. Thomson, D. W. Graham, and P. W Nutter, published in APL Materials, which has been selected as the Editor’s Pick. You can view the paper online here.

Abstract: Spintronic terahertz (THz) emitters, consisting of ferromagnetic (FM)/non-magnetic (NM) thin films, have demonstrated remarkable potential for use in THz time-domain spectroscopy and its exploitation in scientific and industrial applications. Since the discovery that novel FM/NM heterostructures can be utilized as sources of THz radiation, researchers have endeavored to find the optimum combination of materials to produce idealized spintronic emitters capable of generating pulses of THz radiation over a large spectral bandwidth. In the last decade, researchers have investigated the influence of a wide range of material properties, including the choice of materials and thicknesses of the layers, the quality of the FM/NM interface, and the stack geometry upon the emission of THz radiation. It has been found that particular combinations of these properties have greatly improved the amplitude and bandwidth of the emitted THz pulse. Significantly, studying the material properties of spintronic THz emitters has increased the understanding of the spin-to-charge current conversion processes involved in the generation of THz radiation. Ultimately, this has facilitated the development of spintronic heterostructures that can emit THz radiation without the application of an external magnetic field. In this review, we present a comprehensive overview of the experimental and theoretical findings that have led to the development of spintronic THz emitters, which hold promise for use in a wide range of THz applications. We summarize the current understanding of the mechanisms that contribute to the emission of THz radiation from the spintronic heterostructures and explore how the material properties contribute to the emission process.

We have had a new paper “Nanometre imaging of Fe₃GeTe₂ ferromagnetic domain walls” published in Nanotechnology. You can view the paper online here.

Abstract: Fe₃GeTe₂ is a layered crystal which has recently been shown to maintain its itinerant ferromagnetic properties even when atomically thin. Here, differential phase contrast scanning transmission electron microscopy is used to investigate the domain structure in a Fe₃GeTe₂ cross-sectional lamella at temperatures ranging from 95 to 250 K and at nanometre spatial resolution. Below the experimentally determined Curie temperature (T_C) of 191 K, stripe domains magnetised along 〈0001〉, bounded with 180^◦ Bloch type domain walls, are observed, transitioning to mixed Bloch−Néel type where the cross-sectional thickness is reduced below 50 nm. When warming towards T_C, these domains undergo slight restructuring towards uniform size, before abruptly fading at T_C. Localised loss of ferromagnetic order is seen over time, hypothesised to be a frustration of ferromagnetic order from ambient oxidation and basal cracking, which could enable selective modification of the magnetic properties for device applications.

Congratulations to Will Griggs who has his paper “Depth selective magnetic phase coexistence in FeRh thin films” published in APOL materials. You can view the paper online here.

Abstract: We demonstrate the manipulation of magnetic phases in FeRh thin films through atomic displacements and the distribution of structural defects. Atomic scale disorder can be controlled via irradiation with light noble gas ions, producing depth-varying nanoscale phase configurations of distinct antiferromagnetic, ferromagnetic, and paramagnetic regions. Here, we perform a spatial characterization of the magnetic phases and the local magnetic environment around the Fe atoms, as well as the variation of the open-volumes around atomic sites. Thus, a direct correspondence between the existence of the three magnetic phases and lattice defects is revealed. By careful selection of the irradiating fluence, we show that it is possible to produce simple and thermally stable magnetic configurations, such as uniform magnetization or a bilayer phase structure. Furthermore, the thin film surface and interfaces are observed as the nucleation sites for the transitions between the phases. These results demonstrate a sensitive nanoscale manipulation of magnetic properties, shedding light on magnetic ordering in alloy lattices and broadening the scope for applications.

Congratulations to Runze Chen who has had his paper “Skyrmionic Interconnect Device”, by R Chen, Y Li, V. F. Pavlidis, and C Moutafis, published in Physical Review Research. You can view the paper online here.

In this work, an all-magnetic skyrmion-based interconnect device – the so far “missing component” in skyrmionics – is proposed. Sequences of information are encoded by skyrmionic quasi-particles with distinct topological charges in a single device, thereby inherently enabling signal multiplexing. This work underpins a significant paradigm shift from particle-based into topology-based information technologies.

We have a new paper published in Physical Review Applied: “Meronlike Spin Textures in In-Plane-Magnetized Thin Films”, J. Vijayakumar, Y. Li, D. Bracher, C. W. Barton, M. Horisberger, T. Thomson, J. Miles, C. Moutafis, F. Nolting, and C.A.F. Vaz, Phys. Phys. Rev. Appl. 14 (2020), 054031. You can view the published paper here.

Abstract: The interfacing of magnetic materials with nonmagnetic heavy metals with a large spin-orbit coupling, such as $\mathrm{Pt}$, results in an asymmetric exchange interaction at the interface due to the Dzyaloshinskii-Moriya interaction (DMI), which in turn leads to the formation of skyrmions and topological spin structures in perpendicularly magnetized multilayers. Here, we show that out-of-plane spin domains with lateral dimensions from 200 nm to 2 $\mu m$ are stabilized in in-plane magnetized $\mathrm{Ta}$/$\mathrm{Co}$/$\mathrm{Pt}$ trilayers. We show that these spin textures are largely insensitive to the direction of the in-plane magnetization switched by either magnetic fields or electric fields applied across a ${\mathrm{Si}}_3{N}_4$ gate dielectric. The results of micromagnetic simulations indicate that the DMI is required for the stabilization of such out-of-plane domains and that the presence of surface roughness helps to stabilize larger structures, in agreement with experimental results. We identify these spin structures as meronlike topological textures, characterized by a perpendicular spin texture in an uniformly in-plane magnetized system.

We have a new paper published in Physical Review Applied: “Nanoscale room-temperature multilayer skyrmionic synapse for deep spiking neural network”, E. R. Chen, C. Li, Y. Li, J. Miles, G. Indiveri, S. Furber, V. F. Pavlidis, and C. Moutafis, Phys. Phys. Rev. Appl. 14 (2020), 014096. You can view the published paper here.

Abstract: Magnetic skyrmions have attracted considerable interest, especially after their recent experimental demonstration at room temperature in multilayers. The robustness, nanoscale size and non-volatility of skyrmions have triggered a substantial amount of research on skyrmion-based low-power, ultra-dense nanocomputing and neuromorphic systems such as artificial synapses. Room-temperature operation is required to integrate skyrmionic synapses in practical future devices. Here, we numerically propose a nanoscale skyrmionic synapse composed of magnetic multilayers that enables room-temperature device operation tailored for optimal synaptic resolution. We demonstrate that when embedding such multilayer skyrmionic synapses in a simple spiking neural network (SNN) with unsupervised learning via the spike-timing-dependent plasticity rule, we can achieve only a ~78% classification accuracy in the MNIST handwritten data set under realistic conditions. We propose that this performance can be significantly improved to ~98.61% by using a deep SNN with supervised learning. Our results illustrate that the proposed skyrmionic synapse can be a potential candidate for future energy-efficient neuromorphic edge computing.

We have a new paper published in Physical Review Materials: “Anisotropy-induced spin reorientation in chemically modulated amorphous ferrimagnetic films”, E. Kirk, C. Bull, S. Finizio, H. Sepehri-Amin, S. Wintz, A. K. Suszka, N. S. Bingham, P. Warnicke, K. Hono, P. W. Nutter, J. Raabe, G. Hrkac, T. Thomson, and L. J. Heyderman, Phys. Rev. Materials, 5 (2020), 074403. You can view the published paper here.

Abstract: The ability to tune the competition between the in-plane and out-of-plane orientation of magnetization provides a means to construct thermal sensors with a sharp spin reorientation transition at specific temperatures. We have observed such a tuneable, temperature-driven spin reorientation in structurally amorphous, ferrimagnetic rare-earth transition-metal alloy thin films using scanning transmission x-ray microscopy and magnetic measurements. The nature of the spin reorientation transition in FeGd can be fully explained by a nonequilibrium, nanoscale modulation of the chemical composition of the films. This modulation leads to a magnetic domain pattern of nanoscale speckles superimposed on a background of in-plane domains that form Landau configurations in µm-scale patterned elements. It is this speckle magnetic structure that gives rise to a sharp two-step reversal mechanism that is temperature dependent. The possibility to balance competing anisotropies through the temperature opens opportunities to create and manipulate topological spin textures.

We have had a new paper published in Physical Review Research: “Tunable terahertz oscillation arising from Bloch-point dynamics in chiral magnets”, Y. Li, L. Pierobon, M. Charilaou, H-B Braun, N. R. Walet, J. F. Löffler, J. J. Miles, and C. Moutafis, Phys Rev Research 2 (2020), 033006. You can view the published paper here.

Skyrmionic textures are being extensively investigated due to the occurrence of novel topological magnetic phenomena, and their promising applications in a new generation of spintronic devices take advantage of the robust topological stability of their spin structures. The development of practical devices relies on a detailed understanding of how skyrmionic structures can be formed, transferred, detected, and annihilated. In this work our considerations go beyond static skyrmions and theoretically show that the formation/annihilation of both skyrmions and antiskyrmions is enabled by the transient creation and propagation of topological singularities (magnetic monopolelike Bloch points). Critically, our results predict that during the winding/unwinding of skyrmionic textures, the Bloch-point propagation will give rise to an emergent electric field with a substantial amplitude and in the terahertz frequency range. We also demonstrate ways for controlling Bloch-point dynamics, which directly enable the tunablility on generation of this signal, as well as its frequency and amplitude. Our studies provide a concept of directly exploiting topological singularities for terahertz skyrmion-based electronic devices.