Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session X18: Bulk and Artificial SkyrmionsFocus
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Sponsoring Units: GMAG DMP Chair: Gong Chen, Berkeley National Laboratory Room: 317 |
Friday, March 18, 2016 8:00AM - 8:12AM |
X18.00001: Emergence of skyrmions from rich parent phases in the molybdenum nitrides Jiadong Zang, Wei Li, Chiming Jin, Renchao Che, Wensen Wei, Langsheng Lin, Lei Zhang, Haifeng Du, Mingliang Tian We report a new family of skyrmion materials originated from the antisymmetric Dzyaloshinskii-Moriya (DM) interactions. Based on the symmetric tensor technique, the molybdenum nitrides with the $\beta $-manganese structure, A2Mo3N with A$=$Fe, Co, Rh, are predicted to support the skymion phase. This predication is directly proved in doped FexCo$_{1-x}$Rh$_{0.5}$Mo$_{3}$N components by high resolution Lorentz transmission electron imaging. Interestingly, the parent compounds Fe2Mo3N, Co2Mo3N, and Rh2Mo3N exhibit ferromagnetic, anti-ferromagnetic, and even superconducting orderings respectively. Magnetism in these parent phases is theoretically clarified by the first principle calculations, where the corrected nature of the magnetism is revealed.. [Preview Abstract] |
Friday, March 18, 2016 8:12AM - 8:24AM |
X18.00002: Magnetic phase diagram of doped MnSi Chetan Dhital, Mojammel Kahn, Adam P. Phelan, David P. Young, Rongying Jin, John DiTusa The noncentrosymmetric chiral structure of cubic \textit{B20} compound MnSi favors Dzyaloshinskii-Moria (DM) interaction ($D)$ where the broken inversion symmetry determines direction of $D$ and the strength of spin-orbit coupling determines the magnitude of $D$. This relatively weak DM interaction $D$ competes against the exchange interaction ($J)$ resulting in rather unusual long wavelength helimagnetism with the period set by the ratio ($J/D)$. Previous work has shown that both helimagnetic period ($\lambda )$ and the transition temperature ($T_{C})$ are reduced as a result of either Fe or Co substitutions on Mn site [1, and references therein]. Recently we have started to investigate the effect on the helimagnetic/Skyrmion structure and the transition temperature by chemical substitutions on the Si site. I will present our preliminary magnetization and neutron scattering results where we have found clear evidence of increase in both $T_{C}$ and helimagnetic period as a result of doping. The possible connection between carrier doping and the strength of DM interaction strength will be discussed. \begin{enumerate} \item Bauer, A., et al. "Quantum phase transitions in single-crystal Mn $_{1-\, x}$ Fe $_{x}$ Si and Mn $_{1-\, x}$ Co $_{x}$ Si: Crystal growth, magnetization, ac susceptibility, and specific heat."~\textit{Physical Review B}~82.6 (2010) . \end{enumerate} [Preview Abstract] |
Friday, March 18, 2016 8:24AM - 8:36AM |
X18.00003: First-principles simulation and low-energy effective modeling of three-dimensional skyrmion in MnGe Hongchul Choi, Yuan-Yen Tai, Jian-Xin Zhu The skyrmion spin textures are mostly observed in two-dimensional (2D) space, which can be topologically mapped onto the surface of the sphere with an integer multiple of topological winding number. Recently, MnGe has been reported as a candidate of 3D skyrmion crystal, showing the variation of the skyrmion size along the z-direction [1,2]. We have performed the first-principles simulation and constructed a tight-binding model with calculated electronic-structure information to investigate the 3D skyrmion phase in MnGe. Our first-principles study within density functional theory shows that the calculated magnetic moment is larger than that for MnSi (with different lattice constant), implying the possibility of a multiple magnetic transition under pressure [3]. We have also found that the small-sized skyrmion could be stabilized in a 2D structure. Such a high density of the skyrmion is in good agreement with the experimental finding of large topological Hall effect [1]. Finally, we will extend our study to consider the 3D skyrmion structure based on the constructed tight-binding model. [1] Y. Shiomi et al., Phys. Rev. B 88, 064409 (2013); [2] T. Tanigaki et al., Nano Lett. 15, 5438 (2015); [3] M. Deutsch et al., Phys. Rev. B 89, 180407 (2014). [Preview Abstract] |
Friday, March 18, 2016 8:36AM - 8:48AM |
X18.00004: Study of topological spin texture in B20 crystalline FeGe films Emrah Turgut, Albert Park, Kayla Nguyen, Robert Hovden, Lena Kourkoutis, David Muller, Gregory Fuchs The possibility of efficient and robust information storage in B20-hellimagnet systems has been attracted significant interest. Although there have been promising transmission electron microscopy (TEM) and transport studies on bulk B20 crystalline materials, the development of applications motivates study of thin-film samples grown with scalable techniques such as magnetron sputtering. Here we report transport and characterization measurements of FeGe thin films grown on Si \textless 111\textgreater by magnetron co-sputtering. We obtain well-oriented but polycrystalline FeGe films with the B20 crystalline phase after post-growth annealing. Low temperature TEM imaging reveals that the lattice mismatch between the Si substrate and FeGe film introduces disordered helical magnetic phases. In addition, bulk susceptibility measurements of a continuous film and AMR measurements of micron-size wires indicate helical, conical, and ferromagnetic phases, but not an obvious skyrmion phase. Similar to recent reports, our measurements confirm that the observations of additional contributions to Hall effect measurements in B20 materials are not necessarily proof of magnetic skyrmion phase, and that more careful experimental studies are needed to understand thin film properties of B20 materials. [Preview Abstract] |
Friday, March 18, 2016 8:48AM - 9:00AM |
X18.00005: Skyrmion domains in Cu2OSeO3: Short-Range Order and Domain Wall Formation Shilei Zhang, Andreas Bauer, Helmuth Berger, Stavros Komineas, David Burn, Christian Pfleiderer, Gerrit van der Laan, Thorsten Hesjedal Cu$_2$OSeO$_3$ is a chiral ferrimagnetic insulator carrying a long-range order magnetic skyrmion phase. Here, we report a short-range ordered equilibrium skyrmion state on the surface of Cu$_2$OSeO$_3$ single crystal, studied by resonant soft x-ray scattering. Soft x-ray scattering at the $L_{2,3}$ edge of $3d$ compounds is an ideal tool to probe the magnetic order, and is only sensitive to ~60-70 unit cells in depth of Cu$_2$OSeO$_3$. Our results show that under the arbitrary magnetic field directions that deviate from the cubic main axes, the six-fold-symmetric skyrmion order breaks into domains, and the initial, anisotropy-governed pinning of propagation directions is completely unwound. We show that uniform $360^\circ$ Bloch domain walls form between the skyrmion domains. Our findings provide a new way to manipulate and engineer the skyrmion state locally, or even individually, on the surface which will enable applications in the future. [Preview Abstract] |
Friday, March 18, 2016 9:00AM - 9:12AM |
X18.00006: Transverse field muon-spin rotation in skyrmion-hosting materials Tom Lancaster, Fan Xiao, Robert Williams, Zaher Salman, Stephen Blundell, Francis Pratt, Iorwerth Thomas, Monica Ciomaga Hatnean, Geetha Balakrishnan, Shilei Zhang, Thorsten Hesjedal We present the results of transverse field (TF) muon-spin rotation ($\mu^{+}$SR) measurements on examples of materials that host a skyrmion lattice (SL) phase. In measurements on bulk Cu$_{2}$OSeO$_{3}$, we measure the response of the TF $\mu^{+}$SR signal in the SL phase along with the surrounding ones, and suggest how the phases might be distinguished using the results of these measurements. Dipole field simulations support the conclusion that the muon is sensitive to the SL via the TF lineshape and, based on this interpretation, our measurements suggest that the SL is quasistatic on a timescale $\tau > 100$ ns. We also discuss TF $\mu^{+}$SR measurements on an epitaxially grown 40 nm-thick film of MnSi on Si(111) in the region of the field-temperature phase diagram where a skyrmion phase has been observed in the bulk. We identify changes in the quasistatic magnetic field distribution sampled by the muon, along with evidence for magnetic transitions around $T\approx 40$ K and 30 K. Our results suggest that the cone phase is not the only magnetic texture realized in film samples for out-of-plane fields. [Preview Abstract] |
Friday, March 18, 2016 9:12AM - 9:24AM |
X18.00007: Antiferromagnetic skyrmions Oleg Tretiakov, Joseph Barker Skyrmions are topologically protected entities in magnetic materials which have the potential to be used in spintronics for information storage and processing. However, skyrmions in ferromagnets have some intrinsic difficulties which must be overcome to use them for spintronic applications, such as the inability to move straight along current. We show that skyrmions can also be stabilized and manipulated in antiferromagnetic materials. An antiferromagnetic skyrmion is a compound topological object with a similar but of opposite sign spin texture on each sublattice, which e.g. results in a complete cancelation of the Magnus force. We find that the composite nature of antiferromagnetic skyrmions gives rise to different dynamical behavior, both due to an applied current and temperature effects. [Preview Abstract] |
Friday, March 18, 2016 9:24AM - 9:36AM |
X18.00008: Creating Skyrmions Using Spin Transfer Torque Jennifer Grab, Alison Rugar, David MacNeill, Giovanni Finocchio, Robert Buhrman, Daniel Ralph Finding efficient methods to read and write individual skyrmions under ambient conditions is an important first step toward realizing skyrmion-based applications, such as high density information storage and racetrack memory. Of recent interest experimentally are heavy metal /ferromagnet bilayers with a strong interfacial Dzyaloshinskii-Moriya Interaction and perpendicular magnetic anisotropy (PMA), which favor the formation of helical spin textures. Micromagnetic simulations of these materials suggest that an out of plane spin polarized current could be used to write isolated skyrmions and excite skyrmion breathing modes. In this project, we attempt to create skyrmions using a spin valve like device. Our devices consist of a PMA nanopillar on top of a bulk Co/Pt bilayer. The nanopillar generates a spin polarized current, which is expected to locally reverse the magnetization of the film underneath the pillar via spin transfer torque. We report measurements of the DC current and field dependence of the pillar resistance. By monitoring the magnetization state of the bilayer independently, these measurements can be used to isolate the resistive signatures of skyrmion formation. [Preview Abstract] |
Friday, March 18, 2016 9:36AM - 9:48AM |
X18.00009: Generation of magnetic skyrmion bubbles by inhomogeneous spin-Hall currents Olle Heinonen, Wanjun Jiang, Hamoud Somaily, Suzanne G.E. te Velthuis, Axel Hoffmann Recent experiments have shown that magnetic skyrmion bubbles can be generated and injected at room temperature in thin films$^1$. Here, we demonstrate, using micromagnetic modeling, that such skyrmions can be generated by an inhomogeneous spin Hall torque in the presence of Dzyaloshinskii- Moriya interactions (DMIs). In the experimental Ta-Co$_{20}$Fe$_{60}$B$_{20}$ thin films, the DMI is rather small; nevertheless, the skyrmion bubbles are stable, or at least metastable on observational time scales. We identify two different mechanisms, one in a low-current regime and the other in a high current regime, that destabilize a domain wall injected from a narrow region into an expanding region with inhomogeneous spin-Hall torque. In the first, asymmetric torques on the domain wall lead to a cascade of bubble formation and subsequent fragmentation. In the second, an approximately steady-state texture is injected into the wide region. When the current is turned off, the inhomogeneous spin texture relaxes and regions can coalesce into bubbles that attain a definite chirality because of the DMI. [1] W. Jiang {\it et al.}, Science {\bf 349}, 283 (2015). [Preview Abstract] |
Friday, March 18, 2016 9:48AM - 10:00AM |
X18.00010: Tailoring the chiral magnetic interaction between two individual atoms J. Wiebe, A. A. Khajetoorians, M. Steinbrecher, M. Ternes, M. Bouhassoune, M. dos Santos Dias, S. Lounis, R. Wiesendanger Chiral magnets are a promising route toward dense magnetic storage technology due to their inherent nano-scale dimensions and energy efficient properties. Engineering chiral magnets requires atomic-level control of the magnetic exchange interactions, including the Dzyaloshinskii-Moriya interaction, which defines a rotational sense for the magnetization of two coupled magnetic moments. Here we show that the indirect conduction electron mediated Dzyaloshinskii-Moriya interaction between two individual magnetic atoms on a metallic surface can be manipulated by changing the interatomic distance with the tip of a scanning tunneling microscope. We quantify this interaction by comparing our measurements to a quantum magnetic model and ab-initio calculations yielding a map of the chiral ground states of pairs of atoms depending on the interatomic separation. The map enables tailoring the chirality of the magnetization in dilute atomic-scale magnets. [Preview Abstract] |
Friday, March 18, 2016 10:00AM - 10:12AM |
X18.00011: Collapse and control of the MnAu$_{2}$ spin spiral state through pressure and dopin James Glasbrenner MnAu$_{2}$ is a spin spiral material with ferromagnetic Mn layers that rotate from plane to plane. The spiral angle $\theta$ decreases with pressure and collapses to a ferromagnetic state above a critical threshold, although different experiments do not agree on whether the collapse is first or second order. To resolve this contradiction, we employ density functional theory to calculate magnetic energies in the spiral state under both pressure and charge doping and fit the results to the $J_{1} - J_{2} - J_{3} - J_{4}$ Heisenberg model, which predicts either first or second order phase transitions depending on the set of exchange parameters. At ambient pressure, MnAu$_{2}$ sits very close to a dividing line separating first and second order transitions, and applying either pressure or electron doping shifts the system towards the second order region of parameter space. Our findings show how variations in material quality can impact how the spiral state collapses, which resolves the contradiction in pressure experiments. Our results also suggest that MnAu$_{2}$ is amenable to engineering via chemical doping and to controlling $\theta$ using pressure and gate voltages, which holds potential for integration in spintronic devices. [Preview Abstract] |
Friday, March 18, 2016 10:12AM - 10:24AM |
X18.00012: Neutron Study of Spiral Magnetism in Au$_{2}$Mn I-LIN LIU, MARIA PASCALE, NICHOLAS BUTCH The binary compound Au$_{2}$Mn is known to order magnetically with a spiral structure. The pitch of the spiral has been previous shown to be dependent upon temerature and pressure. We will discuss the results of neutron diffraction measurements, in which we study the low temperature behavior of the spiral to higher pressure. [Preview Abstract] |
Friday, March 18, 2016 10:24AM - 10:36AM |
X18.00013: Chiral Magnets Under a Tilted Magnetic Field: Noncircular Skyrmions Avadh Saxena, Shi-Zeng Lin The equilibrium and dynamical properties of skyrmions in thin films of chiral magnets are studied in the presence of oblique magnetic fields. The shape of an individual skyrmion is found to be noncircular and the skyrmion density decreases with the tilt angle (from the normal of the film). Consequently, the interaction between two skyrmions depends on the relative angle between them besides their separation. Under a perpendicular magnetic field a triangular lattice of skyrmions is formed which is distorted into a centered rectangular lattice when the magnetic field is tilted. For low skyrmion densities a chainlike structure of skyrmions is formed. The dynamical response (including the Hall angle of motion) of the noncircular skyrmions is found to depend on the direction of external currents. [Preview Abstract] |
Friday, March 18, 2016 10:36AM - 10:48AM |
X18.00014: Anisotropy of Skyrmion Lattice in Mn$_{0.9}$Fe$_{0.1}$Si probed by magnetic field orientation dependence of the topological Hall effect and magnetoresistance Peter Siegfried, Andrew Treglia, Alexander Bornstein, Thomas Wolf, Minhyea Lee We report the magnetic field orientation dependence of the topological Hall effect (THE) and magnetoresistance (MR) of Mn$_{0.9}$Fe$_{0.1}$Si in the $A$-phase within the applied magnetic field ($H$) – temperature ($T$) phase diagram. In the $A$-phase a two dimensional Skyrmion lattice is formed in the plane perpendicular to the direction of $H$, which is responsible for the observed THE signal. At a given $T$ within the $A$-phase, we investigated the angular dependence of THE and MR at a fixed $H$ to probe the boundaries of the $A$-phase region. We find the MR signal exhibits a unique $H$-direction dependence at the entering and exiting of the $A$-phase, whereas, in the middle $H$ range, i.e. in the core of $A$-phase, the angular dependence is consistent with what is expected from a perfect 2D Skyrmion lattice. However, THE signals show extreme sensitivity upon entering the $A$-phase and unexpected angular dependence, yet did not leave any trace through exiting. The discrepancy between the angular dependence of MR and THE signals at the $A$-phase boundaries indicates a crucial role of Fe impurities as pinning centers for the Skyrmions. We will discuss further our $H$-orientation dependence of the THE, compared to sweeping $H$ at a fixed angle in Fe doped MnSi. [Preview Abstract] |
Friday, March 18, 2016 10:48AM - 11:00AM |
X18.00015: Critical phenomena of emergent monopoles in a chiral magnet Xiao-Xiao Zhang, Naoto Nagaosa A three-dimensional cubic Skyrmion crystal in the bulk, which is simultaneously a lattice of monopole-antimonopole pairs predicted theoretically, has been recently identified experimentally in MnGe. Adopting appropriate temperature Green's function technique for optical conductivity and devising a solvable phonon-magnon interaction, we systematically developed the theory of coupling spin-waves to both itinerant electrons and mechanical degrees of freedom in this chiral magnet, describing the latest experimental observations including anomalies and critical phenomena in magnetotransport and magnetoelasticity, which are identified as hallmarks of fluctuations of the emergent monopolar fields upon the nontrivial monopole dynamics and especially a topological phase transition signifying strong correlation. As a whole, they speak for a crucial role played by the monopole defects and hence the real-space spin topology in this material. [Preview Abstract] |
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