Experimental determination of Rashba and Dresselhaus parameters and g*- factor anisotropy via Shubnikov-de Haas oscillations

The spin splitting of conduction band electrons in inversion-asymmetric InGaAs/InP quantum wells (QWs) is studied by Shubnikov-de Haas measurements combining the analysis of beating patterns and coincidence measurements in doubly tilted magnetic fields. The method allows us to determine the absolute values of the Rashba and linear Dresselhaus spin-orbit interaction (SOI) coefficients, their relative sign and the full Lande g-tensor. This is achieved by analyzing the anisotropy of the beat node positions with respect to both polar and azimuthal angles between the magnetic field direction and the QW normal. Weshow that the SOI is dominated by a large Rashba coefficient together with a linear Dresselhaus coefficient that is 10% of the Rashba coefficient. Their relative sign is found to be positive. The g-tensor is found to have a marked out-of-plane anisotropy and a smaller but distinct in-plane anisotropy due to SOI.

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

Transport properties of cluster-assembled magnetic nanostructures

Giulia Di Domenicantonio

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.

EPFL2008

The role of magnetic anisotropy in the Kondo effect

Harald Brune, Markus Ternes

In the Kondo effect, a localized magnetic moment is screened by forming a correlated electron system with the surrounding conduction electrons of a non-magnetic host1. Spin S D 1=2 Kondo systems have been investigated extensively in theory and experiments, but magnetic atoms often have a larger spin2. Larger spins are subject to the influence of magnetocrystalline anisotropy, which describes the dependence of the magnetic moment’s energy on the orientation of the spin relative to its surrounding atomic environment3,4. Here we demonstrate the decisive role of magnetic anisotropy in the physics of Kondo screening. A scanning tunnelling microscope is used to simultaneously determine the magnitude of the spin, the magnetic anisotropy and the Kondo properties of individual magnetic atoms on a surface. We find that a Kondo resonance emerges for large-spin atoms only when the magnetic anisotropy creates degenerate ground-state levels that are connected by the spin flip of a screening electron. The magnetic anisotropy also determines how the Kondo resonance evolves in a magnetic field: the resonance peak splits at rates that are strongly direction dependent. These rates are well described by the energies of the underlying unscreened spin states.