Abstract
Temperature-dependent magnetic properties of a core/shell nanostructure are reported employing magnetometry and electron magnetic resonance (EMR) spectroscopy. Structural characterization of the sample synthesized by NaBH 4 reduction of FeCl 3 was done by x-ray diffraction, TEM and Mössbauer spectroscopy and showed a core/shell nanostructure with a core of diameter D≃20 nm consisting of α-Fe and amorphous Fe–B alloy and a shell of 7 nm thickness made up of principally γ-Fe 2O 3. Temperature-dependent EMR studies at 9.28 GHz show a narrow line with g≃2.01 superimposed on a broad line with g≃2.20. The narrow line assigned to the oxide shell disappears below about 60 K, in agreement with a blocking temperature T B≃30 K measured in SQUID magnetometry. The EMR parameters of the broad EMR line are similar to those reported for α-Fe nanoparticles imbedded in amorphous SiO 2 matrix. The magnitude of the saturation magnetization M S=70 emu/g of the nanostructure is smaller than that of bulk α-Fe ( M S=220 emu/g) and bulk γ-Fe 2O 3 ( M S=88 emu/g). Size dependence is used to interpret the absence of exchange-bias in the field-cooled sample of the nanostructure.
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