Investigation of magnetic domain structure of Ho(0001)/W(110)

Abstract

Holmium (Ho), a rare earth metal (REM) exhibits rich magnetic phenomena driven by an indi rect RKKY-interaction between 4f magnetic moments. Bulk Ho, with a high magnetic moment (≈ 10µB), transitions to a helical structure below TN = 131 K and a conical state below TC = 20 K. While bulk properties are well understood, this study investigates thin film magnetism, focusing on the interplay between substrate, surface, and RKKY interactions, addressing both fundamental questions and potential applications in magnetic devices. The thesis presents an investigation of epitaxial Ho films grown on W(110) using spin-polarized scanning tunneling microscopy (SP-STM). All the measurements are performed at 5 K, which is well below TC. The optimal growth conditions were identified for deposition at room temperature, followed by annealing at 1000 K. The resulting film exhibits double screw dislocations and smooth step edges, formed through glide processes in hcp(0001) films. Spin-polarized dI/dU measurements with in-plane sensitive tips reveal four to six distinct contrast levels, corresponding to the six easy magnetization directions seen for Ho. Large in-plane magnetic domains extending over several hundred nanometers. Domain wall analysis shows domain width ranging from 1.8 to 5.1 nm for wall angles of 60° to 180°. Measurements using canted Ho-tips reveal striped patterns within each large in-plane domain, exhibiting a periodic modulation of approximately 44 nm. This alternating spin component, oscillating between the +c and −c axes, likely arises as a mechanism to minimize stray fields. Additionally, measurements conducted with manganese (Mn)- tips show an enhanced out-of-plane magnetic signal.