(a) XRD spectra of 8 nm, 13 nm, 23 nm, and 50 nm particles annealed at , , , and , respectively. (b) Rietveld plot for 23 nm particles. The recorded data are indicated by open circles, the calculated and difference patterns are shown by solid lines. The Bragg positions of the reflections of and are indicated by vertical lines.
High-resolution TEM photograph of the assembly of particles. The lattice fringes are visible within an isolated particle. Arrows point the particle with size of .
(a) Size dependence of the unit cell volume of nanoparticles. Straight line shows a linear expansion of the unit cell with inverse-value of particle size due to surface effect. (b) Size dependence of lattice parameters of particles. Inset shows the ratio varying linearly with .
(a) Temperature dependence of ZFC and FC magnetization of nanoparticles at 100 Oe. Inset shows the ZFC magnetization in extended scale. (b) Thermo-remanent magnetization vs temperature curves recorded upon heating at after FC in .
Magnetization hysteresis loops of particles measured at 10 K after FC in 50 kOe. Insets show the size dependencies of spontaneous magnetization and of coercive field .
vs temperature curves for nanoparticles measured at 10 kOe. The straight lines are a Curie–Weiss fit with two fitting parameters and . Insets show the size dependencies of paramagnetic Curie temperature and Co effective magnetic moment .
[(a), (b), (c), and (d)] Temperature dependencies of the in-phase component of ac susceptibility of nanoparticles measured at probing field 10 Oe for different frequencies. (e) vs plot for 8 nm particles in extended scale. (f) The vs linear dependence for 8 nm particles and the best fit line.
Temperature dependence of ZFC magnetization of 8 nm particles measured at different magnetic fields. Inset shows the temperature of magnetization maximum vs magnetic field ; solid line is the best fit of the AT line giving the freezing temperature .
(a) Temperature dependence of FC magnetization measured for 23 and 8 nm particles at 100 Oe and at ambient pressure and under pressure of about 10 kbar. (b) FC magnetization hysteresis loops measured for 23 nm particles at 100 K at ambient pressure and under pressure 10.4 kbar. A half-loop only was recorded at 10 K because of large coercivity. (c) FC magnetization hysteresis loops measured for 8 nm particles at 40 K at ambient pressure and under pressure 9.4 kbar. A half-loop was recorded at 10 K because of large coercivity.
Results of the Rietveld refinement: lattice parameters, volume of unit cell, for oxygen site position in rhombohedral lattice , Co–O–Co angle, and agreement factors for nanoparticles with different particle size.
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