Vortices in ferromagnetic elements with perpendicular anisotropy
C. Moutafis, S. Komineas, C.A.F. Vaz, J.A.C. Bland,
Cavendish Laboratory,
Madingley Road, Cambridge CB3 0HE, United Kingdom
P. Eames
NVE Corporation, 11409 Valley View Road, Eden Prairie, MN 55344
Abstract
We study equilibrium properties of disc-shaped ferromagnetic elements
with perpendicular anisotropy and quality factor less than unity.
A vortex is a static state for sufficiently thin particles.
For thicker particles we find a continuous transition to
a bidomain state which is stable for a range of particle diameters.
This bidomain state is akin to a magnetic ``bubble'' where the
magnetisation is directed along the symmetry axis at the
centre of the disc and it is oppositely magnetized at the periphery.
The magnetization profile of the bidomain state presents the same winding
around the symmetry axis as the vortex.
Its signature is its magnetostatic field above the particle top surface
which consists of two concentric regions of oppositely signed field.
Higher order states of multiple concentric domains with alternating
magnetization direction are also found for particles of sufficiently large
lateral dimensions.
Finally, we explain how these ideas apply to particles
with a very small perpendicular anisotropy.