Transport properties of cluster-assembled magnetic nanostructures

The aim of this thesis is to get a deeper understanding of magnetic properties of small clusters when they are embedded in a host material. The study was motivated by the observation of the very peculiar properties that clusters present in the gas phase or when deposited on a substrate. The intriguing perspective is that those properties would be preserved in a solid state sample that could be used for technological applications. Preformed and well characterized clusters are used as building blocks to produce nanostructured films in which they are embedded in a host material in order to preserve them from chemical and thermal degradation when the sample is exposed to ambient conditions. For this purpose an innovative set-up has been constructed that allows the independent control of cluster size, concentration and chemical composition. In particular samples containing small magnetic clusters embedded in a non magnetic matrix have been produced in order to study the evolution of the magnetic behaviour of such cluster assembled materials. These samples have been subsequently studied by means of magnetotransport, in particular by measuring their magneto-resistance and Hall voltage as a function of temperature in magnetic fields of up to 5T. Additionally, a novel measurement protocol detecting the derivative of the resistance with respect to temperature as a function of magnetic field has been used to characterize the samples. Magnetotransport is known to be an important tool for the investigation of the magnetic behaviour as the conduction electrons can be considered as microscopic probes of the state of the sample. However, even though these properties have been studied extensively, a complete and universally accepted theory on the mechanisms giving rise to the observed magneto-resistance in cluster assembled materials is still to be found. In this work, the high quality of the produced samples allows an unequivocal identification of all the aspects that are not correctly described by the theories in use. In particular it is observed that, in the case of small clusters, the effect of magnetic interactions cannot be neglected even at very low concentrations. The consequence is twofold since both the superparamagnetic model of magnetization and the well known (1 – m2) expression for the magneto-resistance fail if a correlation between magnetic moment exists. Furthermore we claim that the Mott hypothesis of two parallel currents adopted for electrical conduction is not sufficient to describe the details of transport in granular systems. In fact the particular differential resistance measurement introduced in this work allows an underlining of the importance of the spin-channel mixing mechanisms that are not taken into account in the Mott picture.

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

Propriétés magnétiques de nanostructures de cobalt adsorbées

Nicolas Weiss

This thesis reports results on magnetic properties of supported cobalt nanostructures. The nanostructures were grown on single crystal metal surface by Molecular Beam Epitaxy in an Ultra High Vacuum chamber. Our experimental setup is based on two in situ measurement techniques. The first is the Scanning Tunneling Microscope (STM) allowing to investigate the nanostructure morphology. The second, called Surface Magneto-Optical Kerr Effect (SMOKE), probes the magnetism of the nanostructures. By combining the data obtained with both techniques we were able to investigate the magnetism of 2D nanoparticles down to the atomic level. The first aim of the thesis consisted in disclosing the role played by the differently coordinated atoms in determining the Magnetic Energy Anisotropy in 2D nanostructures. To this purpose, we performed experiments on the Co/Pt(111) system, chosen as a model system since the Pt surface is well known to increase the out-of-plane uniaxial anisotropy as well as the magnetic moment of the Co islands. A preliminary work has been devoted to the growth of the Co islands as a function of the deposition and annealing temperatures. As a result, we learned to grow three different island's shapes : ramified, compact and double-layer islands. We then concentrated on the magnetism of these islands. Due to the nonlinear relationship between perimeter length and surface area, we were able to distinguish the different contributions to the anisotropy energy of surface and perimeter atoms. We found that the magnetic anisotropy in 2D Co nanostructures is predominantly due to the low-coordinated edge atoms, for which the anisotropy energy can be as large as 20 times the bulk value. This finding opens new possibilities to separately tune the anisotropy and moment of nanostructures. To exemplify this, and to illustrate once more the role of perimeter atoms, we produced Co island with a non-magnetic Pt core. As expected, these bimetallic islands have identical anisotropy and lower magnetic moment to their equally shaped pure Co counterpart. The second aim we focused on was the investigation of the magnetic properties of ultra-high density arrays of nanostructures created by self-assembly. We chose Co/Au(788) as a model system since the nanoscopic Co dots were found to have a very narrow size distribution and to be placed onto a lattice which is phase coherent on a macroscopic length scale. Moreover, the array has a density of 26 Tdot/in2 which allows the investigation of inter-particle interaction in ultra high density arrays. We showed that the narrow size distribution of the dots corresponds to an unprecedented narrow Magnetic Anisotropy Energy distribution with 35% width at half of the maximum. Moreover, we demonstrated the weak magnetic coupling between the Co dots despite their high density. In parallel to these fundamental researches, we developed a new experimental setup that will replace the current one. We designed a UHV chamber equipped with a homemade variable temperature STM combined with a SMOKE experiment. A quadrupole, focused on the sample, generates a 0.35 T magnetic field. This field is one order of magnitude larger than the present one and can be uniformly rotated in the plane of the quadrupole. Furthermore, the design of the new chamber allows STM imaging in this magnetic field. In this report, we present the adopted solutions as well as the first tests of the components (STM, SMOKE, cryostat).

EPFL2004

Magnetotransport studies of superconducting Pr4Fe2As2Te1-xO4

László Forró, Richard Gaal, Janusz Andrzej Karpinski, Sergiy Katrych, Bálint Náfrádi, Andrea Pisoni, Péter Szirmai

We report a study of the electrical transport properties of single crystals of Pr(4)Fe(2)As(2)Te(1-)xO(4), a recently discovered iron-based superconductor. Resistivity, Hall effect, and magnetoresistance are measured in a broad temperature range revealing the role of electrons as dominant charge carriers. The significant temperature dependence of the Hall coefficient and the violation of Kohler's law indicate multiband effects in this compound. The upper critical field and the magnetic anisotropy are investigated in fields up to 16 T, applied parallel and perpendicular to the crystallographic c axis. Hydrostatic pressure up to 2 GPa linearly increases the critical temperature and the resistivity residual ratio. A simple two-band model is used to describe the transport and magnetic properties of Pr4Fe2As2Te1-xO4. The model can successfully explain the strongly temperature-dependent negative Hall coefficient and the high magnetic anisotropy, assuming that the mobility of electrons is higher than that of holes.