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Register a new userA new class of chiral materials hosting magnetic skyrmions beyond room temperature
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Skyrmions, topologically protected vortex-like nanometric spin textures in magnets, have been attracting increasing attention for emergent electromagnetic responses and possible technological applications for spintronics. In particular, metallic magnets with chiral and cubic/tetragonal crystal structure may have high potential to host skyrmions that can be driven by low electrical current excitation. However, experimental observations of skyrmions have so far been limited to below room temperature for the metallic chiral magnets, specifically for the MnSi-type B20 compounds. Toward technological applications, it is crucial to transcend this limitation. Here we demonstrate the formation of skyrmions with unique spin helicity both at and above room temperature in a family of cubic chiral magnets: beta-Mn-type Co-Zn-Mn alloys with a different chiral space group from that of B20 compounds. Lorentz transmission electron microscopy (LTEM), magnetization, and small angle neutron scattering (SANS) measurements unambiguously reveal the formation of a skyrmion crystal under the application of magnetic field (H<~1 kOe) in both thin- plate (thickness<150 nm) and bulk forms.
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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.
Propagation character of spin wave was investigated for chiral magnets FeGe and Co-Zn-Mn alloys, which can host magnetic skyrmions near room temperature. On the basis of the frequency shift between counter-propagating spin waves, the magnitude and sign of Dzyaloshinskii-Moriya (DM) interaction were directly evaluated. The obtained magnetic parameters quantitatively account for the size and helicity of skyrmions as well as their materials variation, proving that the DM interaction plays a decisive role in the skyrmion formation in this class of room-temperature chiral magnets. The propagating spin-wave spectroscopy can thus be an efficient tool to study DM interaction in bulk single-phase compounds. Our results also demonstrate a function of spin-wave diode based on chiral crystal structures at room temperature.
Temperature gradient in a ferromagnetic conductor may generate a spontaneous transverse voltage drop in the direction perpendicular to both magnetization and heat current. This anomalous Nernst effect (ANE) has been considered to be proportional to the magnetization, and thus observed only in ferromagnets, while recent theories indicate that ANE provides a measure of the Berry curvature at the Fermi energy $E_{rm F}$. Here we report the observation of a large ANE at zero field in the chiral antiferromagnet Mn$_3$Sn. Despite a very small magnetization $sim 0.002$ $mu_{rm B}/$Mn, the transverse Seebeck coefficient at zero field is $sim 0.35~mu$V/K at room temperature and reaches $sim 0.6~mu$V/K at 200 K, comparable with the maximum value known for a ferromagnetic metal. Our first-principles calculation reveals that the large ANE comes from a significantly enhanced Berry curvature associated with the Weyl points nearby $E_{rm F}$. The ANE is geometrically convenient for the thermoelectric power generation, as it enables a lateral configuration of the modules to efficiently cover the heat source. Our observation of the large ANE in an antiferromagnet paves a way to develop a new class of thermoelectric material using topological magnets to fabricate an efficient, densely integrated thermopile.
Writing, erasing and computing are three fundamental operations required by any working electronic devices. Magnetic skyrmions could be basic bits in promising in emerging topological spintronic devices. In particular, skyrmions in chiral magnets have outstanding properties like compact texture, uniform size and high mobility. However, creating, deleting and driving isolated skyrmions, as prototypes of aforementioned basic operations, have been grand challenge in chiral magnets ever since the discovery of skyrmions, and achieving all these three operations in a single device is highly desirable. Here, by engineering chiral magnet Co$_8$Zn$_{10}$Mn$_2$ into the customized micro-devices for in-situ Lorentz transmission electron microscopy observations, we implement these three operations of skyrmions using nanosecond current pulses with a low a current density about $10^{10}$ A/m$^2$ at room temperature. A notched structure can create or delete magnetic skyrmions depending on the direction and magnitude of current pulses. We further show that the magnetic skyrmions can be deterministically shifted step-by-step by current pulses, allowing the establishment of the universal current-velocity relationship. These experimental results have immediate significance towards the skyrmion-based memory or logic devices.
We observe the signature of zero-field ground-state skyrmions in BiSb topological insulator / MnGa bi-layers by using the topological Hall effect (THE). We observe a large critical interfacial Dzyaloshinskii-Moriya-Interaction energy (5.0 pJ/m) at the BiSb/MnGa interface that can be tailored by controlling the annealing temperature of the MnGa template. The THE was observed at room temperature even under absence of an external magnetic field, which gives the strong evidence for the existence of thermodynamically stable skyrmions in MnGa/BiSb bi-layers. Our results will give insight into the role of interfacial DMI tailored by suitable material choice and growth technique for generation of stable skyrmions at room temperature.
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