Remarkably enhanced photovoltaic effects have been observed in the heterostructures of p-type A-site Nd3+-doped BiFeO3 (Bi0.9375Nd0.0625)FeO3 (or BFONd) polycrystalline ceramics and the n-type ITO thin film. The maximum power conversion is ~0.82%, which is larger than 0.015% in BiFeO3 (BFO) under blue-ultraviolet irradiation of wavelength λ = 405 nm. The current-voltage (I-V) characteristic curve suggests a p-n junction interface between the ITO thin film and BFO (or BFONd) ceramics. The band gaps calculated from first-principles for BFO and BFONd are respectively 2.25 eV and 2.23 eV, which are consistent with the experimental direct band gaps of 2.24 eV and 2.20 eV measured by optical transmission spectra. The reduction of the band gap in BFONd can be explained by the lower electronic Fermi level due to acceptor states revealed by first-principles calculations. The optical calculations show a larger absorption coefficient in BFONd than in BFO.
Highly
ordered L10 FePt films with the highest degree
of [001] preferred orientation were prepared by multilayer deposition
on amorphous oxidized Si substrates and subsequent rapid thermal annealing
(RTA). We identify how to synthesize the preferred orientation structure
on the amorphous substrate, which we find to be critical to the optimal
formation of the [001] preferred orientation. This was achieved by
controlling the heating rate of the RTA process. We also identify
how the influence of the beam power, sputtering working pressure,
and annealing time affect the structural and magnetic properties of
FePt films. For example, a L10 preferred orientation and
increased magnetic anisotropy of the FePt films have been attained
by composition and in-plane tensile stress adjustments. The perpendicular
field coercivity was enhanced by increasing the RTA heating rate,
which increased the dislocation densities induced by strain. We also
found that the strain fields of dislocations in FePt L10 films can act as the dominant pinning sites that restrict domain
wall motion, increasing the coercivity. We find that this is due to
the spatial range of the strain fields associated with dislocations
being similar to the domain wall width in the films.
FePd (001) films, prepared by an electron beam deposition system on MgO(100), exhibit a perpendicular magnetic anisotropy (1.7 × 107 erg/cc) with a high order parameter (0.92). The relation between stacking faults induced by the strain relaxation, which act as strong domain wall pinning sites, and the perpendicular coercivity of (001) oriented L10 FePd films prepared at different temperatures have been investigated. Perpendicular coercivity can be apparently enhanced by raising the stacking fault densities, which can be elevated by climbing dissociation of total dislocation. The increased stacking fault densities (1.22 nm−2) with large perpendicular coercivity (6000 Oe) are obtained for samples prepared at 650 °C. This present work shows through controlling stacking fault density in FePd film, the coercivity can be manipulated, which can be applied in future magnetic devices.
The diamagnetic semimetal CoSi presents unanticipated ferromagnetism as CoSi/SiO2 nanowires (NWs). Using first-principles calculations, we offer physical insights into the origins of this unusual magnetism. Due to the distorted and dangling bonds near the NW surface with different bond lengths, the transition metal (Co) d-orbital electron spin up and spin down populations become asymmetric from the exchange interactions, providing the mechanism for some of the measured magnetization. However, the distorted and dangling bonds are clearly not the only factor contributing to the magnetization of the NWs. The transmission electron microscopy selected area electron diffraction analysis of the CoSi region suggested a superlattice structure existed in the cubic CoSi, and defects existing as ordered vacancies in the CoSi were present. The simulation's results for the Co moment in the CoSi NWs without these ordered vacancies, but incorporating the surface and internal spin moments, is only 0.1638 μ(B)/atom Co, which is a ∼80% shortfall compared to the experimental value of 0.8400 μ(B)/atom Co. When the effects of ordered vacancies are incorporated into the simulation, 0.7886 μ(B) per surface Co atom, a much better match with the experimental value (within ∼6%), indicating that the internal ordered vacancies in the CoSi NWs are the dominant mechanism of ferromagnetism.
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