Magnetic properties of Fe/Fe$_{3}$O$_{4}$ core/shell nanostructure

Magnetic properties of a core/shell nanostructure with spherical core of Fe/FeB and a shell of Fe$_{3}$O$_{4}$/$\gamma $-Fe$_{2}$O$_{3}$ are reported employing magnetometry, electron magnetic resonance (EMR) and M\"{o}ssbauer spectroscopy. This nanostructure was produced by reducing FeCl$_{3}$$\cdot$6H$_{2}$O with NaBH$_{4}$. Combining the results from XRD, TEM and M\"{o}ssbauer spectroscopy showed the nanostructure to consist of a core of diameter D$\simeq $20 nm containing both $\alpha $-Fe with D$\simeq $7 nm and amorphous Fe-B alloy and a shell of thickness 5 nm containing Fe$_{3}$O$_{4}$/$\gamma $-Fe$_{2}$O$_{3}$. Measurements of the magnetization M vs. temperature (2 K to 370 K) and in H upto 65 kOe show a blocking temperature T$_{B}\simeq $30 K associated with the oxide shell and ferromagnetism upto 370 K with nearly temperature-independent saturation M$_{S}\simeq $70 emu/g and coercivity H$_{C}\simeq $100 Oe. In EMR studies at 9.28 GHz, two lines are observed: a narrower line with linewidth $\Delta $H$\simeq $600 Oe and g$\simeq $2 and a broader line with $\Delta $H$\simeq $4200 Oe and g$\simeq $2.2. These parameters of the narrower line combined with its disappearance below 50 K suggests its origin to be the oxide shell whereas the broader line is due to Fe/FeB core. Research supported by U. S. Dept. of Energy, Contract {\#}DE-FC26-05NT42456.