Tunable spin polarization and superconductivity in engineered oxide interfaces

Advances in growth technology of oxide materials allow single atomic layer control of heterostructures. In particular delta doping, a key materials' engineering tool in today's semiconductor technology, is now also available for oxides. Here we show that a fully electric-field-tunable spin-polarized and superconducting quasi-2D electron system (q2DES) can be artificially created by inserting a few unit cells of delta doping EuTiO3 at the interface between LaAlO3 and SrTiO3 oxides(1,2). Spin polarization emerges below the ferromagnetic transition temperature of the EuTiO3 layer (T-FM = 6-8 K) and is due to the exchange interaction between the magnetic moments of Eu-4f and of Ti-3d electrons. Moreover, in a large region of the phase diagram, superconductivity sets in from a ferromagnetic normal state. The occurrence of magnetic interactions, superconductivity and spin-orbit coupling in the same q2DES makes the LaAlO3/EuTiO3/SrTiO3 system an intriguing platform for the emergence of novel quantum phases in low-dimensional materials.

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

Magnetocrystalline anisotropy energy of Co and Fe adatoms on the (111) surfaces of Pd and Rh

Harald Brune, Markus Etzkorn, Pietro Gambardella, Anne Lehnert, Géraud Moulas, Stefano Rusponi

We performed a combined theoretical and experimental investigation of the orbital magnetism and magnetocrystalline anisotropy of isolated Co and Fe adatoms on Pd(111) and Rh(111). Theoretical calculations of the spin and orbital moments are based on ab initio spin-polarized density-functional theory (DFT) including a self-consistent treatment of spin-orbit coupling. The calculations use a slab model to represent the adsorbate/substrate complex and allow for a complete structural relaxation leading to a strong inward displacement of the adatom and modest vertical and lateral relaxations in the substrate atoms. Compared to an idealized geometry where the atoms are kept on bulk lattice positions up to the surface, relaxation leads to a much stronger adatom/ligand hybridization. This is also reflected in the results for orbital moments and magnetocrystalline anisotropy energy (MAE). The enhanced hybridization leads to strong quenching of the adatom orbital moments but also to the formation of large induced spin and orbital moments in the substrate. As a consequence, we find that the substrate contribution to the MAE is much more important than estimated before on the basis of studies using an idealized geometry. We also find the surprising result that the MAE strongly depends on the adsorption site. The magnitude and even the sign of the MAE change for adatoms on face-centered cubic with respect to the ones on hexagonal close-packed hollow sites on the (111) surface. The dependence of the MAE on the combination of adatom and substrate has been analyzed in terms of the electronic structure, leading to a sound physical picture of the origin of the MAE. A fundamental problem, however, is the correct prediction of the size of the orbital moments of the adatoms. We suggest that this problem can be solved only via post-DFT corrections introducing an orbital dependence of the exchange potential. The theoretical results are compared to site-averaged, element-specific x-ray magnetic circular dichroism (XMCD) measurements. Low-temperature XMCD spectra and magnetization curves reveal weak out-of-plane anisotropy for Fe adatoms on both substrates. Interestingly, Co adatoms on Rh(111) present in-plane anisotropy with MAE of about -0.6 meV, contrary to the known out-of-plane anisotropy of Co on Pd(111) and Pt(111). The orbital to spin magnetic-moment ratio measured by XMCD shows that the Co adatoms present much stronger orbital magnetization components compared to Fe. The connection between orbital moments and MAE is discussed at the theoretical level including the contribution of the induced substrate magnetization.

2010

Magnetic anisotropy of Fe and Co adatoms and Fe clusters magnetically decoupled from Ni_{3}Al(111) by an alumina bilayer

Harald Brune, Markus Etzkorn, Anne Lehnert, Stefano Rusponi

The magnetic properties of individual Fe and Co atoms as well as of small Fe clusters adsorbed on two atomic layers of Al2O3 grown on Ni3Al(111) have been investigated by x-ray absorption spectroscopy and x-ray magnetic circular dichroism. We find ratios of the orbital over the effective spin magnetic moments of r=0.53 +/- 0.09 for Fe and r=0.91 +/- 0.06 for Co, which are both very close to the free-atom values of r=0.5 and 1, respectively. The magnetization curves acquired at the Fe and Ni edges demonstrate a distinctly different magnetization reversal of the transition-metal nanostructures and the substrate excluding magnetic dipolar or exchange coupling through the alumina film. Our data reveal an out-of-plane easy magnetization axis for Fe and Co originating from the spatially anisotropic hybridization of the 3d states of the adatom, with the 2p states of the oxygen terminated Al2O3 surface. We conclude that the alumina film effectively decouples the magnetic adatoms from the underlying metal substrate while providing a crystal-field environment giving rise to high magnetic anisotropy.

2010

Magnetocrystalline anisotropy energy of Co and Fe adatoms on the (111) surfaces of Pd and Rh

Harald Brune, Markus Etzkorn, Pietro Gambardella, Anne Lehnert, Géraud Moulas, Stefano Rusponi

2010

Transport properties of cluster-assembled magnetic nanostructures

Giulia Di Domenicantonio

EPFL2008