Magnetic properties of fluffy Fe@α-Fe 2 O 3 core-shell nanowires
© Cao et al.; licensee Springer. 2013
Received: 30 August 2013
Accepted: 28 September 2013
Published: 17 October 2013
Novel fluffy Fe@α-Fe2O3 core-shell nanowires have been synthesized using the chemical reaction of ferrous sulfate and sodium borohydride, as well as the post-annealing process in air. The coercivity of the as-synthesized nanowires is above 684 Oe in the temperature range of 5 to 300 K, which is significantly higher than that of the bulk Fe (approximately 0.9 Oe). Through the annealing process in air, the coercivity and the exchange field are evidently improved. Both the coercivity and the exchange field increase with increasing annealing time (T A ) and reach their maximum values of 1,042 and 78 Oe, respectively, at T A = 4 h. The magnetic measurements show that the effective anisotropy is increased with increasing the thickness of theα-Fe2O3 by annealing. The large values of coercivity and exchange field, as well as the high surface area to volume ratio, may make the fluffy Fe@α-Fe2O3 core-shell nanowire a promising candidate for the applications of the magnetic drug delivery, electrochemical energy storage, gas sensors, photocatalysis, and so forth.
In recent decades, the synthesis and properties of nanostructures have been greatly motivated both by a large number of potential applications and by fundamental questions about the physics of nanoscale magnetism. Comparing with other nanostructures, nanowires, especially ferromagnetic metal nanowires, have attracted more attention owing to their fundamental importance for various fields such as environmental remediation [1, 2], biomedicine , magnetic sensors , and magnetic storage devices [5–7], etc. Furthermore, due to the special morphology, it usually exhibits many novel and unique physical characters, including magnetoimpedance (MI) effect , nanoscale confinement , and nanomagnetism , etc.
As the most commonly used magnetic element, iron (Fe)-based nanostructures have stimulated great interest for researchers in the past few decades [11, 12]. However, one of the crucial problems in obtaining Fe nanostructures is that they commonly burn up when they are put into contact with air due to the strong activity of Fe. To avoid such a situation, encapsulating Fe nanostructures through the passivation with a Fe-oxide layer is adopted to both protect and stabilize the Fe nanostructures and thus form the core-shell morphology [13–15]. As a result, strong exchange magnetic coupling between the iron core and the oxide shell alters the magnetic anisotropy, giving rise to the modifications of the coercivity (H C ) and the appearance of the exchange-bias (EB) effect [16–18]. The EB was first observed by Meiklejohn and Bean in oxide-coated Co particles in 1956 . It is characterized by the horizontal shift of the hysteresis loops after the hybrid magnetic systems cooled down through the critical temperature in an external field . For example, for the typical ferromagnetic (FM)/antiferromagnetic (AFM) hybrid magnetic system, the EB appears when the sample is cooled down from above the AFM N éel temperature in an external field. Up to now, the EB effect of Fe-based nanostructures, for example, zero-dimensional core-shell NPs of Fe/ γ-Fe2O3 , FeO/Fe3O4 , and Fe/Fe3O4  have been systematically investigated. However, the physical origin of EB is still poorly understood. For the one-dimensional nanowires, the magnetic properties are even more complicated. The large aspect ratio, the high surface area to volume ratio, the shape anisotropy, and the interface play important roles in the magnetization dynamics of the core-shell structured systems. Therefore, the synthesis of one-dimensional Fe-based nanostructures and varying the magnetic properties via chemical control over the components could be important for the understanding of EB at the nanoscale level.
In this paper, Fe@α-Fe2O3 core-shell nanowires with novel fluffy-like α-Fe2O3 covered on the surface were synthesized. The structure, morphologies, and magnetic properties of the resulted nanowires have been comprehensively studied. It is found that the coercivity and the EB of the nanowires have been improved evidently by forming the Fe@α-Fe2O3 core-shell structure.
The Fe@α-Fe2O3 nanowires were synthesized by a reaction between ferrous sulfate and sodium borohydride proposed by Tong et al. previously . All reagents, such as ferrous sulfate heptahydrate (FeSO 4·7H2O, AR) and sodium borohydride (NaBH4, AR), were obtained from commercial suppliers and were used without any further purification. A solution of 30.0 mL of 0.70 M NaBH4 was added into 60.0 mL of 0.050 M FeSO4 solution in a reaction flask while the solution was vigorously stirred. The reaction mixture was maintained at 60°C for up to 30 min with continuous stirring. The resulting black precipitates were separated from the solution by centrifugation at 4,000 rpm for 5 min, washed several times with deionized water and ethanol, and then dried in vacuum at 40°C for 24 h to obtain the as-synthesized product of the Fe@α-Fe2O3 nanowire. Annealing is the final heat treatment procedure. The annealing procedure was performed in a tube furnace under air atmosphere with a 6°C/min heating rate, and the sample was allowed to annealing at 380°C for 2, 4, 6, and 8 h, respectively. After the annealing process, the sample was cooled down to room-temperature. The cooling rate is also 6°C/min.
Structural analysis was performed by X-ray powder diffraction (XRD, D/max-2500) using the Cu Ka radiation (λ = 1.5406 Å). The microstructures, morphologies, and the elemental distribution of the nanowires were characterized by transmission electron microscopy (TEM, JEOL 2200F, Akishima-shi, Japan) operating at 200 kV. The magnetic properties were measured by a superconducting quantum interference device magnetometer (MPMS-5S) in magnetic fields up to 50 kOe and over the temperature range of 5 to 300 K.
Results and discussion
The H E of the as-synthesized sample is only approximately 30 Oe measured at 5 K after a 10 kOe magnetic field cooling process. Similar to that of H C , H E is also improved by annealing. The 4-h annealed sample shows the largest H E of approximately 78 Oe at 5 K.
Figure 5b displays the temperature dependence of H E for different nanowires measured under the cooling magnetic field of 10 kOe. It can be seen that for all samples, H E decreases monotonically with increasing temperature and becomes negligibly small above the temperature of 50 K. At a certain temperature, H E increases first with increasing T A and then decreases with further increasing T A , exhibiting a maximum at T A = 4 h. The enhancement of H E with increasing T A may be mainly because of the increase of the thickness of AFM Fe2O3 shell at the surface of the nanowires [18, 32]. While the decrease of the H E for 6-h annealed sample is rather complicated. This may depend on the microstructure, for example, the change of the AFM domain structure . This phenomenon has also been found in other exchange bias systems [32–34].
In conclusion, the Fe@α-Fe2O3 nanowires have been synthesized using the chemical method. Some novel fluffy-like α-Fe2O3 grows on the surface of the nanowires through the post-annealing in air. The coercivity of the as-synthesized nanowires is above 684 Oe in the temperature range of 5 to 300 K, which is significantly higher than that of the bulk Fe. Through the annealing process in air, the coercivity and the exchange field are evidently improved. Both the coercivity and the exchange field increase with increasing T A and reach their maximum values of 1,042 and 78 Oe, respectively, at T A = 4 h. The magnetic measurements show that the effective anisotropy is increased with increasing the thickness of the α-Fe2O3 by annealing. The large values of coercivity and exchange field, as well as the high surface area to volume ratio, may make the fluffy Fe@α-Fe2O3 core-shell nanowire a promising candidate for the applications of the magnetic drug delivery, electrochemical energy storage, gas sensors, photocatalysis, and so forth.
This work was supported by the National Natural Science Foundation of China (nos. 51101088, 51171082, and 11204161), Tianjin Key Technology R&D Program (no. 11ZCKFGX01300), Tianjin Natural Science Foundation of Youth (no. 13JCQNJC02800), and Specialized Research Fund for the Doctoral Program of Higher Education (no. 20110031110034).
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