Elements of cellular domain patterns in magnetic garnet films.

Abstract

Cellular patterns of magnetic domains observed in uniaxial garnet films are comprised of three elemental domain structures: stripe segments, threefold vertices that join these segments, and pentagonal structures that join five segments and contain trapped magnetic bubbles. We report observations of the stability and dynamics of these structures and show how they govern the evolution of cellular patterns in an external bias field ${H}_{B}$. Energy localized in the stripe segments acts as tension that drives the domain motion. A simple extension of the conventional model of stripe domains incorporates domain interactions in sparse patterns and gives access to the bias and configuration dependence of the stripe tension. We apply this formulation to an array of stripe domains to characterize the tension that arises in nonequilibrium patterns. Comparison with experiment indicates that sparse, disordered (mazelike) stripe patterns maintain equilibrium as ${H}_{B}$ is monotonically increased and shows the expected divergence in stripe spacing at ${H}_{B}$=${H}_{\mathrm{RI}}$, where ${H}_{\mathrm{RI}}$ is the run-in field for isolated stripe domains. In contrast, cellular patterns persist to ${H}_{B}$>${H}_{\mathrm{RI}}$, where the patterns are far from equilibrium.Vertex propagation is observed when the tensions in the adjoining stripe segments are unbalanced and leads to a reduction in total stripe length and cell density. The vertices are destroyed at a critical bias field ${H}_{V}$ (=0.79\ifmmode\times\else\texttimes\fi{}4\ensuremath{\pi}M=151 Oe for our garnet sample) when the stripes are severed near the vertices. ${H}_{V}$ is the saturation field for cellular patterns and is significantly larger than that of any other observed domain configuration. Pentagonal bubble traps are also mobile and are destroyed by the collapse of the trapped bubble. A divergence of the average cell area which is limited by coercive friction occurs at the collapse field ${H}_{5}$ (=0.54 \ifmmode\times\else\texttimes\fi{}4\ensuremath{\pi}M=103 Oe) of an isolated bubble trap. Nonequilibrium cellular states arise in the regime ${H}_{\mathrm{RI}<{H}_{B}<{H}_{5}}$ when bubble traps resist collapse and obstruct the topological evolution. Coercive drag on the domain motion also results in nonequilibrium configurations and in some cases alters the pattern topology. We employ an ac field component to mitigate the effects of coercivity, and find that an amplitude several times larger than the coercive field ${H}_{c}$ is required to produce smooth dynamics and ensure reproducible, metastable, stationary states.