Surface controlled synthesis of MFe2O4 (M = Mn, Fe, Co, Ni and Zn) nanoparticles and their magnetic characteristics

Engineered magnetic nanoparticles, MFe2O4 (where M = Mn, Fe, Co, Ni and Zn), of mean size 2-16 nm (standard deviation sigma <= 15%) with controlled magnetic properties have been synthesized through a solventless thermolysis technique. In this 'energy-efficient' facile approach, a long-chain amine is utilized as the solvent, reducing and surface-functionalizing agent. The particle sizes between 2-9 nm are tuned by simple manipulation of the amine to precursor molar ratio while the larger sizes (12-16 nm) are attained by seed-mediated growth. The synthesized nanoparticles are magnetic and show cubic-spinel structure as characterized by HRTEM, SAED, XRD and Raman spectroscopy. The spin-orbit coupling, which induces larger magnetocrystalline anisotropy energy, leads to an increase of the blocking temperature in an order of Zn, Ni, Mn, Fe and Co. Additionally the saturation magnetization (magnetization at 2 T) of nanoparticle decreases in an order as per the periodic arrangement i.e. Mn > Fe > Co > Ni > Zn. This trend is better understood in terms of a cationic distribution of varying magnetic moments in these spinel oxides. For all the ferrites, the magnetization saturates to the bulk value at sizes >= 12 nm, indicating strict control over the dead layer thickness. The effect of surface coordination on the magnetic properties of the ferrite nanoparticles is analyzed. In contrast to trioctylphosphine oxide (TOPO) and oleic acid (OA) functionalized nanoparticles the saturation magnetization of amine protected nanoparticles is higher, which suggests a strong coupling of amine molecules to cations at the particle interface.