Coordination dependent magnetic properties of 3d and 4d metal nano-structures

In this thesis, the magnetic properties of self-assembled 3d and 4d metal nano-structures supported on surfaces have been investigated. The atomic coordination within the nano-structures was found to profoundly affect important quantities such as the magnetic moment and the magnetic anisotropy. The use of thin, atomically flat insulating Xe spacers of 1-15 monolayers (ML) thickness allowed for a study of coordination effects in the two limits of strong and weak coupling with an underlying metal substrate. The systems were characterized by surface-sensitive methods, based on synchrotron radiation (X-ray magnetic circular dichroism, and X-ray scattering/diffraction) and variable temperature scanning tunneling microscopy (VT-STM). The VT-STM was developed and implemented during this PhD work. First, the magnetism of Rh nano-structures on a Xe buffer layer has been investigated. Rh is non-magnetic in bulk but shows a finite magnetic moment upon reducing cluster sizes to below 100 atoms. Within this work a small, non-zero magnetic moment was found for Rh nano-structures situated on Xe. The effect of intra-cluster Rh-Rh coordination was observed to affect both the spin and orbital part of the magnetic moment, leading to strongly oscillating values at smallest cluster sizes. Further, the analysis of the spectroscopic data suggests an interpretation for the absence of magnetism in directly deposited Rh on Ag(100) that is based on the formation of a kinetically promoted Ag-Rh alloy. Second, the buffer layer assisted growth (BLAG) was studied for sub-monolayer Co nano-clusters on Ag(111) and Pt(111) surfaces. The observation of the cluster formation process in the very early stages of BLAG revealed the paramount importance of the substrate in determining both magnetism and structural properties of the nano-clusters. On Ag(111), a weakly interacting substrate, the clusters form on the buffer layer independently from the metal substrate and show no magnetic anisotropy at this stage. As soon as the Xe is desorbed by sample annealing an in-plane anisotropy forms, as a consequence of the contact with the substrate. X-ray scattering and diffraction data support this interpretation and also show that in the limit of a single monolayer of Xe on Ag(111) the BLAG is a 'simple' atomic diffusion process, with a very high mobility of Co atoms on Xe. On a thick Xe buffer layer instead, due to a lower Xe-Xe binding energy and to the higher surface energy of Co compared to Xe, the deposition of Co provokes a re-arrangement of the Xe atoms. On the other hand, on Pt(111) the BLAG process fails to ensure a cluster formation process ontop the buffer layer and independent of the metal substrate. Here in fact, electric dipolar interactions occurring between Co atoms and the substrate through the Xe layer, are strong enough to destroy the Xe ML order and bring the Co atoms in direct contact with the Pt(111) before Xe atoms are thermally desorbed. This complex process becomes evident from VT-STM investigations and by the occurrence of perpendicular magnetic anisotropy right after Co deposition on the Xe ML/Pt(111). In a detailed discussion it is shown that magnetic properties like magnetic anisotropy and orbital/spin moments are strongly entangled with their morphology. Both morphology and magnetism are determined by the interaction with the environment. This opens the way to more complex systems, where the interaction with the medium is tuned such as to gain nano-structures with a pre-defined structure and function. Third, the knowledge about the cluster-substrate interactions during BLAG was exploited to build highly ordered arrays of Co nano-structures on a patterned template substrate. In this case the hexagonal boron nitride (h-BN) nanomesh on Rh(111) was used. These systems have been employed to study the effect of hybridization of the Co d band with capping layers such as Pt, Au, Al2O3 and MnPt on the magnetic moment of Co. It was found that in all these cases Co clusters have no remanence, due to the small size and weak coupling with the h-BN atoms. However, it could be shown that capping the clusters strongly influence the clusters magnetization, in a non-trivial way

Magnetism of cobalt nanoclusters on graphene on iridium

The structure and magnetic properties of Co clusters, comprising from 26 to 2700 atoms, self-organized or not on the graphene/Ir(111) moir\'e, were studied in situ with the help of scanning tunneling microscopy and X-ray magnetic circular dichroism. Surprisingly the small clusters have almost no magnetic anisotropy. We find indication for a magnetic coupling between the clusters. Experiments have to be performed carefully so as to avoid cluster damage by the soft X-rays

Modeling Ferro- and Antiferromagnetic Interactions in Metal-Organic Coordination Networks

Magnetization curves of two rectangular metal-organic coordination networks formed by the organic ligand TCNQ (7,7,8,8-tetracyanoquinodimethane) and two different (Mn and Ni) 3d transition metal atoms [M(3d)] show marked differences that are explained using first principles density functional theory and model calculations. We find that the existence of a weakly dispersive hybrid band with M(3d) and TCNQ character crossing the Fermi level is determinant for the appearance of ferromagnetic coupling between metal centers, as it is the case of the metallic system Ni-TCNQ but not of the insulating system Mn-TCNQ. The spin magnetic moment localized at the Ni atoms induces a significant spin polarization in the organic molecule; the corresponding spin density being delocalized along the whole system. The exchange interaction between localized spins at Ni centers and the itinerant spin density is ferromagnetic. Based on two different model Hamiltonians, we estimate the strength of exchange couplings between magnetic atoms for both Ni- and Mn-TCNQ networks that results in weak ferromagnetic and very weak antiferromagnetic correlations for Ni- and Mn-TCNQ networks, respectively.Comment: 27 pages, 6 figures, accepted for publication; Journal of Physical Chemistry C (2014

Heavy hole states in Germanium hut wires

Hole spins have gained considerable interest in the past few years due to their potential for fast electrically controlled qubits. Here, we study holes confined in Ge hut wires, a so far unexplored type of nanostructure. Low temperature magnetotransport measurements reveal a large anisotropy between the in-plane and out-of-plane g-factors of up to 18. Numerical simulations verify that this large anisotropy originates from a confined wave function which is of heavy hole character. A light hole admixture of less than 1% is estimated for the states of lowest energy, leading to a surprisingly large reduction of the out-of-plane g-factors. However, this tiny light hole contribution does not influence the spin lifetimes, which are expected to be very long, even in non isotopically purified samples

Analysis of Performance Instabilities of Hafnia-Based Ferroelectrics Using Modulus Spectroscopy and Thermally Stimulated Depolarization Currents

The discovery of the ferroelectric orthorhombic phase in doped hafnia films has sparked immense research efforts. Presently, a major obstacle for hafnia's use in high-endurance memory applications like nonvolatile random-access memories is its unstable ferroelectric response during field cycling. Different mechanisms are proposed to explain this instability including field-induced phase change, electron trapping, and oxygen vacancy diffusion. However, none of these is able to fully explain the complete behavior and interdependencies of these phenomena. Up to now, no complete root cause for fatigue, wake-up, and imprint effects is presented. In this study, the first evidence for the presence of singly and doubly positively charged oxygen vacancies in hafnia–zirconia films using thermally stimulated currents and impedance spectroscopy is presented. Moreover, it is shown that interaction of these defects with electrons at the interfaces to the electrodes may cause the observed instability of the ferroelectric performance

Europium Underneath Graphene on Ir(111): Intercalation Mechanism, Magnetism, and Band Structure

The intercalation of Eu underneath Gr on Ir(111) is comprehensively investigated by microscopic, magnetic, and spectroscopic measurements, as well as by density functional theory. Depending on the coverage, the intercalated Eu atoms form either a $(2 \times 2)$ or a $(\sqrt{3} \times \sqrt{3})$R$30^{\circ}$ superstructure with respect to Gr. We investigate the mechanisms of Eu penetration through a nominally closed Gr sheet and measure the electronic structures and magnetic properties of the two intercalation systems. Their electronic structures are rather similar. Compared to Gr on Ir(111), the Gr bands in both systems are essentially rigidly shifted to larger binding energies resulting in n-doping. The hybridization of the Ir surface state $S_1$ with Gr states is lifted, and the moire superperiodic potential is strongly reduced. In contrast, the magnetic behavior of the two intercalation systems differs substantially as found by X-ray magnetic circular dichroism. The $(2 \times 2)$ Eu structure displays plain paramagnetic behavior, whereas for the $(\sqrt{3} \times \sqrt{3})$R$30^{\circ}$ structure the large zero-field susceptibility indicates ferromagnetic coupling, despite the absence of hysteresis at 10 K. For the latter structure, a considerable easy-plane magnetic anisotropy is observed and interpreted as shape anisotropy.Comment: 18 pages with 14 figures, including Supplemental Materia

Reconfigurable Si Nanowire Nonvolatile Transistors

Reconfigurable transistors merge unipolar p- and n-type characteristics of field-effect transistors into a single programmable device. Combinational circuits have shown benefits in area and power consumption by fine-grain reconfiguration of complete logic blocks at runtime. To complement this volatile programming technology, a proof of concept for individually addressable reconfigurable nonvolatile transistors is presented. A charge-trapping stack is incorporated, and four distinct and stable states in a single device are demonstrated

Coupling of single, double, and triple-decker metal-phthalocyanine complexes to ferromagnetic and antiferromagnetic substrates

We report a survey of the magnetic properties of metal-organic complexes coupled to ferromagnetic and antiferromagnetic surfaces. Using element-resolved X-ray magnetic circular dichroism, we investigate the magnetism of single, double, and triple-decker phthalocyanines focusing on MnPc, TbPc, and TbPc deposited on Ni, Mn, and CoO thin films. Depending on the number of Pc ligands, we find that the metal ions within the molecules couple either parallel or antiparallel to a ferromagnetic substrate. Whereas single-decker complexes such as MnPc form a unique magnetic entity with ferromagnetic films, the intrinsic single molecule magnet properties of TbPc and TbPc remain largely unaltered. TbPc deposited on perpendicularly magnetized Ni films exhibits enhanced magnetic stability compared to TbPc in molecular crystals, opposite to TbPc deposited on in-plane magnetized Ni. Depending on the competition between uniaxial anisotropy, superexchange, and Zeeman interaction, the magnetic moment of TbPc can be aligned parallel or antiparallel to that of the substrate by modulating the intensity of an external magnetic field. This occurs also for TbPc, but the substrate-induced exchange coupling in triple-decker molecules is found to be short-ranged, that is, limited to the Tb ion closer to the ferromagnetic surface. Finally, we discuss the conditions required to establish exchange bias between molecules and antiferromagnetic substrates. We show that TbPc deposited on antiferromagnetic Mn thin films exhibits both exchange bias and enhanced coercivity when field cooled parallel to the out-of-plane easy axis. However, exchange bias does not extend to all molecules on the surface. On oxide antiferromagnets such as CoO we find no evidence of exchange bias for either TbPc or MnPc

Graphene-Induced Magnetic Anisotropy of a Two-Dimensional Iron Phthalocyanine Network

A single layer of flat-lying iron phthalocyanine (FePc) molecules assembled on graphene grown on Ir(111) preserves the magnetic moment, as deduced by X-ray magnetic circular dichroism from the Fe L2,3 edges. Furthermore, the FePc molecules in contact with the graphene buffer layer exhibit an enhancement of the magnetic anisotropy, with emergence of an in-plane easy magnetic axis, reflected by an increased orbital moment of the FePc molecules in contact with the C atoms in the graphene sheet. The origin of the increased magnetic anisotropy is discussed, considering the absence of electronic state hybridization, and the breaking of symmetry upon FePc adsorption on graphene

Structure and magnetism of atomically thin Fe layers on flat and vicinal Pt surfaces

Ultrathin Fe films on Pt substrates have been investigated under ultrahigh vacuum conditions by scanning tunneling microscopy, low energy electron diffraction, magneto-optical Kerr effect, x-ray magnetic circular dichroism measurements, and Kerr microscopy. We present a comparison between Fe films on flat Pt(111) and stepped Pt(997), with particular focus on the magnetic anisotropy in the submonolayer thickness range below 0.2 monolayer coverage, and above the spin reorientation transition at 3 monolayer thickness. The comparison of structure and magnetism suggests that the perpendicular easy axis found for films thinner than three monolayers is due to dominating contributions from both film interfaces to the anisotropy energy. The Fe-Pt interface contribution has its origin in the hybridization of the Fe 3d with the Pt 5d band. The in-plane magnetic anisotropy above 3 atomic layers film thickness can be correlated directly with peculiarities of the film structure