Bulletin of the American Physical Society
APS March Meeting 2018
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session S19: Magnetic Nanoparticles: Dynamics, Domains and VorticesFocus

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Sponsoring Units: GMAG DMP Chair: Junjia Ding, Argonne National Laboratory Room: LACC 308A 
Thursday, March 8, 2018 11:15AM  11:27AM 
S19.00001: Mechanics of magnetic solitons Oleg Tchernyshyov The dynamics of magnetization in a ferromagnet is governed by the LandauLifshitz equation, a nonlinear PDE that is rarely solvable exactly. To approximate soliton dynamics, we introduced a method of collective coordinates [1] generalizing Thiele's rigidtranslation approximation [2] by including arbitrary deformations. The method has been applied to a variety of solitons (domain walls, skyrmions, vortices), extended to include the effects of spintransfer torque, and adopted to antiferromagnetic solitons [3]. I will describe our recent applications of the method to several new problems: an extended domain wall in a thin ferromagnetic film behaves as a nonreciprocal string with transverse waves propagating on it with unequal speeds; annihilation of a vortexantivortex pair in a thin film; and the propulsion of antiferromagnetic solitons under combined magnetic field and spinpolarized current. 
Thursday, March 8, 2018 11:27AM  11:39AM 
S19.00002: Annihilation of VortexAntivortex Pairs in Thin Ferromagnetic Films Derek Reitz , Oleg Tchernyshyov We study theoretically the annihilation of a vortexantivortex pair in a twodimensional easyplane ferromagnet. The motion is governed by the interplay of three types of forces: attraction between the vortices mediated by exchange interaction, a gyroscopic force proportional to the vortex's velocity and orthogonal to it, and viscous forces. For opposite outofplane orientations of the vortex cores, the two solitons orbit each other. In the presence of finite damping, the radius of the orbit gradually decreases until the cores overlap. We show that the viscous forces involve several components. In addition to the expected viscous force proportional to the vortex's velocity and opposing it, there is mutual drag exerted by vortices on each other. In addition, image vortices, created by the sample edge, also generate appreciable drag on the real ones. We compare predictions of analytical theory to results of micromagnetic simulations. 
Thursday, March 8, 2018 11:39AM  11:51AM 
S19.00003: Ferromagnetic domain wall as a nonreciprocal string Shu Zhang , Oleg Tchernyshyov The collective coordinate method has been a great success in analyzing the dynamics of pointlike topological defects, such as vortices in two dimensions and a domain wall in one dimension. To study a domain wall in a twodimensional ferromagnetic film, we generalize the discrete collective coordinates into a continuous field describing the displacement of the wall. The domain wall moves like a string; however, its dynamics differs significantly from the familiar case of a taut string thanks to an additional Berryphase term reflecting the precessional nature of the spins. The Berry phase gives rise to nonreciprocal dynamics, where the waves traveling left and right have different speeds. Given an initial deformation, the nonreciprocal string with free ends exhibits uniform translational motion in addition to oscillations, as if it is selfdriven, which is allowed by the broken timereversal symmetry. This can be understood from the view of the conserved transverse momentum derived from Noether’s theorem. It is then evident how the domain wall evolves in the presence of an external magnetic field and dissipation. 
Thursday, March 8, 2018 11:51AM  12:03PM 
S19.00004: Spintransfer torque, stressenergy tensor, and Magnus force Sayak Dasgupta , Oleg Tchernyshyov In micromagnetics, equations of motion for the magnetic moments m(r,t) are given by the Landau Lifshitz Gilbert equation, which can be equivalently derived from a Lagrangian. The Lagrangian for ferro and antiferromagnets contains terms with a spatial derivative of the order parameter in the presence of a spin carrying electric current, (u●▽)n. Here u is the electron drift velocity proportional to the current density j. In terms of collective coordinates (q) this term appears as a vector potential coupling to u, A(q)●u. It is free of time derivatives and thus appears to be a form of potential energy. But it is explicitly gauge dependent and should not directly represent a physical quantity like a potential energy density. Treating it like one yields incorrect generalized forces and a gauge dependent stress energy tensor. Motivated by this anomaly we outline the procedure to correctly define the energy density and energy flow caused by the electric current interacting with the soliton and provide a conjectural form of a gauge independent stress energy tensor. We illustrate with examples of a 1D ferromagnetic wire and an antiferromagnetic vortex. 
Thursday, March 8, 2018 12:03PM  12:15PM 
S19.00005: Influence of Interplay of Volume and Interface Spin Transfer Torques Affecting Current Induced Domain Wall Transport in Atomically Engineered Multilayered Racetracks Panagiotis Filippou , Jaewoo Jeong , Yari Ferrante , SeeHun Yang , Mahesh Samant , Teya Topuria , Stuart S Parkin Advances in our understanding of the current controlled motion of domain walls in perpendicularly magnetized multilayer stacks has led to very high domain wall velocities. The motion is controlled by several spin orbit derived phenomena including, notably, the Spin Hall Effect, and the DzyaloshinskiiMoriya interaction. Here we study atomically engineered multilayered racetracks in which interface and volume derived spin transfer torques compete with each other. Of especial significance is the dependence of the domain wall motion on the magnitude and direction of in plane magnetic fields that compete with the DzyaloshinskiiMoriya exchange fields. The various contributions to the interface and volume torques are independently varied to allow their respective contributions to be separated. Insight is provided by 1D analytical modeling of the domain wall motion to extract relevant parameters. 
Thursday, March 8, 2018 12:15PM  12:27PM 
S19.00006: Temperature Dependent Magnetization Reversal Process on a Single Magnetic Vortex Sergi Lendinez , Junjia Ding , Tomas Polakovic , John Pearson , Axel Hoffmann , Valentyn Novosad Magnetic vortices can be the ground state of certain ferromagnetic systems. They have been subject of study for the past decade because they are good candidates for a variety of applications. Magnetic vortices can be nucleated in micron and submicron sized soft ferromagnetic disks because geometry constraints. Previous studies have shown the important effect that pinning sites can have in the magnetization reversal process of an ensemble of disks and in the dynamics of single disks. Moreover, low temperature measurements of single disks showed thermal dependence of the nucleation and annihilation fields. However, a detailed study of domain wall motion at low temperatures in a single disk is missing. 
Thursday, March 8, 2018 12:27PM  12:39PM 
S19.00007: Subnanosecond magnetization reversal of magnetic nanoparticle driven by chirp microwave field pulse Md. Torikul Islam , X. S. Wang , Y. Zhang , Xiangrong Wang We investigate the magnetization reversal of singledomain magnetic nanoparticle driven by linear downchirp microwave magnetic field pulse. Numerical simulations based on the LandauLifshitzGilbert equation reveal that solely downchirp pulse is capable of inducing subnanosecond magnetization reversal. With certain range of initial frequency and chirp rate, the field amplitude required is much smaller than that of constant frequency microwave field. The fast reversal is because the downchirp microwave field acts as energy source and sink for the magnetic particle before and after crossing over the energy barrier respectively. Applying a spinpolarized current additively to the system further reduces the microwave field amplitude. Our findings provide a new way to realize lowcost and fast magnetization reversal. 
Thursday, March 8, 2018 12:39PM  12:51PM 
S19.00008: Chiral Edge Mode in the Coupled Dynamics of Magnetic Solitons in a Honeycomb Lattice Se Kwon Kim , Yaroslav Tserkovnyak Motivated by a recent experimental demonstration of a chiral edge mode in an array of spinning gyroscopes, we theoretically study the coupled gyration modes of topological magnetic solitons, vortices and magnetic bubbles, arranged as a honeycomb lattice [1]. The soliton lattice under suitable conditions is shown to support a chiral edge mode like its mechanical analogue, the existence of which can be understood by mapping the system to the Haldane model for an electronic system. The direction of the chiral edge mode is associated with the topological charge of the constituent solitons, which can be manipulated by an external field or by an electriccurrent pulse. The direction can also be controlled by distorting the honeycomb lattice. Our results indicate that the lattices of magnetic solitons can serve as reprogrammable topological metamaterials. 
Thursday, March 8, 2018 12:51PM  1:03PM 
S19.00009: Influence of Domain Wall Pinning Centers on Stabilization of Vortex State in Cobalt Nanorings Manohar Lal , S. Sakshath , PS Anil Kumar At the mesoscopic scale, magnetostatic interactions play an important role in the equilibrium magnetic domain configuration of magnetic elements. The nanoring geometry has attracted much attention because it can exhibit a vortex state (stray field free), which ensures lower magnetostatic interactions between adjacent ring elements in high packing density memory devices. We fabricated cobalt nanorings in contact with a nanowire whose free end is attached to a square pad and performed the magnetoresistance (MR) measurements by applying an inplane magnetic field (Η). We show that the vortex state can be stabilized in this engineered device. Easy nucleation of domain walls (DWs) results in a decrease of switching field corresponding to the reversal of the nanowire. This leads to an increase in the range of fields, where the vortex state exists. In addition, angular dependence of the switching behaviour indicates that the vortex state can be stabilized at all inplane orientations of Η. The results of our micromagnetic simulations are in a good agreement with the experimental results. 
Thursday, March 8, 2018 1:03PM  1:15PM 
S19.00010: Micromagnetic simulations for modulated permalloy cylindrical nanowires Eugenio Vogel , Guidobeth Sáez , Pablo Díaz , Eduardo Cisternas , Juan Escrig , J.P. Burr , Eduardo Saavedra Modulated magnetic nanowires attract the attention of both experimental and theoretical 
Thursday, March 8, 2018 1:15PM  1:27PM 
S19.00011: Coercive field enhancement in singledomain to vortex switching in magnetic Co nanodisks Ciro Fernando Gélvez Pedroza , Edgar Patino The impressive magnetic properties at the nanoscale have given special attention to the magnetic nanodisks (ND). Among the most prominent applications stand data storage or magneto plasmonic devices. For the present work, we have focused in Cobalt (HCP), using the colloidal lithography technique we have obtained circular ND varying the Cobalt layer from 515nm and diameters of 170nm. Here we have studied experimentally, by vibrating sample magnetometry (VSM). Theoretically studies of nanodisks by micromagnetic simulations (OOMMF) have been performed in a wide range of thicknesses between 515 nm and diameters between 100500 nm. From the hysteresis loop measurements, the coercive field as a function of thickness shows an unexpected peak at a critical thickness. Simulations predict the appearance of such peak and indicate that is strongly dependent on the dimensions of the Co layer. The fabrication and subsequent magnetic measurements confirm those results, such as the presence of magnetic single domain and vortex states in the ND. Finally the critical thickness where the hysteresis loop shows the highest coercive field turns out to be the value of thickness which separates the single domain state from the vortex state, due to dipolar exchange energies, and shape anisotropy. 
Thursday, March 8, 2018 1:27PM  1:39PM 
S19.00012: Magnetization switching by manipulating the thickness of FM&HM layers Byong Sun Chun , Dongseuk Kim , Changsoo Kim , Dae Hyun Kim , Kyoung Woong Moon , Chanyong Hwang Magnetization switching with spin–orbit torque has attracted lots of attention due to its fast switching characteristics and lowpower consumption. Increasing the conversion efficiency from charge current to spin current is one of hot topics in this area. We examine a method to find the optimum layer thickness of the ferromagnet and of the normal metal for efficient magnetization switching. By controlling the thickness of the ferromagnet and of the normal metal, we can change the sign and magnitude of the overall fieldlike torque, which enables faster switching. Our results suggest a method for fast switching in spinorbit torqueinduced cases, where the film thickness of the ferromagnet and the normal metal are controllable parameters to provide an efficient way of implementing spinorbitronics devices. 
Thursday, March 8, 2018 1:39PM  1:51PM 
S19.00013: YIG/Permalloy spin torque oscillator Bassim Arkook , Chris Safranski , Rodolfo Rodriguez , Xian Wang , Houchen Chang , Mingzhong Wu , Ilya Krivorotov , Igor Barsukov We grow 20 nm thick epitaxial YIG films on GGG substrates and deposit 5 nm Py capping layers, using sputter deposition. Nanowires of 280 nm width and 2.2 μm length are fabricated using singlestep ebeam lithography and ionmilling. We investigate the spinwave spectrum of the nanowires using spintorque ferromagnetic resonance and micromagnetic simulations. By applying a direct bias current to the nanowire, magnetic damping can be tuned by injecting spin current from the Py layer. We discover that with increasing bias current, the damping can be fully compensated in this system. Above a critical current, the nanowire enters the autooscillatory regime and emits a microwave signal. We find that using Py as a spin injector allows us to achieve microwave emission power levels (pWscale) that significantly exceed those observed in the YIG/Pt systems (fWscale). We discuss the physics of currentdriven autooscillatory dynamics, spinorbit and thermal spin torques in nanodevices based on bilayer ferromagnets. 
Thursday, March 8, 2018 1:51PM  2:03PM 
S19.00014: Magnonic Architectures for Quantum Circuits and Measurements. Alexy Karenowska , Arjan Van Loo , Sandoko Kosen , Richard Morris The development of 2 and 3dimensional circuit architectures for experiments at millikelvin temperatures aimed at exploring and exploiting magnonic systems at the quantum level is fast becoming an established area of magnetics research. Magnons are readily excited over the frequency range used by established solidstate quantum circuit technology and couple readily to EM fields. These facts, in combination with the fieldtunable dispersion of the excitations make them an interesting proposition in the context of quantum device design. In this talk, we discuss recent progress made in the combination of planar superconducting circuit technology with magnon systems. We discuss the technical and materials requirements of successful magnonic experiments at millikelvin temperatures. We go on to describe experimental investigations including the study of spinwave propagation in magnetic waveguides at the single magnon level [1, 2], the investigation of magnon modes in spherical magnetic resonators [3], the measurement of magnonic crystals, and the development of systems incorporating Josephson junction based qubits. 
Thursday, March 8, 2018 2:03PM  2:15PM 
S19.00015: Relation between switching time distribution and damping constant JungHwan Moon , Tae Young Lee , ChunYeol You Magnetization dynamics is strongly influenced by the damping process, which is energy dissipation of the magnetization. Especially for spintransfer torque magnetic randomaccess memories (STTMRAMs) applications, the critical current density for magnetization switching is strongly affected by the damping constant because the spintransfer torque must overcome the damping torque for the switching. However, it is not easy to determine the damping constant of nanoscale patterned samples. 
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