Domain walls with alternating magnetic order in a model with dipolar coupling

Abstract

A model for a one-dimensional chain of elongated nano-scale magnetic islands with dipole interactions is analyzed here for the properties of its static domain walls with site-by-site alternating order. The anisotropic magnetic islands on a nonmagnetic substrate have their longer axes oriented transverse ($y$-direction) to the chain direction ($x$-direction), in a transverse applied magnetic field. The islands' magnetic dipoles $\vecμ_n$ are represented as macrospins of fixed length $μ$. The nearest-neighbor (NN) dipole interactions drive transverse alternating order, allowing for doubly-degenerate, uniform, static, $y$-alternating states, where the dipoles alternately point transverse to the chain direction, as in $\vecμ_n = \pm(-1)^n μ\hat{y}$. Assuming only NN interactions, the domain walls connecting these two alternating states are found with numerical relaxation simulations and analyzed in a two-sublattice continuum theory. As the uniaxial anisotropy constant $K_1$ descends from larger values until it closely approaches the NN dipolar coupling constant $D$, the domain wall width grows indefinitely, and the large-$x$ continuum solutions closely approach the lattice numerical solutions. The applied field produces a very slight canting of the dipoles towards the field, maximum in the center of the domain wall. The dipoles on the two sublattices are found to rotate in {\em opposite senses} as one scans along the lattice, resulting in a large longitudinal magnetic moment of the domain wall. A much smaller transverse magnetic moment has a topological contribution that depends on whether the chain length is odd or even.