Agglomeration Dynamics of Magnetite Nanoparticles at Low Magnetic Field Gradient

Agglomeration of magnetite nanoparticles in the aqueous solution is studied at the low magnetic field gradients of 23-34 T/m by monitoring the temporal change of magnetic weight. A conventional electronic balance is used to measure the magnetic weight that is the magnetic force on the magnetic sample by the magnetic field gradient. The magnetic weight grows slowly following the stretched exponential after the instantaneous jump by the Neel and Brown relaxation. Magnetization of the magnetite nanoparticles is estimated from the magnetic weight and compared with the Langevin function. The magnetization is close to the saturation in the studied magnetic field range and the saturation magnetization of the agglomerate of nanoparticles is about 60% of that of the bulk magnetite. Kinetic parameters of the stretched exponential show little the magnetic field dependence in the investigated range. Complex energy landscape is involved in the agglomeration as the stretched exponential dynamics indicates. The half-life of the response function for the magnetic weight change suggests that the pathways of low energy barriers are activated by magnetic field at the early stage of agglomeration.