Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session G48: Focus Session: Spin Transport and Magnetization Dynamics in Metal-Based Systems: Skyrmion I |
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Sponsoring Units: DMP FIAP GMAG Chair: J.-Y. Lin, National Chiao Tung University Room: Mile High Ballroom 1A |
Tuesday, March 4, 2014 11:15AM - 11:27AM |
G48.00001: The skyrmion phase in Mn$_{x}$Si revealed by the specific heat J.-Y. Lin, F.C. Chou We have measured the low temperature specific heat of Mn$_{x}$Si single crystals with two manganese contents, $x$ = 0.998 and 0.945. The skyrmion phase is manifested in the specific heat data. More importantly, it was found that the skyrmion phase is stabilized at $H$= 0 for $x$ = 0.998. [Preview Abstract] |
Tuesday, March 4, 2014 11:27AM - 11:39AM |
G48.00002: Electrical Detection of a Skyrmion State in Sputter Deposited MnSi Thin Films Brian Youngblood, Ilya Krivorotov We report a simple method of growing highly ordered B20-phase MnSi films on Si(111) by magnetron sputtering of a stoichiometric target and demonstrate that these films support a skyrmion phase. By measuring the DC magnetoresistance as a function of temperature and applied magnetic field we are able to map a portion of the phase diagram of the MnSi film. For a ~15nm film we find an extended region of stability (20K-35K) above 400G for the skyrmion phase, similar to that reported for MBE deposited films. The critical (Curie) temperature (~30K) at zero magnetic field is greater than that of thin free-standing single crystal samples and comparable to that of bulk MnSi. We also report measurements of the transverse conductivity in these films, including the topological Hall contribution due to the skyrmions. We further report the observation of temperature hysteresis of the resistance, indicating frustrated order coexisting with the skyrmion phase. Our work simplifies the fabrication of spintronic devices based on MnSi and the helimagnons and skyrmions it hosts. The skyrmions in particular are highly technologically relevant due to the ease with which they can be manipulated by injected angular momentum and their toplogically protected stability. [Preview Abstract] |
Tuesday, March 4, 2014 11:39AM - 11:51AM |
G48.00003: Magnetotransport properties of Fe doped MnSi under pressure Benjamin Chapman, Maxwell Grossnickle, Thomas Wolf, Minhyea Lee In an exotic magnetic phase known as a skyrmion lattice, the coupling between spin texture and conduction electrons produces enormous emergent magnetic fields, experimentally detectable with electrical transport measurements as evidenced in MnSi and similar B20 structured magnets. We present magnetotrasport data from a helimagnet Mn$_1-x$Fe$x$Si ($x$ = 0.09) under hydrostatic pressure. In this system, both pressure and Fe doping suppress long-range magnetic order while their effects on the saturated magnetic moment and helical pitch length are quite different. We discuss the size and characteristics of the topological Hall effect, a signature of these emergent magnetic fields, in pure and 9\% Fe doped MnSi under pressure, and compare to other samples with different Fe compositions at ambient pressure. [Preview Abstract] |
Tuesday, March 4, 2014 11:51AM - 12:03PM |
G48.00004: Non-Fermi liquid phase in metallic Skyrmion crystals Haruki Watanabe, Siddharth Parameswaran, Srinivas Raghu, Ashvin Vishwanath Motivated by reports of a non-Fermi liquid state in MnSi, we examine the effect of coupling phonons of an incommensurate skyrmion crystal (SkX) to conduction electrons. We find that non-Fermi liquid behavior emerges in both two and three dimensions over the entire phase, due to an anomalous electron-phonon coupling that is linked to the net skyrmion density. A small parameter, the spiral wave vector in lattice units, allows us to exercise analytic control and ignore Landau damping of phonons over a wide energy range. At the lowest energy scales the problem is similar to electrons coupled to a gauge field. The best prospects for realizing these effects is in short period skyrmion lattice systems such as MnGe or epitaxial MnSi films. We also compare our results with the unusual $T^{3/2}$ scaling of temperature dependent resistivity seen in high pressure experiments on MnSi. [Preview Abstract] |
Tuesday, March 4, 2014 12:03PM - 12:15PM |
G48.00005: Elastic properties of skyrmion crystal in MnSi Yoichi Nii, Akiko Kikkawa, Yasujiro Taguchi, Yoshihiro Iwasa, Yoshinori Tokura Recently magnetic skyrmion, discovered in several chiral magnets, has attracted strong attention. The particle-like objects crystallize in the form of triangular lattice, in analogy with magnetic flux lattice in type-II superconductors. Here, we report the elastic properties of MnSi by means of ultrasonic measurement. We have succeeded in detecting the skyrmion crystal (SkX) phase from elastic anomalies. A clear elastic hardening in the SkX phase signifies the elastic stiffness of skyrmion lattice on the background of crystal lattice stiffness. Moreover large ultrasonic absorption was confirmed at phase boundaries between SkX and intermediate phases. From these data, we mapped out characteristic phase diagram in the skyrmion system. [Preview Abstract] |
Tuesday, March 4, 2014 12:15PM - 12:27PM |
G48.00006: Thermally Driven Ratchet Motion of Skyrmion Microcrystal and Topological Magnon Hall Effect Masahito Mochizuki, Xiuzhen Yu, Shinichiro Seki, Naoya Kanazawa, Wataru Koshibae, Jiadong Zang, Maxim Mostovoy, Yoshinori Tokura, Naoto Nagaosa By means of the Lorentz TEM, we have found that micron-sized crystals of topologically nontrivial spin textures so called skyrmions in thin specimens of MnSi and Cu2OSeO3 show unidirectional rotations. Our numerical simulation based on a stochastic Landau-Lifshitz-Gilbert equation demonstrates that these rotations are driven sheerly by thermal fluctuations in the presence of temperature gradient. We show that hidden behind the skyrmion rotation is the rotational flow of magnons deflected by the effective magnetic field of skyrmions. [Preview Abstract] |
Tuesday, March 4, 2014 12:27PM - 1:03PM |
G48.00007: Theoretical Studies on Dynamical Phenomena of Magnetic Skyrmions Invited Speaker: Masahito Mochizuki We discuss our recent results of theoretical studies on the dynamical phenomena of magnetic skyrmions in chiral-lattice magnets. The topics are; \begin{enumerate} \item electric-current-induced dynamics of skyrmions \item resonant excitations and microwave diode effect of skyrmions \item electric-field generation of skyrmions. \end{enumerate} [Preview Abstract] |
Tuesday, March 4, 2014 1:03PM - 1:15PM |
G48.00008: Current-induced skyrmion dynamics in constricted geometries Junichi Iwasaki, Masahito Mochizuki, Naoto Nagaosa Skyrmion is a vortex-like swirling spin structure with quantized topological number realized in chiral magnets, which can be driven by ultralow current density. For the skyrmion-based spintronic devices, we studied skyrmion motions in confined geometries by micromagnetic simulations. We found that the current-driven motion of skyrmions under the influence of geometrical boundaries is completely different from that in an infinite plane. In a channel of finite width, the confinement transverse to the direction of the current gives the steady-state characteristics of the skyrmion velocity as a function of current that are similar to those of domain walls in ferromagnets, whereas the transient behaviour depends on the initial distance of the skyrmion from the boundary and is distinct to skyrmion. Furthermore, we show that the creation of a single skyrmion can be controlled by an electric current in a simple constricted geometry comprising a plate-shaped specimen of suitable size and geometry.\\[4pt] [1] J. Iwasaki, M. Mochizuki and N. Nagaosa, Nature Nanotech. 8, 742 (2013). [Preview Abstract] |
Tuesday, March 4, 2014 1:15PM - 1:27PM |
G48.00009: Skyrmion motion induced by the spin Seebeck effect and ac current generation in chiral magnetic insulators Avadh Saxena, Shi-Zeng Lin, Cristian Batista, Charles Reichhardt Stable topological excitations such as domain walls, and vortices are ubiquitous in condensed matter systems and are responsible for many emergent phenomena. Recently a new mesoscopic spin texture called skyrmion with radius about 10 $\sim$ 100 nm was discovered experimentally in certain conducting as well as insulating chiral magnets. In the temperature-magnetic field phase diagram, skyrmions form a triangular lattice in the low temperature and intermediate magnetic field regime in thin films. Because of the low dissipation and the existence of magnetoelectric coupling, skyrmions in insulators have attracted considerable interests. In this work, we show that a thermal gradient can be used to move magnetic skyrmions in insulating chiral magnets: the induced magnon flow from the hot to the cold region drives the skyrmions in the opposite direction via a magnonic spin transfer torque. We also show that a temperature gradient induces an ac electric current in multiferroic insulators when the sample is embedded in a circuit. Both results are combined to compute the effect of skyrmion motion on the ac current generation. We demonstrate that skyrmions in insulators are a promising route for spin caloritronics applications. [Preview Abstract] |
Tuesday, March 4, 2014 1:27PM - 1:39PM |
G48.00010: Magnetization process of chiral magnet: creation and annihilation of skyrmions and anti-skyrmions Wataru Koshibae, Naoto Nagaosa By applying a magnetic field to the single-$q$ helical state in the chiral magnet, the skyrmions appear. The topology of the skyrmion is characterized by the skyrmion number $N_{sk}$ defined as $N_{sk} = \int \frac{d^2 r}{4 \pi} {\vec n}_{\vec r} \cdot [ ({ \partial {\vec n}_{\vec r}}/{\partial x})\times ({ \partial {\vec n}_{\vec r}}/{\partial y}) ],$ where ${\vec n}_{\vec r}$ is the unit vector along magnetic moment at ${\vec r}$, assuming the two-dimensional configuration. The single-$q$ helical state is a topologically trivial magnetic texture, i.e., $N_{sk}=0$. Therefore, within the continuous deformation, there is no way to realize the skyrmions from the single-$q$ helical state. We find, by a numerical simulation of Landau-Lifshitz-Gilbert equation, the pair nucleation of skyrmion and anti-skyrmion occurs, and they annihilate to reach the skyrmion crystal-like state or the ferromagnetic state. We show the lives of skyrmion and anti-skyrmion in the dynamics of a quenched chiral magnet. [Preview Abstract] |
Tuesday, March 4, 2014 1:39PM - 1:51PM |
G48.00011: Skyrmions: a showcase for non-Newtonian kinematics Aron Beekman, Naoto Nagaosa Consisting of hundreds or thousands of spins, skyrmions in magnets can nevertheless be regarded as individual particles that keep their identity due to topological protection. These particles interact with externally imposed waves like electric current or magnons. We have recently shown that they do this in a completely counterintuitive, non-Newtonian way. For instance, elastic scattering causes the skyrmion to move in the direction opposite to the incoming wave. The underlying reason is that the skyrmion momentum is descendant from the ferromagnetic dynamics, such that the skyrmion center of mass coordinates are each other's canonical conjugates. Here we argue that this is a general feature of dynamics of excitations in media with broken time-reversal symmetry, and is strongly related to the existence of Berry phases. [Preview Abstract] |
Tuesday, March 4, 2014 1:51PM - 2:03PM |
G48.00012: Topological Hall Effect in Skyrmions: A Nonequilibrium Coherent Transport Approach Gen Yin, Jiadong Zang, Roger Lake Skyrmion is a topological spin texture recently observed in many materials with broken inversion symmetry. In experiments, one effective method to detect the skyrmion crystal phase is the topological Hall measurement. At adiabatic approximation, previous theoretical studies show that the Hall signal is provided by an emergent magnetic field, which explains the topological Hall effect in the classical level. Motivated by the potential device application of skyrmions as digital bits, it is important to understand the topological Hall effect in the mesoscopic level, where the electron coherence should be considered. In this talk, we will discuss the quantum aspects of the topological Hall effect on a tight binding setup solved by nonequilibrium Green's function (NEGF). The charge distribution, Hall potential distribution, thermal broadening effect and the Hall resistivity are investigated in detail. The relation between the Hall resistance and the DM interaction is investigated. Driven by the spin transferred torque (SST), Skyrmion dynamics is previously studied within the adiabatic approximation. At the quantum transport level, this talk will also discuss the non-adiabatic effect in the skyrmion motion with the presence of the topological Hall effect. [Preview Abstract] |
Tuesday, March 4, 2014 2:03PM - 2:15PM |
G48.00013: Spin-Orbit Torques and Anisotropic Magnetization Damping in Skyrmion Crystals Kjetil Hals, Arne Brataas We theoretically study the effects of reactive and dissipative homogeneous spin-orbit torques and anisotropic damping on the current-driven skyrmion dynamics in cubic chiral magnets. Our results demonstrate that spin-orbit torques play a significant role in the current-induced skyrmion velocity. The dissipative spin-orbit torque generates a relativistic Magnus force on the skyrmions, whereas the reactive spin-orbit torque yields a correction to both the drift velocity along the current direction and the transverse velocity associated with the Magnus force. The spin-orbit torque corrections to the velocity scale linearly with the skyrmion size, which is inversely proportional to the spin-orbit coupling. Consequently, the reactive spin-orbit torque correction can be the same order of magnitude as the non-relativistic contribution. More importantly, the dissipative spin-orbit torque can be the dominant force that causes a deflected motion of the skyrmions if the torque exhibits a linear or quadratic relationship with the spin-orbit coupling. In addition, we demonstrate that the skyrmion velocity is determined by anisotropic magnetization damping parameters governed by the skyrmion size. [Preview Abstract] |
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