Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session J48: Focus Session: Spin Transport and Magnetization Dynamics in Metal-Based Systems: Skyrmion II |
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Sponsoring Units: DMP FIAP GMAG Chair: Jacob Gayles, Texas A&M University Room: Mile High Ballroom 1A |
Tuesday, March 4, 2014 2:30PM - 2:42PM |
J48.00001: Helicity and size dependence of skyrmions in Mn$_{1-x}$Fe$_x$Ge mediated by the Dzyaloshinskii-Moriya interaction Jacob Gayles, Frank Freimuth, Giovanna Lani, Rembert Duine, Jairo Sinova, Yuriy Mokrousov, Stefan Bl{\"u}gel We carry out first-principles electronic structure calculations for bulk alloys of Mn$_{1-x}$Fe$_x$Ge and MnSi in the B20 compound where skyrmions are seen to vary in size and chiral order. We utilize the Virtual Crystal Approximation to vary the concentration of Fe/Mn atoms within the unit cell. Using a first order perturbation approach with spin-orbit coupling applied to spin-spiral calculations we observe the Dzyaloshinski-Moriya vector changes sign and magnitude with the concentration of the transition metal ion. [Preview Abstract] |
Tuesday, March 4, 2014 2:42PM - 2:54PM |
J48.00002: The size and helicity of skyrmions in B20-type chiral magnet Mn$_{1-x}$Fe$_x$Ge Kiyou Shibata, Xiuzhen Yu, Toru Hara, Daisuke Morikawa, Naoya Kanazawa, Koji Kimoto, Shintaro Ishiwata, Yoshio Matsui, Yoshinori Tokura A magnetic skyrmion is a topologically-stable spin vortex structure observed in chiral-lattice helimagnets. Skyrmions and their crystallized state, skyrmion crystal, have been attracting much attention because of the emergent electromagnetic properties. However, crystal engineering in terms of controlling the skyrmion crystal structure itself is not well established. Here, we report on the correlation between skyrmion helicity and crystal chirality in alloys of $B$20-type chiral-lattice helimagnet Mn$_{1-x}$Fe$_x$Ge with varying compositions by Lorentz transmission electron microscopy and convergent-beam electron diffraction over a broad range of compositions ($x$ = 0.3 - 1.0)$^{[1]}$. The skyrmion lattice constant or the skyrmion size shows non-monotonous variation with the composition $x$, with a divergent behavior around $x$ = 0.8, where the correlation between magnetic helicity and crystal chirality changes sign. This originates from continuous variation of the spin-orbit coupling strength and its sign reversal in the metallic alloys as a function of $x$. Controllable spin-orbit coupling may offer a promising way to tune skyrmion size and helicity. [1] K. Shibata et al., Nat. Nanotech. 8, 723 (2013). [Preview Abstract] |
Tuesday, March 4, 2014 2:54PM - 3:06PM |
J48.00003: Resonant x-ray scattering from a skyrmion lattice S. Roy, M.C. Langner, S.K. Mishra, J.C.T. Lee, X.W. Shi, M.A. Hossain, Y.-D. Chuang, S.D. Kevan, R.W. Schoenlein, S. Seki, Y. Tokura Topologically protected novel phases in condensed matter systems are a current research topic of tremendous interest due to both the unique physics and their potential in device applications. Skyrmions are a topological phase that in magnetic systems manifest as a hexagonal lattice of spin-swirls. We report the first observation of the skyrmion lattice using resonant soft x-ray diffraction in Cu$_2$OSeO$_3$, a cubic insulator that exhibits degenerate helical magnetic structures along <100> axes in zero magnetic field. Within a narrow window of temperature and applied magnetic field we observed the six fold symmetric satellite peaks due to the skyrmion lattice around the (001) lattice Bragg peak. As a function of incident photon energy a rotational splitting of the skyrmion satellite peaks was observed that we ascribe to the two Cu sublattices of Cu$_2$OSeO$_3$, with different magnetically active orbitals. The splitting implies a long wavelength modulation of the skyrmion lattice. [Preview Abstract] |
Tuesday, March 4, 2014 3:06PM - 3:18PM |
J48.00004: Tailoring Artificial Skyrmions in single-crystalline Co/Ni/Cu(001) system Jia Li, Ali Tan, Z.Q. Qiu, Tony Young, Matthew Marcus, Andrew Doran, Elke Arenholz, Padraic Shafer Magnetic Skyrmions, which correspond to a topological spin texture pattern, were recently realized in several experimental systems as a result of Dzyaloshinsky-Moriya interactions (DMI). An alternative approach is to produce non-collinear spins in magnetic vortex states. With this motivation, we fabricated single crystalline Co disks on perpendicularly magnetized Ni/Cu(001) film to create artificial Skyrmions whose topology can be tailored by changing the relative orientation between the vortex core polarity and the surrounding perpendicular magnetization. In this way, we studied the topological effect of the Skyrmion using Photoemission Electron Microscopy (PEEM). By applying an in-plane magnetic field of various strength, we find strong evidence that the annihilation of the vortex core depends on the Skyrmion number of the system. [Preview Abstract] |
Tuesday, March 4, 2014 3:18PM - 3:30PM |
J48.00005: Dynamic Phases of Skyrmions in Chiral Magnets Driven over Random and Periodic Pinning Arrays Charles Reichhardt, Shizeng Lin, Cristian Batista, Cynthia Olson Reichhardt, Dipanjan Ray Skyrmions in chiral magnets have been generating tremendous excitement since their recent discovery, both for the intrinsic science and for possible applications of skyrmions. Skyrmions can be driven with an applied spin-polarized current and appear to have many similarities to vortices in type-II superconductors. Here we numerically simulate skyrmions driven over random and periodic arrays of defects or pinning using a combination of particle-based models and continuum models. We find that for weak pinning, the skyrmions depin elastically, while for strong pinning, the skyrmions depin plastically. In both cases, there are distinct features in the resulting transport curves and we show that in the presence of pinning the Hall angle continuously changes as a function of drive. In samples where plastic depinning occurs, at high drives there is a transition to a dynamically ordered state which we compare to the dynamical reordering observed for driven vortices in type-II superconductors. With periodic pinning, the Hall angle changes in discrete steps for increasing drive as the skyrmion motion locks to different symmetry directions of the underlying pinning array. [Preview Abstract] |
Tuesday, March 4, 2014 3:30PM - 3:42PM |
J48.00006: Dynamics of an Insulating Skyrmion under a Temperature Gradient Jiadong Zang, Lingyao Kong Skyrmion is a topological spin texture in which local magnetic moments wrap the unit sphere an integer number of times. The study of Skyrmion dynamics is not only an important physics issue, but also application oriented. On the other side, dynamics of the insulating Skyrmions is also an interesting subject. In this talk, I will briefly review my previous work on current driven Skyrmion motion based on an emergent gauge field. We study the Skyrmion dynamics in thin films under a temperature gradient. Our numerical simulations show that both single and multiple Skyrmions in a crystal move towards the high temperature region, which is contrary to particle diffusion. Noticing a similar effect in the domain wall motion, we employ a magnon pulling mechanism to explain this counterintuitive phenomenon. Unlike the temperature driven domain wall motion, the Skyrmion's topological charge plays an important role, and a transverse Skyrmion motion is observed. Our theory turns out to be in agreement with numerical simulations, both qualitatively and quantitatively.\\[4pt] [1]Jiadong Zang, Maxim Mostovoy, Jung Hoon Han, and Naoto Nagaosa, Phys. Rev. Lett. \textbf{107}, 136804 (2011). \\[0pt] [2] Lingyao Kong, and Jiadong Zang, Phys. Rev. Lett. \textbf{111}, 067203 (2013). [Preview Abstract] |
Tuesday, March 4, 2014 3:42PM - 3:54PM |
J48.00007: Spirals and skyrmions in two dimensional oxide heterostructures Xiaopeng Li, W. Vincent Liu, Leon Balents A symmetry-based general free energy governing long-wavelength magnetism in two-dimensional oxide heterostructures will be presented. This leads, in the relevant regime of weak but non-negligible spin-orbit coupling, to a rich phase diagram containing in-plane ferromagnetic, spiral, cone, and skyrmion lattice phases, as well as a nematic state stabilized by thermal fluctuations. The general conclusions are vetted by a microscopic derivation for a simple model with Rashba spin-orbit coupling. [Preview Abstract] |
Tuesday, March 4, 2014 3:54PM - 4:06PM |
J48.00008: Orbital Dzyaloshinskii-Moriya Exchange Interaction Panjin Kim, Jung Hoon Han A superexchange calculation is performed for multi-orbital band models with broken inversion symmetry. Orbital-changing hopping terms allowed by the symmetry breaking electric field lead to a new kind of orbital exchange interaction closely resembling the Dzyaloshinskii-Moriya spin exchange. Inversion symmetry breaking as present in surfaces and interfaces and a strong on-site repulsion, but not the spin-orbit interaction, are the requirements to observe the proposed effect. Mean-field phase diagram exhibits a rich structure including anti-ferro-orbital, ferro-orbital, and both single and multiple spiral-orbital phases in close analogy with the Skyrmion spin crystal phase recently discovered in thin-film chiral magnets. [Preview Abstract] |
Tuesday, March 4, 2014 4:06PM - 4:18PM |
J48.00009: Spin-orbit coupling, compass anisotropy and skyrmions in 2D chiral magnets Sumilan Banerjee, Onur Erten, James Rowland, Mohit Randeria Spin-orbit coupling (SOC) gives rise to the chiral Dzyaloshinskii-Moriya (DM) interaction in systems that lack inversion symmetry like non-centrosymmetric helimagnets, and two-dimensional magnetism at surfaces and interfaces. We explore here the role of SOC in several microscopic exchange mechanisms -- superexchange, double exchange and RKKY -- in insulating and itinerant electron systems. We show that, in addition to giving rise to the DM interaction, SOC generically leads to compass anisotropy terms. Although seemingly negligible, the compass terms are energetically comparable to DM and play a crucial role in deciding the fate of the magnetic ground state. We demonstrate that the compass terms act as an effective easy-plane anisotropy in 2D chiral magnets and lead to extremely large region of stable skyrmion crystal (SkX) phase in a perpendicular magnetic field. We discuss the electronic properties of SkX in this hitherto unexplored region of the anisotropy-field plane for itinerant systems. We also comment on the possibility of realizing such SkX phase in the oxide interfaces [1]. [1] S. Banerjee, O. Erten and M. Randeria, Nature Physics 9, 626 (2013). [Preview Abstract] |
Tuesday, March 4, 2014 4:18PM - 4:30PM |
J48.00010: Easy-plane anisotropy stabilizes skyrmions in 2D chiral magnets James Rowland, Sumilan Banerjee, Mohit Randeria Experiments on two-dimensional (2D) chiral magnetic materials, like thin films of non-centrosymmetric helimagnets and metallic magnetic layers, have revealed interesting spatially modulated spin textures such as spirals and skyrmions. Motivated by this we study the ground-state phase diagram for a 2D chiral magnet in a magnetic field using a Ginzburg-Landau model, with Dzyaloshinskii-Moriya (DM) term, anisotropic exchange and single-ion anisotropy. The easy-axis anisotropy region of the phase diagram has been well-studied [1], whereas the easy-plane region has not been discussed. In the easy-plane region, we find an unexpectedly large stable skyrmion crystal (SkX) phase in a perpendicular magnetic field. We find re-entrant transitions between ferromagnetic and SkX phases, and intriguing internal structure of the skyrmion core with two-length scales. We argue that such an easy-plane anisotropy arises naturally from the compass terms induced by spin-orbit coupling that is also responsible for the DM term, as proposed recently in the context of oxide interfaces [2]. We also discuss the phase diagram in a tilted field configuration, relevant for torque magnetometry experiments. [1] Robler et. al. Phys.: Conf. Ser. 303, 012105 (2011). [2] Banerjee et. al. Nat. Phys. 9, 626 (2013). [Preview Abstract] |
Tuesday, March 4, 2014 4:30PM - 4:42PM |
J48.00011: Controlling the dynamical modes of the chiral magnetic structures by spin Hall effect Ronghua Liu, Weng-Lee Lim, Sergei Urazhdin Recently, pure spin currents generated due to spin Hall effect have been proved as an efficient approach to reverse the magnetization, modify the dynamical relaxation rates, and excite magnetization oscillations in the heavy metal/ferromagnetic heterostructures. In addition, the Dzyaloshinskii-Moriya interaction (DMI) can also induce chiral magnetization configurations and rich dynamics in these asymmetrical heterostructures$.$ We controllably excited several distinct dynamical modes in spin Hall oscillator based on Pt/ [CoNi] magnetic multilayer with perpendicular anisotropy. At low current, a quasi-linear Slonczewski-like propagating spin wave mode was excited. This mode transforms to a localized soliton mode above a certain threshold current. At large fields, this mode can be identified as the spin wave `bullet' mode. At small fields, the localized mode is transformed to the topological structure of the `droplet' mode, which comes from the oscillations of the chiral domain walls forming the boundary of the bubble domain due to DMI. Our measurements demonstrate a straightforward route for emission of spin waves by nano-oscillators controlled either by current or by the applied magnetic field. [Preview Abstract] |
Tuesday, March 4, 2014 4:42PM - 4:54PM |
J48.00012: Chirality of symmetry broken spin-orbit systems Kyoung-Whan Kim, Hyun-Woo Lee, Kyung-Jin Lee, Mark Stiles Recently, structures consisting of an ultrathin magnetic layer adjacent to a heavy metal layer with strong atomic spin-orbit coupling have received a considerable attention. Their unexpected behavior not only stimulates scientific interest but also makes them promising candidates for spintronic devices. Strong spin-orbit coupling of two kinds, bulk spin Hall effect and interfacial spin-orbit coupling, play important roles on magnetization dynamics. In this work, we propose a unified theory of magnetic systems with interfacial spin-orbit coupling up to linear order starting from a two dimensional Rashba model which includes structural inversion symmetry breaking and time reversal symmetry breaking. The combination of both broken symmetries makes the system chiral. In our theory, conventional terms in the equation of motion each give rise to a linear chiral effect; this relationship is captured by replacing the usual spatial derivative with a chiral derivative. Introducing the chiral derivative not only captures previously reported results but also reveals previously unreported chiral aspects of the Rashba model such as the Dzyaloshinskii-Moriya interaction. It also clarifies the one-to-one correspondence between interfacial spin-orbit effects and conventional effects without spin-orbit coupling. [Preview Abstract] |
Tuesday, March 4, 2014 4:54PM - 5:06PM |
J48.00013: Tuning Interfacial Dzyaloshinskii-Moriya Interactions in Ta/CoFe/MgO through Annealing Parnika Agrawal, Satoru Emori, David Bono, Geoffrey Beach Out-of-plane magnetized ultrathin ferromagnets interfaced between a heavy metal and an oxide exhibit anomalously efficient current-induced domain wall (DW) motion. In these ultrathin ferromagnets, the interfacial Dzyaloshinkii-Moriya interaction (DMI) stabilizes N\'{e}el DWs with a fixed chirality [1] which permits the Spin Hall Effect (SHE) to drive the DWs uniformly. The magnitude and direction of DMI is a strong function of the material composition and thickness of the heavy metal underlayer, sharpness of the interface, temperature and other processing parameters [2,3]. Here we quantify the DMI effective field in Ta/CoFe/MgO films by studying the asymmetry of expansion [4] of a circular domain in the presence of in-plane bias fields. We show that while the DMI is relatively weak, it can be enhanced by annealing, and we describe the correlation between DMI and interfacial perpendicular magnetic anisotropy as a function of annealing conditions. These results provide new insights into the interfacial origin of the DMI. [1] A. Thiaville, et al., EPL \textbf{100} 57002 (2012) ;[2] S. Emori, et al.,~arXiv:1308.1432v1 (2013) ; [3] S. Emori, et al., Nat. Mat.~\textbf{12}, 611 (2013) ; [4] S.-G. Je, et al., \underline {arXiv:1307.0984v1}~ (2013) [Preview Abstract] |
Tuesday, March 4, 2014 5:06PM - 5:18PM |
J48.00014: Spin to charge conversion using Rashba coupling at the interface between non-magnetic materials J.C. Rojas Sanchez, L. Vila, G. Desfonds, S. Gambarelli, J.P. Attane, J.M. De Teresa, C. Magen, A. Fert The Rashba effect is an interaction between the spin and the momentum of electrons induced by the spin-orbit coupling (SOC) in surface or interface states. Its potential for conversion between charge and spin currents has been theoretically predicted but never clearly demonstrated for surfaces or interfaces of metals. Here we present experiments evidencing a large spin-charge conversion by the Bi/Ag Rashba interface. We use spin pumping to inject a spin current from a NiFe layer into a Bi/Ag bilayer and we detect the resulting charge current. As the charge signal is much smaller (negligible) with only Bi (only Ag), the spin to charge conversion can be unambiguously ascribed to the Rashba coupling at the Bi/Ag interface. This result demonstrates that the Rashba effect at interfaces can be used for efficient charge-spin conversion in spintronics [Preview Abstract] |
Tuesday, March 4, 2014 5:18PM - 5:30PM |
J48.00015: Magnetotransport Properties of the Highly Anisotropic Helimagnet Cr$_{1/3}$NbS$_2$ Alexander Bornstein, Nirmal Ghimire, David Mandrus, David Parker, Minhyea Lee Unusual electrical transports properties such as the topological Hall Effect in non-trivial spin textures have demonstrated great potential for controlling electrical properties via underlying spin degree of freedom. In particular, magnetic systems with no-inversion symmetry in their crystal structure are promising candidates to search for these effects due to their tendency to support non-collinear spin configurations, a requirement for non-trivial spin texture. Here, we study the in-plane magnetotransport properties in the chiral helimagnet Cr$_{1/3}$NbS$_2$, which falls in such a category and has larger crystalline anisotropy relative to other known systems (e.g. MnSi). At low temperature ($T \ll T_C$), we find that the in-plane magnetoresistance with applied field perpendicular to plane is suppressed up to three times more than with the field in-plane. Concurrently, Hall voltage, which is also taken with B field perpendicular to the plane, displays unique B field dependence. We discuss these results in the light of the role of the anisotropy in Cr$_{1/3}$NbS$_2$'s magnetic structure and band structure. [Preview Abstract] |
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