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Register a new userGyrotropic resonance of individual Neel skyrmions in Ir/Fe/Co/Pt multilayers
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Added by
Anjan Soumyanarayanan
Publication date
2018
fields
Physics
and research's language is
English
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Magnetic skyrmions are nanoscale spin structures recently discovered at room temperature (RT) in multilayer films. Employing their novel topological properties towards exciting technological prospects requires a mechanistic understanding of the excitation and relaxation mechanisms governing their stability and dynamics. Here we report on the magnetization dynamics of RT Neel skyrmions in Ir/Fe/Co/Pt multilayer films. We observe a ubiquitous excitation mode in the microwave absorption spectrum, arising from the gyrotropic resonance of topological skyrmions, and robust over a wide range of temperatures and sample compositions. A combination of simulations and analytical calculations establish that the spectrum is shaped by the interplay of interlayer and interfacial magnetic interactions unique to multilayers, yielding skyrmion resonances strongly renormalized to lower frequencies. Our work provides fundamental spectroscopic insights on the spatiotemporal dynamics of topological spin structures, and crucial directions towards their functionalization in nanoscale devices.
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Magnetic skyrmions are nanoscale topological spin structures offering great promise for next-generation information storage technologies. The recent discovery of sub-100 nm room temperature (RT) skyrmions in several multilayer films has triggered vigorous efforts to modulate their physical properties for their use in devices. Here we present a tunable RT skyrmion platform based on multilayer stacks of Ir/Fe/Co/Pt, which we study using X-ray microscopy, magnetic force microscopy and Hall transport techniques. By varying the ferromagnetic layer composition, we can tailor the magnetic interactions governing skyrmion properties, thereby tuning their thermodynamic stability parameter by an order of magnitude. The skyrmions exhibit a smooth crossover between isolated (metastable) and disordered lattice configurations across samples, while their size and density can be tuned by factors of 2 and 10 respectively. We thus establish a platform for investigating functional sub-50 nm RT skyrmions, pointing towards the development of skyrmion-based memory devices.
Skyrmions are topologically protected, two-dimensional, localized hedgehogs and whorls of spin. Originally invented as a concept in field theory for nuclear interactions, skyrmions are central to a wide range of phenomena in condensed matter. Their realization at room temperature (RT) in magnetic multilayers has generated considerable interest, fueled by technological prospects and the access granted to fundamental questions. The interaction of skyrmions with charge carriers gives rise to exotic electrodynamics, such as the topological Hall effect (THE), the Hall response to an emergent magnetic field, a manifestation of the skyrmion Berry-phase. The proposal that THE can be used to detect skyrmions needs to be tested quantitatively. For that it is imperative to develop comprehensive understanding of skyrmions and other chiral textures, and their electrical fingerprint. Here, using Hall transport and magnetic imaging, we track the evolution of magnetic textures and their THE signature in a technologically viable multilayer film as a function of temperature ($T$) and out-of-plane applied magnetic field ($H$). We show that topological Hall resistivity ($rho_mathrm{TH}$) scales with the density of isolated skyrmions ($n_mathrm{sk}$) over a wide range of $T$, confirming the impact of the skyrmion Berry-phase on electronic transport. We find that at higher $n_mathrm{sk}$ skyrmions cluster into worms which carry considerable topological charge, unlike topologically-trivial spin spirals. While we establish a qualitative agreement between $rho_mathrm{TH}(H,T)$ and areal density of topological charge $n_mathrm{T}(H,T)$, our detailed quantitative analysis shows a much larger $rho_mathrm{TH}$ than the prevailing theory predicts for observed $n_mathrm{T}$.
Magnetic skyrmions are chiral spin structures that have recently been observed at room temperature (RT) in multilayer thin films. Their topological stability should enable high scalability in confined geometries - a sought-after attribute for device applications. While umpteen theoretical predictions have been made regarding the phenomenology of sub-100 nm skyrmions confined in dots, in practice their formation in the absence of an external magnetic field and evolution with confinement remain to be established. Here we demonstrate the confinement-induced stabilization of sub-100 nm RT skyrmions at zero field (ZF) in Ir/Fe(x)/Co(y)/Pt nanodots over a wide range of magnetic and geometric parameters. The ZF skyrmion size can be as small as ~50 nm, and varies by a factor of 4 with dot size and magnetic parameters. Crucially, skyrmions with varying thermodynamic stability exhibit markedly different confinement phenomenologies. These results establish a comprehensive foundation for skyrmion phenomenology in nanostructures, and provide immediate directions for exploiting their properties in nanoscale devices.
Skyrmions are nanoscale spin configurations with topological properties that hold great promise for spintronic devices. Here, we establish their Neel texture, helicity, and size in Ir/Fe/Co/Pt multilayer films by constructing a multipole expansion to model their stray field signatures and applying it to magnetic force microscopy (MFM) images. Furthermore, the demonstrated sensitivity to inhomogeneity in skyrmion properties, coupled with a unique capability to estimate the pinning force governing dynamics, portends broad applicability in the burgeoning field of topological spin textures.
We report a study of the magnetization reversals and skyrmion configurations in two systems - Pt/Co/MgO and Ir/Fe/Co/Pt multilayers, where magnetic skyrmions are stabilized by a combination of dipolar and Dzyaloshinskii-Moriya interactions (DMI). First Order Reversal Curve (FORC) diagrams of low-DMI Pt/Co/MgO and high-DMI Ir/Fe/Co/Pt exhibit stark differences, which are identified by micromagnetic simulations to be indicative of hybrid and pure Neel skyrmions, respectively. Tracking the evolution of FORC features in multilayers with dipolar interactions and DMI, we find that the negative FORC valley, typically accompanying the positive FORC peak near saturation, disappears under both reduced dipolar interactions and enhanced DMI. As these conditions favor the formation of pure Neel skyrmions, we propose that the resultant FORC feature - a single positive FORC peak near saturation - can act as a fingerprint for pure Neel skyrmions in multilayers. Our study thus expands on the utility of FORC analysis as a tool for characterizing spin topology in multilayer thin films.
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