Probing dynamical magnetization pinning in circular dots as a function of the external magnetic field orientation (original) (raw)
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Physical Review B, 2011
We investigate the magnetization dynamics in circular Permalloy dots with spatially separated magnetic vortices interconnected by domain walls (double vortex state). We identify a novel type of quasi one-dimensional (1D) localized spin wave modes confined along the domain walls, connecting each of two vortex cores with two edge half-antivortices. Variation of the mode eigenfrequencies with the dot sizes is in quantitative agreement with the developed model, which considers a dipolar origin of the localized 1D spin waves or so-called Winter's magnons [J. M. Winter, Phys. Rev. 124, 452 (1961)]. These spin waves are analogous to the displacement waves of strings and could be excited in a wide class of patterned magnetic nanostructures possessing domain walls, namely in triangular, square, circular, or elliptic soft magnetic dots.
Magnetic properties of submicron circular permalloy dots
IEEE Transactions on Magnetics, 2002
Both the static and the dynamical magnetic properties of a square array of circular permalloy dots, characterized by a magnetic vortex configuration of the magnetization, have been investigated by means of magneto-optical Kerr effect and of Brillouin light scattering (BLS) from thermally excited spin waves. The measured hysteresis loop can be satisfactorily reproduced by micromagnetic simulations, showing that the vortex configuration is stable over a wide range of applied field. The high frequency response of the dots was analyzed by BLS measurements performed under external magnetic field intensity large enough to uniformly magnetize the dots. Evidence is given of a marked discretization of the spin wave spectrum with respect to the case of the continuous permalloy film, where only one peak, corresponding to the Damon-Eshbach mode, was detected. The experimental frequencies have been compared to those calculated using a recently developed analytical model for a flat uniformly magnetized cylindrical dot.
Journal of Applied Physics, 2003
͑Presented on 15 November 2002͒ Ferromagnetic resonance at 9.2 GHz (X band͒ was used to characterize the uniaxial magnetic anisotropies in rectangular arrays of submicron circular Ni dots. The in-plane anisotropy, originated from interdot interactions in the rectangular lattice, and the perpendicular anisotropy, due to individual dot shape and magnetostriction, were explored. For in-plane dependencies of the resonance field (H r ), the main resonance mode angular dependence was well described by the standard Kittel formula. As the interdot distances decreased from 800 to 50 nm, the in-plane uniaxial anisotropy field changed from 5 to 130 Oe, in reasonable agreement with calculations. Simultaneously, the position of perpendicular H r increased from 6.38 to 6.83 kOe, also following Kittel's formula.
Magnetostatic interdot coupling in arrays of circular ferromagnetic dots
Journal of Magnetism and Magnetic Materials, 2002
The effect of interdot magnetostatic coupling on magnetization reversal due to nucleation, displacement and annihilation of magnetic vortices in arrays of circular permalloy dots has been studied experimentally. The magnetostatic coupling leads to decreases in both, the vortex nucleation and annihilation fields, and to an increase in susceptibility. The experimental data is interpreted using micromagnetic numerical calculations.
Localized spin modes in ferromagnetic cylindrical dots with in-plane magnetization
Journal of Physics-condensed Matter, 2007
A study of spin localized excitations in ferromagnetic tangentially magnetized dots of cylindrical shape and of nanometric size is presented. A recently formulated variational theory permits us to study the most representative localized spin modes of the spectrum. One of these, the fundamental mode, is mainly localized in the central part of the dot endfaces and is analogous to the Kittel uniform mode in ellipsoids. We also investigate the dynamical properties of spin modes localized in the lateral part of the dot endfaces along the direction of the applied magnetic field, studying the dependence of their localization on the variational parameter and the applied magnetic field. Finally, a comparison of the calculated frequencies of some of these localized modes with available experimental data is performed.
Physical Review B, 2013
We report a combined experimental and theoretical study of the quasistatic hysteresis and dynamic excitations in large-area arrays of NiFe nanodisks forming a hexagonal lattice with the lattice constant of 390 nm. Arrays were fabricated by patterning a 20-nm-thick NiFe film using the etched nanosphere lithography. We have studied a close-packed (edge-to-edge separation between disks d cp = 65 nm) and an ultraclosed packed (d ucp = 20 nm) array. Hysteresis loops for both arrays were qualitatively similar and nearly isotropic, i.e., independent on the in-plane external field orientation. The shape of these loops revealed that magnetization reversal is governed by the formation and expulsion of vortices inside the nanodisks. When we assumed that the nanodisks' magnetization significantly decreases near their edges, micromagnetic simulations with material parameters deter-mined independently from continuous film measu-rements could satisfactorily reproduce the hysteresis. Despite the isotropic hysteresis, significant in-plane anisotropy of the dynamic response of the ultraclose-packed array was found experimentally by the all-electrical spin-wave spectroscopy and Brillouin light scattering. Dynamical simulations could successfully reproduce the difference between excitation spectra for fields directed along the two main symmetry axes of the hexagonal lattice. Simulations revealed that this difference is caused by the magnetodipolar interaction between nanodisks, which leads to a strong variation of the spatial distribution of the oscillation power both for bulk and edge modes as a function of the bias field orientation. Comparison of simulated and measured frequencies enabled the unambiguous identification of experimentally observed modes. Results of this systematic research are relevant both for fundamental studies of spin-wave modes in patterned magnetic structures and for the design of magnonic crystals for potential applications as, e.g., spin-wave guides and filters.
Origin of fourfold anisotropy in square lattices of circular ferromagnetic dots
Physical Review B, 2006
We discuss the four-fold anisotropy of in-plane ferromagnetic resonance (FMR) field Hr, found in a square lattice of circular Permalloy dots when the interdot distance a gets comparable to the dot diameter d. The minimum Hr, along the lattice 11 axes, and the maximum, along the 10 axes, differ by ∼ 50 Oe at a/d = 1.1. This anisotropy, not expected in uniformly magnetized dots, is explained by a non-uniform magnetization m(r) in a dot in response to dipolar forces in the patterned magnetic structure. It is well described by an iterative solution of a continuous variational procedure.
Magnetization reversal and configurational anisotropy of dense permalloy dot arrays
Applied Physics Letters, 2002
Electron beam patterned permalloy circular dots of 700 nm diameter with small separations were studied by magnetic force microscopy (MFM) in the presence of an in situ magnetic field. Images in the demagnetized state show that the dot is in a vortex state with a vortex core (singularity) in the center. Local hysteresis loops, measured by cantilever frequency shift in an external field, indicate that the magnetization reversal of individual disks is a vortex nucleation and annihilation process. By carefully doing MFM, nucleation and annihilation fields without MFM tip stray field distortions are obtained. Configurational anisotropy originated from magnetostatic coupling is found through hysteresis loops.
Reorientational magnetic transition in high-density arrays of single-domain dots
Applied Physics Letters, 2000
A theoretical study on the reorientational transition from in-plane to out-of-plane magnetized state is performed for two-dimensional magnetic dot arrays coupled by magnetostatic interaction. The square lattice of nanoscale cylindrical dots is considered with the assumptions that the dots are magnetically soft and they have uniform magnetization. The present study predicts that the interdot magnetostatic coupling determines the reorientation transition for close-packed arrays of such magnetic dots. Recent experimental results on the nanometer-scale single-domain dot arrays are discussed in light of the present calculation.