Field Dependence of the Magnetization at Increasing Size: An Empirical Approach for Cobalt Ferrite Nanoparticles

This paper reports on the synthesis and characterization (morphological and magnetic) of cobalt ferrite nanoparticles in alkaline medium at increasing co-precipitation temperature (30 °C, 50 °C, and 95 °C). Transmission electron microscopy reveals morphological mean diameter (diameter dispersity) increasing (decreasing), with the increase of the co-precipitation temperature, from 6.9 to 10.4 nm (0.19-0.12) while describing the particle size histogram using a log-normal distribution function. Hysteresis curves (M versus H) recorded at 5, 100, and 300 K reveal cycles progressively closing while increasing the temperature. In the range of temperature of our investigation, the M versus H data can be successfully curve fit by a phenomenological model including both ferrimagnetism and superparamagnetism, from which the content of the latter (p), the coercivity (H
_C
), the squareness ratio (M
_R
/M
_S
^FM
), and the magnetic mean diameter (diameter dispersity) can be extracted. The magnetic mean diameter (diameter dispersity) increases (decreases), with the increase of the co-precipitation temperature, from 5.2 to 9.1 nm (0.39-0.26) while averaging out the superparamagnetic contribution using a log-normal distribution function.