Micromagnetic structures and microscopic magnetization-reversal processes in epitaxial Fe/GaAs(001) elements

The in-plane size and orientation-dependent micromagnetic structures of thin epitaxial Fe(001) elements were studied by Lorentz electron microscopy. It is found that the single-domain remanent state supported by continuous epitaxial films with in-plane anisotropy decays into a multidomain configuration upon reducing the film lateral dimensions. For 150-Angstrom-thick Fe(001) elements, such drastic changes in the remanent domain structure and reversal processes occur when the element size is reduced to similar to 10 mu m. This transition can be explained as a consequence of the in-plane dipolar (shape anisotropy) contribution to the total energy becoming comparable with that of the magnetocrystalline anisotropy at this size. Due to the interplay between in-plane shape and magnetocrystalline anisotropies, novel micromagnetic phenomena were observed. Distinct microscopic reversal processes arise according to not only the crystallographic direction along which the field is applied but also the orientation of the element edges. For magnetization reversal along the in-plane [100] directions (easy axes), domains nucleate at either element edges or corners depending on the orientation of element edges. For applied fields aligned along the in-plane [110] directions (hard axes), a fine-scale stripe (width less than or equal to 200 nm) domain structure develops upon reducing the applied field from saturation. In addition to coherent rotation and domain-wall displacement, a 90 degrees coherent jump reversal process has been observed for the elements with edges parallel to the [110] directions. The micromagnetic behavior of these epitaxial elements is substantially different from those of either continuous epitaxial Fe(001) films [E. Gu et al., Phys. Rev. B 51, 3596 (1995), C. Daboo et al., Phys. Rev. B 51, 15 964; (1995)] or polycrystalline elements in which the magnetocrystalline anisotropy is negligibly small. As the relative contributions of the in-plane shape and magnetocrystalline anisotropies can be modified by varying the element size, shape and orientation, these mesoscopic epitaxial elements not only offer an ideal model to study the roles of anisotropies in determining the micromagnetic structures but also allow the magnetic spin configuration to be controlled which could be useful for device applications, e.g., spin-polarized injection contacts and magnetic memory elements.