Magnetic nanoparticles experience magnetic force under the magnetic field. The magnetic force can be measured as the magnetic weight of the magnetic nanoparticle sample with a conventional electronic balance. The instantaneous jump of the magnetic weight by the Neel and the Brown mechanisms is observed when the magnetic sample is placed under the magnetic field. The magnetic weight increases slowly thereafter as the nanoparticles agglomerate. During the slow increase of the magnetic weight, the magnetic weight fluctuates with the change of temperature. When the temperature rises, the magnetic weight decreases, and vice versa, which suggests that the magnetic weight could be used as a thermometer. The thermal fluctuation of the magnetic weight results from the structural relaxation of the agglomerate by the kinetic energy of nanoparticles. We propose a simple equilibrium model for the thermal fluctuation of the magnetic weight, which explains the fluctuation in terms of the free energy difference between the agglomerated and the dispersed nanoparticles. The structural relaxation of the agglomerate is a complex process because many different clusters are formed during the agglomeration. In order to understand the details of the thermal fluctuation of the magnetic weight, the energy landscape picture with the distribution of the involved free energy is required.Keywords: Magnetic nanoparticles, magnetic weight, thermal fluctuation, equilibrium model
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