Reorientational magnetic transition in nanostructured epitaxial cobalt patches (abstract)

Tiny magnets are currently studied for their fundamental and technological properties in the form of particles or patterned submicron dots in close interaction with a substrate. While very small monodomain Co particles (a few tens of atoms) do not show any hysteretic behavior at room temperature, bigger particles, relevant to high-density data storage materials, on the contrary, build up anisotropies that lock the magnetic moment in a given direction. We will discuss here how the geometrical constraints affect the magnetic domain structure in the case of particles dimensions comparable to the characteristic nanoscopic magnetic length scales. We present a study of magnetic dot array 0.5 μm lateral dimension and 1 μm periodicity fabricated out of an epitaxial (0001) hcp cobalt film. While the magnetic properties of the overall particles were determined by conventional magnetometry, the domain structure of each patch was studied by local magnetic force microscopy. This study shows the effects induced by the nanostructuration of the cobalt films in which the patch height determines the domain size and the patch shape controls the pattern of the magnetic domain structure.¹ How domain confinement proceeds is particularly interesting in the 25 nm thick cobalt dot array in which the magnetization curves are characteristic of mostly in-plane magnetization with small alternately up and down perpendicular components. The in-plane magnetostatic energy reduced for in-plane magnetic moments parallel to the edges of the dots results in concentric magnetic domain pattern. This implies that a singularity occurs at the center of the dot where the in-plane magnetization reorients fully perpendicular to form a so-called vortex structure. When the temperature is decreased from room temperature, interestingly we show that a reorientational transition occurs at 200 K from the quasi-in-plane magnetization to perpendicularly oriented domain structure. This reorientational transition is attributed to an increase of the perpendicular anisotropy and gives rise to an elongated stripe domain structure.