High coercive single-domain CoFe 2 O 4 nanoparticles with the minimal average size (7.6–12.8 nm) were synthesized by polyethylene glycol (PEG) assisted sol–gel method and subsequent annealing at different temperatures. The prepared samples were characterized by XRD, TEM, TG-DSC, and FTIR techniques. The XRD and TEM studies indicated the size and shape of the particles are highly dependent on annealing temperature. Magnetic properties of the developed cobalt ferrite nanoparticles were found to be dependent on their size and shape. The particles annealed at lower temperatures (about 400 °C) are found to be near spherical in shape and as the annealing temperature is increased from 400 to 800 °C, the shape of the particles is observed to be transformed from spherical to octahedron through intermediate cubic shape. Magnetic parameters viz., saturation magnetization ( M s ) and remnant magnetization ( M r ) exhibited a study increase with increase of the particle size. The coercive field, H c exhibited a non-monotonic behavior with distinct maximum at about 700 °C and suggested the transition from single-domain to multi-domain state. The magneto-crystalline anisotropy constant, K , determined from Stoner–Wohlfarth relation, exhibited the maximum value, 10.74 × 10 6 erg/cm 3 for the samples annealed at 800 °C. In addition, the magnitude of (BH) max (which is considered as the efficiency of cobalt ferrite nanoparticles for using as permanent magnets) exhibited the maximum value (1.04 × 10 6 GOe). Such higher values of these two parameters suggest the possible applications of the studied material in magnetic recording, high density digital recording disks and in magnetic sensors.

中文翻译：

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聚乙二醇辅助溶胶-凝胶法合成超细钴铁氧体纳米粒子磁性的关键研究

具有最小平均尺寸（7.6-12.8 nm）的高矫顽力单畴CoFe 2 O 4 纳米粒子是通过聚乙二醇（PEG）辅助溶胶-凝胶法和随后在不同温度下退火合成的。通过XRD、TEM、TG-DSC和FTIR技术对制备的样品进行表征。XRD 和 TEM 研究表明颗粒的大小和形状高度依赖于退火温度。发现开发的钴铁氧体纳米颗粒的磁性取决于它们的尺寸和形状。发现在较低温度（约 400 °C）下退火的颗粒形状接近球形，随着退火温度从 400 °C 增加到 800 °C，观察到颗粒的形状通过中间体从球形转变为八面体。立方体形状。磁参数，即 饱和磁化强度（ M s ）和剩余磁化强度（ M r ）随着颗粒尺寸的增加而增加。矫顽场 H c 在约 700 °C 时表现出具有明显最大值的非单调行为，并表明从单域状态到多域状态的转变。由 Stoner-Wohlfarth 关系确定的磁晶各向异性常数 K 表现出最大值，10.74 × 10 6 erg/cm 3 对于在 800 °C 下退火的样品。此外，(BH) max 的大小（被认为是钴铁氧体纳米颗粒用作永磁体的效率）表现出最大值（1.04 × 10 6 GOe）。这两个参数的如此高的值表明所研究的材料在磁记录、高密度数字记录盘和磁传感器中的可能应用。