Seminars Archive

Atomic-scale engineering of magnetic anisotropy of nanostructures through interfaces

Abstract
The fine-tuning of the magnetic anisotropy in magnetic nanoclusters and thin films represents a key issue for several technological fields such as magnetic information storage and spintronic. The conventional way to achieve this purpose consists in combining the magnetic material with other elements in alloys which properties can be controlled by the relative concentration of the constituents. Here I present an alternative approach, namely the creation of nanostructures with atomically sharp two dimensional interfaces and one-dimensional interlines between the different constituents. They exhibit unexpectedly high magnetization reversal energy with values and directions of the easy magnetization axes strongly depending on chemistry and texture. The first part will be focused on the characterization of magnetic bimetallic nanoclusters with the ultimate goal of enhancing their magnetic hardness, which is of particular interest for magnetic information storage [1]. Using self-assembly at single crystal metal surfaces we were able to control the morphology, chemical composition and interfaces textures to the atomic level. Most of the element combinations employed in this study follow the classical rule, where magnetic 3d elements are combined with 4d and 5d elements with high spin orbit coupling (SOC) and magnetic polarizability. However, we find a number of surprises. Interfacing two elements with low SOC, namely Fe and Co, leads to high out-of-plane interline anisotropy such that onion-type alternations of the two elements with shell thicknesses of five atoms or less are giving higher hardness than a homogeneous alloy. For all elements we find an unexpected strong dependence of the interface anisotropy on crystallographic orientation and dimension. Ab-initio calculations reproduce these results and unravel their electronic origin. The second part will be focused on the characterization graphene/ferromagnetic thin film epitaxial hybrid systems, which are grown by intercalation of the magnetic material between the graphene and its substrate. The intercalation allows for a layer-by-layer growth of the thin film resulting in an atomically sharp interface between the graphene and the magnetic material [2]. This interface is of particular importance for spintronic applications since it has been shown that the graphene protects the ferromagnetic material from oxidation and also, stabilizes the out of plane magnetization of thin magnetic films [2-3]. In order to perform the ferromagnet intercalation, the sample has to be heated to elevated temperatures. As a result, the ferromagnet/substrate lower interface might also change and induce peculiar magnetic behavior in the system. In this case it is still not clear if the observed magnetic response is entirely due to the graphene or also to the modified lower interface [3]. We investigate the structural properties of graphene/Co/Ir(111) hybrid systems by surface x-ray diffraction measurements. We demonstrate that the modification Co/Ir interface due to the sample heating plays an important role in the overall magnetic behavior of the system. Moreover the presence of the graphene layer, and so the intercalation process, strongly promotes the Co/Ir alloying with respect to the simple case of Co directly deposited on the Ir(111) surface [4]. References [1] S. Ouazi*, S.Vlaic*, S. Rusponi, G. Moulas, P. Buluschek , K. Halleux, S. Bornemann, S. Mankovsky, J. Minár, J. Staunton, H. Ebert, and H. Brune, Nat. Commun. 3, 1313 (2012). [2] J. Coraux, A. T. NDiaye, N. Rougemaille, C. Vo-Van, A. Kimouche, H.-X. Yang, M. Chshiev, N. Bendiab, O. Fruchart, and A. K. Schmid, J. Phys. Chem. Lett. 3, 2059 (2012). [3] N. Rougemaille, A.T. N’Diaye, J.Coraux, C. Vo-Van, O. Fruchart, and A. K. Schmid, Appl. Phys. Lett.101, 142403 (2012). [4] J. Drnec, S. Vlaic, I. Carlomagno, C; J. Gonzalez, H. Isern, F. Carlà, R. Fiala, N. Rougemaille, J. Coraux, R. Felici, Carbon, 94, 554 (2015)