# Stray Magnetic Field Variations and Micromagnetic Simulations: Models for Ni0.8Fe0.2 Disks Used for Microparticle Trapping

## TL;DR

This paper explores how magnetic fields around tiny Ni0.8Fe0.2 disks can trap microparticles, using simulations and fitting methods to improve efficiency.

## Contribution

A novel fitting method is introduced to speed up stray field calculations from micromagnetic simulations of magnetic microdisks.

## Key findings

- Stray fields from micromagnetic simulations of Ni0.8Fe0.2 disks vary significantly based on vortex or non-vortex states.
- A fitting method is proposed to reduce computational time for field calculations.
- Field strengths from different micromagnetic states differ by approximately a factor of two.

## Abstract

Patterned micro-scale thin-film magnetic structures, in conjunction with weak (~few tens of Oe) applied magnetic fields, can create energy landscapes capable of trapping and transporting fluid-borne magnetic microparticles. These energy landscapes arise from magnetic field magnitude variations that arise in the vicinity of the magnetic structures. In this study, we examine means of calculating magnetic fields in the local vicinity of permalloy (Ni0.8Fe0.2) microdisks in weak (~tens of Oe) external magnetic fields. To do this, we employ micromagnetic simulations and the resulting calculations of fields. Because field calculation from micromagnetic simulations is computationally time-intensive, we discuss a method for fitting simulated results to improve calculation speed. Resulting stray fields vary dramatically based on variations in micromagnetic simulations—vortex vs. non-vortex micromagnetic results—which can each appear despite identical simulation final conditions, resulting in field strengths that differ by about a factor of two.