175 research outputs found
Probing phase coexistence and stabilization of the spin-ordered ferrimagnetic state by Calcium addition in the YBa_{1-x}Ca_{x}Co_{2}O_{5.5} layered cobaltites using neutron diffraction
In this article we study the effects of a partial substitution of Ba with the
smaller cation Ca in the layered cobaltites YBaCo_2O_{5+\delta} for \delta
\approx 0.5. Neutron thermodiffractograms are reported for the compounds
YBa_{0.95}Ca_{0.05}Co_2O_{5.5} (x_{Ca}=0.05) and YBa_{0.90}Ca_{0.10}Co_2O_{5.5}
(x_{Ca}=0.10) in the temperature range 20 K \leq T \leq 300 K, as well as high
resolution neutron diffraction experiments at selected temperatures for the
samples x_{Ca}=0.05, x_{Ca}=0.10 and the parent compound x_{Ca}=0. We have
found the magnetic properties to be strongly affected by the cationic
substitution. Although the "122" perovskite structure seems unaffected by Ca
addition, the magnetic arrangements of Co ions are drastically modified: the
antiferromagnetic (AFM) long-range order is destroyed, and a ferrimagnetic
phase with spin state order is stabilized below T \sim 290 K. For the sample
with x_{Ca}=0.05 a fraction of AFM phase coexists with the ferrimagnetic one
below T \sim 190 K, whereas for x_{Ca}=0.10 the AFM order is completely lost.
The systematic refinement of the whole series has allowed for a better
understanding of the observed low-temperature diffraction patterns of the
parent compound, YBaCo_2O_{5.5}, which had not yet been clarified. A two-phase
scenario is proposed for the x_{Ca}=0 compound which is compatible with the
phase coexistence observed in the x_{Ca}=0.05 sample
High temperature behavior of Sr-doped layered cobaltites Y(Ba1-xSrx)Co2O5.5: phase stability and structural properties
In this article we present a neutron diffraction in-situ study of the thermal
evolution and high-temperature structure of layered cobaltites Y(Ba, Sr)Co2
O5+{\delta}. Neutron thermodiffractograms and magnetic susceptibility
measurements are reported in the temperature range 20 K <= T <= 570 K, as well
as high resolution neutron diffraction experiments at selected temperatures.
Starting from the as-synthesized samples with {\delta} ~ 0.5, we show that the
room temperature phases remain stable up to 550 K, where they start loosing
oxygen and transform to a vacancy-disordered "112" structure with tetragonal
symmetry. Our results also show how the so-called "122" structure can be
stabilized at high temperature (around 450 K) in a sample in which the addition
of Sr at the Ba site had suppressed its formation. In addition, we present the
structural and magnetic properties of the resulting samples with a new oxygen
content {\delta} ~ 0.25 in the temperature range 20 K <= T <= 300 K
Comparison of GC–MS, GC–MRM-MS, and GC GC to characterise higher plant biomarkers in Tertiary oils and rock extracts
Higher plant biomarkers occur in various compound classes with an array of isomers that are challenging to separate and identify. Traditional one-dimensional (1D) gas chromatographic (GC) techniques achieved impressive results in the past, but have reached limitations in many cases. Comprehensive two-dimensional gas chromatography (GC × GC) either coupled to a flame ionization detector (GC × GC–FID) or time-of-flight mass spectrometer (GC × GC–TOFMS) is a powerful tool to overcome the challenges of 1D GC, such as the resolution of unresolved complex mixture (UCM). We studied a number of Tertiary, terrigenous oils, and source rocks from the Arctic and Southeast Asia, with special focus on angiosperm biomarkers, such as oleanoids and lupanoids. Different chromatographic separation and detection techniques such as traditional 1D GC–MS, metastable reaction monitoring (GC–MRM-MS), GC × GC–FID, and GC × GC–TOFMS are compared and applied to evaluate the differences and advantages in their performance for biomarker identification. The measured 22S/(22S + 22R) homohopane ratios for all applied techniques were determined and compare exceptionally well (generally between 2% and 10%). Furthermore, we resolved a variety of angiosperm-derived compounds that co-eluted using 1D GC techniques, demonstrating the superior separation power of GC × GC for these biomarkers, which indicate terrigenous source input and Cretaceous or younger ages. Samples of varying thermal maturity and biodegradation contain higher plant biomarkers from various stages of diagenesis and catagenesis, which can be directly assessed in a GC × GC chromatogram.The analysis of whole crude oils and rock extracts without loss in resolution enables the separation of unstable compounds that are prone to rearrangement (e.g. unsaturated triterpenoids such as taraxer-14-ene) when exposed to fractionation techniques like molecular sieving. GC × GC–TOFMS is particularly valuable for the successful separation of co-eluting components having identical molecular masses and similar fragmentation patterns. Such components co-elute when analysed by 1D GC and cannot be resolved by single-ion-monitoring, which prevents accurate mass spectral assessment for identification or quantification
Current Induced Fingering Instability in Magnetic Domain Walls
The shape instability of magnetic domain walls under current is investigated
in a ferromagnetic (Ga,Mn)(As,P) film with perpendicular anisotropy. Domain
wall motion is driven by the spin transfer torque mechanism. A current density
gradient is found either to stabilize domains with walls perpendicular to
current lines or to produce finger-like patterns, depending on the domain wall
motion direction. The instability mechanism is shown to result from the
non-adiabatic contribution of the spin transfer torque mechanism.Comment: 5 pages, 3 figures + supplementary material
Domain-wall roughness in GdFeCo thin films: crossover length scales and roughness exponents
Domain-wall dynamics and spatial fluctuations are closely related to each
other and to universal features of disordered systems. Experimentally measured
roughness exponents characterizing spatial fluctuations have been reported for
magnetic thin films, with values generally different from those predicted by
the equilibrium, depinning and thermal reference states. Here, we study the
roughness of domain walls in GdFeCo thin films over a large range of magnetic
field and temperature. Our analysis is performed in the framework of a model
considering length-scale crossovers between the reference states, which is
shown to bridge the differences between experimental results and theoretical
predictions. We also quantify for the first time the size of the depinning
avalanches below the depinning field at finite temperatures.Comment: 10 pages, 6 figures. The first two authors contributed equally to
this wor
Substitution site and effects on magnetism in Sr-for-Ca substituted CaBaCo4O7
Cationic substitutions in the novel magnetoelectric compound CaBaCo4O7 lead to profound changes in its magnetic and electric behaviors. In this work, we present a structural study of the isovalent substitution Sr-for-Ca in CaBaCo4O7. X-ray diffraction, as well as neutron powder diffraction experiments, are reported for a series of samples Ca1- xSrxBaCo4O7 with 0 = x = 0.10. Special emphasis is given to the identification of the substitution site, as Sr has also been reported to substitute for Ba in this crystal structure. The solubility limit for Sr at the Ca site is shown to be at x ¿ 0.08. The variation of lattice constants with Sr-doping firmly supports the Sr-for-Ca substitution. Rietveld refinements of the Sr-substituted samples are presented, and used as starting point to analyse the local structure around Sr by means of X-ray absorption spectroscopy at the Sr K-edge. Both the near-edge absorption and the extended absorption fine-structure confirm the substitution of Sr for Ca, giving definite support to the proposed nominal formula. In addition, macroscopic magnetization measurements are presented which reveal the striking effects of Sr-substitution over the magnetic landscape of this puzzling compound
Universal Critical Exponents of the Magnetic Domain Wall Depinning Transition
Magnetic field driven domain wall dynamics in a ferrimagnetic GdFeCo thin
film with perpendicular magnetic anisotropy is studied using low temperature
magneto-optical Kerr microscopy. Measurements performed in a practically
athermal condition allow for the direct experimental determination of the
velocity () and correlation length () exponents of the depinning transition. The whole family of exponents
characterizing the transition is deduced, providing evidence that the depinning
of magnetic domain walls is better described by the quenched Edwards-Wilkinson
universality class.Comment: 6 pages, 3 figures. Ancillary Material with 10 pages and 4 figures is
also include
Transient magnetic domain wall ac dynamics by means of magneto-optical Kerr effect microscopy
The domain wall response under constant external magnetic fields reveals a
complex behavior where sample disorder plays a key role. Furthermore, the
response to alternating magnetic fields has only been explored in limited cases
and analyzed in terms of the constant field solution. Here we unveil phenomena
in the evolution of magnetic domain walls under the application of alternating
magnetic fields within the creep regime, well beyond a small fuctuation limit
of the domain wall position. Magnetic field pulses were applied in ultra-thin
ferromagnetic films with perpendicular anisotropy, and the resulting domain
wall evolution was characterized by polar magneto-optical Kerr effect
microscopy. Whereas the DC characterization is well predicted by the elastic
interface model, striking unexpected features are observed under the
application of alternating square pulses: magneto-optical images show that
after a transient number of cycles, domain walls evolve toward strongly
distorted shapes concomitantly with a modification of domain area. The
morphology of domain walls is characterized with a roughness exponent when
possible and contrasted with alternative observables which result to be more
suitable for the characterization of this transient evolution. The final
stationary convergence as well as the underlying physics is discussed.Comment: 9 pages, 8 figure
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