Observation of propagating edge spin waves modes

The following article appeared in Journal of Applied Physics 114.7 (2013): 213905 and may be found at http://scitation.aip.org/content/aip/journal/jap/114/21/10.1063/1.4839315Broadband magnetization response of equilateral triangular 1000 nm Permalloy dots has been studied under an in-plane magnetic field, applied parallel (buckle state), and perpendicular (Y state) to the triangles base. Micromagnetic simulations identify edge spin waves (E-SWs) in the buckle state as SWs propagating along the two adjacent edges. These quasi one-dimensional spin waves emitted by the vertex magnetic charges gradually transform from propagating to standing due to interference and are weakly affected by dipolar interdot interaction and variation of the aspect ratio. Spin waves in the Y state have a two dimensional character. These findings open perspectives for implementation of the E-SWs in magnonic crystals and thin films.This work has been supported by the Spanish MINECO (MAT2012-32743, CONSOLIDER CSD2007-00010), Comunidad de Madrid (P2009/MAT-1726) and by the U.S. NSF, grant ECCS-0823813 Grants

Treatment with Y-27632, a ROCK Inhibitor, Increases the Proinvasive Nature of SW620 Cells on 3D Collagen Type 1 Matrix

The concept of using tissue density as a mechanism to diagnose a tumor has been around for centuries. However, this concept has not been sufficiently explored in a laboratory setting. Therefore, in this paper, we observed the effects of cell density and extracellular matrix (ECM) density on colon cancer invasion and proliferation using SW620 cells. We also attempted to inhibit ROCK-I to determine its effect on cell invasion and proliferation using standard molecular biology techniques and advanced imaging. Increasing cell seeding density resulted in a 2-fold increase in cell invasion as well as cell proliferation independent of treatment with Y-27632. Increasing collagen I scaffold density resulted in a 2.5-fold increase in cell proliferation while treatment with Y-27632 attenuated this effect although 1.5 fold increase in cell invasion was observed in ROCK inhibited samples. Intriguingly, ROCK inhibition also resulted in a 3.5-fold increase in cell invasion within 3D collagen scaffolds for cells seeded at lower densities. We show in this paper that ROCK-I inhibition leads to increased invasion within 3D collagen I microenvironments. This data suggests that although ROCK inhibitors have been used clinically to treat several medical conditions, its effect largely depends on the surrounding microenvironment

The extracellular matrix microtopography drives critical changes in cellular motility and Rho A activity in colon cancer cells

We have shown that the microtopography (mT) underlying colon cancer changes as a tumor de-differentiates. We distinguish the well-differentiated mT based on the increasing number of "pits" and poorly differentiated mT on the basis of increasing number of "posts." We investigated Rho A as a mechanosensing protein using mT features derived from those observed in the ECM of colon cancer. We evaluated Rho A activity in less-tumorogenic (Caco-2 E) and more tumorigenic (SW620) colon cancer cell-lines on microfabricated pits and posts at 2.5 μm diameter and 200 nm depth/height. In Caco-2 E cells, we observed a decrease in Rho A activity as well as in the ratio of G/F actin on surfaces with either pits or posts but despite this low activity, knockdown of Rho A led to a significant decrease in confined motility suggesting that while Rho A activity is reduced on these surfaces it still plays an important role in controlling cellular response to barriers. In SW620 cells, we observed that Rho A activity was greatest in cells plated on a post microtopography which led to increased cell motility, and an increase in actin cytoskeletal turnover

Vortex dynamics in confined stratified conditions

We report on linear spin dynamics in the vortex state of the Permalloy dots subjected to stratified (magnetic) field. We demonstrate experimentally and by simulations the existence of two distinct dynamic regimes corresponding to the vortex stable and metastable states. Breaking cylindrical symmetry leads to unexpected eigenmodes frequency splitting in the stable state and appearance of new eigenmodes in the metastable state above the vortex nucleation field. Dynamic response in the metastable state strongly depends on relative orientation of the external rf pumping and the bias magnetic fields. These findings may be relevant for different vortex states in confined and stratified conditions

Diffracted magneto-optical Kerr effects of permalloy ring arrays

Diffracted magneto-optical Kerr effect (MOKE) technique was used to study the quasistatic magnetization switching of thin-film permalloy ring structures. Two different switching mechanisms were observed, dependent on the width of the annular portion of the ring. For small widths, a two-step switching is found, from an initial onion state to a state of circulation magnetization around the ring to a final onion state. In the wider rings, the switching from the circulation state to the onion state takes place via nucleation of a vortex within the annulus. The presence of the vortex is determined by the higher spatial frequency information revealed by diffracted MOKE, and through comparison with micromagnetic simulation.NRC publication: Ye

Broadband spin dynamics of the magnetic vortex state: Effect of the pulsed field direction

The dynamic spin modes observed in magnetic vortex structures are shown to depend strongly on the nature of the initial excitation by a transient pulse field. In submicrometer-sized Permalloy disks, when a uniform perpendicular transient field is used to perturb the magnetization, radial standing-wave modes are excited; whereas if an in-plane transient field is used, angular or azimuthal modes are formed. The existence of the vortex core is responsible for a frequency splitting of the azimuthal modes, as demonstrated through comparison to micromagnetic simulations of a ring geometry.NRC publication: Ye

Broadband spin dynamics of Permalloy rings in the circulation state

The spin dynamics of the remanent state of circulating magnetization in micrometer-scale Permalloy rings is studied by broadband ferromagnetic resonance. A number of resonance modes are observed, depending upon the geometries of coupling the transient excitation to the ring and of magneto-optically monitoring the response. The two dominant modes involve precession with uniform phase around the ring, or in a circularly antisymmetric standing wave pattern with two nodes. Other magnetic normal modes are also excited but with much smaller amplitude.Peer reviewed: NoNRC publication: Ye

X-ray resonant magnetic scattering study of magnetization reversals in a nanoscale spin-valve array

We present an x-ray resonant magnetic scattering study that uses the periodicity of a patterned array of trilayer (Co/Cu/NiFe) elements to determine not only layer-dependent magnetic hysteresis, but, more importantly, to extract the magnetization reversal in different sections of the picture-frame-shaped structure. Spatially resolved and layer-resolved magnetization measurements have revealed that magnetic switching mechanism is very distinct in different regions of the structure and results from a balancing of the shape anisotropy and strong interlayer dipolar coupling. These results demonstrate how spatially averaged measurements are not sufficient to resolve the nature of the reversal mechanism within the structure