Leonardo Lari

Properties of GaN Nanowires Grown by Molecular Beam Epitaxy

On Si(1 1 1) and Si(0 0 1), GaN nanowires (NWs) form in a self-induced way without the need for any external material. On sapphire, NW growth is induced by Ni collectors. Both types of NWs exhibit the wurtzite crystal structure and grow in the Ga-polar C-direction perpendicular to the substrate. The NW sidewalls are M-plane facets, although on the Ni-induced NWs also A-plane segments form, if the growth temperature is low. Both self-induced and collector-induced NWs are free of strain and epitaxially aligned to the substrate, but in particular the former show a significant spread in tilt and twist caused by a mostly amorphous interfacial layer of Si-N. The self-induced NWs are virtually free of extended defects, but the collector-induced NWs contain many stacking faults. The photoluminescence of the former is significantly brighter and sharper. The spectra of single, dispersed, self-induced NWs contain extremely sharp excitonic lines. Significant emission is caused by excitons bound to donors close to the surface whose binding energy is reduced compared to the bulk value. In comparison, both the microstructure and optical properties of the self-induced NWs are superior. The limited material quality of the collector-induced NWs can be explained by detrimental effects of the collector.

Enhanced oxidation of nanoparticles through strain-mediated ionic transport

Geometry and confinement effects at the nanoscale can result in substantial modifications to a materials properties with significant consequences in terms of chemical reactivity, biocompatibility and toxicity. Although benefiting applications across a diverse array of environmental and technological settings, the long-term effects of these changes, for example in the reaction of metallic nanoparticles under atmospheric conditions, are not well understood. Here, we use the unprecedented resolution attainable with aberration-corrected scanning transmission electron microscopy to study the oxidation of cuboid Fe nanoparticles. Performing strain analysis at the atomic level, we reveal that strain gradients induced in the confined oxide shell by the nanoparticle geometry enhance the transport of diffusing species, ultimately driving oxide domain formation and the shape evolution of the particle. We conjecture that such a strain-gradient-enhanced mass transport mechanism may prove essential for understanding the reaction of nanoparticles with gases in general, and for providing deeper insight into ionic conductivity in strained nanostructures.

Ferromagnetic InMnSb multi-phase films study by aberration-corrected (scanning) transmission electron microscopy

In this work we report a structural and compositional study of ferromagnetic In0.78Mn0.22Sb films correlated to the magnetic properties as determined by superconducting quantum interference device magnetometer. The epilayers grown by metalorganic vapor phase epitaxy on GaAs(001) substrates showed two active magnetic components with Curie temperatures of approximately 300 K and in excess of 570 K. Secondary phases driven by the high manganese concentration (10 at. %) were identified by high-resolution (scanning) transmission electron microscopy imaging and energy dispersive X-ray spectroscopy. Most of the Mn was found to be incorporated in metallic manganese nanoprecipitates surrounded by an InMnSb matrix with Mn at 1 at. % concentration. The origin of the two Curie temperatures of the film is associated with the presence of three magnetic components: hexagonal MnSb nanoprecipitates, non-stoichiometric MnAsSb, and the InMnSb matrix.

Exchange Bias in Fe@Cr Core-Shell Nanoparticles

We have used X-ray magnetic circular dichroism and magnetometry to study isolated Fe@Cr core-shell nanoparticles with an Fe core diameter of 2.7 nm (850 atoms) and a Cr shell thickness varying between 1 and 2 monolayers. The addition of Cr shells significantly reduces the spin moment but does not change the orbital moment. At least two Cr atomic layers are required to stabilize a ferromagnetic/antiferromagnetic interface and generate the associated exchange bias and increase in coercivity.

Origin of anomalous magnetite properties in crystallographic matched heterostructures: Fe3O4(111)/MgAl2O4(111)

Magnetite films grown on crystallographically matched substrates such as MgAl2O4 are not expected to show anomalous properties such as negative magnetoresistance and high saturation fields. By atomic resolution imaging using scanning transmission electron microscopy we show direct evidence of anti-phase domain boundaries (APB) present in these heterostructures. Experimentally identified 1/4<101> shifts determine the atomic structure of the observed APBs. The dominant non-bulk superexchange interactions are between 180° octahedral-Fe/O/octahedral-Fe sites which provide strong antiferromagnetic coupling across the defect interface resulting in non-bulk magnetic and magnetotransport properties.

Magnetism and magnetotransport in symmetry matched spinels: Fe3O4/MgAl2O4

In this work, we show that Fe3O4 films grown by oxygen plasma assisted molecular beam epitaxy have anomalous magnetic properties such as negative magnetoresistance and high saturation magnetic fields. The film substrate mismatch of 3% is relieved by the formation of misfit dislocations at the interface. Transmission electron microscopy results show that misfit dislocations are not the cause of antiphase domain boundary (APB). This suggests that in this system APB formation is a property of the three dimensional Fe3O4 growth.

Structural study of Fe3O4(111) thin films with bulk like magnetic and magnetotransport behaviour

Post-annealing of Fe3O4 films in a CO/CO2 atmosphere results in a significant improvement in magnetic and magnetotransport properties with values close to the single crystal bulk of Ms ∼ 480 emu/cm3 and negative magnetoresistance of 0.05% in a field of 1 T. By using atomic resolution Z-contrast transmission electron microscopy, we show that improved magnetic properties in the annealed films are due to improved structural ordering as a result of the annealing process.

Origin of reduced magnetization and domain formation in small magnetite nanoparticles

The structural, chemical, and magnetic properties of magnetite nanoparticles are compared. Aberration corrected scanning transmission electron microscopy reveals the prevalence of antiphase boundaries in nanoparticles that have significantly reduced magnetization, relative to the bulk. Atomistic magnetic modelling of nanoparticles with and without these defects reveals the origin of the reduced moment. Strong antiferromagnetic interactions across antiphase boundaries support multiple magnetic domains even in particles as small as 12–14 nm.

Correlations between atomic structure and giant magnetoresistance ratio in Co2(Fe,Mn)Si spin valves

We show that the magnetoresistance of Co2FexMn1?xSi-based spin valves, over 70% at low temperature, is directly related to the structural ordering in the electrodes and at the electrodes/spacer (Co2FexMn1?xSi/Ag) interfaces. Aberration-corrected atomic resolution Z-contrast scanning transmission electron microscopy of device structures reveals that annealing at 350??C and 500??C creates partial B2/L21 and fully L21 ordering of electrodes, respectively. Interface structural studies show that the Ag/Co2FexMn1?xSi interface is more ordered compared to the Co2FexMn1?xSi/Ag interface. The release of interface strain is mediated by misfit dislocations that localize the strain around the dislocation cores, and the effect of this strain is assessed by first principles electronic structure calculations. This study suggests that by improving the atomic ordering and strain at the interfaces, further enhancement of the magnetoresistance of CFMS-based current-perpendicular-to-plane spin valves is possible.

Atomic and electronic structure of twin growth defects in magnetite

We report the existence of a stable twin defect in Fe3O4 thin films. By using aberration corrected scanning transmission electron microscopy and spectroscopy the atomic structure of the twin boundary has been determined. The boundary is confined to the (111) growth plane and it is non-stoichiometric due to a missing Fe octahedral plane. By first principles calculations we show that the local atomic structural configuration of the twin boundary does not change the nature of the superexchange interactions between the two Fe sublattices across the twin grain boundary. Besides decreasing the half-metallic band gap at the boundary the altered atomic stacking at the boundary does not change the overall ferromagnetic (FM) coupling between the grains.