94 research outputs found
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
Low temperature irreversibility induced by thermal cycles on two prototypical phase separated manganites
We have studied the effect of irreversibility induced by repeated thermal
cycles on the electric transport and magnetization of polycrystalline samples
of La0.5Ca0.5MnO3 and La0.325Pr0.3Ca0.375MnO3. An increase of the resistivity
and a decrease of the magnetization at different temperature ranges after
cycling is obtained in the temperature range between 300 K and 30 K. Both
compounds are known to exhibit intrinsic submicrometric coexistence of phases
and undergo a sequence of phase transitions related to structural changes.
Changes induced by thermal cycling can be partially inhibited by applying
magnetic field and hydrostatic pressure.
Our results suggest that the growth and coexistence of phases with different
structures gives rise to microstructural tracks and strain accommodation,
producing the observed irreversibility. Irrespective of the actual ground state
of each compound, the effect of thermal cycling is towards an increase of the
amount of the insulating phase in both compounds.Comment: to appear in Journal of Alloys and Compounds (2003
Persistent magnetoresistive memory in phase separated manganites
We have studied magnetic and transport properties on different manganese
oxide based compounds exhibiting phase separation: polycrystalline
La5/8-yPryCa3/8MnO3 (y=0.3) and La1/2Ca1/2Mn1-zFezO3 (z = 0.05), and single
crystals of La5/8-yPryCa3/8MnO3 (y=0.35). Time dependent effects indicate that
the fractions of the coexisting phases are dynamically changing in a definite
temperature range. We found that in this range the ferromagnetic fraction f can
be easily tuned by application of low magnetic fields (< 1 T). The effect is
persistent after the field is turned off, thus the field remains imprinted in
the actual value of f and can be recovered through transport measurements. This
effect is due both to the existence of a true phase separated equilibrium state
with definite equilibrium fraction f0, and to the slow growth dynamics. The
fact that the same global features were found on different compounds and in
polycrystalline and single crystalline samples, suggests that the effect is a
general feature of some phase separated media.Comment: Proceedings of the MISM-2002 Moscow International Symposium on
Magnetism, Moscow, 20-24 June 2002. To appear in Journal of Magnetism and
Magnetic Material
Discontinuous properties of current-induced magnetic domain wall depinning
The current-induced motion of magnetic domain walls (DWs) confined to nanostructures is of great interest for fundamental studies as well as for technological applications in spintronic devices. Here, we present magnetic images showing the depinning properties of pulse-current-driven domain walls in well-shaped Permalloy nanowires obtained using photoemission electron microscopy combined with X-ray magnetic circular dichroism. In the vicinity of the threshold current density (J th = 4.2 × 10 11 â.A.m-2) for the DW motion, discontinuous DW depinning and motion have been observed as a sequence of "Barkhausen jumps". A one-dimensional analytical model with a piecewise parabolic pinning potential has been introduced to reproduce the DW hopping between two nearest neighbour sites, which reveals the dynamical nature of the current-driven DW motion in the depinning regime
Exceptional preservation of palaeozoic steroids in a diagenetic continuum
The occurrence of intact sterols has been restricted to immature Cretaceous (~125 Ma) sediments with one report from the Late Jurassic (~165 Ma). Here we report the oldest occurrence of intact sterols in a Crustacean fossil preserved for ca. 380 Ma within a Devonian concretion. The exceptional preservation of the biomass is attributed to microbially induced carbonate encapsulation, preventing full decomposition and transformation thus extending sterol occurrences in the geosphere by 250 Ma. A suite of diagenetic transformation products of sterols was also identified in the concretion, demonstrating the remarkable coexistence of biomolecules and geomolecules in the same sample. Most importantly the original biolipids were found to be the most abundant steroids in the sample. We attribute the coexistence of steroids in a diagenetic continuum-ranging from stenols to triaromatic steroids-to microbially mediated eogenetic processes
Thermochemical sulfate reduction in fossil Ordovician deposits of the Majiang area: Evidence from a molecular-marker investigation
The main reservoirs of Majiang fossil deposits consist of the Silurian Wengxiang group, dominantly sandstones, and the Ordovician Honghuayuan formation, dominantly carbonate rocks, and the Lower Cambrian Niutitang Formation mudstones serve as the major source rocks. Thermochemical sulfate reduction (TSR) might have taken place in the Paleozoic marine carbonate oil pools, as indicated by high concentrations of dibenzothiophenes in the extracts (MDBT=0.27-4.32 µg/g extract, and MDBT/MPH= 0.71-1.38). Hydrocarbons in the Pojiaozhai Ordovician carbonate reservoirs have undergone severe TSR and are characterized by higher quantities of diamondoids and MDBT and heavier isotopic values (δ13C=-28.4‰). The very large amounts of dibenzothiophenes might be products of reactions between biphenyls and sulfur species associated with TSR
Citizen science as seen by scientists: Methodological, epistemological and ethical dimensions
Citizen science as a way of communicating science and doing public engagement has over the past decade become the focus of considerable hopes and expectations. It can be seen as a win–win situation, where scientists get help from the public and the participants get a public engagement experience that involves them in real and meaningful scientific research. In this paper we present the results of a series of qualitative interviews with scientists who participated in the ‘OPAL’ portfolio of citizen science projects that has been running in England since 2007: What were their experiences of participating in citizen science? We highlight two particular sets of issues that our participants have voiced, methodological/epistemological and ethical issues. While we share the general enthusiasm over citizen science, we hope that the research in this paper opens up more debate over the potential pitfalls of citizen science as seen by the scientists themselves
- …