Berlin 2008 – scientific programme

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O: Fachverband Oberflächenphysik

O 60: Symposium: Bimetallic Nanosystems: Tuning Physical and Chemical Properties I (Invited Speakers: Harald Brune, Michael Hilgendorff, Konstantin Neyman)

O 60.1: Invited Talk (no funding)

Thursday, February 28, 2008, 09:30–10:00, MA 005

Interface vs. Alloy Contribution to Magnetic Anisotropy in Bi-Metallic Nanostructures — •Harald Brune — Swiss Federal Institute of Technology (EPFL), Lausanne, Switzerland

We investigate the magnetic anisotropy of atoms located at atomically sharp interfaces between two metals and of those situated in a homogeneous alloy. The numbers we derive enable to estimate whether homogeneous alloys or onion type alternations of two metals lead to higher anisotropies for a given size. The model systems are two-dimensional bi-metallic islands on Pt(111). The magnetic properties are determined by means of magneto-optical Kerr effect for island ensembles for which the morphology is derived from STM.

The blocking temperature of Fe_xCo_1−x alloy islands is highest for x = 0.5 and with T_b = 160 K two times higher than the one of pure Co (T_b = 90 K) or pure Fe islands (T_b = 80 K) of identical size and shape. This yields to an alloy contribution to the barrier for thermally induced magnetization reversal of E_alloy = 0.14 meV/atom. Co-core–Fe-shell islands reveal a steep increase of T_b, starting with minute amounts of Fe and ending at a shell being only 2 atomic rows wide. Further addition of Fe leads to a much more shallow increase of T_b, similar to the one of pure Fe or Co. From this behavior we infer that the interface between Co and Fe contributes by E_int = 0.9 meV/pair. These values suggests that abrupt 1D interfaces between two metals have significantly higher anisotropies than homogeneous alloys. For Co islands decorated by Pd we find very different anisotropies for lateral compared with vertical interfaces.