It’s great to see research students from the group graduating. Well done to Harry Waring, Charley Bull and Will Griggs for graduating!
The nano engineering and spintronic technologies (NEST) group focuses on understanding the physical processes and developing the fundamental understanding necessary to create the computational and data storage devices of the future.
Our research includes exploring recent device ideas in non-conventional computing and encompasses a broad range of activities to explore the spin of electrons in nanoscale magnetic structures. The group interacts strongly with the Henry Royce Institute for advanced materials and the National Graphene Institute. We regularly use large scale facilities in the UK and Europe for neutron and X-ray scattering and have wide range of collaborators in the UK and Europe.
In addition to the group's leading edge deposition, device fabrication and characterisation facilities we have full access to the state-of-the-art facilities at the National Graphene Institute and the Royce institute.
Well done to Michale Grimes for having his latest research published in AIP Advances: “X-ray investigation of long-range antiferromagnetic ordering in FeRh” by M. Grimes, N. Gurung, H. Ueda, D. G. Porter, B. Pedrini, L. J. Heyderman, T. Thomson, and V. Scagnol. You can view the paper online here.
Abstract: We demonstrate a probe of long-range antiferromagnetic (AF) order in FeRh thin films using non-resonant magnetic x-ray scattering. In particular, x-rays at energies below the Fe K-edge have been used for the observation of magnetic Bragg peaks. Due to the low efficiency of the magnetic scattering, a grazing incidence geometry was used to optimise the diffracted intensity from the thin film samples. Based on Scherrer analysis, we estimate a coherence length similar to previous reports from x-ray magnetic linear dichroism (XMLD) experiments, indicating that domain sizes are limited to 40 nm which is consistent with the grain size. The temperature dependent behaviour of the AF order shows an inverse correlation with the emergence of the ferromagnetic (FM) moment, as expected from the phase diagram.
It was great to be able to attend a face-to-face conference for the first time in a while as the group members descended on York to attend Magnetism 2022, which took place at the University of York on 28th-29th March. We have staff and students presenting talks and displaying posters. We all enjoyed the conference dinner on the evening of the 28th!
We have had a research paper “Repeatable and deterministic all electrical switching in a mixed phase artificial multiferroic” by W. Griggs, and T. Thomson, published in Nature Scientific Reports. You can view the paper online here.
Abstract: We demonstrate a repeatable all-electric magnetic switching behaviour in a PMN-PT/FeRh thin film artificial multiferroic. The magnitude of the effect is significantly smaller than expected from conventional thermomagnetic switching of FeRh thin films and we explore properties of the PMN-PT/FeRh system in order to understand the origin of this reduction. The data demonstrate the importance of the crystallographic phase of PMN-PT and show how a phase transition at ~ 100 °C modifies the magneto-electric coupling. We demonstrate a large strain remanence effect in the PMN-PT substrate, which limits the magnetoelectric coupling on successive cycling of the applied electric field..
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 (TC) 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 TC, these domains undergo slight restructuring towards uniform size, before abruptly fading at TC. 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.