A remarkably high saturation magnetization of ~0.4mu_B/Fe along with room temperature ferromagnetic hysteresis loop has been observed in nanoscale (4-40 nm) multiferroic BiFeO_3 which in bulk form exhibits weak magnetization (~0.02mu_B/Fe) and an antiferromagnetic order. The magnetic hysteresis loops, however, exhibit exchange bias as well as vertical asymmetry which could be because of spin pinning at the boundaries between ferromagnetic and antiferromagnetic domains. Interestingly, like in bulk BiFeO_3, both the calorimetric and dielectric permittivity data in nanoscale BiFeO_3 exhibit characteristic features at the magnetic transition point. These features establish formation of a true ferromagnetic-ferroelectric system with a coupling between the respective order parameters in nanoscale BiFeO_3.Comment: 13 pages including 4 figures; pdf only; submitted to Appl. Phys. Let

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We have measured the resistivity of PrFeAsO 1−x F y samples over a wide range of temperature in order to elucidate the role of electron-phonon interaction on normal-and superconducting-state properties. The linear T dependence of ρ above 170 K followed by a saturationlike behavior at higher temperature is a clear signature of strong electron-phonon coupling. From the analysis of T dependence of ρ, we have estimated several normal-state parameters useful for understanding the origin of superconductivity in this system. Our results suggest that Fe-based oxypnictides are phonon mediated BCS superconductors like Chevrel phases and A15 compounds.

We report the observation of an unusual B dependence of transverse magnetoresistance (MR) in the PrFeAsO, one of the parent compound of pnictide superconductors. Below the spin density wave transition, MR is large, positive and increases with decreasing temperature. At low temperatures, MR increases linearly with B up to 14 T. For T ≥40 K, MR vs B curve develops a weak curvature in the low-field region which indicates a crossover from B linear to B 2 dependence as B→0. The B linear MR originates from the Dirac cone states and has been explained by the quantum mechanical model proposed by Abrikosov.The Fe-pnictide high-temperature superconductors have attracted much interest because of the interplay of multi-band structure of Fermi surface and antiferromagnetism mediated by the magnetic Fe ions 1 . The undoped compounds RFeAsO (R=La, Ce, Pr, Nd and Sm) and AFe 2 As 2 (A=Ba, Ca, Sr, Eu) are semimetal and exhibit a structural phase transition at T S followed by an antiferromagnetic spin-density wave (SDW) magnetic ordering at T N 2 . This SDW has been argued to arise from Fermi surface nesting of the hole and electron Fermi surface sheets, and/or from one-electron band effects 3 . The theoretical calculation shows that the combination of the physical symmetry and topology of the band structure naturally stabilizes a gapless nodal SDW state with Dirac nodes for the parent compounds of pnictide 4,5 . Angle-resolved photoemission spectroscopy measurement on BaFe 2 As 2 confirms a Dirac-type dispersion in the antiferromagnetic phase 6 .Dirac type dispersion was first observed in graphene, where the energy spectrum is linear in momentum at the corners of the first Brillouin zone, so that the electrons at low energies can be described by the Dirac equation 7,8 . Such a unique energy band structure gives rise to several characteristic transport properties 9 . In this letter, we report the investigation on the magnetotransport properties of PrFeAsO sample, one of the parent compound of 1111 oxypnictides. We observe a large positive magnetoresistance (MR) below the SDW transition temperature. At low temperature, the MR follows a linear field dependence up to 14 T magnetic field. For T ≥40 K, MR vs B curve develops a weak curvature in the lowfield region which indicates a crossover from B linear to B 2 dependence. The B linear dependence of MR is analyzed by the quantum mechanical model developed by Abrikosov 10,11 .Polycrystalline PrFeAsO sample has been prepared by standard solid state reaction method as described in our a) Electronic mail: dilipkumar.bhoi@saha.ac.in b) Electronic mail: prabhat.mandal@saha.ac.in earlier reports 12 . The phase purity of the sample was checked by powder X-ray diffraction method and no trace of the impurity phase has been detected. The scanning electron microscope (SEM) (SUPRA, 35 VP, Carl Zeiss) image reveals well connected platelet crystallites. The energy dispersive X-ray spectra obtained from grains of different size and morphology reveal that the stoichiometric ratio Pr:Fe:As:O is close t...

The saturation magnetization (M S ), antiferromagnetic transition point (T N ), and the off-center displacements of Bi and Fe ions have been measured as a function of particle size in nanoscale BiFeO 3 . T N decreases down to $550 K for particles of size $5 nm from $653 K in bulk, while M S rises by more than an order of magnitude. Analysis of crystallographic structure from Rietveld refinement of x-ray diffraction patterns shows significant rise in off-center displacements of Bi (d Bi ) and Fe (d Fe ) ions within a unit cell with the decrease in particle size. The net unit-cell polarization P S too, is found to be larger in nanoscale regime.

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Using the results of x-ray and neutron diffraction experiments, we show that the ferroelectric polarization, in ~22 nm particles of BiFeO3, exhibits a jump by ~30% around the magnetic transition point T_N (~635 K) and a suppression by ~7% under 5T magnetic field at room temperature (<

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