alexa Magnetic properties of nearly defect-free maghemite nanocrystals
Materials Science

Materials Science

Journal of Nanomedicine & Nanotechnology

Author(s): P Dutta, A Manivannan

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Temperature variations of the magnetization M and the electron magnetic resonance (EMR) parameters of maghemite (γ−Fe2O3) nanocrystals are reported for the 4K–300K range. Transmission electron microscopy of the nanocrystals shows them to be nearly spherical samples (aspect ratio a∕b=1.15) with diameter D=7(1)nm, and analysis of x-ray diffraction lines yields D=6.4nm with negligible strain. M versus T data show blocking temperatures TB≃101K, 89K, and 68K in measuring fields H=50, 100, and 200Oe respectively. M versus H data for T>TB fits the modified Langevin function M=MsL(μpH/kBT)+χaH with μp=8000(500)μB/particle and Ms=80emu∕g, identical to Ms for bulk γ−Fe2O3. It is argued that this large value of Ms, the small value of coercivity Hc≃20Oe at 5K, the lack of exchange bias in a field-cooled sample, and negligible strain point to nearly defect-free nanocrystals. In the EMR studies, two resonance lines are observed, one with the resonance field Hr greater than that for the free-electron value and the other smaller. From the temperature variations of Hr and the linewidths of the two lines, it is argued that the two lines are, respectively, due to nanocrystals with easy-axis aligned perpendicular and parallel to the applied field. The relatively narrow intrinsic linewidths (≃400Oe) of the two lines in nearly defect-free nanocrystals facilitated their observation.

This article was published in Physical Review B and referenced in Journal of Nanomedicine & Nanotechnology

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