Uniaxial 2D Superlattice of Fe-4 Molecular Magnets on Graphene

We demonstrate that electrospray deposition enables the fabrication of highly periodic self-assembled arrays of Fe4H single molecule magnets on graphene/Ir(111). The energetic positions of molecular states are probed by means of scanning tunneling spectroscopy, showing pronounced long- and short-ranged spatial modulations, indicating the presence of both locally varying intermolecular as well as adsorption-site dependent molecule substrate interactions. From the magnetic field dependence of the X-ray magnetic circular dichroism signal, we infer that the magnetic easy axis of each Fe4H molecule is oriented perpendicular to the sample surface and that after the deposition the value of the uniaxial anisotropy is identical to the one in bulk. Our findings therefore suggest that the observed interaction of the molecules with their surrounding does not modify the molecular magnetism, resulting in magnets that retain their bulk magnetic properties.

Magnetism of Single Adatoms and Small Adsorbed Clusters Investigated by Means of Low-Temperature STM

Quentin Dubout

This thesis works reports on the magnetic properties of single transition metal atoms and hydrogenated compounds adsorbed on crystal surfaces. It is an experimental study, based on scanning tunneling microscopy (STM), initiated by the project of demonstrating the existence of magnetic remanence for single adsorbed atoms (adatoms). All the measurements have been performed with a scanning tunneling microscope operating at 0.4 K and equipped with 8.5 T superconducting coils. The investigated systems are cobalt atoms, pure and hydrogenated (cobalt hydrides), adsorbed on Pt(111) and graphene on Pt(111) surfaces. In the first case, we have immediately been confronted to the presence of cobalt hydrides on the surface. Three types of hydrides (CoH, CoH2, and CoH3) have been identified, quite different from the clean adatoms, and readily distinguished by their tunnel spectrum (STS). Twoofthesehydridesdisplaylow-energyvibrationalmodes(1−30meV),identifiedbytheir absence of response to a magnetic field, the effect of an isotopic substitution, and by comparison with known systems. The three hydrides can be deprotonated applying a sufficient voltage between tip and sample. CoH2 on Pt(111) presents a Kondo resonance, due to a half-integer spin (S = 3/2) and a peculiar magnetic anisotropy. This system corresponds to the first demonstration of the appearance of the Kondo effect upon hydrogen adsorption. The results of several attempts to observe remanence for single cobalt adatoms on Pt(111) with spin-polarized scanning tunneling microscopy (SP-STM) are also presented and discussed. The second system differs from the first one in the presence of a graphene layer on the Pt surface. The first experimental determination of the magnetic moment and magnetic anisotropy of adatoms on graphene is reported in this thesis. A magnetic moment of (2.2 ± 0.4) μB was measured by spin-excitation spectroscopy (SES), as well as a hard magnetization axis with an anisotropy energy of 8.1 ± 0.4 meV. This value, comparable to the record value of single cobalt atoms on Pt(111), has been mainly attributed to the strong hybridization between cobalt and graphene. Ab initio calculations confirmed our observations. Graphene thus represents an extremely promising substrate for future applications in nanoscale magnetism and spintronics. This system also exhibits three different cobalt hydrides in addition to the clean atoms. Their respective chemical composition, CoH, CoH2, and CoH3, was determined by direct comparison between high resolution STM images and simulated images obtained from ab initio calculated atomic structures. A controlled deprotonation method is presented. CoH3 is the only hydride to display spin excitations. Its magnetic moment is comparable to that of clean cobalt, it presents a hard magnetization axis as well, however, its magnetic anisotropy energy is sensibly lower, 1.7 ± 0.05 meV. The other two hydrides display vibrational excitations only. The effect of hydrogen on the magnetic properties of metal adatoms is therefore very strong in this system as well, and must be taken into account in future studies, hydrogen being a very common contaminant in ultra-high vacuum set-ups. Finally, a detailed study of the structure graphene mono- and bilayers on Ru(0001) was realized. Two different stackings, AB and AA, have been identified for the bilayers, the last one not observed to date for this system. The moiré structure of monolayer graphene and of this last type of bilayer has been determined: (11.57×11.57)R4.3◦ and (10.54×10.54)R4.7◦, respectively. Moreover, C-C bond distortions of more than 10%, observed in the STM images, have been studied in detail. A quantitative comparison with ab initio calculations allowed to identify their origin as the tilt of the graphene π-orbitals. These distortions are therefore purely artificial. This is an important result for future scanning tunneling microscopy studies of graphene.

EPFL2013

Investigation and modification of the magnetism of epitaxial Fe structures

Diego Repetto

In this thesis, the magnetic and structural properties of layered epitaxial film systems and clusters are presented. A main achievement is the investigation of the direct correlation between magnetism and structural or morphological details of ultrathin films. For this purpose, the film structure has been accessed directly by means of surface science methods. The effect of structural changes on the magnetism were observed in-situ by magneto-optical techniques. First, epitaxial Fe films grown at low temperature (∼ 120K) on Cu(100) have been investigated. Such layers assume an fcc-crystal structure due to their interaction with the underlying Cu substrate. Within this work it is shown that the morphology of these films can be altered by annealing treatments. The thermally-induced morphological changes have direct consequences for domain wall propagation and result in increased coercivity and in modified surface anisotropy. Flat fcc-Fe layers on Cu(100) have been used as templates to form fcc-Fe/Cu/Fe trilayers with coupled perpendicular magnetization. Such systems might be of relevance for data storage applications. The perpendicular coupling is ascribed to the realignment of the magnetization of the bottom Fe layer within the stray field produced by the top Fe layer magnetization. The magnetic coupling has been investigated as a function of the thickness of the top and the bottom Fe layer as well as the Cu spacer thickness. It was found that the magneto-static interaction is a direct consequence of the fcc-structure of both Fe layers and the interface roughness. It is demonstrated that Fe films with fcc-structure and perpendicular magnetization can also be prepared on other fcc-templates, such as Pt substrates. Monatomic steps on the substrate surface are exploited to steer the growth of the film and with it the magnetism. Despite significant differences in the morphology and the atomic structure of Fe films on flat Pt(111) and stepped Pt(997), the critical thickness for the spin reorientation transition is similar on the two surfaces. However, the local atomic arrangement above three monolayer film thickness determines in-plane easy magnetization axis parallel to the step edges. For all layered systems investigated in this thesis, epitaxial strain is a the driving force which influences the film structure and triggers structural transitions. The strain dependence of the morphology and the magnetism was probed by a comparative study of Fe deposited on Pt substrates with and without a noble gas buffer layer. The formation of relaxed Fe clusters during Xe layer desorption is favored in the absence of the overlayer-substrate interaction. Striking differences in the magnetic anisotropy between the systems are attributed to magneto-elastic contributions. Measurements of the blocking temperature of the Fe clusters allow estimating the cluster size. All samples have been investigated under ultra high vacuum conditions. The study of the sample magnetic properties study was done by in-situ magneto-optical Kerr effect measurements and Kerr microscopy. Structural characterization was done by low energy electron diffraction and, in collaboration with other scientists, by scanning tunnelling microscopy.

EPFL2006

Magnetism of individual adatoms and of epitaxial monolayers

Anne Lehnert

This thesis reports on the magnetism of low coordinated Fe and Co atoms in different chemical and electronic environments. The objective of this research was to contribute to the fundamental understanding of the magnetic properties, such as magneto-crystalline anisotropy energy (MAE), orbital and spin magnetic moment, of surface supported transition metal (TM) atoms and one atomic layer thick epitaxial TM films. A detailed knowledge on the interplay of electronic hybridization and magnetic properties is mandatory to elaborate new materials with high perpendicular magnetic anisotropy and acceptable writing field for future use in magnetic recording devices. The magnetic properties of the different experimental systems were investigated by x-ray absorption spectroscopy, x-ray magnetic circular dichroism, and magneto-optical Kerr effect (MOKE) and they were correlated to the morphology obtained by scanning tunneling microscopy (STM). The first part focuses on the effect of electronic hybridization between isolated 3d TM atoms and different kinds of substrates. First, we considered 4d and 5d TM substrates known for their high magnetic polarizability due to the close onset of ferromagnetism. Deposition of single Fe and Co atoms on Rh(111), Pd(111), and Pt(111) produces an increasing MAE going from Rh to Pd and finally to Pt that we directly correlate to the increasing spin-orbit constant of the substrate interlinking the induced magnetic moment and the MAE. Further we investigated single Fe and Co atoms on two different insulating layers, namely Al2O3/Ni3Al(111) and Cu2N/Cu(001), providing an efficient decoupling of the TM 3d states from the conduction electrons of the substrates. In case of the alumina film the absorption spectra of Fe and Co show a pronounced multiplet structure. On Cu2N the multiplet features of the adatoms are less defined due to the lower thickness of the screening layer. On both insulating layers we find a common out-of-plane easy axis, highly unquenched orbital moments, and giant MAE values of several meV/atom which are due to the formation of strong anisotropic bonds with the O and N atoms, respectively. The second part is dedicated to single atom thick layers of Fe and Co deposited on Pt(111) and Rh(111). We find the MAE to be strongly enhanced compared with bulk and we establish a direct proportionality between the induced magnetic moment of the topmost substrate layer and the MAE. The study of bimetallic Fe1-xCox alloys on platinum reveal a material with high MAE (0.5 meV at the equiatomic composition), large saturation magnetization (3μB to 2μB going from Fe to Co), and perpendicular magnetization easy axis. These characteristics hint at new possibilities for the further miniaturization of the grain size in recording devices. In the last part the theoretical description of the MOKE for ultra-thin films is extended to the transverse geometry and improved analytic formulae for the polar and longitudinal geometry are presented. This finding was used to optimize the Kerr signal for a new MOKE setup which is fully integrated in an ultrahigh vacuum chamber combining in situ MOKE and STM. The setup performance was demonstrated for a continuous film of 0.9 monolayers (ML) Co/Rh(111) with in-plane easy axis and for a superlattice of nanometric double layer Co islands on Au(11,12,12) with out-of-plane easy axis. The detection limit is 0.5 ML for transverse and 0.1 ML for polar Kerr geometry corresponding for island superlattices with the density of Co/Au(11,12,12) to islands composed of about 50 atoms. Moreover, the setup holds two independent ways to measure the MAE which is either by thermally induced magnetization reversal or by applying a torque onto the magnetization by turning the magnetic field. Both ways yield similar results for 1.1 ML Co/Au(11,12,12).

EPFL2009