Superparamagnetic behaviour and T₁, T₂ relaxivity of ZnFe₂O₄ nanoparticles for magnetic resonance imaging

Abstract

In the present study, ZnFe₂O₄ nanoparticles were synthesized by the chemical co-precipitation followed by calcinations at 473 and 673 K for 4 h. Particle sizes obtained were 4 and 6 nm for the calcination temperatures of 473 and 673 K, respectively. To study the origin of system’s low temperature spin dynamic behaviour, temperature dependence of susceptibility was investigated as a function of particle size and frequency. Slight increase in the grain size from 4 nm at 473 K to 6 nm at 673 K has led to a peak shift of temperature dependence of susceptibility measured at a constant frequency of 400 Hz. Temperature dependence of at different frequencies also resulted in peak shift. Relaxation time dependence of peak temperature obeys a power law, which provides the fitting parameters within the range of superparamagnetic nature of the particles. Further, dependence of relaxation time and peak temperature obeys Vogel–Fulcher law rather than Néel–Brown equation demonstrating that the particles follow the behaviour of superparamagnetism of slightly interacting system. Spin–lattice, T ₁ and spin–spin, T ₂ relaxivity of proton of the water molecule in the presence of chitosan-coated superparamagnetic ZnFe₂O₄ nanoparticle yields the values of 0.002 and 0.360 s⁻¹ per ppm.

Keywords:

• ZnFe₂O₄
• superparamagnetism
• T ₁ and T ₂ relaxivity
• nuclear magnetic resonance
• transmission electron microscopy

Acknowledgements

Authors are indebted to Department of Materials Engineering, Indian Institute of Science for support. One of the authors, Dr S. Manjura Hoque gratefully acknowledges Indian Institute of Science for awarding Centenary Visiting Fellowship under which present work has been carried out. Authors also acknowledge the support of NMR Centre and AFMM of Indian Institute of Science for providing NMR and electron microscope facilities.