Magnetic anisotropy from single atoms to large monodomain islands of Co/Pt(111)

Abstract

By studying the magnetic behavior of self-assembled Co islands on a single-crystal metal surface, Pt(111), we show how the magnetic anisotropy evolves from isolated atoms to monolayer islands and films. Single Co adatoms are found to have a giant magnetocrystalline anisotropy energy of E a = 9.3 ± 1.6 meV/atom arising from the combination of partly preserved orbital moments (m L = 1.1 µ B ) and strong spin-orbit coupling induced by the Pt substrate. Combined scanning tunneling microscopy and X-ray magnetic circular dichroism experiments performed for differently sized small two-dimensional Co islands establish a clear connection between E a and m L , both quantities decrease sharply with the lateral coordination of the magnetic atoms. In accordance with this, Kerr magnetometry experiments, again performed in conjunction with scanning tunneling microscopy, reveal that the anisotropy energy of large two-dimensional Co islands is almost entirely determined by the relatively low number of perimeter atoms, having E a = 0.9 ± 0.1 meV/atom. These results confirm theoretical predictions and are of fundamental value the understanding of how the magnetic anisotropy develops in finite-size magnetic particles. Identification of the role of perimeter and surface atoms opens up new opportunities for engineering the anisotropy and the moment of a magnetic nanostructure. RésuméAnisotropie magnétique de l'atome isolé aux îlots monocouches de Co/Pt(111). En étudiant les propriétés magnétiques de particules de Co auto assemblées sur une surface d'orientation (111) d'un monocristal de Pt, nous montrons comment l'anisotropie magnétique évolue de l'atome isolé aux îlots monocouches et aux films ultraminces. Nous trouvons que les atomes de Co isolés adsorbés en surface ont une énergie d'anisotropie magnétocrystalline de E a = 9.3 ± 1.6 meV/atome provenant de la combinaison d'un moment orbital conservé (m L = 1.1 µ B ) et d'un fort couplage spin-orbite induit par le substrat de Pt. Des expériences combinées de microscopie à effet tunnel et de dichroïsme circulaire magnétique à rayons X, effectués sur des îlots de Co de petit taille établissent une connexion claire entre E a et m L , chacune de ces quantités décroissant rapidement avec la coordination latérale des atomes magnétiques. En conformité avec ceci, nous trouvons en employant la magnétométrie par effet Kerr et le microscope à effet tunnel que l'énergie d'anisotropie de grands îlots bidimensionnels de Co est presque entiè-rement déterminée par le nombre relativement faible d'atomes au périmètre, ayant E a = 0.9 ± 0.1 meV/atome. Ces résultats confirment les prédictions théoriques et sont d'un intérêt fondamental pour comprendre comment l'anisotropie magnétique se développe dans les particules magnétiques de taille finie. L'identification du rôle des atomes au périmètre et des atomes de surface ouvre de nouvelles opportunités pour l'ingénierie de l'anisotropie et du moment de nanostructures magnétiques. Pour citer cet article : P. Gambardella et al., C. R. Physique 6 (200...

Magnetism in reduced dimensions

“…Upon annealing Ostwald ripening sets in, yielding islands of well-controlled size and narrow size distribution. Thus, for Co in contact with Pt the variation of MAE from single atoms to bulk was fully spanned for the first time [32,33] (Figure 1c) [34]. These studies confirm that the magnitude of Es increases dramatically from surfaces, to steps, then to kinks or atoms.…”

Magnetic anisotropy of deposited transition metal clusters

“…A system which has been intensively investigated experimentally [3,4,5,6,7,8,9,10] as well as theoretically [11,12,13,14,15] in recent years is Co/Pt(111) (used here as a short notation for Co clusters or nanostructures, respectively, deposited on a Pt(111) substrate) as this is a prototype to study the requirements on new high-density magnetic storage materials. Earlier theoretical works studied only rather small Co clusters or Co chains on Pt(111) [11,12,13,14], whereas only recently first qualitative results and trends based on a parameterised tight-binding approach were published for deposited structures of up to 37 Co atoms [15].…”

Orbital moment anisotropy of Pt/Co/AlOxheterostructures with strong Rashba interaction

“…Following early predictions of interface-induced PMA by Néel [1] and subsequent confirmations in NiFe/Cu, Co/Pd, Co/Pt, Co/Au and Fe/Ag [2][3][4][5][6], a host of layered systems with surface-induced PMA has been proposed and studied [6][7][8]. New phenomena were discovered in these systems, such as enhanced spin and orbital magnetic moments [8][9][10][11][12][13][14]. It was demonstrated that PMA is driven by the orbital moment anisotropy, which couples the symmetry axes of the system to the spin magnetic moment [13,[15][16][17][18].…”