The various applications of Fe-filled carbon nano-
tubes(CNTs) such as magnetic perpendicular recording media, magnetic sensor, carrier in biomedical have resulted in the demand that Fe-filled should have corresponding coercivity. Because the coercivity of Fe-filled CNTs originates from encapsulated Fe nano-
particles (Fe-NPs), changing the corresponding stru-
cture of encapsulated Fe-NPs is a possible method to understand and tune the coercivity of Fe-filled CNTs.
In this study, Fe-filled CNT arrays with different mean Fe-NPs aspect ratios were prepared, and the in-
fluence of the shape anisotropy of encapsulated Fe-NPs on the coercivity of Fe-filled CNT arrays was in-
vestigated.
Four Fe-filled CNT-array samples were prepared by catalytic pyrolysis of acetylene using ferrocene as catalyst. In order to vary the aspect ratios of the Fe-NPs, the growth parameters, such as the evapora-
tion temperature of ferrocene, the growth temperat-
ure of CNT, and the gas flow rates are optimized. The results of transmission electron microscopy(TEM) characterization shows that the mean aspect ratios of Fe-NPs of four Fe-filled CNT arrays are 1.6, 3.2, 4.5 and 6.0, respectively.
The result of magnetic measurements shows that the coercivity of the Fe-filled CNT arrays increases from ~300 to ~800 Oe at room temperature, when the mean aspect ratio of the encapsulated Fe-NPs changes from 1.6 to 6.0. It is found that most encapsulated Fe-NPs are single-domain particles, and the possible magnetization-reversal mode for most of the encapsu-
lated Fe-NPs is coherent rotation. Therefore, the Stoner-Wohlfarth model may be applied to describe the effect of the shape anisotropy of the Fe-NPs on the coercivity of the Fe-filled CNT array. The aspect
ratio dependence of the observed coercivities of the Fe-filled CNT arrays are very analogous to those of the calculated coercivities. This analogy suggests that the shape anisotropy of encapsulated Fe-NPs has a predominant effect on the coercivity of Fe-filled CNT array. The relationship between the structure and the magnetic properties indicates that changing the shape anisotropy of the encapsulated Fe-NPs may be an effective method to tune the coercivity of the Fe-filled CNTs.
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