Particle Size Dependence of Magnetic Properties in Cobalt Ferrite Nanoparticles Jun Hee Cho 1, Sang Gil Ko 1, Yang kyu Ahn 1, Eun Jung Choi 2 * 1 Department of Nanochemistry & Biochemistry, Konyang University, Nonsan, Chungnam, Korea Department of opthalmic Optics, Konyang University Daejeon , Korea

Experimental Procedure 1. Synthesis : Microemulsion method 2. X-ray diffraction pattern (XRD) 3. Transmission electron microscope (TEM) 4. Mössbauer spectroscopy DEPARTMENT OF OPTHALMIC OPTICS KONYANG UNIVERSITY

Synthesis of nanosize CoFe 2 O 4 particles Metal Ion Aqueous Solution (CoCl 2 /FeCl 3 0.1M) Drying & Aging (600K) Decantation & Washing with Solvent & Aalcohol Coprecipitation (Cobalt-Iron Hydrixide Carbonate Particles with Surfactant) Mixing & Stirring at R.T. Nanosize Co-Ferrite Particles Alkali Aqueous Solution (Na 2 CO 3 0.2M) Microemulsions 0.2M AOT in iso-Octane Microemulsions 0.2M AOT in iso-Octane Water drops in oil Surfactant stabilized particle Exchange of water content Precipitation & Particle growth MFe(OH)n DEPARTMENT OF OPTHALMIC OPTICS KONYANG UNIVERSITY

X-ray diffraction pattern of CoFe 2 O 4 nanoprticles 1. Cubic spinel structure 2. Lattice parameter α 0 =8.39 Å 3. Average size : 88 Å DEPARTMENT OF OPTHALMIC OPTICS KONYANG UNIVERSITY

TEM image and Particle size distribution of CoFe 2 O 4 nanoparticles Cobalt ferrite paricles average size calculated : 88 Å ― calculated : log-normal distribution Log-normal distribution DEPARTMENT OF OPTHALMIC OPTICS KONYANG UNIVERSITY

Magnetic Hysteresis (SQUID) of cobalt ferrite DEPARTMENT OF OPTHALMIC OPTICS KONYANG UNIVERSITY

Mössbauer spectrum at various temperatures ▶ a typical paramagnetic doublet → paramagnetic behavior of the particles ▶ Isomer-shift value : 0.35 mm/s ▶ Quadrupole splitting value : 0.78 mm/s (R.T.) DEPARTMENT OF OPTHALMIC OPTICS KONYANG UNIVERSITY

Cation distribution Co 1-x Fe x )[Co x Fe 2-x ]O 4 Area ratio of the A and B subspectra for the above distribution Where ∱ A and ∱ B represent the recoil-free fraction of A and B site Fe ions, respectively. Recoil Free Fraction ∱ Where E R is the recoil energy of 57 Fe for the 14.4 keV gamma ray. θ and k B represent the Debye temperature and the Boltzmann constant. (Co 0.66 Fe 0.34 )[Co 0.34 Fe 1.66 ]O 4 DEPARTMENT OF OPTHALMIC OPTICS KONYANG UNIVERSITY

Line broadening from the distribution of quadrupole splittings Mössbauer line shift E Q from the pure magnetic lines due to the quadrupole interaction where θ and Ф are the polar and azimuth angles of the magnetic hyperfine field vector relative to the principal axes of the electric-field- gradient tensor. On assuming that the maximum electric field gradient q and the asymmetry parameter ŋ are independent of θ and Ф, the average value of E Q taken over all directions vanishes, i.e. DEPARTMENT OF OPTHALMIC OPTICS KONYANG UNIVERSITY

The line broadening from this random distribution is not zero: that is, Another source of broadening can be estimated from the linewidth, 0.47 mm/s, of the quadrupole dooublet above the Curie temperature. In view of the 0.25 mm/s linewidth of an iron foil, a line broadening of 0.22 mm/s of the doublet is the result of distributions of the isomer shift and magnitudes of α and ŋ in addition to the thickness effect of the absorber. Thus, the combined broadening from distribution of quadrupole splitting and isomer shift can not be negligible. DEPARTMENT OF OPTHALMIC OPTICS KONYANG UNIVERSITY

▶ Nanosize CoFe2O4 particle have been synthesized using microemulsion method. ▶ Cubic spinel structure ▶ the lattice constant : α 0 =8.39 Å ▶ Average size : 88 Å ▶ Combined broadening from distribution of quadrupole splitting and isomer shift is not negligible. ▶ Cation distribution (Co 0.66 Fe 0.34 )[Co 0.34 Fe 1.66 ]O 4 ▶ Neel temperature : 330 K Conclusions DEPARTMENT OF OPTHALMIC OPTICS KONYANG UNIVERSITY