Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session W14: Focus Session: Magnetic Domains 
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Sponsoring Units: GMAG DMP Chair: Andrew Balk, National Institute of Standards and Technology Room: 316 
Thursday, March 21, 2013 2:30PM  2:42PM 
W14.00001: Observation and Control of exotic magnetic domain structures in ferromagnetic CeRu$_{2}$Ga$_{2}$B Jeehoon Kim, R. Baumbach, O. AyalaValenzuela, K. Barros, K. Yasuyuki, I. Martin, L. Civale, E. Bauer, F. Ronning, J. D. Thompson, R. Moshovich The structure of magnetic domains in a single crystal of CeRu$_{2}$Ga$_{2}$B was investigated using lowtemperature magnetic force microscopy (MFM) over a wide range of fields and temperatures. The low Curie temperature (T$_{C}\approx $16 K) allows for extensive tunability, revealing~a rich variety of magnetic states including branched stripes, bubble domains, and fingerlike domains. In addition to~the higher~spatial resolution, the advantage of MFM over optical imaging techniques~is~the~ability~to~manipulate~magnetic domains. In particular, we are able to manipulate (move and destroy) individual circular domains~with~the MFM tip, which suggests that we observe unusual spherical `bubble' domains (as opposed to~cylindrical ones, with round terminations~at the surface). Our results~clarify the origins and illustrate the diversity of the domain structures in nearly ferromagnetic compounds. [Preview Abstract] 
Thursday, March 21, 2013 2:42PM  2:54PM 
W14.00002: Distribution of nonadiabatic and adiabatic torques in domain wall systems Elizabeth Golovatski, Michael Flatt\'e Spin torque and the subsequent motion of domain walls caused by coherent carrier transport[1] is an important aspect in the development of spintronic devices[2]. We model spin torque in N\'eel walls[3] in various configurations using a piecewise linear transfermatrix method[4] and calculate the spin torque distribution[5] throughout the system. We find a large nonadiabatic component to the spin torque throughout the system, that oscillates with position in the wall, as if it would introduce an outofplane twist in the magnetization. This twisting effect is especially pronounced in a domain sandwiched between two domain walls, where the nonadiabatic torque increases almost linearly from a large negative value to a large positive value across the domain. We also note differences in the adiabatic torques across materials: ferromagnetic semiconductors have symmetry of the adiabatic torques around the wall center that is lost when considering a magnetic metal. Work supported by an ARO MURI. [1] M. Yamanouchi et al., Nature 428, 539 (2004) [2] S. Parkin et al., Science 320, 190 (2008) [3] G. Vignale and M. Flatt\'e, Phys. Rev. Lett. 89, 098302 (2002) [4] E. Golovatski and M. Flatt\'e, Phys. Rev. B, 84, 115210 (2011) [5] J. Xiao et al., Phys. Rev. B, 73, 054428 (2006) [Preview Abstract] 

W14.00003: ABSTRACT WITHDRAWN 
Thursday, March 21, 2013 3:06PM  3:18PM 
W14.00004: Large scale magnetic domain wall fluctuations in ultrathin cobalt films Andrew Balk, John Unguris Controlling anisotropy through ion bombardment is a convenient method for manipulating domain walls in perpendicularly magnetized films. In ultrathin (\textless 1nm) cobalt deposited on platinum, exposure to 50eV argon ions reduces the perpendicular magnetic anisotropy until the magnetization lies in plane.~ Just before this inplane transition, the domain wall energy and pinning strength are reduced such that zerofield Barkhausenlike domain wall jumps become observable at zero field and room temperature. The domain wall jumps are large enough (\textgreater 100nm) to be measured optically. In this work we use magnetooptic Kerr effect to measure how these fluctuations depend on the film thickness and applied magnetic field. Furthermore, we observe magnetostatic correlations between fluctuations in nearby domain walls. [Preview Abstract] 
Thursday, March 21, 2013 3:18PM  3:30PM 
W14.00005: Voltage Control of Domain Wall Motion in Perpendicular Magnetic Anisotropy Materials Uwe Bauer, Satoru Emori, Geoffrey S. D. Beach Highperformance solidstate operation of a wide variety of spintronic devices requires efficient electrical control of domain walls (DWs). In this work we examine DW dynamics in ultrathin Co films under the influence of an electric field applied across a gadolinium oxide gate dielectric. By measuring the velocity scaling with temperature, driving field, and gate voltage, we verify domain expansion via thermallyactivated creep dynamics. We show that an electric field linearly modulates the activation energy barrier $E_{\mathrm{A}}$ that governs DW creep, leading to an exponential dependence of DW velocity on gate voltage. As a consequence, significant voltageinduced velocity enhancement can be achieved in the lowvelocity regime, but the efficiency is diminished at high velocities where $E_{\mathrm{A}}$ is correspondingly small. We overcome this limitation by engineering novel device structures with significantly larger voltage induced effects on magnetic anisotropy and demonstrate voltage modulation of the DW propagation field by hundreds of Oe. Implementation into magnetic nanowire devices allows us to engineer gate voltage controlled DW traps which are nonvolatile and robustly switchable for many cycles. [Preview Abstract] 
Thursday, March 21, 2013 3:30PM  3:42PM 
W14.00006: Direct Imaging the Thermally Excited Magnon Driven Domain Wall Motion in Magnetic Insulators Wanjun Jiang, Pramey Upadhyaya, Yabin Fan, Jing Zhao, Robert Schwartz, Kang L. Wang Experimental demonstrations of domain wall (DW) motion induced by the thermally excited magnons in YIG are revealed by applying spatial/temporal resolved polar MOKE imaging in the presence of various temperature gradients. These results include: (1) the DW moves from the cold regime towards the hot regime (for both positive and negative temperature gradients); (2) a threshold temperature gradient (5 K/mm), $i$.$e$., a minimal temperature gradient required to induce DW motion; (3) the linear relation of the average DW velocity with the (positive/negative) temperature gradients. Our results suggest that DWs in insulating magnetic materials can be effectively manipulated by a magnonic STT simply by applying small temperature gradients. Further efforts are required to understand this exciting phenomenon, such as quantifying the thermally excited spin wave spin current $J_{m}$, resolving the reflection, and transmission of $J_{m}$ across the DW. Nevertheless, our observations demonstrate that, by incorporating thermal effect into DW engineering, insulating magnetic materials could potentially enable many devices for information processing and other applications in spin caloritronics. [Preview Abstract] 
Thursday, March 21, 2013 3:42PM  3:54PM 
W14.00007: Mechanical manipulation of magnetic domains in continuous and patterned magnetostrictive FeGa thin films Paris Alexander, Sean Fackler, Ichiro Takeuchi, John Cumings The controlled and reversible switching of magnetic domains using static electric fields has been previously demonstrated via magnetoelectric (ME) coupling in a multiferroic system [T. Brintlinger, Nano Lett. 10, 1219(2010)]. In these systems, enhanced magnetostriction allows for magnetic switching in response to an electrically induced deformation. Here we demonstrate the nature of magnetic switching using mechanical stress alone. Magnetostrictive irongallium (Fe$_{\mathrm{70}}$Ga$_{\mathrm{30}})$ thin films are deposited on flexible freestanding membranes, and patterned to square arrays. Using a mechanically manipulated tip a strain is directly applied to the film. We observe the resulting magnetization dynamics using Lorentzforce transmission electron microscopy (LTEM). The varied hysteretic behaviors under applied magnetic and strain fields will be presented for both continuous and patterned films. [Preview Abstract] 
Thursday, March 21, 2013 3:54PM  4:06PM 
W14.00008: Magnoninduced motion of magnetic domain wall in a nanowire with nonuniaxial anisotropy JaeHo Han, HyunWoo Lee Magnons propagating along a nanowire may interact with a magnetic domain wall (DM) and shift the DW position. Often, the DW shift direction is opposite to the magnon propagation direction, which can be explained by the angular momentum conservation when the wire has only uniaxial anisotropy. We studied a nanowire with nonzero perpendicular anisotropy constant, in which the angular momentum conservation argument is broken. Additional to the term comes from the angular momentum conservation, we found new term in the DW shift which comes from the rotation of the DW plane during the magnon pass through the DW. The rotation direction gives DW shift in the opposite direction to the magnon propagation direction, and same for two types of transvers DW: headtohead or tailtotail. The magnitude of this term can be comparable to that comes from the angular momentum conservation when the large perpendicular anisotropy and the small magnon wavelength compare to the DW width. [Preview Abstract] 
Thursday, March 21, 2013 4:06PM  4:18PM 
W14.00009: CurrentInduced Dynamics in Antiferromagnetic Metal: Domain Wall Dynamics and Spin Wave Excitation Ran Cheng, Qian Niu When a spinpolarized current flows through a ferromagnetic (FM) metal, angular momentum is transferred to the magnetization via spin transfer torque. However, corresponding theory is absent in antiferromagnetic (AFM) metals due to the absence of spin conservation. We solve this problem via effective gauge theory without the necessity of spin conservation. By identifying the adiabatic dynamics of conduction electrons as a nonAbelian gauge theory on degenerate band, we derive the AFM version of LandauLifshitzGilbert equation with currentinduced dynamics from a microscopic point of view. Quite different from its FM counterpart, currentinduced dynamics in AFM materials does not behave as a torque, but a driving force triggering second order derivative of local staggered order with respect to time. Its physical consequences are studied in two examples: 1. A domain wall is accelerated to a terminal velocity without a Walker's threshold; 2. A sufficiently large spin current will generate spin wave excitation. [Preview Abstract] 
Thursday, March 21, 2013 4:18PM  4:30PM 
W14.00010: Domain wall propagation through spin wave emission Xiansi Wang, Peng Yan, Yuhua Shen, G.E.W. Bauer, X. R. Wang We theoretically study fieldinduced domain wall (DW) motion in an electrically insulating ferromagnet with hard and easyaxis anisotropies. Different from the common wisdom, we prove that a DW in a dissipationless wire with a finite transverse magnetic anisotropy can propagate along the wire. The DW subjected to an external magnetic field undergoes a periodic transformation that excites SWs. The energy carried away must be compensated by the Zeeman energy that is released by DW propagation. Thus, a domain wall propagation mode through spin wave emission is revealed. The DW propagation locked into the known soliton velocity at low fields. In the presence of small damping, the usual Walker rigidbody propagation mode may become unstable for magnetic fields below the Walker breakdown. [Preview Abstract] 
Thursday, March 21, 2013 4:30PM  4:42PM 
W14.00011: Enhanced controllability of domainwall pinning by asymmetric control of domainwall injection SungMin Ahn Recently, using magnetostatic interactions via the magneticcharge distributions, a few ideas to effectively and selectively manipulate the DW pinning without additional alterations to the nanowire have been suggested. Even though the DW pinning via the magnetostatic interaction is locally controlled, the pinning strength is insufficient to reliably manipulate the DW propagation in the real DWmediated device. Here, it is experimentally studied that depinning fields of domain walls (DWs) under an interaction between magnetic charges distributed at a nanobar and at a notch can be enhanced by controlling injection fields for injecting DWs into the ferromagnetic nanowire with an asymmetrical nucleation pad. The DWs injected from the asymmetrical pad show an asymmetrical dependence of the injection field on the saturation angle and are pinned by the notch with the nanobar vertical to it. We have found that the shape of the pinning potential energy experienced by the DW depends on the magnetized direction of the nanobar and the level of that is lifted by the injection field leading to an increase in the depinning field with respect to the saturation angle. This is consistent with our estimation based on micromagnetic simulation. [Preview Abstract] 
Thursday, March 21, 2013 4:42PM  4:54PM 
W14.00012: Domain Wall Trajectory Determined by its Fractional Topological Edge Defects Aakash Pushp, Timothy Phung, Charles Rettner, Brian Hughes, SeeHun Yang, Luc Thomas, Stuart Parkin A domain wall in a ferromagnetic nanowire is composed of elementary topological bulk and edge defects with integer and fractional winding numbers, respectively. The spatial arrangement of the defects reflects the chiral internal structure of the domain wall. By breaking the symmetry across the width of the nanowire we show that we can control the formation of these topological defects and thereby can form domain walls of a given chirality with high fidelity. Utilizing this capability, we show that the fractional topological edge defects of the domain wall determine its trajectory in branched nanowire networks. Our results can account for the motion of domain walls in complex networks of magnetic nanowires such as ``Artificial Spin Ice'' systems, explaining the formation of Dirac strings, and may also lead to faulttolerant domain wall based memory and logic devices. [Preview Abstract] 
Thursday, March 21, 2013 4:54PM  5:06PM 
W14.00013: Dynamics of topological defects in a 2D magnetic stripe pattern David Venus, Nidal AbuLibdh The magnetic stripe domain patterns formed in perpendicularlymagnetized ultrathin films are one example of pattern formation in 2D systems with shortrange attractive (exchange) and longrange repulsive (dipole) interactions. Topological pattern defects (dislocations) play a key role in the evolution of the pattern. The magnetic susceptibility due to domain wall motion is very sensitive to the presence of the topological defects, and can be used to study their energetics and population dynamics. The total energy density of the domain pattern is altered by the contribution from the concentration of topological defects, changing the average domain density and magnetic ``stiffness'' in a characteristic way. These changes can be directly monitored in the magnetic susceptibility peak, where the peak location and shape can be related quantitatively to the defect concentration. These ideas are confirmed using recently published data for perpendicularlymagnetized Fe/ 2 ML Ni/W(110) films, and allows the extraction of the characteristic time scale, lifetime, and activation energy for the annihilation of topological defects. In addition, it is possible to quantify the proportion of the domain system energy density that is due to the topological defects. [Preview Abstract] 
Thursday, March 21, 2013 5:06PM  5:18PM 
W14.00014: Topological defects and misfit strain in magnetic stripe domains of lateral multilayers with perpendicular magnetic anisotropy Maria Velez, A. HierroRodriguez, R. Cid, G. RodriguezRodriguez, J.I. Martin, L.M. AlvarezPrado, J.M. Alameda Stripe domain patterns are characteristic of magnetic films with perpendicular magnetic anisotropy (PMA). In this work, PMA amorphous NdCo films have been nanostructured with a periodic thickness modulation that induces the lateral modulation of magnetic stripe periods and inplane magnetization. Confinement effects of stripe domains within the nanostructured regions are combined with coupling effects between nearby elements through elastic interactions within the magnetic stripe pattern. The resulting ``lateral'' magnetic superlattice is the 2D equivalent of a strained superlattice controlled by interfacial misfit strain within the magnetic stripe structure and shape anisotropy: misfit dislocations appear in the stripe pattern at the boundaries between nanostructured regions and, during magnetization reversal, a 2D variable angle grain boundary is observed within the magnetic stripe pattern. Beautiful patterns appear at the point of maximum misfit strain due to the decay of dislocations in the magnetic stripe pattern into 1/2 disclination pairs. The link between topological defects in the magnetic stripe patterns and domain walls for the inplane magnetization component allow us to tailor the whole magnetization reversal process. [1] A.HierroRodriguez et al, PRL 109(2012)117202 [Preview Abstract] 
Thursday, March 21, 2013 5:18PM  5:30PM 
W14.00015: Currentdriven domain wall dynamics in ultrathin heavymetal/ferromagnet/oxide submicron strips Satoru Emori, SungMin Ahn, Geoffrey Beach Recent studies have reported efficient currentdriven domain wall (DW) motion and magnetization switching in outofplane magnetized structures consisting of an ultrathin (\textless 1 nm) ferromagnetic Co layer embedded between a heavymetal Pt underlayer and an oxide overlayer such as AlOx [1, 2] and GdOx [3]. These phenomena have been attributed to ``spinorbit'' torques arising from the metaloxide interface (Rashba effect) and in the heavymetal underlayer (spin Hall effect). We investigate currentdriven DW motion in submicronwide strips of ultrathin Ta/CoFe/MgO and Pt/CoFe/MgO. DWs move in the direction of electron flow in Ta/CoFe/MgO, whereas they move against electron flow in Pt/CoFe/MgO. Measurements of the DW propagation field and velocity reveal large spin torque efficiencies exceeding 100 Oe/10$^{\mathrm{11}}$ A/m$^{\mathrm{2}}$ in both structures. Because the signs of the spin Hall angles of Ta and Pt are opposite, the spin Hall effect may partially explain such efficient currentdriven DW motion whose directionality differs with the heavymetal underlayer. [1] I.M. Miron et al, Nat. Mater. 10, 419 (2011). [2] L. Liu et al, arXiv:1110.6846 (2011). [3] S. Emori et al, Appl. Phys. Lett. 101, 042405 (2012) [Preview Abstract] 
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