Session P16: Focus Session: Magnetic Nanostructures, Vortices & Domain Walls

Tuneable remote pinning of domain walls in magnetic nanowires

Domain wall (DW) motion in ferromagnetic nanowires has received much attention for its potential technological applications and for probing fundamental physics. The role of DW pinning in nanowires is crucial for these investigations however it is in general a complex process. Distortions of the DW shape make quantitative agreement between modelling and experiment difficult. Here we demonstrate pinning using nanometre scale localised stray fields. This type of interaction gives well characterised, tailorable potential landscapes that do not appreciably distort the DW. Our experimental results are in excellent quantitative agreement with an Arrhenius-N\'{e}el model of depinning - a result only possible when the modelled potential profile agrees fully with that experienced by the DW.   

Measurements of nanoscale domain wall flexing in a ferromagnetic thin film

We use the anomalous Hall effect to probe the nanoscale behavior of a single magnetic domain wall (DW) in (Ga,Mn)As thin film devices with out-of-plane magnetic anisotropy. Video-rate magneto-optical Kerr microscopy is also used to confirm the variation of the AHE with DW position. Our all-electrical technique allows us to observe a low field flexing regime of DW motion, distinct from the stochastic creep regime that occurs at higher fields. This flexing regime is characterized by a larger DW mobility, linear response to applied field, and non-hysteretic motion which is repeatable within our $\sim 5$ nm experimental resolution. We then analyze the flexing and depinning behavior of the DW to estimate the density and strength of pinning sites. Supported by the ONR MURI program.   

Observation of two step magnetization reversal in Fe$_0.25$TaS$_2$

Understanding magnetic coercivity mechanisms in strong ferromagnets is crucial for new technologies. We studied domain wall pinning in a highly anisotropic ferromagnet of single crystalline Fe$_0.25$TaS$_2$ by utilizing variable temperature magnetic force microscopy (VT-MFM). Magnetic domain structure and the magnetization reversal were investigated in magnetic fields up to 8 tesla at several temperature. Our results revealed the existence of two step magnetization reversal in Fe$_0.25$TaS$_2$. The real space images of magnetic domains, showing this intriguing phenomenon, will be presented.   

Measurement of Annihilation Barriers for Magnetic Vortices

Measurements of the susceptibility of an array of 2 micrometer diameter Permalloy discs are made using the AC magneto-optical Kerr effect. Employing an extended version of the rigid vortex model, saturation magnetization as a function of temperature is extracted from the data. The model also allows extraction of the switching distribution of the array as the discs transition from the vortex state to the quasi single-domain state. Tuning of temperature or sweep rate shows shifts in the distribution peak that confirm vortex annihilation is governed by a thermally activated mechanism. Using the measured saturation magnetization data in conjunction with the measured peak shifts, quantitative extraction of energetic parameters used in semi-empirical models of the annihilation energy barrier is possible. Several models are considered in the context of qualitative observations made in the experiment.   

Pinning Mechanisms for Vortices in Ferromagnetic Films

In ferromagnetic materials, domain wall motion is generally discontinuous and stochastic in the presence of pinning sites. The pinning energy is typically quantified via a single experimental parameter - the coercivity of the hysteresis loop. We show here that in magnetic structures supporting a vortex, the vortex dynamics provide quantitative information about both the strength and range of the interaction between the vortex and individual pinning sites. Using time-resolved Kerr microscopy, we have measured the defect-induced pinning energy and length scales for magnetic vortices in micron-sized NiFe disks. We find that the pinning length scale matches the size of vortex core, and is insensitive to film thickness and growth conditions. This suggests that the dominant mechanism of vortex pinning is directly associated with the core region. The pinning energy however, is strongly dependent on microstructure. Specifically, we observe large pinning energies in NiFe films that have large roughness on lateral length scales commensurate with the core size (10 nm). The dependence of pinning energy on thickness provides further insight into the relative role of surface roughness versus bulk disorder. The strength as well as the spatial distribution of pinning sites suggest that roughness at this length scale is the dominant source of pinning in these films.   

Enhanced current-induced domain wall motion by tuning perpendicular magnetic anisotropy

The effect of perpendicular magnetic anisotropy (PMA) on current-induced domain wall (DW) motion is investigated by micromagnetic simulations. The critical current density Jc to drive DWs into periodic transformation and continuous motion by adiabatic spin transfer torque decreases with increasing PMA. Also, with optimized PMA that almost exactly compensates the demagnetizing field, the adiabatic displacement of DWs driven by currents less than Jc is strongly enhanced. Since PMA can be controlled easily in multilayer films (e.g. Co/Pt), this technique of enhancing current-induced DW motion may be practical for device applications.   

Geometrically Confined Spin Vortices: from Fundamental Physics to Biomedical Applications

The magnetic ground state of magnetically soft thin film ferromagnets in confined geometries (on the micrometer scale) consists of a curling spin configuration, known as a magnetic vortex state. We have recently demonstrated that the magnetic vortex microdisks can be successfully used as multifunctional magnetic carriers for biomedicine [1]. In particular, we will report on successful interfacing of ferromagnetic nanomaterials with a spin vortex ground state and biomaterials (antibody, whole cell). Namely, the gold-coated lithographically defined microdisks with an Fe-Ni magnetic core were biofunctionalized with anti-human-IL13a2R antibody for specifically targeting human glioblastoma cells. When an alternating magnetic field is applied the vortices shift, leading to the microdisks oscillation that causes a mechanical force to be transmitted to the cell. Cytotoxicity assays, along with optical and atomic force microscopy studies, show that the spin vortex-mediated stimulus creates two dramatic effects: (a) membrane disturbance and compromising, and (b) cellular signal transduction and amplification, leading to robust DNA fragmentation and, finally, programmed cell death [2]. The experiments reveals that by employing biofunctionalized magnetic vortex microdisks the magnetic fields of low frequency of ~a few tens of Hz ~and of small amplitude of $<$ 100 Oe applied during only 10 minutes was sufficient to achieve $\sim $90{\%} cancer cells destruction. \\[4pt] [1] E. A. Rozhkova, et al., J. Appl. Phys. Vol. 105, (2009) 07B306. \\[0pt] [2] D.-H. Kim, et al., Nature Materials, vol. 9, pp. 165 - 171 (2010).   

Fast transport of superparamagnetic beads by field-driven magnetic domain walls

The manipulation of superparamagnetic (SPM) beads with magnetic domain walls (DWs) is of interest for biomedical applications [1, 2]. We present data supporting fast, continuous transport of SPM beads by field-driven DWs along straight magnetic nanowires. If the magnetostatic binding force (F$_{b}$) between a DW and an SPM bead exceeds the Zeeman force (F$_{Z}$) from a driving field, DW velocity is limited by the hydrodynamic drag force on the bead [3], and a wall-bead pair can be propelled at high speeds. We have combined micromagnetic simulations and numerical calculations to determine F$_{b}$, covering the parameter space of bead radius, wire width and thickness, and domain wall type. Comparing F$_{b}$ and F$_{Z}$ for different applied fields, we find that the field, H$_{crit}$, at which the Zeeman force separates the wall from the bead, is maximized by the same wire width, independent of bead size. Optimal conditions for continuous bead transport are achieved with 150 nm wide wires, which can transport 500 nm radius beads in driving fields up to 90 Oe, corresponding to transport velocities of up to 8 mm/s. These results suggest that fast, long-distance transport of SPM beads is possible using simple linear magnetic guide-wire structures. [1] M. Donolato, et al., Nanotechnology 20 (2009) [2] G. Vieira et al., Phys. Rev. Lett. 103, 128101 (2009) [3] M.T. Bryan et al., Appl. Phys. Lett. 96,192503 (2010)   

Domain wall pinning in magnetic structures with perpendicular magnetic anisotropy

An experimental technique has been designed to trap domain walls in ferromagnetic nanostructures. Spin valve nanowires and nanopillars with perpendicularly magnetized free and reference layers were engineered with lithographically defined notches of varying depths and lengths. The influence of notch geometry in domain wall pinning has been compared with intrinsic domain wall pinning sites. Thermally activated jumping between metastable states has been observed under rf excitation along with telegraph noise. Coercive fields have been determined to vary linearly with applied direct currents.   

Breather states in magnetic domain wall racetrack memory samples

Proposed magnetic domain wall(DW) racetrack memory [1] exploits controlled motion of magnetic DWs along magnetic nanowires, and the sequence of DWs encodes the bit states. Here we investigate the possibility of the existence of dynamically bound states of pairs of DWs. We show that by the choice of suitable initial conditions for two DWs in a racetrack geometry, such dynamical states can be prepared by a suitable applied field. The breather states correspond to two DWs which have the same chirality and which oscillate around their common center of mass.\\[4pt] [1] S.S.P. Parkin, Science 320, 190 (2008)   

Reversible helicity and higher harmonics in spin textures: stripes and skyrmions

The magnetic bubbles viewed as skyrmions have long been attracting attention because of possible application to spintronics. The bubble configuration has been revealed by versatile microscopic techniques such as magnetic-force microscopy, scanning Hall microscopy, and Lorentz transmission electron microscopy (TEM). However, their topological properties, such as topological spin texture and helicity, have not been sufficiently unraveled in spite of possibly important implication in the novel magneto-transport phenomena. In this study, we have scrutinized the spin texture of the thin films of Sc-doped hexagonal barium ferrite with controlled magnetic anisotropy; we have demonstrated the generation of the bubble lattice under external magnetic fields which are applied perpendicular to the film plane. The magnetic component distributions in strips, bubbles and Bloch lines have been successfully achieved by means of high-resolution Lorentz TEM observations and quantitative analyses of the local magnetizations. The results indicate the reversible helicity and higher harmonics in spin textures of stripy and bubble domains.   

Domain Growth Behavior in the Compressible Ising Model

We perform large scale Monte Carlo simulations to study long-time domain growth behavior in a compressible, spin-exchange, two-dimensional triangular-lattice Ising model with continuous particle positions and zero total magnetization. To investigate the effects of compressibility on domain growth behavior, we include an elastic energy term in the Hamiltonian of our model to adjust the rigidity. The system is quenched below the critical temperature from a homogenous disordered state to an ordered phase where multiple domains coexist. Theory expects the domain size $R(t)$ grow as a power law $R(t)=A+Bt^{n}$, where $t$ is the time after quench, and $n$ is the domain growth exponent. Lifshitz and Slyozov have predicted $n$ to be $\frac{1}{3}$ at late-time, but earlier studies\footnote{S. J. Mitchell and D. P. Landau, Phys. Rev. Lett. \textbf{97}, 025701(2006).} suggested that $n$ could be affected by compressibility. We observe the domain growth exponent to be significantly smaller than the Lifshitz-Slyozov value of $n=\frac{1}{3}$.