科研之友
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摘要
High density Fe-based ferromagnetic nanoparticle (NP) assembly is expected to have unique magnetic properties, such as superferromagnetism and super-spin-glass, different from magnetically isolated NP systems due to strong dipole interactions among the NPs. A high dipole interaction field, H-dip, of similar to 3.5 kOe can result in a high effective internal field to the magnetic moment of the NP, expecting for ultra-fast magnetic response, that is, a high magnetic resonance frequency, f(r), of similar to 10 GHz. However, for a simply molded Fe NP assembly, a low f(r) was observed due to inhomogeneous distribution of the internal field, implying the necessity of a unidirectional state of H-dip for higher f(r). In this study, we fabricated a columnar Fe NP assembly for realizing the unidirectional state of H-dip by applying our uniquely developed external field-induced agglomeration method for monodispersed Fe NPs (13 nm in average size) as a function of the field (0-30 kOe) and volume fraction of the Fe NPs (0.5%-51%) in a polymer matrix with dimensions of 4 mm x 4 mm x 0.7 mm(t). A columnar-structured Fe NP assembly was successfully achieved along an in-plane direction (defined as the x-axis) under optimized conditions. From static magnetization curves, induced uniaxial magnetic anisotropy was observed according to the shape of the columnar structure of the Fe NP assembly, where easy and hard axes of magnetization were realized along the parallel (x-axis) and normal directions (in-plane y-axis and z-axis in the thickness direction) to the external field during the process, respectively. Interestingly, this fabricated columnar-structured Fe NP assembly exhibited very high f(r) in the range from 3 to 11 GHz judging from the complex susceptibility spectra obtained. The f(r) values were well-scaled by a modified Snoek's-limit-law using demagnetization factors quantitatively estimated from the static magnetization curves. Thus, shape-induced anisotropy originating from the unidirectional state of H-dip in the columnar structure of the Fe NP assembly plays an important role for high frequency magnetic response in the GHz-band.
- 出版日期2014-5-7
