Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session G30: Focus Session: Skyrmions III |
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Sponsoring Units: GMAG DMP FIAP Chair: Sujoy Roy, Lawrence Berkeley National Laboratory Room: 206B |
Tuesday, March 3, 2015 11:15AM - 11:27AM |
G30.00001: Tailoring magnetic skyrmions at transition-metal interfaces Bertrand Dup\'e, Markus Hoffman, Charles Paillard, Stefan Heinze Skyrmions in magnetic materials offer attractive perspectives for future spintronic applications [1] since they are localized, topologically stabilized spin structures which can be manipulated at electric current densities which are by orders of magnitude lower than those required for moving domain walls. Recently, it has been discovered that due to the broken inversion symmetry at surfaces magnetic skyrmions can also occur in ultra-thin transition metal films at surfaces [2,3]. Here, we use first-principles electronic structure theory to show how transition-metal interfaces can be modified such that they exhibit skyrmion phases and to explain the observation of individual skyrmions in an ultra-thin film composed of Pd and Fe on the Ir(111) surface [3,4]. We determine the magnetic interactions in this system using density functional theory and explain the occurrence of skyrmion phases in an external magnetic field using Monte-Carlo simulations. Our work paves the way to tailor the properties of skyrmions at transition-metal interfaces. \\[4pt] [1] A. Fert et al Nature Nanotech. {\bf 8}, 152 (2013). \\[0pt] [2] S. Heinze et al Nature Phys. {\bf 7}, 713 (2011). \\[0pt] [3] N. Romming et al Science {\bf 341}, 636 (2013). \\[0pt] [4] B. Dup\'e et al Nature Comm., {\bf 5}, 4030 (2014). [Preview Abstract] |
Tuesday, March 3, 2015 11:27AM - 11:39AM |
G30.00002: Oxygen-enable control of Dzyaloshinskii-Moriya Interaction in Fe/Ir(001) bilayers Abderrezak Belabbes, Gustav Bihlmayer, Stefan Bl\"ugel, Aurelien Manchon Using relativistic first principles calculations, we demonstrate that the magnitude and sign of the Dzyaloshinskii-Moriya inter action(DMI) of Fe/Ir(001) interface can be controlled by tuning the coverage of the oxygen capping layer, which changes the spin-wave length and the depth of the energy minimum. In addition, we explain how the magnetic interactions [Exchange inter action, DMI, and the magnetocrystalline anisotropy (MAE)]at such transition metal interface are modified in the presence of Oxygen, which might prevent any stable magnetic order due to the small energy scale. We observe a change sign of the DMI when the coverage exceeds 50\%. In particular, we found that due to the C4 breaking symmetry and the large spin-orbit interaction of the Ir substrate the DMI exceeds a critical strength and competes with the exchange interaction and causes homochiral magnetic structures. This study reveals that in realistic systems capped by an oxide, such as HM/F/MOx (HM is a heavy metal, F a ferromagnet an MOx=MgOx, CoOx, TaOx etc., the DMI can be tuned by changing the oxidation conditions of the capping layer, offering a convenient way to control it. Therefore, understanding of the these phenomena may have impact in the context of facilitating applications in spintronics. [Preview Abstract] |
Tuesday, March 3, 2015 11:39AM - 11:51AM |
G30.00003: Imprinting topological domain structure in epitaxial Ni/Fe/Co/Cu(001) Ali Tan, Jia Li, Zi Qiang Qiu, Elke Arenholz, Andreas Scholl, Chanyong Hwang A vortex state can be stabilized in magnetic thin films by reducing the lateral dimension of the thin film such that the shape anisotropy imposes flux-closure on the magnetic domains. In the language of skyrmions, a vortex state has a topological skyrmion charge Q $= \pm 1/2$, with vorticity w $= +$1 and helicity $\gamma = \pm \pi /2$. By tuning the interlayer coupling strength, various domain structures can be imprinted on an adjacent ferromagnetic layer. We investigated domain imprinting by cobalt (Co) vortices on nickel (Ni) layer through a face-centered-cubic (fcc) iron (Fe) interlayer in a Ni/Fe(wedge)/Co(disks)/Cu(001) trilayer system. Using element-specific X-ray Magnetic Circular Dichroism, we observed a strong antiferromagnetic IEC for 5 ML thick Fe interlayer. From the domain images of each elements obtained using Photoemission Electron Microscopy (PEEM), we observed that the relative strength of the bilinear and biquadratic exchange coupling changes as a function of Fe interlayer thickness, leading to non-collinear coupling between Ni and Co around 5.5 ML of Fe. The resulting Ni domain structures have topological skyrmion charge Q $= \pm 1/2$, with vorticity w $= +$1 but varying helicity $\gamma $. [Preview Abstract] |
Tuesday, March 3, 2015 11:51AM - 12:03PM |
G30.00004: Biskyrmion bubble lattice in Fe/Gd alloy thin films James Lee, Xiaowen Shi, Jordan Chess, Sergio Montoya, Shrawan Mishra, Lev Sakharov, Daniel Parks, Ben McMorran, Steven Kevan, Eric Fullerton, Sujoy Roy Magnetic bubbles with topologically non-trivial twists, called ``skyrmion bubbles,'' exhibit particle-like properties and novel magnetic interactions with each other. They are seen in non-centrosymmetric crystals, such as MnSi, and monolayers of Fe on Ir(111) substrates. Our study considers whether skyrmion bubbles can also form in soft ferrimagnetic alloys with perpendicular anisotropy. Using resonant x-ray scattering at the Fe L$_3$ and Gd M$_5$ transition edges, we show that triangular lattices of skyrmion bubbles form in Fe/Gd thin films in a limited temperature and magnetic field range. Uniaxial anisotropy in the resonant scattering pattern indicates the lattice unit cell contains two skyrmions. Lorentz TEM images reveal that the repeating unit is a bound pair of bubbles called \textit{biskyrmions}. Adjusting the composition of the films can shift the temperature range of the biskyrmion lattice by 100 K, allowing the lattice to form at room temperature. Fe/Gd thin films may prove a promising material for spintronics. [Preview Abstract] |
Tuesday, March 3, 2015 12:03PM - 12:15PM |
G30.00005: Investigation of the Stripe-Bubble Phase in La2-2xSr1+2xMn2O7(x=0.32) by Magnetic Force Microscopy Juyoung Jeong, Jinho Yang, Yunwon Kim, Ilkyu Yang, Jianshi Zhou, Neliza Leon, Roman Movshovich, Alex de Lozanne, H.W. Yeom, J.B. Goodenough, Nestor Haberkorn, Jeehoon Kim We constructed a home-built low temperature magnetic force microscope (LTMFM) with the fiber interferometer detection scheme: The base temperature is 4 K and magnetic field range is up to 6 T. The MFM system is applied to investigate unconventional magnetism of La$_{2-2x}$Sr$_{1+2x}$Mn$_2$O$_7$(x=0.32) in a wide span of temperature and magnetic field. We imaged spin reorientation transition and found two types of bubble domains. The behavior of bubble domains shows the same upon field cycle, indicating a reversible magnetization property reported previously in the bulk magnetization measurement. The origin of the two types of the bubble domains will be discussed by comparing MFM and bulk magnetization data. [Preview Abstract] |
Tuesday, March 3, 2015 12:15PM - 12:27PM |
G30.00006: Metastable multi-domain state in ultrathin films with Dzyaloshinskii-Moriya Interaction Parnika Agrawal, Seonghoon Woo, Geoffrey Beach Helical spin structures such as skyrmions and chiral domain walls are stabilized in magnetic films with strong Dzyaloshinskii Moriya interaction (DMI) [1,2]. The chiral spin state is the ground state when the ratio of the effective DMI field to anisotropy field is greater than 2/pi [3]. However, even when the DMI is too weak to generate a chiral ground state, such states can be metastable if the uniform state is appropriately perturbed [1,4].Here, we show that an in-plane applied field reduces the energy barrier for domain wall formation, and provides a simple technique to generate a multidomain state in uniform magnetic films. Further, we identify that the threshold between the stable single-domain state and the metastable multidomain state can be controlled by two parameters-demagnetizing energy and geometrical confinement. We use these parameters to create isolated geometrically confined magnetic bubbles in patterned discs of Pt/Co/GdOx multilayers. These bubbles may provide insight into the mechanism of creation of skyrmions in magnetic thin films with strong DMI.\\[4pt] [1] A. Fert et al., Nat. Nanotech., 8, 152-156 (2013);\\[0pt] [2] S. Emori et al., Nature Materials 12,61-616 (2013);\\[0pt] [3] S. Emori, et al., arXiv:1308.1432v2(2013);\\[0pt] [4] N. Perez, et al., Appl. Phys. Lett.104, 092403(2014) [Preview Abstract] |
Tuesday, March 3, 2015 12:27PM - 1:03PM |
G30.00007: Tailoring the topology of an artificial magnetic skyrmion Invited Speaker: Jia Li A skyrmion is a topological twist of a continuous field that was first proposed by Skyrme to describe discrete nucleons. In condensed-matter physics, skyrmions emerge as topological invariant spin textures in a two dimensional Heisenberg spin lattice. Evidence of skyrmions in condensed matter physics appeared after the discovery of the Quantum Hall Effect in which the lowest energy charged excitations can be mapped onto two-dimensional magnetic skyrmion states. Despite theoretical predictions, it remains an experimental challenge to realize an artificial magnetic skyrmion whose topology can be well controlled and tailored so that its topological effect can be revealed explicitly in a deformation of the spin textures. Here we report epitaxial magnetic thin films in which an artificial skyrmion is created by embedding a magnetic vortex into an out-of-plane aligned spin environment. By changing the relative orientation between the central vortex core polarity and the surrounding out-of-plane spins, we are able to control and tailor the system between two skyrmion topological states. An in-plane magnetic field is used to annihilate the skyrmion core by converting the central vortex state into a single domain state. Our result shows distinct annihilation behaviour of the skyrmion core for the two different skyrmion states, suggesting a topological effect of the magnetic skyrmions in the core annihilation process. [Preview Abstract] |
Tuesday, March 3, 2015 1:03PM - 1:15PM |
G30.00008: Interlayer Exchange Coupling: A route to stabilize skyrmions in magnetic multilayers Ashis Kumar Nandy, Nikolai S. Kiselev, Stefan Bl\"ugel ~ Magnetic skyrmion is a topologically nontrivial spin texture with particle like properties, which may emerge under an appropriate applied magnetic field in any magnetic thin layer or multilayer with surface or interface induced Dzyaloshinskii-Moriya interaction. However, magnetic fields required to stabilize skyrmions can be extremely large. We present an approach, which allows the stabilization of skyrmions in such magnetic multilayers even at zero magnetic field. It is based on fine-tuning the interplay between internal and interfaces induced interactions by adjusting the thicknesses and interface compositions of multilayers. Our multiscale approach is based on DFT calculations and atomistic spin dynamic simulations, which predicts the existence of a skyrmion lattice and isolated skyrmions in a thin film of a transition-metal monolayer grown on a heavy metal substrate. The simulated skyrmions exhibit high stability in an applied magnetic field and temperature. We provide a description for the complex phases occurring in such systems and present a magnetic phase diagram for a prototype example of Mn/W(001). [Preview Abstract] |
Tuesday, March 3, 2015 1:15PM - 1:27PM |
G30.00009: Novel rare-earth free magnetic nanostructures Bhaskar Das, Balamurugan Balasubramanian, Pinaki Mukherjee, Priyanka Manchanda, Ralph Skomski, George Hadjipanayis, David Sellmyer Magnetic nanostructuring including stabilization of novel structures without critical elements, easy-axis alignment, and self-assembly are important for creating new magnetic materials. We use a single-step process in a cluster-deposition system to fabricate rare-earth-free nanostructured magnetic materials with potential for high-energy magnet or spintronics applications.\footnote{B. Balamurugan, B. Das, D. J Sellmyer et al., \textit{Advanced Materials}, \textbf{25}, 6090 (2013).} For example, Mn$_{5}$Si$_{3}$ nanoclusters of diameter about 8 nm form hexagonal D8$_{8}$ structure and show strong ferromagnetic properties with a high $T_{c} =$ 590 K, an appreciable $K_{1} =$ 11 Mergs/cm$^{3}$, and a high $J_{s} = $12.5 kG. This result is in a sharp contrast to the antiferromagnetic ordering observed in bulk Mn$_{5}$Si$_{3}$ with $T_{N} =$ 100 K, and is supported by DFT calculations. On the other hand, MnSi nanoclusters form B20-type cubic crystal structure and are ferromagnetic below T $=$ 25 K. Skyrmion-type spin structures have been observed in MnSi thin films and evidence for such structures in nanoclusters will be discussed. [Preview Abstract] |
Tuesday, March 3, 2015 1:27PM - 1:39PM |
G30.00010: Growth and Properties of Skyrmionic MnSi Nanowires and Thin Film on Silicon Zheng Gai, Jieyu Yi, Siwei Tang, Ivan.I. Kravchenko, Guixin Cao, David Mandrus Magnetic skyrmion lattice, a vortex-like spin texture recently observed in chiral magnets, is of great interest to future spin-electronic data storage and other information technology applications. The skyrmion lattice in MnSi appears in a small region (known as the A phase) of the H-T phase diagram in bulk samples, but in 2D samples like thin films the skyrmion phase is much more robust. If skyrmion ordering can persist in one-dimensional MnSi nanowires and 2D films, then these systems may be very promising for spintronics applications as the magnetic domains and individual skymions could be manipulated with small currents. We have systematically explored the synthesis of single crystal MnSi nanowires via controlled oxide-assisted chemical vapor deposition and observed a characteristic signature of skyrmion magnetic ordering in MnSi nanowires. The SiO$_{2}$layer pla ys a key role for the high yield, correct stoichiometric and crystalline growth of the B20 MnSi nanowires. A growth phase diagram was constructed. For the thin films, an unique growth receipt was developed for the growth of high quality of thin films. The structure and magnetic properties of the films at different thickness were studied. [Preview Abstract] |
Tuesday, March 3, 2015 1:39PM - 1:51PM |
G30.00011: Manipulating Topological States by Imprinting Non Collinear Spin Textures Peter Fischer, Robert Streubel, Luyang Han, Mi-Young Im, Florian Kronast, Ulrich K. Roessler, Florin Radu, Radu Abrudan, Gungun Lin, Oliver G. Schmidt, Denys Makarov Topological magnetic states, such as chiral skyrmions, are of great scientific interest and show huge potential for novel spintronics applications, provided their topological charges can be fully controlled. So far skyrmionic textures have been observed in noncentrosymmetric crystalline materials with low symmetry and at low temperatures. We propose theoretically and demonstrate experimentally the design of spin textures with topological charge densities that can be tailored at ambient temperatures. Tuning the interlayer coupling in vertically stacked nanopatterned magnetic heterostructures, such as a model system of a Co/Pd multilayer coupled to Permalloy, the in-plane non-collinear spin texture of one layer can be imprinted into the out-of-plane magnetised material. We observe distinct spin textures, e.g. vortices, magnetic swirls with tunable opening angle, donut states and skyrmion systems of Dn symmetry. We show that applying a small magnetic field, a reliable switching between topologically distinct textures can be achieved at remanence. [Preview Abstract] |
Tuesday, March 3, 2015 1:51PM - 2:03PM |
G30.00012: Dzyaloshinki-Moriya interaction mediated by combined exchange and Rashba bands Anirban Kundu, Shufeng Zhang Domain wall structure determined by the competition among exchange, anisotropy, and magnetostatics does not have a preferred wall chirality, i.e., the clockwise and anti-clockwise spin rotations of the domain wall are equally probable. Dzyaloshinskii-Moriya interaction (DMI) has been identified as the dominant mechanism for the observed chiral domain walls in ultrathin ferromagnetic CoNi films.\footnote{Gong Chen {\em et al}., Nature Communication {\bf 4}, 2671 (2013).} We show that the DMI arises from the interplay between the ferromagnetic exchange coupling and the interface Rashba spin-orbit coupling; these two couplings generally exists for ultrathin films with perpendicular magnetic anisotropy. The DMI displays an oscillatory dependence on the distance of two magnetic ions. In the limit that the Rashba coupling is much smaller than the exchange coupling, the strength of the DMI is linear with respect to the Rashba coupling. In the opposite limit, the DMI depends on the quadratic Rashba coupling. We apply our results to study the chiral dependence of the domain walls. In particular, we quantitatively relate the Rashba coupling strength to the wall structure and map out the preferred Neel or Bloch walls with definite chirality. The results agree with the experiment.\footnote{Chen {\em et al}.} [Preview Abstract] |
Tuesday, March 3, 2015 2:03PM - 2:15PM |
G30.00013: Observation of Skyrmions in Mn-Pt-Sn based Heusler material Ajaya Kumar Nayak, Roshnee Sahoo, Johannes Wild, Daniel Ebke, Josef Zweck, Stuart Parkin, Claudia Felser Mn2YZ based Heusler materials often exhibit collinear spin alignment[1]. However, in a recent work we found Mn2RhSn displays a non-collinear magnetic structure[2]. The presence of noncollinear spin alignment with lack of inversion symmetry sets up a perfect condition for skyrmions in Mn2RhSn. However, experimentally it is still to be verified. To find skyrmions in Heusler system we search for materials with large spin-orbit coupling and lack of inversion symmetry. Here we present the experimental evidence of skyrmions in Mn-Pt-Sn Heusler compound. This sample shows an order-disorder transition around 400 K, followed by a spin-reorientation transition at low temperature. Our magnetization and ac-susceptibility measurements display a similar type of behavior that generally found in the skyrmion system. We have performed high resolution Lorentz transmission electron microscopy measurements to get a direct evidence of skyrmions in Mn-Pt-Sn sample. Our results show evolution of skyrmions in particular fields and temperature intervals.\\[4pt] [1] A. K. Nayak, et al., Phys. Rev. Lett. 110, 127204 (2013).\\[0pt] [2] O. Meshcheriakova, et al., Phys. Rev. Lett. 113, 08720(2014). [Preview Abstract] |
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