A wide range of intriguing magnetic phenomena that are of fundamental and technological importance can occur in ultra-thin films due to their small dimensions. Thin films have also attracted attention because their magnetic properties can be influenced by the interaction with its supporting substrate surface. Ordered surface alloys may be interesting substrates because their properties can be quite different from the bulk surfaces of their constituent elements. This outlook is encouraged by the prospect of tailoring properties through a careful selection of alloy constituents. In the present work, we explored the possibility of controlling magnetic interfaces by investigating the influence of surface alloys on the magnetic properties of ultra-thin Fe films. Among the numerous surface al...[ Read more ]

A wide range of intriguing magnetic phenomena that are of fundamental and technological importance can occur in ultra-thin films due to their small dimensions. Thin films have also attracted attention because their magnetic properties can be influenced by the interaction with its supporting substrate surface. Ordered surface alloys may be interesting substrates because their properties can be quite different from the bulk surfaces of their constituent elements. This outlook is encouraged by the prospect of tailoring properties through a careful selection of alloy constituents. In the present work, we explored the possibility of controlling magnetic interfaces by investigating the influence of surface alloys on the magnetic properties of ultra-thin Fe films. Among the numerous surface alloy systems that are known, our investigations were focused on c(2×2) ordered alloys that are induced by various noble metals on the W(100) refractory metal surface. These investigations were carried out by examining and comparing the magnetic properties of Fe films on the bare W(100) and W(100)-M (M=Au, Ag, Cu) c(2×2) surface alloys. Magnetic ordering was studied by probing the magnetic state of the film continuously during Fe deposition at room temperature using spin polarized low energy electron microscopy. Ferromagnetism is suppressed in the thinnest films due to finite size effects. We find that the onset of ferromagnetism with increasing thickness during growth occurs in films that are slightly thicker on the W(100)-Au c(2×2) surface alloy than on the bare surface, while the W(100)-Ag and W(100)-Cu c(2×2) surface alloy accelerate the occurrence of magnetic onset. First principle calculations based on density function theory indicates earlier onset on all three surface alloy substrates. With reference to first principles calculations of structure, magnetic moments and magnetic ordering, we attribute the experimental observations to competing effects of magnetic enhancement on the surface alloys and suppression due to several possible reasons, including interfacial intermixing and the inherent island-terrace roughness of the surface alloys. Besides ordered surface alloys, step bunches and noble metal overlayer are also found to induce magnetic moments at room temperature. Evolution of the magnetization easy axis with increasing film thickness is observed to be different on the clean surface and the surface alloys. On clean W(100) surface, Fe films up to 6 ML are magnetized along the [011̅] direction without external magnetic field. On W(100)-M (M=Au, Ag, Cu) c(2×2) surface alloys, the spontaneous magnetization direction in Fe films go through a transition of [011] → [001] → [010]. The first spin reorientation transition (SRT) happens right after magnetic onset, being the result of the four-fold magnetic anisotropy change. The second SRT is discontinuous. The easy axis flipping between [001] and [010] indicates a break in the four-fold symmetry. A sandwiched system Fe/Au/Fe/W(100) experience a SRT from [001] to [011]. Accompanying the transition, the magnetic domains first break into much smaller ones and re-form a large one afterwards.

The experiments were done in the Hong Kong University of Science and Technology. The theoretical calculations were done in collaboration with Prof. Li Huang and Mr. Zhe Wang from the South University of Science and Technology of China.