Domain Walls, Bloch-Line Vortices and Their Resonances Imaged in Garnet Films Using Cotton-Mouton Magneto-Optics

Abstract

Microscopic images of domain walls, Bloch-lines, magnetic vortices, and their resonances in iron-garnet films are formed using the magneto-optical Cotton-Mouton effect (CM). Results reported in 1984 which were the first to image Bloch-lines optically, are summarized. In our sample epitaxial garnet film of Eu_0.7Tm_0.5Ga_0.85:YIG, the domain moments lie in the [100] film plane. A pattern created with ion irradiation stabilizes the in-plane domains along fourfold “easy axes” [110]. CM microphotographs of 180 ° Neel walls show that multiple Bloch-line vortices may be present. A single vortex occurs at the intersection of two 90 ° Neél walls. For two 90 ° walls intersecting orthogonally, the motions observed at resonance induced with in-plane rf fields reveal the vortex motion is circular. The frequency of resonance depends on the length of the intersecting walls. Resonances appear at 24, 15.5 and 13 MHz for wall lengths 14, 28 and 35 µm respectively. Slonczewski estimated theoretically that the contribution of the effective areal mass of the 90 ° walls to the resonance frequency is negligible. His model resonances based on restoring forces arising from stray field energy when walls displace from equilibrium, are found in reasonable agreement with the experiment. The circular motion at resonance essentially involves precessions of just those electron spins within the invisibly small vortex-core region, and it confirms experimentally the existence of a Magnus force orthogonal to the velocity of a simple quantized magnetic vortex. The vortex core in our film carries magnetic film-normal flux equal 3% of a flux quantum, ch/2e. Very recently, Park, et al [Phys. Rev. B, vol. 67, 020403(R)] formed MFM images of single vortices isolated in Permalloy nano particles and measured their time-resolved impulse response. We briefly compare the magneto dynamics they observed with our observations in the garnet film when the vortex is part of a wall network.