Magnetic domain evolution in permalloy mesoscopic dots (original) (raw)

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.

Anisotropic magnetic coupling of permalloy micron dots forming a square lattice

Applied Physics Letters, 1997

Static magnetic and spin wave properties of square lattices of permalloy micron dots with thicknesses of 500 Å and 1000 Å and with varying dot separations have been investigated. A magnetic fourfold anisotropy was found for the lattice with dot diameters of 1 µm and a dot separation of 0.1 µm. The anisotropy is attributed to an anisotropic dipole-dipole interaction between magnetically unsaturated parts of the dots. The anisotropy strength (order of 10 5 erg/cm 3) decreases with increasing in-plane applied magnetic field.

Micromagnetism in mesoscopic epitaxial Fe dot arrays

Journal of Applied Physics, 2000

The domain structures of epitaxial Fe ͑20 nm͒/GaAs͑100͒ circular dot arrays ͑diameters from 50 to 1 m͒ were studied with magnetic force microscopy. A transition from a single domain to a multidomain remanent state was observed upon reducing the dot diameter beneath 10 m in dot arrays with the separation twice the dot diameter. When the separation is reduced to half the dot diameter, the single domain states were found to ''collapse'' into stripe-like multidomain states due to local dipole coupling between dots. Micromagnetic simulations further suggest that for ultrathin Fe dots of less than about 2 nm thickness the diameter does not have a significant influence on the domain structures due to a dramatic reduction of the dipole energy.

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.

Interactions and switching behavior of anisotropic magnetic dots

Journal of Applied Physics, 2004

The magnetic properties of collections of three soft magnetic nanodots with various aspect ratios are investigated. Permalloy films are first produced by dc magnetron sputtering. Focused ion beam milling is then used to mill dots, each with different shape anisotropy. We find that each of the three dots in the system has a unique switching field, and that there is significant magnetostatic coupling. Micromagnetic simulations suggest that for dot separations of less than 50 nm there exists strong interdot interaction, leading to the possibility of controlled switching of neighboring dots. This switching behavior is of interest in magnetic information processing.

Micromagnetic simulations of Permalloy double-dot structures

Current Applied Physics, 2017

In this paper we investigate by micromagnetic simulations the magnetic properties of Permalloy doubledot structures as a function of the center-to-center distance between the dots. Generally, a well-defined neck appears in the hysteresis curve, evidencing that the process of magnetization reversal occurs by nucleation and propagation of vortices. However, although the processes of magnetization reversal are similar for different distances between the dots, the coercivities are quite different, primarily due to the different energy contributions that dominate the system. The contribution of the shape anisotropy is important for Permalloy double-dot structures where the distance between the dots is less than their diameters, while the magnetostatic interaction between the two dots predominates if the distance between the dots is greater than their diameters. Finally, there is a critical distance for which the influence of magnetostatic interaction decreases sufficiently so that the interacting system can be considered as an isolated system.

Magnetization anisotropy of Ni dots with several tens of nanometer diameter

Solid State Communications, 2009

We have studied the magnetization of Ni dot with 50 to 70 nanometer diameter and 12 nanometer thickness using a magnetic force microscopy with an in-plane magnetic field. The Ni dots were prepared using self-assembled dot patterns with poly (styrene-b-methyl mathacrylate) diblock copolymers on Ni film and ion etching. It was found that the remanent magnetization direction of the dot was perpendicular to the plane as prepared. From the vibrating sample magnetometer measurement, a hysteresis loop was found in the perpendicular magnetization. When an in-plane external magnetic field was applied, the magnetization was rotated into a horizontal direction with low coercivity along the field direction.

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.

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.