Growth and magnetism of 2D bimetallic nanostructures

This thesis addresses the growth and magnetic characterization of 2D bimetallic nanostructures deposited by atomic beam epitaxy (ABE) on Pt(111). These structures possess both tunable high perpendicular magnetic anisotropy (PMA) and magnetic moment. These properties make them appealing as model systems in order to learn how to control the properties of the futures media used in magnetic data storage. Our study combined two in situ measurements techniques : variable-temperature scanning tunneling microscopy (VT-STM) as a local probe allows insight on the morphology of nanostructures, while magneto-optic Kerr effect (MOKE) is a spatially integrating technique giving access to the variation of the overall magnetization of a sample. The first part of this work focuses on the growth of iron on the Pt(111) surface. Growth was investigated on the atomic scale as a function of the substrate temperature in the case of low coverages. We have fitted the mean cluster size as a function of the annealing temperature with mean field rate equations for diffusion-controlled growth. The activation parameters for monomer, dimer and trimer diffusion could be inferred from this procedure. The formation of monatomic Fe wires has also been evidenced on the temperature range 160 K–260 K. The origin of their formation was discussed. In a second part, we have made use of our knowledge on the growth of cobalt and iron on Pt(111) in order to fabricate "core-shell" Co nanostructures of which density, size and shape were controlled. Therefore, we could realized both compact and ramified structures within the size range 800–1800 atoms. The study of the mechanism of magnetization reversal of these model structures has revealed a strong size and shape dependence. This is due to the shape-induced non-uniformity of the local magnetization and the number of pinning centers. The conclusions are that ramified structures with arms longer than 150 Å reverse their magnetization by nucleation and domain-wall motion while compact structures reverse coherently their magnetization. The third part deals with the magnetic properties of one monolayer thick bimetallic "Co core-shell" nanostructures on Pt(111). The blocking temperature TB marks the transition between superparamagnetic and blocked states and is inferred from the magnetic anisotropy. Here, we performed magnetic zero-field susceptibility measurements so as to determine TB in our samples. From our experiments, we show the possibility to make up a fine tuning of the nanostructure magnetic anisotropy and overall magnetization. In the case of the FexCo1-x alloy, TB adopt a bell-shape with x and exhibit a maximum at x = 0:5. The various lateral and vertical interfaces between Co from one side and Fe, Pt or Pd from the other side are at the origin of substantial TB variation. Those variations are inferred from the symmetry breaking and the strong hybridization between the d orbitals of these elements.

Magnetism and Atomic Scale Structure of Bimetallic Nanostructures at Surfaces

Sergio Vlaic

his thesis reports on the magnetic properties of bi- and tri-metallic nanostructures at surfaces. The main goal is to build the smallest nanostructure ferromagnetic at room temperature and the assembly of those structures in high density arrays. To this purpose we have developed an approach consisting in a fine engineering of the nanostructure chemical structure at the atomic level. We grow core-shell nanostructures with atomically sharp interfaces between the different constituents and we investigate the effects produced by these interfaces as a function of their chemistry and structure. This approach is then used to grow organized arrays of bimetallic nanostructures that represent model systems for next generation high density magnetic storage media. The magnetic properties of the different experimental systems were investigated by magneto-optical Kerr effect (MOKE) and they were correlated to the morphology obtained by scanning tunneling microscopy (STM). The first part focuses on the understanding of the effects of atomically sharp interfaces among several elements. Two dimensional cobalt nanostructures have been grown on Pt(111) single crystal surface and subsequently their edges have been decorated by Fe, Pt or Pd. This one-dimensional decoration produces element dependent variations of the magnetic anisotropy energy with strong enhancement for Fe decoration and reduction for Pt and Pd. Furthermore we observe a crystallographic direction dependence on the effect of Pt and Pd decoration since Co/Pt or Co/Pd staking with {111}-orientation strongly enhances the magnetic hardness of the nanostructures. The effect of atomically sharp Co/Fe one-dimensional interface has been compared with direct alloying of the two elements finding the first one to give the highest effect for shell thicknesses smaller than 5 atoms. With the help of fully relativistic ab-initio we were able to reproduce the experimental results and unravel their pure electronic origin. The electronic hybridizations at the interface between the elements gives rise to "hot spots" in the electronic band structure responsible of a strong variation of the magnetocrystalline anisotropy resulting in the enhanced magnetic hardness. By growing Co-core Fe-shell Pd-capped nanostructures we were able to enhance the magnetization thermal stability of nanostructures by almost a factor of 3 with respect single-metal nanostructures of the same size. In the second part of this thesis, the first attempt of growing organized array of bimetallic nanostructures is presented. Taking advantage of the well-known self organized growth of Co nanodots on Au(111)-vicinal surfaces, we were able to grow Co-core Fe-shell nanodots on Au(11,12,12) single crystal. The thermal stability enhancement produced by the atomically sharp Co/Fe interface is higher than the direct alloying of the two elements as in the previous case. We argue that with this method, combined with three-dimensional growth of nanostructures arrays, it will be possible to produce multimetallic nanodots composed by ≈ 3000 atoms with magnetization thermal stability suitable for magnetic storage media. In the last part another model system for magnetic nanostructure superlattices is presented. Fe nanostructures have been grown on the Pd-seeded and unseeded alumina thin film formed by oxidation of a Ni3Al(111) single crystal surface at high temperature. We found that the sample stoichiometry near the surface evolves with the preparation cycles. This leads to the formation of a Ni-rich region, close to the alumina surface, ferromagnetic up to 250 K. By means of X-ray magnetic circular dichroism measurements we were able to estimate the amount of Ni in excess. Fe clusters deposited on the alumina surface are magnetically coupled to the Ni-rich region independently on the presence of Pd. Nevertheless, the system behavior cannot be explained by a simple ferromagnetic or antiferromagnetic coupling. We argue that the Fe nanostructures are coupled to the substrate by an interlayer exchange coupling across the alumina thin film. The coupling constant is expected to change sign depending on the distance between the Fe clusters and the Ni-rich region resulting in a peculiar magnetic behavior of the system.

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