Sm2Bi2Fe4O12 ferromagnetic semiconductor
Description
Samples of Sm2Bi2Fe4O12 were solid reacted. Structural characterization through the X-ray diffraction technique evidences that the synthesized ceramic crystallizes in orthorhombic structures (Pnma, #62 space group). Micrographs reveal the granular and densified feature on the surface and inside of the samples. Two micrometric and submicrometric grain sizes were mostly observed. X-ray energy dispersion spectra allowed establishing the stoichiometric composition suggested by the chemical formula of the compound. The electrical behavior was studied by means of I-V curves and electrical permittivity as a function of temperature. Results reveal semiconductor-like behaviors with strong influences on the polycrystalline character of the material and a Maxwell-Wagner-type dielectric trend. Diffuse reflectance spectra corroborate the semiconductor feature with optical band gap Eg=2.62 eV. Temperature-dependent magnetization curves for applied fields of 500, 2000 and 10000 Oe show a ferromagnetic characteristic, with strong evidence of disorder causing magnetic irreversibility between the Zero Field Cooling and Field Cooled measurement procedures. The ferromagnetic type ordering was established from the hysteretic character of magnetization curves measured at 50, 200 and 300 K temperatures. Ab-initio calculations of the electronic density of states show the occurrence of a mean semiconducting gap of 2.38 eV produced by hybridizations of orbitals of the nonmetallic cations with the 2p oxygen electrons. 3d-Fe orbitals are responsible by the strong asymmetry between the spin-up and -down polarized states, which gives rise to a high magnetic moment. The increase in applied pressure and temperature produces substantial changes in the thermophysical properties of the material. At low temperatures, the specific heat reveals a trend to the Dulong-Petit limit, which converges to 490.2 J/mol.K. In this temperature regime, changes occur in all the thermophysical properties studied because of the effect of pressure and temperature on the wave behavior of the crystal lattice. At higher temperatures, a strong variation in Debye temperature and divergence between the behaviors of entropy, thermal expansion and Grüneisen parameter for different applied pressures are observed. Based on these results, the material is classified as a ferromagnetic semiconductor with low coercive field and medium saturation magnetization that would have eventual technological implications in the spintronic industry. To see the full article, search DOI: 10.1039/D0TC02935A
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Samples were synthesized by means of the conventional solid-state method, by the reaction of the high purity precursor oxides produced by Aldrich: Sm2O3 (99.9%), Bi2O3 (99.9%) and Fe3O4 (99.8%). Calcining thermal process was performed at T=550 °C for 36 h. The material was then homogenize it in an agate mortar for 30 min, after which it was crushed in an acetone binder environment and under an axial pressure of 471.6 Mpa for 15 min, obtaining 2 disc-shaped samples with 1.0 mm thickness and 9.0 mm diameter each. Finally, sintering processes were carried out at temperatures of 600, 715 and 810 °C for 24 hours each. The final sintering temperature of 810 °C was chosen because this value is lower than the melting temperature of bismuth oxide, ensuring no degradation or percolation of bismuth, although this low temperature does not facilitate high densification of the polycrystalline material. The crystal structure of prepared samples was characterized by X-ray diffraction (XRD) experiments in a PANalytical X'pert-Pro Multipurpose Powder Diffractometer, following the Bragg-Brentano configuration, with stepsize of 0.001° and 2θ degree between 10° and 90° in a continuous scan using CuKα radiation with λ=1.540598 Å. The study of the morphology of the prepared samples was performed from micrographs of scanning electron microscopy (SEM), which were acquired in a TESCAN Vega 3 SB microscope with secondary (SE) and backscattering (BSE) detectors. The composition of the samples was semiquantitatively analyzed through the Energy-dispersive X-ray spectroscopy technique (EDS), using a Bruker X-ray cannon coupled to the electron microscope. For the measurement of the electrical response it was necessary to cut one of the samples with a diamond disk, obtaining rectangular prisms of 5.55 mm large, 2.45 mm wide and 1.00 mm thick. Subsequently, gold electrodes were deposited on both sides of 5.55 mm x 2.45 mm. Complex impedance measurement as a function of temperature was carried out using an Agilent-4194A phase gain and impedance analyzer, a Janis Research cryogenic system (VPF-475 model) and a Lake Shore 332 temperature controller. These measurements were performed at a temperature variation rate of 1.7 K/min and in a frequency range between 102 and 107 Hz. The I-V curve at room temperature was elaborated from the data measured in a Keithley-6517A DC electrometer and a sample holder with gold-plated silver contacts designed by the authors, which was adapted to the cryogenic system and the temperature controller. The magnetic behavior of the samples was studied by means of a vibrating sample magnetometer in temperature susceptibility and field magnetization measurements.