Colossal Reduction in Curie Temperature Due to Finite-Size Effects in CoFe2O4 Nanoparticles

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• 作者：Victor Lopez-Dominguez ; Joan Manel Hern脿ndez ; Javier Tejada ; Ronald F. Ziolo
• 刊名：Chemistry of Materials
• 出版年：2013
• 出版时间：January 8, 2013
• 年：2013
• 卷：25
• 期：1
• 页码：6-11
• 全文大小：387K
• 年卷期：v.25,no.1(January 8, 2013)
• ISSN：1520-5002

文摘

In this work, we show the enormous size effect on the ordering transition temperature, T_O, in samples of CoFe₂O₄ nanoparticles with diameters ranging from 1 to 9 nm. Samples were characterized by HRTEM and XRD analyses and show a bimodal particle size distribution centered at 3 nm and around 6 nm for 鈥渟mall鈥?and 鈥渓arge鈥?particles, respectively. The results and concomitant interpretation were derived from studies of the magnetization dependence of the samples on temperature at low and high magnetic fields and relaxation times using both dc and ac fields. The large particles show a typical superparamagnetic behavior with blocking temperatures, T_B, around 100 K and a Curie temperature, T_C, above room temperature. The small particles, however, show a colossal reduction of their magnetic ordering temperature and display paramagnetic behavior down to 10 K. At lower temperatures, these small particles are blocked and show both exchange and anisotropy field values above 5 T. The order of magnitude reduction in T_O demonstrates a heretofore unreported magnetic behavior for ultrasmall nanoparticles of CoFe₂O₄, suggesting its further study as an advanced material.

Keywords:

magnetic nanoparticles; Curie temperature