Uniaxial Garnet Films Research Articles

Elements of cellular domain patterns in magnetic garnet films.

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.

Direct optical observation of Bloch lines and their motion in uniaxial garnet films using a polarizing light microscope

Recently, the first direct optical observation of vertical Bloch lines using a laser scanning microscope has been reported (A. Thiaville, L. Arnaud, F. Boileau, and G. Sauron, EMMA Conference, Salford, Great Britain, 1987, paper AD-19). Here we describe a different way of direct observation that uses a polarizing light microscope. This method has the advantage of lower thermal disturbance of the domain structure and the ability of real-time observation. To obtain one-sided dark-field illumination, the incoming light beam is coupled into the garnet film by means of a glass microprism. On examining the Bi-containing film, the Bloch lines can be directly observed through the eyepiece of the microscope. Movement of Bloch lines caused by in-plane field pulses, and also by asymmetric bias field pulses, is observed.

On the structure of a moving domain wall subjected to an inplane magnetic field

For (111)-oriented uniaxial garnet films the structure of stationary moving planar domain walls subjected to an inplane magnetic field was investigated numerically and experimentally. Considering an inplane field dependent tilt of magnetization in the domains adjacent to the wall two main types of wall structure were found: 1) the structures with a parallel deflection of magnetic moments, 2) structures containing horizontal Bloch lines. The stability range of these structures as a function of an inplane magnetic field Hy and the wall velocity v is given. Experimentally found critical velocities marking the transition from a winded to an unwinded wall profile agree quite well with our data obtained numerically.

The effect of the cubic anisotropy on the behaviour of uniaxial garnet films

Measurements of the relative volumes of the two sets of 180 degrees domains in a garnet film show a sixfold component dependence on the orientation of the in-plane field. The calculation which yields such a dependence is in good agreement with the data and leads to estimates of the uniaxial and cubic anisotropy constants.

Interpretation of domain nucleation measurements: Neel phase theory and micromagnetism

The observed conditions for homogeneous second-order stripe domain nucleation in uniaxial garnet films has previously been interpreted by a generalization of Neel's 1946 phase theory that yields values of the anisotropy constants from the observations, We show that a micromagnetic description of the phenomena yields additional detailed predictions of the domain orientation. These can be correlated with the films' anisotropic and magnetoelastic properties. The predicted behavior is consistent with earlier observations reported in the literature, and provides a good description of new measurements reported here.