Session F50: Spin-Orbit Mediated Chiral Spin Textures I

Tunable Room Temperature Skyrmions in Ir/Fe/Co/Pt Multilayers

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[1]. 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 further investigate their electrical signature using a combination of transport and imaging experiments, and explore their nucleation and stability in patterned nanostructures down to 100 nm. We thus establish a platform for investigating functional RT skyrmions, pointing towards the development of skyrmion-based memory devices. [1] A. Soumyanarayanan et al., arXiv:1606.06034 (2016).   

Theoretical study of interacting skyrmions in bilayer systems

Magnetic skyrmion behaves as an emergent particle serving as an ideal laboratory to study "particle" in field theory, and also has large potentiality for applications to memory devices. Due to the recent advances on the skyrmions in multilayer system with interfaces, the skyrmions interacting along the direction perpendicular to the plane become realistic. As a most fundamental model of this issue, here we study the two skyrmions on different two-dimensional Dzyaloshinskii-Moriya (DM) magnets, i.e., bilayer system. The skyrmions show rich and complex dynamics depending on the sign of the interlayer exchange interaction and also the relative signs of the DM interactions on two layers. Especially, the collision dynamics and the bound state formation are revealed. In the case of ferromagnetic interlayer exchange interaction, the two skyrmions sometimes shows the rotating motion around each other, the direction of which can change, and eventually form the bound state or are detached depending on the strength of the current flow driving the skyrmion. In the case of antiferromagnetic interlayer interaction, they shows the parallel motion and the velocity is enhanced when the bound state is formed. Based on this result, we propose a new colossal spin-transfer-torque device.   

Fabrication of magnetic heterostructures for imaging skyrmions via nitrogen-vacancy center magnetometry

Thin film magnetic heterostructures have long been known to exhibit chiral magnetic order due to breaking of inversion symmetry in the system and the resulting interfacial Dzyaloshinskii-Moriya interaction (DMI).\footnote{M. Bode et. al., \textit{Nature} \textbf{447}, 190-193 (2007)} By tuning the strength of the interfacial interaction between magnetic and nonmagnetic thin films, we can manipulate the zero-field domain patterns in these materials, creating labyrinth domains and skyrmion lattices.\footnote{S. Woo et. al., \textit{Nature Materials} \textbf{15}, 501-506 (2016)} Imaging these spin structures has presented a significant challenge to the field; we have shown that nitrogen-vacancy (NV) magnetometry provides a flexible, room temperature, and variable field method for imaging and reconstructing magnetic spin structures.\footnote{Y. Dovzhenko$^*$, F. Casola$^*$, et. al., arXiv: 1611.0067v1 (2016)} We present an alternative fabrication method of Pt/Co/Ta multilayers for imaging within the experimental constraints presented by NV scanning magnetometry. We further present unique deposition techniques for creating and imaging stable skyrmions within a single layer of Pt/CoFeB/MgO.   

Reconstructing Magnetization Patterns with an Atomic-Sized Sensor

Establishing magnetic structures in systems of reduced dimensionality is notoriously challenging. We have used the single spin of a Nitrogen Vacancy (NV) center in diamond to perform scanning magnetometry of skyrmions in Pt/Co/Ta multilayers under ambient conditions [1]. Our atomic-sized sensor can record 2D maps of the stray magnetic field produced by the non-collinear ordered state, at a distance of $\sim30$ nm from the surface. We show how to systematically obtain a set of solutions for the underlying spin texture compatible with the measured local field. Such solutions can be reconstructed by fixing the magnetic helicity of the configuration. We then select physically allowed patterns by requiring the topological number for the spin structure to be an integer. Our work is the first instance of spin reconstruction performed by a single scanning NV center. We will discuss possible applications to other two-dimensional ordered states, e.g. in complex oxide interfaces, novel 2D materials, and van der Waals heterostructures. [1] Y.~Dovzhenko$^*$, F.~Casola$^*$, S.~Schlotter, T.~X.~Zhou, F.~B\"uttner, R.~L.~Walsworth, G.~S.~D.~Beach and A. Yacoby, arXiv:1611.00673v1 (2016).   

Imaging Magnetic Skyrmions Under Ambient Conditions with an Atomic-Sized Sensor

Magnetic skyrmions are particle-like topologically-protected spin structures, which commonly crystallize in chiral magnets at cryogenic temperatures. Recently stable room-temperature skyrmions were reported in stacks of thin magnetic films[1,2]. Establishing the microscopic structure of these skyrmions in the presence of external magnetic fields is a key experimental challenge, calling for a quantitatve room-temperature approach. We use a scanning Nitrogen-Vacancy (NV) center to image skyrmions at a Pt/CoFeB interface as well as in Pt/Co/Ta multilayers in ambient conditions[3]. We perform full vector magnetometry of the local magnetic fields produced by the films. We establish the presence of a N\'{e}el-type skyrmion. In addition to static magnetic signal, we discover a modulation of the NV spin transition linewidth suggesting the presence of thermal fluctuations of the spin structure. Our results identify NV magnetometry as a promising local probe for both static magnetization structures and spin fluctuations in a variety of low-dimensional condensed matter systems. [1] Woo, S. \textit{et al.} \textit{Nat. Mater.} \textbf{15,} 501--506 (2016). [2] Moreau-Luchaire, C. \textit{et al.} \textit{Nat. Nanotechnol.} \textbf{11,} 444--448 (2016). [3] Dovzhenko*, Y., Casola*, F. \textit{et al.} arxiv:1611.00673 (2016).   

Domain Wall Structure of Thin Film Magnetic Skrymions Investigated with a Single-Spin Sensor

The nitrogen-vacancy (NV) center in diamond is an atomic scale defect that is capable of sensing magnetic fields with nanometer-scale spatial resolution and single electron spin sensitivity. By incorporating an individual NV center into a robust scanning probe microscope, we have recently constructed a versatile, NV-based magnetic imaging tool that operates from room temperature down to 5 K. We have imaged skyrmion structures in magnetic thin films with an interfacial Dzyaloshinskii-Moriya interaction (DMI) and perpendicular magnetic anisotropy. Magnetic skyrmions are topologically stabilized magnetization structures that are appealing for use in future high-density, low-power memory and logic devices. Utilizing the high spatial resolution of the NV microscope, we determine the domain wall structure of skyrmions in two classes of magnetic thin films: Ta/CoFeB/Pt/MgO/Ta and Pt/Co/W/Pt. The details of the domain wall structure in these materials has important implications for the functionality of magnetic bubbles in real devices, in particular for the stability of the bubbles and the current density required for their manipulation.   

Topological Hall and Spin Hall Effects in Disordered Skyrmionic Textures

We carry out a throughout study of the topological Hall and topological spin Hall effects in disordered skyrmionic systems: the dimensionless (spin) Hall angles are evaluated across the energy band structure in the multiprobe Landauer-B\"uttiker formalism and their link to the effective magnetic field emerging from the real space topology of the spin texture is highlighted. We discuss these results for an optimal skyrmion size and for various sizes of the sample and found that the adiabatic approximation still holds for large skyrmions as well as for few atomic size-nanoskyrmions. Finally, we test the robustness of the topological signals against disorder strength and show that topological Hall effect is highly sensitive to momentum scattering.   

Magnetoelectrical control of nonreciprocal microwave response in a multiferroic helimagnet

Control of physical property in terms of external fields is essential for contemporary technologies. The conductance can be controlled by a gate electric field in a field effect transistor, which is a main component of the integrated circuit. Optical phenomena induced by an electric field such as electroluminescence and electrochromism are useful for display and other technologies. Control of microwave propagation seems also imperative for future wireless communication technology. Microwave properties in solids are dominated mostly by magnetic excitations, which cannot be easily controlled by an electric field. One of the solutions for this problem is utilizing magnetically induced ferroelectrics (multiferroics). Here we show that microwave nonreciprocity, which is difference between oppositely propagating microwaves, can be reversed by the external electric field in a multiferroic helimagnet Ba$_{2}$Mg$_{2}$Fe$_{12}$O$_{22}$. This result offers a new avenue for the electrical control of microwave properties.   

Effect of chemical substitution on magnetic and charge transport behavior of MnSi.

The cubic non-centrosymmetric \textit{B20} compound MnSi has gained renewed interest after the discovery of skyrmion lattice within a small range of temperature and magnetic field [1]. This unique spin texture results from the specific reorientation of helixes where the period of one such helix is determined from two energy scales, the Dzyaloshinskii-Moriya interaction strength ($D)$ and the Heisenberg exchange interaction ($J)$ i.e. $\lambda $\textit{\textasciitilde J/D }[1]. We are currently exploring the tuning of these energy scales and the stabilizing the skyrmion lattice by chemical substitutions on either the Mn or Si site. Our combined charge transport, magnetization and neutron scattering investigation indicates that significant change in the magnetic and charge transport behavior can be induced by simple chemical substitutions in MnSi [2]. I will discuss our results indicating the readjustments of these energy scales thereby influencing the size of the helix and consequently the region of field and temperature stability of the skyrmion lattice phase. 1. M\"{u}hlbauer, S., et al. "Skyrmion lattice in a chiral magnet."~\textit{Science}~323.5916 (2009): 915-919. 2. Dhital, C., et al. ``Effect of Negative Pressure on the Prototypical Itinerant Magnet MnSi''.~\textit{arXiv preprint arXiv:1609.08181}.   

Gigantic Dzyaloshinskii-Moriya interaction in the MnBi ultrathin films

The magnetic skyrmion, a swirling-like spin texture with nontrivial topology, is driven by strong Dzyaloshinskii-Moriya (DM) interaction originated from the spin-orbit coupling in inversion symmetry breaking systems. Here, based on first-principles calculations, we predict a new material, MnBi ultrathin film, with gigantic DM interactions. The ratio of the DM interaction to the Heisenberg exchange is about 0.3, exceeding any values reported so far. Its high Curie temperature, high coercivity, and large perpendicular magnetoanisotropy make MnBi a good candidate for future spintronics studies. Topologically nontrivial spin textures are emergent in this system. We expect further experimental efforts will be devoted into this systems.   

Interfacial Control of Dzyaloshinskii--Moriya Interaction in Heavy Metal/Ferromagnetic Metal Thin Film Heterostructures

The interfacial Dzyaloshinskii--Moriya Interaction (DMI) in ultrathin magnetic thin film heterostructures provides a new approach for controlling spin textures on mesoscopic length scales. Here we investigate the dependence of the interfacial DMI constant $D$ on a Pt wedge insertion layer in Ta/CoFeB/Pt(wedge)/MgO thin films by observing the asymmetric spin wave dispersion using Brillouin light scattering. Continuous tuning of $D$ by more than a factor of three is realized by inserting less than one monolayer of Pt. The observations provide new insights for designing magnetic thin film heterostructures with tailored $D$ for controlling skyrmions and magnetic domain wall chirality and dynamics.   

Magnetic Phase Diagram of a Lacunar Spinel GaV$_{\mathrm{4}}$Se$_{\mathrm{8}}$

The magnetic field ($H$) - temperature ($T$) phase diagram of a lacunar spinel GaV$_4$Se$_8$ is determined by means of AC magnetic susceptibility and magnetoelectric measurements on single crystals and classical Monte Carlo simulation. GaV$_4$Se$_8$ is pyroelectric below the structural phase transition temperature $T_{\mathrm{S}}=41\,\mathrm{K}$ and magnetically ordered below $T_{\mathrm{C}}=17.5\,\mathrm{K}$. AC magnetic susceptibility measurement has revealed that GaV$_4$Se$_8$ undergoes successive magnetic phase transitions with increasing applied magnetic field. Each phase is assigned to cycloidal, skyrmion lattice and forced ferromagnetic phases. Both cycloidal and skyrmion-lattice magnetic orders induce electric polarization up to around $10\,\mu \mathrm{C/m^2}$ compared with ferromagnetic order, suggesting a spin-driven magnetoelectric nature in GaV$_4$Se$_8$. The skyrmion lattice phase seems to be stable down to $T=2\,\mathrm{K}$ and up to $\mu_0 H=370\,\mathrm{mT}$. This enhanced stability of skyrmion lattice in GaV$_4$Se$_8$ compared with GaV$_4$S$_8$ may provide a key to understand the formation mechanism of the skyrmion lattice.   

Spatiotemporally resolved magnetic dynamics in B20 chiral FeGe

Chiral magnetic materials have shown promise for ultra-low-power memory devices exploiting low critical currents for manipulation of spin textures. This motivates systematic studies of chiral dynamics in thin films, both for understanding magnetic properties and for developing devices. We use time-resolved anomalous Nernst effect (TRANE) microscopy to examine ferromagnetic resonance modes in 170 nm thin films of B20 chiral FeGe. Using 3 ps laser pulses with 1.2 $\mu$m resolution to generate a local thermal gradient, we measure the resulting Nernst voltage, which is proportional to the in-plane component of the magnetization. We first characterize and image the static magnetic moment as a function of temperature near the helical phase transition at 273 K. We then excite ferromagnetic resonance with microwave current and study the dynamical modes as a function of temperature, spatial position, and frequency. We identify both the uniform field-polarized mode and the helical spin-polarized mode and study the different spatial structures of the two modes.