Enhancement of magnetic heating efficiency in size controlled MFe2O4 (M = Mn, Fe, Co and Ni) nanoassemblies

The MFe2O4 magnetic nanoparticle nanoassemblies (MNNAs) have been synthesized via thermal decomposition of metal chloride in ethylene glycol (EG) in the presence of ethylenediamine (EDA). The size of the nanoassemblies is controlled in the range of 25-60 nm by manipulation of Fe-precursor mole content to ethylene glycol (EG) content and from 60 to 135 nm by using a bi-solvent mixture of ethylene glycol and polyethylene glycol (PEG-400). In this study, we demonstrate optimization of magnetic fluid heat activation by tailoring the size of MFe2O4 (M = Mn, Fe, Co and Ni) MNNAs. The densely packed nanocrystals within the MNNAs induce strong exchange as well as dipolar interactions between the nanocrystals, which increases the total magnetic moment for MNNAs. Additionally the magnetization (MS, magnetization in a field of 20 kOe) of MNNAs decreases in the order Mn > Fe > Co > Ni due to the cationic distribution of ions with varying magnetic moments in these spinel oxides. A sharp increase of heating efficiency for 25-60 nm assembled particles could be attributed to the collective Neel relaxation of nanocrystals within the assemblies and also due to high particle magnetic moment, which increases with the MNNAs size. Furthermore, among all the MFe2O4 nanoassemblies of various sizes, Fe3O4 MNNAs with an average diameter of 80 nm show an excellent SAR value of 646 W g(-1) of Fe3O4 at 247 kHz with an applied AC magnetic field of 310 Oe, which is 4 times higher than that of the single domain assembled nanoparticles. The moderate anisotropy constant and high MS values of Fe3O4 MNNAs make it a most suitable candidate to produce the highest heating power. These magnetic MNNAs are efficient in killing the cancer cells by the application of an AC magnetic field even for a short treatment time of 30 min.