Change of Magnetic Properties in Ultrathin Fe Films on Pt(111) Induced by Interdiffusion

We have studied magnetic properties of ultrathin Fe films grown on Pt(111) substrate by using the in situ surface magneto-optic Kerr effect (SMOKE), X-ray photoelectron spectroscopy (XPS), and low-energy electron diffraction (LEED). SMOKE measurements show that the Fe layers are not ferromagnetic when the film is thinner than approximately 4.5 ML (monolayers), but the in-plane magnetization is present for a 4.1-ML Fe film on Pt(111) annealed at 550 K. Upon post-annealing at 770 K, a 9.2-ML Fe film does not show any Kerr signal, while a 5.1-ML Fe film has the in-plane Kerr signal with increased coercivity. A diffuse 2 × 2 LEED pattern was observed for both cases, and the average Fe concentration of the intermixed layers caused by the interdiffusion of Fe atoms into the Pt substrate is estimated to be about 50 and 61 at. % Fe for the 9.2- and 5.1-ML Fe films, respectively. Underlying reasons are discussed on the basis of the XPS results. PACS numbers: 78.20.Ls, 79.60.Jv, 68.35.C