41 research outputs found
Combined theoretical and experimental study of the Moir\'e dislocation network at the SrTiO-(La,Sr)(Al,Ta)O interface
Recently a highly ordered Moir\'e dislocation lattice was identified at the
interface between a \ce{SrTiO3} (STO) thin film and the
(LaAlO)(SrTaAlO) (LSAT) substrate. A fundamental
understanding of the local ionic and electronic structure around the
dislocation cores is crucial to further engineer the properties of these
complex multifunctional heterostructures. Here we combine experimental
characterization via analytical scanning transmission electron microscopy with
results of molecular dynamics and density functional theory calculations to
gain insights into the structure and defect chemistry of these dislocation
arrays. Our results show that these dislocations lead to undercoordinated Ta/Al
cations at the dislocation core, where oxygen vacancies can easily be formed,
further facilitated by the presence of cation vacancies. The reduced Ti
observed experimentally at the dislocations by electron energy-loss
spectroscopy are a consequence of both the structure of the dislocation itself,
as well as of the electron-doping due to oxygen vacancy formation. Finally, the
experimentally observed Ti diffusion into LSAT around the dislocation core
occurs only together with cation-vacancy formation in LSAT or Ta diffusion into
STO
Modification of the ferromagnetic anisotropy and exchange bias field of NiFe/CoO/Co trilayers through the CoO spacer thicknesses
Combined Theoretical and Experimental Study of the Moiré Dislocation Network at the SrTiO3-(La,Sr)(Al,Ta)O3 Interface.
Recently, a highly ordered Moiré dislocation lattice was identified at the interface between a SrTiO3 (STO) thin film and the (LaAlO3)0.3(Sr2TaAlO6)0.7 (LSAT) substrate. A fundamental understanding of the local ionic and electronic structures around the dislocation cores is crucial to further engineer the properties of these complex multifunctional heterostructures. Here, we combine experimental characterization via analytical scanning transmission electron microscopy with results of molecular dynamics and density functional theory calculations to gain insights into the structure and defect chemistry of these dislocation arrays. Our results show that these dislocations lead to undercoordinated Ta/Al cations at the dislocation core, where oxygen vacancies can easily be formed, further facilitated by the presence of cation vacancies. The reduced Ti3+ observed experimentally at the dislocations by electron energy-loss spectroscopy is a consequence of both the structure of the dislocation itself and of the electron doping due to oxygen vacancy formation. Finally, the experimentally observed Ti diffusion into the LSAT around the dislocation core occurs only together with cation vacancy formation in the LSAT or Ta diffusion into STO
Unusual ferrimagnetism in CaFe2O4
Incomplete cancellation of collinear antiparallel spins gives rise to
ferrimagnetism. Even if the oppositely polarized spins are owing to the equal
number of a single magnetic element having the same valence state, in
principle, a ferrimagnetic state can still arise from the crystallographic
inequivalence of the host ions. However, experimental identification of such a
state as ferrimagnetic is not straightforward because of the tiny magnitude
expected for M and the requirement for a sophisticated technique to
differentiate similar magnetic sites. We report a synchrotron-based resonant
x-ray investigation at the Fe L2,3 edges on an epitaxial film of CaFe2O4, which
exhibits two magnetic phases with similar energies. We find that while one
phase of CaFe2O4 is antiferromagnetic, the other one is ferrimagnetic with an
antiparallel arrangement of an equal number of spins between two distinct
crystallographic sites with very similar local coordination environments. Our
results further indicate two distinct origins of an overall minute M; one is
intrinsic, from distinct Fe3+ sites, and the other one is extrinsic, arising
from defective Fe2+ likely forming weakly-coupled ferrimagnetic clusters. These
two origins are uncorrelated and have very different coercive fields. Hence,
this work provides a direct experimental demonstration of ferrimagnetism solely
due to crystallographic inequivalence of the Fe3+ as the origin of the weak M
of CaFe2O4.Comment: 14 pages, 8 figure
Hole doping in compositionally complex correlated oxide enables tunable exchange biasing
Magnetic interfaces and the phenomena arising from them drive both the design
of modern spintronics and fundamental research. Recently, it was revealed that
through designing magnetic frustration in configurationally complex entropy
stabilized oxides, exchange bias can occur in structurally single crystal
films. This eliminates the need for complex heterostructures and nanocomposites
in the design and control of magnetic response phenomena. In this work, we
demonstrate through hole doping of a high entropy perovskite oxide that tuning
of magnetic responses can be achieved. With detailed magnetometry, we show
magnetic coupling exhibiting a variety of magnetic responses including exchange
bias and antiferromagnetic spin reversal in the entropy stabilized ABO3
perovskite oxide La1-xSrx(Cr0.2Mn0.2Fe0.2Co0.2Ni0.2)O3 family. We find that
manipulation of the A-site charge state can be used to balance magnetic phase
compositions and coupling responses. This allows for the creation of highly
tunable exchange bias responses. In the low Sr doping regime, a spin frustrated
region arising at the antiferromagnetic phase boundary is shown to directly
couple to the antiferromagnetic moments of the film and emerges as the dominant
mechanism, leading to a vertical shift of magnetization loops in response to
field biasing. At higher concentrations, direct coupling of antiferromagnetic
and ferromagnetic regions is observed. This tunability of magnetic coupling is
discussed within the context of these three competing magnetic phases,
revealing critical features in designing exchange bias through exploiting spin
frustration and disorder in high entropy oxides