Structural, optical, dielectric and magnetic properties of PVP coated magnetite (Fe3O4) nanoparticles
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We report the synthesis of magnetite (Fe3O4) and polyvinylpyrrolidone (PVP) coated Fe3O4 nanoparticles by chemical co-precipitation route. X-ray diffraction (XRD) and transmission electron microscope (TEM) confirm the formation of inverse spinel structure of Fe3O4. XRD peaks of PVP coated Fe3O4 nanoparticles are broad and noisy as compared to Fe3O4. The broadness of peaks is due to small size and large defect density confirmed by energy dispersive spectroscopy (EDS) and scattered area electron diffraction (SAED). Noisy behavior is due to presence of PVP. Average particle size reduced from 10.36 ± 1.97 to 6.91 ± 1.89 nm for Fe3O4 and PVP coated Fe3O4, respectively. From EDS analysis, it is confirmed that the oxygen content reduced from 33.45 to 15.30 at.% with PVP coating. The oxygen content is reduced to half in case of PVP coated Fe3O4 as compared to uncoated Fe3O4. The reduction in oxygen content reveals enhancement in oxygen vacancies. Fourier transform infrared (FTIR) spectroscopy and thermogravimetric analysis (TGA) confirm the PVP coating and the calculated value of thickness of the PVP layer on the surface of Fe3O4 is 2.4601 nm. Dielectric constant (εr) and dielectric loss (tanδ) exhibits the dispersion behavior. Ac conductivity (σac) increases sharply at large frequencies, which is due to enhancement in charge density (liberated charge carriers from defects + conduction charge carriers). The variation in dielectric properties and conductivity is due to Maxwell Wagner interfacial polarization and hopping of charge carriers between Fe2+/Fe3+. Magnetic properties M(T) shows reduction of blocking temperature (TB) from 86 to 75 K for uncoated and PVP coated Fe3O4 nanoparticles. Shifting of TB to lower values is consistent with particle size reduction. M(H) loops at room temperature show typical superparamagnetic behavior. Reduction in saturation magnetization (Ms) is due to the presence of nonmagnetic polymer layer on the surface of Fe3O4 nanoparticles and large number of defects (oxygen vacancies). Field cooled M(H) loops at 5 K show the antisymmetric shift of coercive field along the negative x-axis. The exchange bias field HE enhances to 227 Oe in case of PVP coated Fe3O4 nanoparticles, which is double of the 125 Oe for uncoated nanoparticles. The enhancement in HE is due to smaller sized nanoparticles having large surface to volume ratio having large defect density (oxygen vacancies).
This work is financially supported by the Higher Education Commission of Pakistan under START-UP RESEARCH GRANT PROGRAM (Grant no.: 21-1732/SRGP/R&D/HEC/2017), the Fundamental Research Funds for the HEC Pakistan.
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