Observation of the magnetic domain structure in Cu$_{0.47}$Ni$_{0.53}$ thin films at low temperatures

We report on the first experimental visualization of domain structure in films of weakly ferromagnetic Cu$_{0.47}$Ni$_{0.53}$ alloy with different thickness at liquid helium temperatures. Improved high-resolution Bitter decoration technique was used to map the magnetic contrast on the surface of the films well below the Curie temperature T$_{\rm Curie}$ ($\sim\,60$ K). In contrast to magnetic force microscopy, this technique allowed visualization of the domain structure without its disturbance while the larger areas of the sample were probed. Maze-like domain patterns, typical for perpendicular magnetic anisotropy, were observed. The average domain width was found to be about 100 nm.