Evolution of magnetic state of ultrathin Co films with Volmer-Weber growth

The surface structure and magnetic state of ultrathin Co films with Volmer-Weber growth (V-W growth) on α-Al₂O₃(0001) have been investigated as a function of Co thickness. Due to V-W growth, Co forms particles with a diameter of approximately 5 nm. Originating from the particle structure, the magnetic state of Co is superparamagnetism below a thickness of 1.1 nm, and evolves to ferromagnetism as the Co thickness increases. The transition occurs for a wide thickness range, i.e., 1 to 3 nm. Due to the wide-range transition, it enables to observe the coexistence of superparamagnetism and ferromagnetism at a thickness of around 2 nm. The wide range transition of the magnetic state is explained by the slow coalescence of Co particles. The structural transition would be dominated by the large interface energy between Co and α-Al₂O₂(0001).