Cation-Specific Magnetic Depth Profiles of Ultrathin Fe3O4 Films Obtained by X-Ray Resonant Magnetic Reflectivity (XRMR)

Abstract

Magnetite, Fe3O4, is a frequently studied magnetic oxide growing in the inverse spinel structure Fe3O4 exhibits a ferrimagnetic ground state which stems from the interplay of its three cation species due to super exchange and double exchange depending on site and oxidation state: Octahedrally coordinated Fe2+ and Fe3+ ions, and tetrahedrally coordinated Fe3+ ions. The coupling among octahedral sites is ferromagnetic, while tetrahedral and octahedral sites couple antiferromagnetically. One of the most successful ways to discriminate between the magnetic contributions of these cations are X-Ray Magnetic Circular Dichroism (XMCD) experiments across the Fe L2,3 edges, making use of a slight difference of their L edge energies and the easily resolvable antiferromagnetic coupling between the tetrahedral and the octahedral sites (c.f. Fig. 1).On the other side, X-Ray Resonant Magnetic Reflectivity(XRMR) is a technique combining the depth resolution of X-Ray Reflectivity and the site- and magnetism-sensitivity of XMCD. This enables us to determine the magnetooptical depth profiles at the energies characteristic for Fe2+oct, Fe3+tet, and Fe3+oct in the XMCD of Fe3O4/MgO(100) ultrathin films (c.f. Fig. 2). We find a ~3.9Å layer of enhanced magnetooptical absorption at the surface for both Fe3+ species but not for Fe2+ [1]. The magnetically enhanced Fe3+tet layer is additionally shifted about 3±1.5 Å into the sample compared to the Fe3+oct layer. We attribute this to the first unit cell from the surface containing an excess to Fe3+ cations.