Enhanced magnetism and suppressed magnetoelastic coupling induced by electron doping in Ca1-xYxMnReO6

The Ca₂MnReO₆double perovskite is a spin-orbit-assisted Mott insulator with exotic magnetic properties, including a largely non-collinear Mn²⁺spin arrangement and nearly orthogonal coupling between such spins and the much smaller Re 5dmagnetic moments. Here, the electron-doped compound Ca_1-xY_xMnReO₆(x= 0.1, 0.2 and 0.3) is reported and a detailed investigation is conducted forx= 0.3. Neutron and x-ray powder diffraction confirm that nearly full chemical order is maintained at the Mn and Re sites under the Y substitution at the Ca site. X-ray absorption measurements and an analysis of the Mn-O/Re-O bond distances show that the Mn oxidation state remains stable at +2 whereas Re is reduced upon doping. The electron doping increases the magnetic ordering temperature fromT_c= 121 to 150 K and also enhances significantly the ferromagnetic component of the Mn spins at the expense of the antiferromagnetic component at the base temperature (T= 3 K). The lattice parameter anomalies atT_cobserved in the parent compound are suppressed by the electron doping. The possible reasons for the enhanced magnetism and the suppressed magnetoelastic coupling in Ca_1.7Y_0.3MnReO₆are discussed.

Electronic and magnetic properties of stoichiometric CeAuBi2

We report the electronic and magnetic properties of stoichiometric CeAuBi₂ single crystals. At ambient pressure, CeAuBi₂ orders antiferromagnetically below a Néel temperature (T_N ) of 19 K. Neutron diffraction experiments revealed an antiferromagnetic propagation vector τ ^ = [ 0 , 0 , 1 ∕ 2 ] , which doubles the paramagnetic unit cell along the c axis. At low temperatures several metamagnetic transitions are induced by the application of fields parallel to the c axis, suggesting that the magnetic structure of CeAuBi₂ changes as a function of field. At low temperatures, a linear positive magnetoresistance may indicate the presence of band crossings near the Fermi level. Finally, the application of external pressure favors the antiferromagnetic state, indicating that the 4f electrons become more localized.

Evidence of precursor orthorhombic domains well above the electronic nematic transition temperature in Sr(Fe1-x Co x )2As2

Raman scattering, synchrotron x-ray diffraction, specific heat, resistivity and magnetic susceptibility measurements were performed in Sr(Fe_1-x Co _x )₂As₂ [[Formula: see text]] single crystals with superconducting critical temperature [Formula: see text] K and two additional transitions at 132 and 152 K observed in both specific heat and resistivity data. A quasielastic Raman signal with B _2g symmetry (tetragonal cell) associated with electronic nematic fluctuations is observed. Crucially, this signal shows maximum intensity at [Formula: see text] K, marking the nematic transition temperature. X-ray diffraction shows evidence of coexisting orthorhombic and tetragonal domains between [Formula: see text] and [Formula: see text]  ∼  152 K, implying that precursor orthorhombic domains emerge over an extended temperature range above [Formula: see text]. While the height of the quasielastic Raman peak is insensitive to [Formula: see text], the temperature-dependence of the average nematic fluctuation rate indicates a slowing down of the nematic fluctuations inside the precursor orthorhombic domains. These results are analogous to those previously reported for the LaFeAsO parent oxypnictide (Kaneko et al 2017 Phys. Rev. B 96 014506). We propose a scenario where the precursor orthorhombic phase may be generated within the electronically disordered regime ([Formula: see text]) as long as the nematic fluctuation rate is sufficiently small in comparison to the optical phonon frequency range. In this regime, the local atomic structure responds adiabatically to the electronic nematic fluctuations, creating a net of orthorhombic clusters that, albeit dynamical for [Formula: see text], may be sufficiently dense to sustain long-range phase coherence in a diffraction process up to [Formula: see text].

Atomic pair distribution function at the Brazilian Synchrotron Light Laboratory: application to the Pb1-xLaxZr0.40Ti0.60O3 ferroelectric system

This work reports the setting up of the X-ray diffraction and spectroscopy beamline at the Brazilian Synchrotron Light Laboratory for performing total scattering experiments to be analyzed by atomic pair distribution function (PDF) studies. The results of a PDF refinement for Al₂O₃ standard are presented and compared with data acquired at a beamline of the Advanced Photon Source, where it is common to perform this type of experiment. A preliminary characterization of the Pb_1-xLa_xZr_0.40Ti_0.60O₃ ferroelectric system, with x = 0.11, 0.12 and 0.15, is also shown.

XDS: a flexible beamline for X-ray diffraction and spectroscopy at the Brazilian synchrotron

The majority of the beamlines at the Brazilian Synchrotron Light Source Laboratory (LNLS) use radiation produced in the storage-ring bending magnets and are therefore currently limited in the flux that can be used in the harder part of the X-ray spectrum (above ∼10 keV). A 4 T superconducting multipolar wiggler (SCW) was recently installed at LNLS in order to improve the photon flux above 10 keV and fulfill the demands set by the materials science community. A new multi-purpose beamline was then installed at the LNLS using the SCW as a photon source. The XDS is a flexible beamline operating in the energy range between 5 and 30 keV, designed to perform experiments using absorption, diffraction and scattering techniques. Most of the work performed at the XDS beamline concentrates on X-ray absorption spectroscopy at energies above 18 keV and high-resolution diffraction experiments. More recently, new setups and photon-hungry experiments such as total X-ray scattering, X-ray diffraction under high pressures, resonant X-ray emission spectroscopy, among others, have started to become routine at XDS. Here, the XDS beamline characteristics, performance and a few new experimental possibilities are described.

Two-dimensional magnetic correlations and partial long-range order in geometrically frustrated Sr2YRuO6

Neutron diffraction on the double perovskite Sr(2)YRuO(6) with a quasi-fcc lattice of Ru moments reveals planar magnetic correlations that condense into a partial long-range ordered state with coupled alternate antiferromagnetic (AFM) YRuO(4) square layers coexisting with the short-range correlations below T(N1) = 32 K. A second transition to a fully ordered AFM state below T(N2) = 24 K is observed. The reduced dimensionality of the spin correlations is arguably due to a cancellation of the magnetic coupling between consecutive AFM square layers in fcc antiferromagnets, which is the simplest three-dimensional frustrated magnet model system.

Spin-electron-phonon excitation in Re-based half-metallic double perovskites

A remarkable hardening (~30 cm(-1)) of the normal mode of vibration associated with the symmetric stretching of the oxygen octahedra for the Ba(2)FeReO(6) and Sr(2)CrReO(6) double perovskites is observed below the corresponding magnetic ordering temperatures. The very large magnitude of this effect and its absence for the antisymmetric stretching mode provide evidence against a conventional spin-phonon coupling mechanism. Our observations are consistent with a collective excitation formed by the combination of the vibrational mode with oscillations of Fe or Cr 3d and Re 5d occupations and spin magnitudes.

Co-substitution effects on the Fe valence in the BaFe2As2 superconducting compound: a study of hard x-ray absorption spectroscopy

The Fe K x-ray absorption near edge structure of BaFe(2-x)Co(x)As(2) superconductors was investigated. No appreciable alteration in shape or energy position of this edge was observed with Co substitution. This result provides experimental support to previous ab initio calculations in which the extra Co electron is concentrated at the substitute site and do not change the electronic occupation of the Fe ions. Superconductivity may emerge due to bonding modifications induced by the substitute atom that weakens the spin-density-wave ground state by reducing the Fe local moments and/or increasing the elastic energy penalty of the accompanying orthorhombic distortion.

Unique Coordination of Copper in Hexacyanometallates

Within divalent transition metals hexacyanometallates (III) the copper (2+) salts show unique features. To the copper (2+) salts correspond the shortest unit cell edges, the highest ν(CN) vibration frequencies, the lowest hydration degree and dehydration temperatures, the lowest Mössbauer isomer shift value, and the strongest magnetic exchange interaction (J) between the metal centers. Such unique features were attributed to a particularly strong bond of the copper atom to the N ends of the CN groups. The driving force for such behaviour was ascribed to a combined effect where the copper (2+) shows a high ability to receive electrons in its 3d hole favouring an electronic configuration close to 3d 10 and the CN group complements such ability donating electrons through its 5σ orbital which has certain anti-bonding character. This hypothesis is supported by the obtained structural and spectroscopic data. The occurrence of a cooperative Jahn–Teller effect in this family of materials was discarded. In the pseudo-octahedral coordination for the copper (2+) atom the e g orbital degeneration is initially removed. The collected EPR spectra are characteristic of a cubic environment (isotropic g-values). Analogue evidence was obtained from high resolution X-ray powder patterns recorded in the 12–300K temperature range. All the patterns correspond to a cubic unit cell.

Incipient orbital order in half-metallic Ba2FeReO6

Largely unquenched Re 5d orbital magnetic moments in half-metallic Ba2FeReO6 drive a symmetry lowering transition from a cubic paramagnet to a compressed tetragonal (c/a < 1) ferrimagnet below Tc ~ 305 K, with a giant linear magnetoelastic constant and the spins lying spontaneously along the unique tetragonal axis. The large orbital magnetization and degree of structural deformation indicate proximity to a metal-insulator transition. These results point to an incipient orbitally ordered state in the metallic ferrimagnetic phase.

Unconventional metallic magnetism in LaCrSb3

Neutron-diffraction measurements in LaCrSb3 show a coexistence of ferromagnetic and antiferromagnetic sublattices below T(C)=126 K, with ordered moments of 1.65(4) and 0.49(4)mu(B)/formula unit, respectively (T=10 K), and a spin-reorientation transition at approximately 95 K. No clear peak or step was observed in the specific heat at T(C). Coexisting localized and itinerant spins are suggested.

Dramatic changes in the magnetic coupling mechanism for La-doped CaMnO3

The exchange interactions in polycrystalline samples of Ca1-xLaxMnO3 (0.00< or =x< or =0.05) are studied by means of Raman scattering and electron paramagnetic resonance. Dramatic reductions in the spin-phonon interactions and magnetic correlations are observed for La doping levels as small as approximately 2%-3%. These results show that the charge carriers play an important role in the overall exchange coupling in the electron-doped manganites, even at very low doping levels.

[Prevalence of cutaneous lesions in Freixo de Espada à Cinta]

Incidence or prevalence studies on cutaneous lesions in general populations are not available. The community observed by the authors (Freixo de Espada à Cinta, North-Eastern Portugal) is relatively closed. A pre-tested questionnaire for socio-demographic and clinical information was used and a physical observation was conducted in January and February 1994 by the Dermatology Team of Santo António General Hospital, Oporto. The analysis was supported by independent tests (Pearson and Fisher chi 2, Student-t, Mann-Whitney and Kruskall-Wallis). The Spearman correlation coefficient was also used. The results indicated that 51.9% of 1000 subjects analysed were in phototype III. Concerning specific pathologies, the following percentages were observed: melanocytic nevi 81.2; hemangiomas 26.5; androgenetic alopecia 20.5; deshydrotic eczema 12.7; seborrheic keratosis 11; solar keratosis 9.6; acne vulgaris 9.5; seborrheic dermatitis 6.9; superficial mycosis 6.4; other dermatitis 5.5; vascular spiders 4.4; vulgar psoriasis 1.9; vulgar warts 1.5; pediculosis and scabiosis 1.2.; herpes simplex 0.9; impetigo 0.7; tumours (neoplasias) 0.7; dysplastic nevi 0.6; urticaria 0.3. More than nine nevi were counted in 50% of the subjects observed. A difference was observed (p < 0.05) concerning the presence of nevi, mycosis, hemangiomas, non-specific dermatitis, vascular spider and solar keratosis according to sex. The females had more nevi in number than males (Mann-Whitney, p = 0.03). According to age there was a difference concerning the presence of nevi, superficial mycosis, deshydrotic eczema, seborrheic dermatitis, acne, psoriasis, tumours, hemangiomas, non-specific dermatitis, vascular spider, solar keratosis (with linear augmentation by age) and seborrheic keratosis. Nevi increased with ageing (Rs = 0.10 p < 0.001) and a greater number were observed in age groups 0-9 and 60 or more years. Acne was observed in 9.4% of the females and 9.7% of the males, 46.2% in the 15-29 year-old age group. This study indicated that the population had a mean of nine nevi. Psoriasis was similar to that mentioned in other studies: about 1% in the United States of America. Atopic dermatitis (5.5%) is below the values for the northern population (> = 15%), but above that indicated for the English and American population. Nevertheless, the values of the present study were higher than those in the latter populations, when children until seven years old were considered. The Freixo de Espada à Cinta population above 60 years had solar keratosis in 30.6%, a value three times higher than the previous decade (10.9%); reference values were not found. Contrary to the literature, an association was not established between phototype and solar keratosis. Cutaneous tumours were observed in 0.7% of the cases.