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  • 1 Department of Physics, Graduate School of Engineering, Yokohama National University 79-5 Tokiwadai, Hodogaya, Yokohama 240-8501, Japan
  • 2 Department of Physics, Graduate School of Engineering, Yokohama National University 79-5 Tokiwadai, Hodogaya, Yokohama 240-8501, Japan
  • 3 Department of Physics, Graduate School of Engineering, Yokohama National University 79-5 Tokiwadai, Hodogaya, Yokohama 240-8501, Japan
  • 4 Department of Physics, Faculty of Engineering 79-5 Tokiwadai, Hodogaya, Yokohama 240-8501, Japan
  • 5 Department of Physics, Faculty of Engineering 79-5 Tokiwadai, Hodogaya, Yokohama 240-8501, Japan
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Summary Ni1-xZnxFe2O4 (0≤x≤1) mixed ferrite nanoparticles encapsulated with amorphous-SiO2 were prepared by a wet chemical method. Particle sizes were controlled to range from 2.6 to 33.7 nm by heat treatment, and the particle size dependence of saturation magnetization Ms was investigated for the x=0.5 region. The Ms value decreased abruptly for particle sizes below about 6 nm. From the temperature dependence of the magnetization under field-cooled and zero-field-cooled conditions, blocking temperatures Tb were observed to be between 28 and 245 K depending on the particle size. At the blocking temperature, the superparamagnetic spins in the particle are supposed to be blocked against the thermal fluctuation energy. A smaller particle volume causes a lower blocking temperature; so an extremely small particle would be strongly affected by thermal fluctuation.

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