ABSTRACT
Understanding the origin of magnetic ordering in an undoped semiconductor with native defects is an open question, which is being explored actively in research. In this investigation, the interplay between magnetic ordering and excess induced native defects in undoped anatase TiO2 nanoparticles is explained using an experimental and theoretical approach. It is demonstrated that structurally disordered TiO2 nanoparticles with a high concentration of native defects such as titanium interstitials and oxygen vacancies are synthesized using controlled atmospheric rapid cooling (i.e. quenching) process. The structural disorders in the lattice have been examined using various spectroscopic and microscopic analyses revealed the existence of Ti deficiency in both pristine and quenched TiO2 nanoparticles. A possible origin of magnetic ordering in titanium deficient anatase TiO2 system is elucidated based on first-principle calculations. It was found that the overall magnetic moment of Ti deficient TiO2 system is determined by the distance between Ti interstitials and its neighboring vacancies (i.e. either V Ti or V Os). However, quenched TiO2 nanoparticles possess excess Ti interstitials, Ti and O vacancies and therefore the net magnetic moment of the system is reduced due to anti-ferromagnetically coupled neighboring Tilattice ions.
ABSTRACT
We present a general approach to thin bulk samples to transparency for experiments in the soft x-ray and extreme ultraviolet spectral range. The method relies on mechanical grinding followed by focused-ion-beam milling. It results in a uniformly thin area of high surface quality, suitable for nanoscale imaging in transmission. In a proof-of-principle experiment, nanoscale magnetic bits on a commercial hard drive glass disk are imaged with a spatial resolution below 30 nm by soft x-ray spectro-holography. Furthermore, we demonstrate imaging of a lithographically patterned test object via absorption contrast. Our approach is suitable for both amorphous and crystalline substrates and has been tested for a variety of common epitaxy growth substrates. Lateral thinning areas in excess of 100 µm2 and a remaining substrate thickness as thin as 150 nm are easily achievable. Our approach allows preserving a previously grown thin film, and from nanofocus electron diffraction, we find no evidence for morphological changes induced by the process, in agreement with numerical simulations of the ion implantation depth distributon. We expect our method to be widely applicable and especially useful for nanoscale imaging of epitaxial thin films.
ABSTRACT
Magnetic γ-Fe(2)O(3) nanoparticles with a mean diameter of 20 nm and size distribution of 7% were chemically synthesized and spin-coated on top of a Si-substrate. As a result, the particles self-assembled into a monolayer with hexagonal close-packed order. Subsequently, the nanoparticle array was coated with a Co layer of 20 nm thickness. The magnetic properties of this composite nanoparticle/thin film system were investigated by magnetometry and related to high-resolution transmission electron microscopy studies. Herein three systems were compared: i.e. a reference sample with only the particle monolayer, a composite system where the particle array was ion-milled prior to the deposition of a thin Co film on top, and a similar composite system but without ion-milling. The nanoparticle array showed a collective super-spin behavior due to dipolar interparticle coupling. In the composite system, we observed a decoupling into two nanoparticle subsystems. In the ion-milled system, the nanoparticle layer served as a magnetic flux guide as observed by magnetic force microscopy. Moreover, an exchange bias effect was found, which is likely to be due to oxygen exchange between the iron oxide and the Co layer, and thus forming of an antiferromagnetic CoO layer at the γ-Fe(2)O(3)/Co interface.
