(I) Schematic of Ir/Fe/Co/Pt multilayer stack. The large DMI vectors at Co/Pt (D1) and Fe/Ir (D2) interfaces act in concert to enhance the effective DMI, Deff. (II) By varying the Fe/Co thicknesses, the magnetic interactions in the multilayer stack can be continuously modulated, enabling a platform for tunable skyrmion size, density, and configuration at room temperature. (III) Zero field skyrmions stabilised at intermediate dot sizes, with uniform magnetisation and labyrinth stripe phases for smaller and larger dot sizes, respectively.
Magnetic skyrmions are nanoscale spin structures with topological properties that have been recently discovered in several material systems. Due to their topologically protected spin structure, skyrmions behave like finite-sized magnetic particles that can be (a) packed into dense nanoscale arrays, (b) singly created, switched, and deleted, and (c) moved controllably at low current densities. Skyrmions are thus natural candidates for next-generation memory: forming high-density cells with potential fast switching and low power readout. The discovery of room temperature (RT) skyrmions in technologically relevant sputtered multilayer films triggered an explosion of efforts to modulate their physical properties towards fast-tracking the imminent realisation of skyrmion-based technologies. Acquiring deterministic control over skyrmion formation and properties requires the ability to tailor the parent magnetic interactions. Skyrmions are formed in multilayer thin films due to the presence of the chiral Dzyaloshinskii-Moriya interaction (DMI) at ferromagnet (FM) – heavy metal (HM) interfaces
One of our signature achievements is the world-first realisation of tunable room temperature (RT) skyrmions in multilayer thin film systems. We can tune the magnetic interactions in the system by varying the relative composition of iron and cobalt, leading to corresponding variation of the skyrmions’ sizes and configurations, as shown in Fig. (i). These RT skyrmions have been detected and characterised by three complementary magnetic microscopy techniques (MFM, MTXM, and L-TEM), providing an established recipe for quantitative imaging of single skyrmions in device configurations. Importantly, demonstrable electrical detection of these skyrmions via Hall transport has immediate relevance for device applications
We have also demonstrated confinement-induced skyrmion formation and evolution at zero magnetic field in dot structures fabricated from these multilayers. As seen in Fig. (ii), by tuning the dot width and relative composition of iron and cobalt, sub-100nm skyrmions were observed to be stabilised at zero field. This first realisation at such a small scale makes it immediately applicable in current technological lines especially within MTJ devices, as well as future applications such as microwave detectors, oscillators, and multi-bit devices.
- A.Soumyanarayanan et al., “Emergent phenomena induced by spin–orbit coupling at surfaces and interfaces”, Nature 539, 509–517 (2016).
- A.Soumyanarayanan et al., “Tunable room-temperature magnetic skyrmions in Ir/Fe/Co/Pt multilayers”, Nature Materials 16, 898-904 (2017).
- A.Yagil et al., “Stray field signatures of Néel textured skyrmions in Ir/Fe/Co/Pt multilayer films”, Appl. Phys. Lett. 112, 192403 (2018).
- P. Ho et al., “Geometrically Tailored Skyrmions at Zero Magnetic Field in Multilayered Nanostructures”, Phys. Rev. Applied 11, 024064 (2019).
- M. Raju, A. Yagil et al., “The evolution of skyrmions in Ir/Fe/Co/Pt multilayers and their topological Hall signature”, Nature Communications 10, 696 (2019).
Publicity and Media Coverage
1. EE Times
2. A*STAR Research
3. Young Scientist Awards 2018
Dr. Anjan Soumyanarayanan