Influence of crystallite size on the magnetic properties of Fe3O4 nanoparticles

• Magnetite nanoparticles have been synthesized using chemical precipitation method.
• Smaller magnetite particles below 14 nm in size are in super-paramagnetic state.
• Bigger particles show multi-domain character.
• Magnetite in the size range 14–36 is dominantly single domain particle.
• Smallest particles are spherical while bigger particles are in faceted cubic shape.

Structural and magnetic properties of chemically synthesized magnetite nanoparticles have been studied using X-ray diffraction, Transmission Electron Microscopy and Vibrating Sample Magnetometer. Magnetically the synthesized nanoparticles are ranging from superparamagnetic to multi domain state. Average crystallite size of the synthesized magnetite nanoparticles were determined using X-ray line broadening and are found to be in the range of 9–53 nm. On the other hand, the TEM images show that the size is ranging between 7.9 and 200 nm with the transition from spherical superparamagnetic particles to faceted cubic multi domain particles. Magnetic parameters of the samples show a strong dependence on average crystallite size. The ratio of coercive field at 20 K to that at 300 K (Hc (20 K)/Hc (300 K)) increased sharply with decrease in crystallite size. A critical crystallite diameter of order 36 nm may be inferred as boundary between single domain to multi domain transition. Zero-field-cooled (ZFC) and field-cooled (FC) measurements at 10 Oe field validate the same for smallest and largest size samples, confirming that the anisotropy energy is greater than thermal energy upto 300 K temperature. For 9 nm sample broad ZFC curve with overlapping of FC curve is observed just at 300 K, indicating the effect of strong dipolar field in superparamagnetic system.

We present our study on magnetite nanoparticles. We observed that the synthesized nanoparticles behave like single domain particles in the range of 14 nm–36 nm. They show superparamagnetic properties if particles are smaller than 14 nm and multi-domain properties when the particles are bigger than 36 nm.Figure optionsDownload as PowerPoint slide