Congratulations to Charley Bull on successfully defending her thesis and being awarded a PhD. Charley’s work (thesis: “Development of MTJs and antiferromagnetic materials for spintronic applications”) focused on studying the sputter deposition conditions required to fabricate magnetic tunnel junctions (MTJs) and investigating the thin films properties using a number of characterisation techniques. In addition, Charley investigated the effect of topography and strain on the magnetic properties of FeRh thin fils as the thickness is reduced.
We have had a new paper published in RSC Advances: “Magnetic response of FeRh to static and dynamic disorder”, B. Eggert, A. Schmeink, J. Lill, M.O. Liedke, U. Kentsch, M. Butterling,
A. Wagner, S. Pascarelli, K. Potzger, J. Lindner, T. Thomson, J.
Fassbender, K. Ollefs, W. Keune, R. Bali, H. Wende, RSC Advances 10 (2020), 14386 – 14395. You can view the published paper here.
Abstract: Atomic scale defects generated using focused ion as well as laser beams can activate ferromagnetism in initially non-ferromagnetic B2 ordered alloy thin film templates. Such defects can be induced locally, confining the ferromagnetic objects within well-defined nanoscale regions. The characterization of these atomic scale defects is challenging, and the mechanism for the emergence of ferromagnetism due to sensitive lattice disordering is unclear. Here we directly probe a variety of microscopic defects in systematically disordered B2 FeRh thin films that are initially antiferromagnetic and undergo a thermally-driven isostructural phase transition to a volatile ferromagnetic state. We show that the presence of static disorder i.e., the slight deviations of atoms from their equilibrium sites is sufficient to induce a non-volatile ferromagnetic state at room temperature. A static mean square relative displacement of 9 × 10−4 Å−2 is associated with the occurrence of non-volatile ferromagnetism and replicates a snapshot of the dynamic disorder observed in the thermally-driven ferromagnetic state. The equivalence of static and dynamic disorder with respect to the ferromagnetic behavior can provide insights into the emergence of ferromagnetic coupling as well as achieving tunable magnetic properties through defect manipulations in alloys.
Congratulations to Harry Waring who has had his paper “Zero-field Optic Mode Beyond 20 GHz in a Synthetic Antiferromagnet”, by H J Waring, N A B Johansson, I J Vera-Marun and T Thomson, published in Physical Review Applied, Vol 13, Article 034035 (2020). You can view the published paper here.
Abstract: Antiferromagnets have considerable potential as spintronic materials. Their dynamic properties include resonant modes at frequencies higher than can be observed in conventional ferromagnetic materials. An alternative to single-phase antiferromagnets are synthetic antiferromagnets (SAFs), engineered structures of exchange-coupled ferromagnet/nonmagnet/ferromagnet trilayers. SAFs have significant advantages due to the wide-ranging tunability of their magnetic properties and inherent compatibility with current device technologies, such as those used for Spin-transfer-torque magnetic random-access memory production. Here we report the dynamic properties of fully compensated SAFs using broadband ferromagnetic resonance and demonstrate resonant optic modes in addition to the conventional acoustic (Kittel) mode. These optic modes possess the highest zero-field frequencies observed in SAFs to date with resonances of 18 and 21 GHz at the first and second peaks in antiferromagnetic Ruderman-Kittel-Kasuya-Yosida (RKKY) coupling, respectively. In contrast to previous SAF reports that focus only on the first RKKY antiferromagnetic coupling peak, we show that a higher optic mode frequency is obtained for the second antiferromagnetic coupling peak. We ascribe this to the smoother interfaces associated with a thicker nonmagnetic layer. This demonstrates the importance of interface quality to achieving high-frequency optic mode dynamics entering the subterahertz range.
We have a new PhD opening looking at “Spin wave dynamics for spintronic computational devices“.
Spin waves (magnons) are fundamental excitations found in magnetically ordered materials. Recently spin wave phenomena have been proposed as a new paradigm for dedicated computational tasks due to the inherently low energy of this type of excitation. Spin waves have high potential for nanostructured magnetic devices able to perform a set of important computational tasks such as pattern recognition, convolution, and Fourier transformations. Additionally, the generation, detection and manipulation of spin waves are key enabling technologies for microwave processing, magnetic sensing as well as for unconventional logic.
Figure 1: Concept of spin wave spintronics [1]
In order to understand the potential of spin waves, the physics of new materials needs to be explored. It has very recently been realized that high frequencies, towards THz, can be generated using antiferromagnets and we have already succeeded in demonstrating high frequencies from a synthetic antiferromagnetic system [2] which provides a solid platform for this project. Similarly, exploring the potential of materials with perpendicular anisotropy (PMA) provides exciting avenues for developing spin wave technologies. For example, there is a class of thin films with L10 ordering (MnAl, FePt, MnAlGe, FePtPd) which possess the very high PMA needed to generate high frequencies. Extensions to these ideas are so-called hybrid systems, where in addition to a layer containing perpendicular magnetization other 2D layers with including those with in-plane anisotropy [3].
In this project, the aim is to develop the understanding necessary to create high frequency spin wave devices using the emerging ideas for new atomically engineered materials. This is an experimental project involving depositing and characterization of atomically layered magnetic films, creating devices using lithography and measuring them using ferromagnetic resonance (FMR) and advanced electrical measurements (e.g. non-local geometry) to understand their high frequency properties. The project will use the state-of-the-art instrumentation and facilities for magnetism and nanodevice research in Manchester where we have a wide range of nanoscale magnetism activities. There will also be opportunities to interact with our existing collaborators in laboratories across Europe.
[1] R.L. Stamps et al. “The 2014 Magnetism Roadmap” J. Phys. D: Appl. Phys. 47 (2014) 333001 [2] Waring et al. In preparation. [3] Wohlhüter et al. “Nanoscale switch for vortex polarization mediated by Bloch core formation in magnetic hybrid systems” Nature Comms. 6 (2015) 7836.
We have had a new paper published in Scientific Reports: “Topography dependence of the metamagnetic phase transition in FeRh thin films”, by J L Warren, C W Barton, C Bull and T Thomson, Vol 10, Article 4030 (2020). You can view the published paper here.
The equiatomic alloy FeRh is of great scientific and technological interest due its highly unusual first-order antiferromagnetic (AF) to ferromagnetic (FM) phase transition. Here we report an exploration of the interplay between topography and phase evolution with a comprehensive magnetic force microscopy study of nominal 50 nm thick FeRh thin films and subtractively patterned wires of width 0.2 µm–2 µm. In continuous films where the surface morphology had not been optimised for smoothness, the topographical variation was observed to dominate the distribution of the magnetic transition temperatures and dictates the nucleation and growth of the magnetic phases. This observation was repeated for patterned elements, where the effects of surface morphology were more significant than those of spatial confinement. These results have clear implications for future studies of low-dimensional FeRh films, as surface topography must be considered when analysing and comparing the transition behaviour of FeRh thin films.
We are looking for a student to investigate the generation of THz pulses of radiation from spintronic structures to reveal the physics that governs the properties of these remarkable material systems and optimise spin-based structures for the emission of THz radiation. The work is experimental in nature and encompasses a number of areas, such as the fabrication and characterisation of metallic thin-film emitters and the investigation of the THz characteristics of such systems. The project will support a recently awarded UKRI EPSRC-funded project focused on understanding the THz emission process from spintronic structures.
