Internal structure of magnetic nanoparticle determines magnetic response
This work aims to emphasize that the magnetic response of the single-domain magnetic nanoparticles (NPs) is driven by the NP’s internal structure, and the NP size dependencies of magnetic properties are overestimated. The relation between the degree of the NP’s crystallinity and magnetic response is unambiguously demonstrated in eight samples of uniform maghemite/magnetite NPs and corroborated with results obtained on about 20 samples of spinel ferrite NPs with different degree of crystallinity. The NP samples were prepared by the thermal decomposition of an organic iron precursor subjected to varying reaction conditions, yielding variations in NP size, shape and relative crystallinity. We characterized the samples by using several complementary methods, such as powder X-ray diffraction (PXRD), transmission electron microscopy (TEM), high resolution TEM (HR-TEM) and Mössbauer spectroscopy (MS). We evaluated the NP’s relative crystallinity by comparing the NP sizes determined from TEM and PXRD and further inspecting the NP’s internal structure and relative crystallinity by using HR-TEM. The results of the structural characterization were put in the context of the NP’s magnetic response. In this work, the highest saturation magnetization (Ms) was measured for the smallest but well-crystalline NPs, while the larger NPs exhibiting worse crystallinity revealed lower Ms. Our results clearly demonstrate that the NP crystallinity level that is mirrored in the internal spin order drives the specific magnetic response of the single-domain NPs.