Micromagnetic simulation of ferromagnetic part-spherical particles

Abstract

The paramagnetic size limit for current magnetic storage media, particularly in sputtered grain storage, is being approached rapidly. To further increase media storage density, patterned media can be used which only need a single grain to store one bit of data. Chemical self-assembly techniques offer cost-effective methods to create templates, from which periodic arrays of magnetic structures can be formed. In contrast to systems of dots prepared by standard lithography, which have a cylindrical shape, dots prepared by chemical self-assembly template techniques are often spherical or part spherical in shape. In this article, we investigate the properties of such magnetic shapes using micromagnetic simulations. To represent accurately the geometry produced through chemical self-assembly methods, we attach a partial sphere (lower part) to a small ellipsoidal dome. We compute the hysteresis loops for various dot sizes and compare them with experimental results. In those below a critical diameter (140 nm in nickel), the hysteresis loop is square-like, resembling the uniform rotation of magnetization once the critical field is exceeded. For larger sizes, the hysteresis loop reverses reversibly around zero applied field but shows minor loops, placed symmetrically at the onset of magnetization reversal. These correspond to vortices penetrating and exiting the structure. In summary, we find that the coercive field of the droplets becomes zero above a critical diameter where the magnetization reversal behavior changes from single domain-like to vortex-like. Our results agree with experimental measurements performed on such structures.

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Identifiers

ORCID for Richard P. Boardman: orcid.org/0000-0002-4008-0098

ORCID for Hans Fangohr: orcid.org/0000-0001-5494-7193

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