An evidence of local structural disorder across spin-reorientation transition in DyFeO3: an extended x-ray absorption fine structure (EXAFS) study.

The present work is aimed at exploring the local atomic structure modifications related to the spin reorientation transition (SRT) in DyFeO3 orthoferrite exploiting x-ray absorption fine structure (XAFS) spectroscopy. For this purpose we studied by XAFS the evolution of the local atomic structure around Fe and Dy as function of temperature (10-300 K) in a DyFeO3 sample having the SRT around 50-100 K. For sake of comparison we studied a YFeO3 sample having no SRT. The analysis of the extended region has revealed an anomalous trend of Fe-O nearest neighbour distribution in DFO revealing (i) a weak but significant compression with increasing temperature above the SRT and (ii) a peculiar behavior of mean square relative displacement (MSRD) [[Formula: see text]] of Fe-O bonds showing an additional static contribution in the low temperature region, below the SRT. These effects are absent in the YFO sample supporting these anomalies related to the SRT. Interestingly the analysis of Dy L 3-edge data also reveal anomalies in the Dy-O neighbour distribution associated to the SRT, pointing out a role of magnetic Re ions across [Formula: see text]. These results point out micro-structural modification at both Fe and Dy sites associated to the magnetic transitions in DFO, it can be stated in general terms that such local distortions across [Formula: see text] and magnetic Re3+ may be present in other orthoferrites exhibiting multiferroic nature.

57Fe Mössbauer studies across the spin density wave transition in BaFe2-xRuxAs2.

Mössbauer measurements have been carried out in powdered single crystalline samples of BaFe(2-x)RuxAs(2), for Ru concentration in the x = 0.0-0.5 range. The internal hyperfine field (B(hf)) measured at 5 K is found to decrease with an increase in Ru concentration, consistent with the disappearance of a magnetic ground state with Ru substitution. The temperature dependent Mössbauer measurements have been used to study the evolution of magnetic ordering at the Fe sites, in samples with a Ru fraction of x = 0.1 and 0.5. From the analysis of the data, it is surmised that the isomer shift increases with a decrease in temperature, with a characteristic slope change at the structural transition in both samples studied. In both the x = 0.1 and x = 0.5 samples, a low B(hf) centred around 0.5 Tesla is seen to occur well above the structural transition temperature, the contribution from which is suppressed with a decrease in temperature. Below the structural transition temperature, a bimodal distribution of B(hf) centred at about 3 Tesla and 5 Tesla emerges, the contribution from which increases with a further decrease in temperature. Spin polarized density functional calculations suggest the occurrence of different magnetic moments at the Fe sites in the Ru substituted compounds, and provide a rationale for the experimentally observed multimodel B(hf).

Magnetic and 57Fe Mössbauer study of magneto-electric GaFeO3 prepared by the sol-gel route.

This work reports the preparation of magneto-electric GaFeO(3) by the sol-gel route and its characterization by x-ray diffraction, dc-magnetization, ac-susceptibility, low temperature and high field (57)Fe Mössbauer spectroscopy and dielectric constant measurements. The prepared samples are found to be single phase from x-ray diffraction studies. The crystallite sizes are found to be in the nano-regime for the samples sintered at low temperatures. From the temperature dependent dc-magnetization (M-T) measurements, bifurcation of the zero-field cooled (ZFC)-field cooled data and a cusp in the ZFC data are observed. With the help of low-field ac-susceptibility, (57)Fe Mössbauer and detailed dc-magnetic measurements these features are explained in terms of the magnetic anisotropy of the sample ruling out phenomena like spin-glass and super-paramagnetism as quoted in the literature for this compound. Apart from this, very interesting and different M-H behavior mimicking composite two-phase magnets is observed for the samples sintered at different temperatures. A symmetric M-H loop is observed for samples sintered at low temperatures and a pinched M-H loop is observed for samples sintered at high temperatures. The observed magnetic properties are explained by estimating the Fe cation distribution using high field (57)Fe Mössbauer spectroscopy measurements. An anomaly in the dielectric constant data at the Curie temperature indicates the ME coupling of the samples.

High temperature extended x-ray absorption fine structure study of multiferroic BiFeO3.

Local atomic structure modifications around Fe atoms in polycrystalline multiferroic BiFeO(3) are studied by Fe K edge x-ray absorption spectroscopy as a function of temperature across the Néel temperature (T(N) = 643 K) in order to reveal local structure modifications related to the magnetic transition. This work demonstrates that on crossing T(N) the local structure around Fe shows peculiar changes: the Fe-O bond lengths get shorter, the ligand symmetry increases and the Fe-O bond length disorder (σ(2)) deviates from Debye behaviour. These results suggest that the structural transition at the ferroelectric Curie temperature (T(C) = 1103 K) is anticipated by early local rearrangement of the structure starting already at T(N).

Low temperature Raman and high field 57Fe Mossbauer study of polycrystalline GaFeO3.

The magnetic and phonon properties of polycrystalline magnetoelectric/multiferroic GaFeO(3) are studied. Using high field (57)Fe Mossbauer spectroscopy, occupation of Fe is observed at four cation sites. A Fe population of about 6% is observed at the tetrahedral Ga1 site, which explains the observed pinched-like M-H curve and initial sharp increase of the magnetization. The calculated net magnetization value from Mossbauer data suggests that the Fe moment at the Ga1 site is parallel to Fe1 and opposite to that of Fe2 and Ga2 sites, resulting in ferrimagnetism. From low temperature Raman data, anomalous temperature variation in frequency at T(C) is observed for the mode at ∼700 cm(-1).

Eu doping in multiferroic BiFeO3 ceramics studied by Mossbauer and EXAFS spectroscopy.

Bismuth ferrite ceramics (BiFeO(3)) are multifunctional materials classified as multiferroics for their special magnetic and electric properties that can be modified by substitutional doping at the Bi and/or Fe sites. Understanding the relation between magnetoelectric response and structural/electronic modification upon doping is a relevant issue. In this work, the structure of Eu-doped multiferroic systems (Bi(1-x)Eu(x)FeO(3), x = 0, 0.5, 0.1, 0.15) as well as the valence state of Fe and Eu ions have been investigated combining Mossbauer and x-ray absorption fine structure (XAFS) spectroscopy techniques. The Eu(3+) doping at the Bi site results in better magnetic properties. High temperature (57)Fe Mossbauer data and Fe K-edge XAFS results show that FeO(6) octahedron distortions reduce with Eu(3+) doping. It is conclusively shown that the observed magnetic properties in BiFeO(3) with chemical substitution (Eu) are mainly due to the structural distortions and not due to Fe multiple valence. (151)Eu Mossbauer measurements show that the Eu(3+)(Bi(3+)) site is magnetically inactive in BiFeO(3).

In situ x-ray reflectivity and grazing incidence x-ray diffraction study of L 1(0) ordering in (57)Fe/Pt multilayers.

In situ high temperature x-ray reflectivity and grazing incidence x-ray diffraction measurements in the energy dispersive mode are used to study the ordered face-centered tetragonal (fct) L 1(0) phase formation in [Fe(19 Å)/Pt(25 Å)]( × 10) multilayers prepared by ion beam sputtering. With the in situ x-ray measurements it is observed that (i) the multilayer structure first transforms to a disordered FePt and subsequently to an ordered fct L 1(0) phase, (ii) the ordered fct L 1(0) FePt peaks start to appear at 320 °C annealing, (iii) the activation energy of the interdiffusion is 0.8 eV and (iv) ordered fct FePt grains have preferential out-of-plane texture. The magneto-optical Kerr effect and conversion electron Mössbauer spectroscopies are used to study the magnetic properties of the as-deposited and 400 °C annealed multilayers. The magnetic data for the 400 °C annealed sample indicate that the magnetization is at an angle of ∼50° from the plane of the film.