We have investigated the dynamics of agglomeration of magnetite nanoparticles under magnetic field by measuring the magnetic weight. Round magnetite nanoparticles are synthesized and used in the experiments. Figure 1 shows the experimental setup.[1] The magnetic weight is monitored with a conventional electronic balance. Figure 2 shows a typical change of the magnetic weight. When the magnetite nanoparticle sample is placed under the magnetic field, the magnetic weight jumps instantaneously by the Neel and the Brown mechanism, and then increases slowly as the nanoparticles agglomerate. The temporal change of the magnetic weight fits well to the stretched exponential function, W(t) = W(∞) + [W(0) – W(∞)]exp[-(t/τ)β] where τ is the characteristic relaxation time and the exponent β is smaller than one.[2] The stretched exponential kinetics implies that the activation energy barrier involved in the dynamics has some distribution and that there are many different pathways for the agglomeration. The activation energy spectrum can be determined by the inverse Laplace transformation of the temporal change. Figure 3 shows the fluctuation of the magnetic weight when the laboratory temperature changes considerably in the winter season. Deviation from the stretched exponential function is attributed to thermal effect. Thermal motions of the nanoparticles destabilize the agglomerate of nanoparticles to reduce the magnetic weight. Figure 4 shows the difference of the measured and the fitted magnetic weight, and the laboratory temperature. The magnetic weight difference follows the temperature change exactly after the nanoparticles agglomerate. The fluctuation of the magnetic weight is explained well with the Boltzmann distribution,[3] which suggests that the magnetic weight of the agglomerate of magnetic nanoparticles works as a thermometer. Analysis with the Boltzmann distribution and the stretched exponential growth of the magnetic weight indicate that the agglomeration of magnetite nanoparticles by magnetic field is a complex process.
[1] D. Jin, H. Kim, Bull. Korean Chem. Soc. 2013, 34, 1715.
[2] D. Jin, H. Kim, Bull. Korean Chem. Soc. 2015, 36, 424.
[3] D. Jin, H. Kim, Bull. Korean Chem. Soc. 2016, 37, 962.
Nanomagnetics & magneto-optical nanomaterials , Nanosensors , Nanofluidics
