Magnetic properties of epitaxial Ni (0 0 1) films and sub-micron particles

The magnetic properties of Ni particles with well-defined geometry, prepared by electron lithography from epitaxial Ni (0 0 1) films of thickness 50 and 60 nm were studied. The particles were circles with diameters 0.6 μm and rectangles with sides 0.9 μm and 0.3 μm, that were positioned in square and rectangular lattices, having lattice constants about twice the particle dimensions. Reference samples and particles with the lattices oriented along the [1 0 0] and [1 1 0] directions were prepared. Hysteresis curves were obtained for particles and reference samples, in the field range ±2 T at temperatures between 50 and 300 K. The particles were further imaged by magnetic force microscopy. The coercivities of the particles are about the same as that of the reference samples, being of the order of 10 mT at room temperature and increasing with decreasing temperature. This may be explained by the temperature dependence of the magnetic anisotropy of the Ni film, estimated to K1=-12.5, -12.8 and -87.6 kJ m-3 at 295, 250 and 50 K, respectively. Whereas the hysteresis curves of the particles are governed by the intrinsic properties of the starting film in low fields, the decreased lateral size influences the behaviour in higher fields as demagnetization effects and features characteristic for annihilation and nucleation of domain walls. One of the samples, rectangles with the long axis along the [1 1 0] direction, has a significantly higher remanence and coercivity than the others. The magnetic images show that the demagnetized state of this sample comprises both single-domain and multidomain particles. Corresponding images showed only multidomain particles in all other samples. Thus it was concluded that the actual size (0.9 μmx0.3 μmx50 nm) is close to the critical size for single domains in Ni