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Register a new userAll-electrical detection of skyrmion lattice state and chiral surface twists
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We study the high-temperature phase diagram of the chiral magnetic insulator Cu$_2$OSeO$_3$ by measuring the spin-Hall magnetoresistance (SMR) in a thin Pt electrode. We find distinct changes in the phase and amplitude of the SMR signal at critical lines separating different magnetic phases of bulk Cu$_2$OSeO$_3$. The skyrmion lattice state appears as a strong dip in the SMR phase. A strong enhancement of the SMR amplitude is observed in the conical spiral state, which we explain by an additional symmetry-allowed contribution to the SMR present in non-collinear magnets. We demonstrate that the SMR can be used as an all-electrical probe of chiral surface twists and skyrmions in magnetic insulators.
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We show that skyrmions on the surface of a magnetic topological insulator may experience an attractive interaction that leads to the formation of a skyrmion-skyrmion bound state. This is in contrast to the case of skyrmions in a conventional chiral ferromagnet, for which the intrinsic interaction is repulsive. The origin of skyrmion binding in our model is the molecular hybridization of topologically protected electronic orbitals associated with each skyrmion. Attraction between the skyrmions can therefore be controlled by tuning a chemical potential that populates/depopulates the lowest-energy molecular orbital. We find that the skyrmion-skyrmion bound state can be made stable, unstable, or metastable depending on the chemical potential, magnetic field, and easy-axis anisotropy of the underlying ferromagnet, resulting in a rich phase diagram. Finally, we discuss the possibility to realize this effect in a recently synthesized Cr doped ${left(mathrm{Bi}_{2-y}mathrm{Sb}_{y}right)}_{2}mathrm{Te}_3$ heterostructure.
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.
The spin-polarized surface states in topological insulators offer unique transport characteristics which make them distinguishable from trivial conductors. Due to the topological protection, these states are gapless over the whole surface of the material. Here, we detect the surface states in the topological insulator BiSbTeSe$_{2}$ by electrical means using a non-local transport configuration. We unambiguously probe the spin-momentum locking of the topologically protected surface states by spin-sensitive electrical read-out using ferromagnetic Co/Al$_2$O$_3$ electrodes. We show that the non-local measurement allows to probe the surface currents flowing along the whole surface, i.e. from the top along the side to the bottom surface and back to the top surface along the opposite side. This is in contrast to local transport configurations where only the surface states of the one face being in contact to the electrodes can be measured. Our results furthermore exclude the contribution of the bulk to the non-local transport at low temperatures. Increasing the temperature, on the other hand, increases the interaction between bulk and surface states, which shortens the non-local current path along the surface and hence leads to a complete disappearance of the non-local signal at around 20 K. All this demonstrates that the non-local signal at low temperatures is solely due to the topologically protected surface states.
We formulate and study the general boundary conditions dictating the magnetization profile in the vicinity of an interface between magnets with dissimilar properties. Boundary twists in the vicinity of an edge due to Dzyaloshinskii-Moriya interactions have been first discussed in [Wilson et al., Phys. Rev. B 88, 214420 (2013)] and in [Rohart and Thiaville, Phys. Rev. B 88, 184422 (2013)]. We show that in general case the boundary conditions lead to the magnetization profile corresponding to the Neel, Bloch, or intermediate twist. We explore how such twists can be utilized for creation of skyrmions and antiskyrmions, e.g., in a view of magnetic memory applications. To this end, we study various scenarios how skyrmions and antiskyrmions can be created from interface magnetization twists due to local instabilities. We also show that a judicious choice of Dzyaloshinskii-Moriya tensor (hence a carefully designed material) can lead to local instabilities generating certain types of skyrmions or antiskyrmions. The local instabilities are shown to appear in solutions of the Bogoliubov-de-Gennes equations describing ellipticity of magnon modes bound to interfaces. In one considered scenario, a skyrmion-antiskyrmion pair can be created due to instabilities at an interface between materials with properly engineered Dzyaloshinskii-Moriya interactions. We use micromagnetics simulations to confirm our analytical predictions.
A magnetic skyrmion is a topological object that can exist as a solitary embedded in the vast ferromagnetic phase, or coexists with a group of its siblings in various stripy phases as well as skyrmion crystals (SkXs). Isolated skyrmions and skyrmions in an SkX are circular while a skyrmion in other phases is a stripe of various forms. Unexpectedly, the sizes of the three different types of skyrmions depend on material parameters differently. For chiral magnetic films with exchange stiffness constant $A$, the Dzyaloshinskii-Moriya interaction (DMI) strength $D$, and perpendicular magnetic anisotropy $K$, $kappaequivpi^2D^2/(16AK)=1$ separates isolated skyrmions from condensed skyrmion states. In contrast to isolated skyrmions whose size increases with $D/K$ and is insensitive to $kappall1$ and stripe skyrmions whose width increases with $A/D$ and is insensitive to $kappagg1$, the size of skyrmions in SkXs is inversely proportional to the square root of skyrmion number density and decreases with $A/D$. This finding has important implications in our search for stable smaller skyrmions at the room temperature in applications.
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