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Single domain CoFe 2 O 4 nanoparticles with different amount of defects/strain have been synthesized by varying the growth temperature in the hydrothermal method. Nanoparticles grown at lower temperature are of larger size and exhibit more planar defects and oxygen vacancies as compared to nanoparticles grown at higher temperatures which are of smaller sizes and exhibit less planar defects and oxygen vacancies. The nanoparticles with larger amount of defects also possess a higher value of intrinsic strain as compared to nanoparticles with fewer defects. The presence of intrinsic strain in the nanoparticles is found to shift the cationic distribution at the tetrahedral and octahedral sites. The saturation magnetization ( M s ) of the nanoparticles is found to depend upon both the intrinsic strain and size of the nanoparticles. The M s increases with the decrease in the nanoparticles size from 32 nm to 20 nm, and this is correlated to the inverse of spin canting effect due to decrease in the intrinsic strain which leads to shifting of Co 2+ ions from tetrahedral to octahedral sites. However, with further decrease in the size of the nanoparticles (16 nm), the size effect dominates over the strain effect leading to decrease in M s . The coercivity is found to be higher in the nanoparticles with larger amount of defects/strain and has been attributed to strain induced strong spin canting and pinning due to defect sites. The variation of coercivity with particle size ( D ) exhibits deviation from D 3/2 dependence for the nanoparticles with larger amount of strain/defects. [ABSTRACT FROM AUTHOR]

Two sets of single domain CoFe 2 O 4 (CFO) nanoparticles have been synthesized using hydrothermal technique which are of the same sizes, but with different amount of strain/defects. The nanoparticles synthesized at a lower growth temperature (80 °C) exhibit high density of planar defects and oxygen vacancies as compared to the CFO nanoparticles grown at higher temperature (180 °C). The CFO nanoparticles with high density of defects also possess higher intrinsic strain. The nanoparticles with higher strain/defects exhibit higher coercivity and smaller value of saturation magnetization. The effect of strain on the magnetization characteristics of CFO nanoparticles is simulated using the Object Oriented MicroMagnetic Framework. The observed larger value of coercivity for the CFO nanoparticles with higher intrinsic strain is attributed to strain induced enhanced anisotropy of the nanoparticles. [ABSTRACT FROM AUTHOR]