共沉澱與水熱法製備超順磁性鈷鐵氧體奈米粒子之磁性與高頻應用探討

Synthesis of Superparamagnetic Cobalt Ferrite Nanoparticles via Co-precipitation and Hydrothermal and High-Frequency Applications

本研究以共沉澱結合水熱法製備超順磁性鈷鐵氧體（Cobalt Ferrite，CoFe2O4）奈米粒子，探討水熱法溫度（160°C、180°C、200°C）對結構與磁性的影響。首先利用X光繞射儀與TOPAS軟體精修證實皆為立方反尖晶石單相，晶體尺寸隨溫度升高而增大，高解析穿透式電子顯微鏡與選區電子繞射顯示形貌均勻與結晶度，以及X光光電子能譜儀指出尺寸改變伴隨陽離子反轉與氧空缺調整，進而影響整體磁各向異性，最後以超導量子磁化干涉儀量測磁滯曲線，室溫下矯頑力低、剩磁近於零，呈現超順磁行為，以及零場冷卻與場冷卻的變溫磁化曲線顯示阻隔溫度（TB）約為156 K，且隨粒徑增加而上升。結果證明僅以溫度調控即可同時調整尺寸與缺陷並獲得低損耗磁性，具高頻應用潛力。

This study investigates the influence of hydrothermal temperature (160, 180, and 200°C) on the structural and magnetic properties of cobalt ferrite (CoFe2O4) nanoparticles, synthesized via a two-step method combining co-precipitation with subsequent hydrothermal treatment. TOPAS refinement of X-ray diffraction data confirmed that all samples possess a single-phase cubic inverse spinel structure, with crystallite and particle sizes increasing monotonically with the synthesis temperature. High-resolution transmission electron microscopy and selected-area electron diffraction further corroborated the high crystallinity and uniform morphology of the nanoparticles. According to X-ray photoelectron spectroscopy analysis, the temperature-induced size variation was accompanied by adjustments in cation inversion degree and oxygen vacancy concentration. These modifications directly influenced the magnetic anisotropy. Magnetic characterization using a superconducting quantum interference device magnetometer showed that all samples exhibited room-temperature superparamagnetism, evidenced by hysteresis loops with very low coercivity and near-zero remanence. Zero-field-cooled and field-cooled measurements revealed a blocking temperature (TB) that increased with particle size, starting from ca. 156 K for the 160°C sample. In conclusion, this work demonstrates that facile control of hydrothermal temperature provides an effective strategy to simultaneously tailor the particle size and defect chemistry of CoFe2O4 nanoparticles, thereby enabling the modulation of their magnetic anisotropy and the reduction of magnetic loss for potential high-frequency applications.