78 research outputs found
Estrutura atômica e eletrônica em niquelatos (TRNiO3) através de espectroscopia de nÃveis profundos
Orientador: Helio Cesar Nogueira TolentinoTese (doutorado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb WataghinResumo: Neste trabalho de tese estudamos a estrutura atômica e eletrônica de sistemas de perovskitas de Ni com terras-raras (TRNiO3) utilizando a espectroscopia de absorção de nÃveis profundos. A fonte de luz sÃncrotron foi explorada na região de raios X moles para o estudo das bordas LIII e LII do Ni, e na região de raios X duros para o estudo da borda K do Ni e LIII dos terras raras. A propriedade mais notável nesses sistemas é a ocorrência de uma transição metal-isolante ao variarmos a temperatura da amostra. Essa tem-peratura de transição depende do Ãon terra-rara, e aumenta ao reduzirmos o tamanho do mesmo. Esses sistemas possuem uma estrutura perovskita distorcida, onde os octaedros NiO6 giram para preencher o espaço em torno do Ãon terra-rara. Quanto menor o Ãon, maior a distorção. Observamos assim uma estreita correlação entre a distorção da rede e a transição metal-isolante.Para sistemas com TR variando desde Pr ao Gd foi encontrada uma estrutura cristalográfica de simetria ortorômbica, onde o Ni ocupa um sÃtio simétrico.Baseado nisto, um primeiro modelo proposto sugeria que a transição seria devido ao estreitamento da banda formada pelos orbitais hibridizados Ni-3d e O-2p, quando o ângulo Ni-O-Ni diminui. No entanto, modificações na temperatura de transição metal-isolante pela substituição isotópica do O mostraram que o acoplamento elétron-fônon deveria ser importante. De fato, para sistemas com Ãons TR menores (de Ho a Lu) foi encontrada uma distorção monoclÃnica na fase isolante desses sistemas. Em tal estrutura o Ni ocupa sÃtios distintos com distâncias Ni-O diferentes. Um ordenamento de cargas nesses sÃtios distintos explica a fase isolante, bem como o ordenamento antiferromagnético observado para alguns dos compostos. Nossos resultados mostram que essas duas distâncias Ni-O coexistem para todos os sistemas, independentemente da estrutura cristalográfica de longo alcance, e em ambas fases eletrônicas. O sÃtio maior, de fraca hibridização, é o responsável pela localização eletrônica e coexiste com uma matriz condutora de forte hibridização. A transição metal isolante é explicada pelas modificações na proporção entre esses dois sÃtios. Dentro deste contexto, um forte acoplamento dos elétrons de condução com a rede é esperado, bem como a supressão da fase isolante sob pressãoAbstract: In this work, we studied the atomic and electronic structure of Ni perovskite systems (TRNiO3, TR=rare earth) using core level absorption spectroscopy. The synchrotron light source was exploited in the soft X-ray range to study Ni LIII and LII edges and in the hard X-ray range to study Ni K edge and rare earths LIII edges. The most remarkable property in these systems is a metal to insulator transition with temperature. This transition temperature depends on the rare-earth ion, increasing its value as the rare earth size is re-duced. These systems have a distorted perovskite structure, where the Ni O6octahedra rotate to fill the empty space left around the rare-earth ion. The smaller the ion, the larger the distortion. This indicates a straight correlation between the net distortion and the metal-insulator transition. For systems with TR varying from Pr to Gd it was found a crystallographic structure with orthorhombic symmetry, where Ni occupies a very symmetric site. Based on these results, it was proposed a model suggesting that the bandwidth would decrease due to a smaller hybridization between Ni3d and O2p bands caused by a decrease at the Ni-O-Ni angle. However, modifications in the transition temperature by the O isotope substitution, showed that the electron-phonon coupling plays an important role. Indeed, for systems with smaller TR ions (from Ho to Lu) it was found a monoclinic distortion in the insulating phase. In such structure Ni occupies two different sites with different Ni-O distances. A charge ordering associated to these different sites explains the insulating phase, as well as the antiferromagnetic ordering observed for some systems. Our results show that these two Ni-O distances coexist in all systems, inde-pendent of its long range crystallographic structure, and in both electronic phases. The site with longer Ni-O distance, which is weakly hybridized, is the responsible for the electronic localization and it is immersed in a con-ducting matrix with stronger hybridization. The metal-insulator transition is explained by the modifications in the proportion between these two Ni sites. In this context, a strong electron-phonon coupling is expected, as well as the suppression of the insulating phase under pressureDoutoradoFÃsicaDoutor em Ciência
An electron hole doping and soft x-ray spectroscopy study on La1-xSrxFe0.75Ni0.25O3-{\delta}
The conductivity of the electron hole and polaron conductor
La1-xSrxFe0.75Ni0.25O3-{\delta}, a potential cathode material for intermediate
temperature solid oxide fuel cells, was studied for 0 <x < 1 and for
temperatures 300 K <T < 1250 K. In LaSrFe-oxide, an ABO3 type perovskite,
A-site substitu-tion of the trivalent La3+ by the divalent Sr2+ causes
oxidation of Fe3+ towards Fe4+, which forms conducting electron holes. Here we
have in addition a B-site substitution by Ni. The compound for x = 0.5 is
identified as the one with the highest conductivity ({\sigma} ~ 678 S/cm) and
lowest activation energy for polaron conductivity (Ep = 39 meV). The evolution
of the electronic structure was monitored by soft x-ray Fe and oxygen K-edge
spectroscopy. Homogeneous trend for the oxida-tion state of the Fe was
observed. The variation of the ambient temperature conductivity and activation
energy with relative Sr content (x) shows a correlation with the ratio of
(eg/eg+t2g) in Fe L3 edge up to x=0.5. The hole doping process is reflected by
an almost linear trend by the variation of the pre-peaks of the oxygen K-edge
soft x-ray absorption spectra
Evidence for Coexistence of Bulk Superconductivity and Itinerant Antiferromagnetism in the Heavy Fermion System CeCo(InCd)
In the generic phase diagram of heavy fermion systems, tuning an external
parameter such as hydrostatic or chemical pressure modifies the superconducting
transition temperature. The superconducting phase forms a dome in the
temperature-tuning parameter phase diagram, which is associated with a maximum
of the superconducting pairing interaction. Proximity to antiferromagnetism
suggests a relation between the disappearance of antiferromagnetic order and
superconductivity. We combine muon spin rotation, neutron scattering, and x-ray
absorption spectroscopy techniques to gain access to the magnetic and
electronic structure of CeCo(InCd) at different time scales.
Different magnetic structures are obtained that indicate a magnetic order of
itinerant character, coexisting with bulk superconductivity. The suppression of
the antiferromagnetic order appears to be driven by a modification of the
bandwidth/carrier concentration, implying that the electronic structure and
consequently the interplay of superconductivity and magnetism is strongly
affected by hydrostatic and chemical pressure.Comment: Article + Supplementary information 33 pages, 13 figure
The impact of the near-surface region on the interpretation of x-ray absorption spectroscopy
Transition metal oxides (TMOs) exhibit a broad spectrum of electronic,
magnetic, and optical properties, making them intriguing materials for various
technological applications. Soft x-ray absorption spectroscopy (XAS) is widely
used to study TMOs, shedding light on their chemical state, electronic
structure, orbital polarization, element-specific magnetism, and more.
Different XAS acquisition modes feature different information depth regimes in
the sample. Here, we employ two XAS acquisition modes, having surface-sensitive
versus bulk probing depths, on the prototypical TMO SrVO3. We illustrate and
elucidate a strong apparent discrepancy between the different modes,
emphasizing the impact of the near-surface region on the interpretation of XAS
data. These findings highlight the importance of the acquisition mode selection
in XAS analysis. Moreover, the results highlight the role of the near-surface
region not only in the characterization of TMOs, but also in the design of
future nanoscale oxide electronics
Phonon activity and intermediate glassy phase of YVO₃
We show that in YVO₃ additional hard phonons gradually become zone center infrared active below ∼210 K, verifying that a lattice phase transition takes place at about that temperature. Their gradual increment in intensity between ∼210 and ∼77 K is associated with a "glassy" behavior found in the temperature-dependent V K edge pseudoradial distribution. This translates into an increase in the Debye-Waller factors ascribed to the appearance of V local structural disorder below ∼150 K. Conflicts between various ordering mechanisms in YVO₃ bring up similarities of the intermediate phase to known results in dielectric incommensurate systems, suggesting the formation of commensurate domains below 116 K, the onset temperature of G-type antiferromagnetism. We propose that ∼210 and ∼77 K be understood as the temperatures where the commensurate-incommensurate and incommensurate-commensurate "lock-in" phase transitions take place. We found support for this interpretation in the inverted λ shapes of the measured heat capacity and in the overall temperature dependence of the hard phonons.Facultad de Ciencias ExactasCentro de QuÃmica Inorgánic
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
Reversed ageing of FeO nanoparticles by hydrogen plasma
Magnetite (Fe3O4) nanoparticles suffer from severe ageing effects when exposed to air even when they are dispersed in a solvent limiting their applications. In this work, we show that this ageing can be fully reversed by a hydrogen plasma treatment. By x-ray absorption spectroscopy and its associated magnetic circular dichroism, the electronic structure and magnetic properties were studied before and after the plasma treatment and compared to results of freshly prepared magnetite nanoparticles. While aged magnetite nanoparticles exhibit a more γ-Fe2O3 like behaviour, the hydrogen plasma yields pure Fe3O4 nanoparticles. Monitoring the temperature dependence of the intra-atomic spin dipole contribution to the dichroic spectra gives evidence that the structural, electronic and magnetic properties of plasma treated magnetite nanoparticles can outperform the ones of the freshly prepared batch
Paramagnetic Nd sublattice and thickness-dependent ferromagnetism in Nd<sub>2</sub>NiMnO<sub>6</sub> double perovskite thin films
We investigate ferromagnetic and insulating thin films of the B-site ordered double perovskite Nd2NiMnO6 (NNMO) grown by radio frequency off-axis magnetron sputtering. The films grow epitaxially strained on a selection of substrates and display a strain-independent and bulklike TC of 200K at a thickness of 30 unit cells. We explore the thickness dependence of the NNMO/SrTiO3(001) system and find ferromagnetism down to ultralow thicknesses of only 3 unit cells (∼1.2nm). Below 10 unit cells, the magnetic properties deteriorate due to an interfacial charge transfer caused by the polar discontinuity at the NNMO/SrTiO3 interface. A detailed x-ray magnetic circular dichroism study allows us to separate the magnetic components into a robust ferromagnetic Ni/Mn sublattice and a paramagnetic Nd sublattice.</p
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