On the origin of torque generation in superparamagnetic particles

The torsion properties of proteins are being studied on the single-molecule level using multicore magnetic particles[1]. A previous model for the maximum particle torque assumed an angular distribution of the nanometer-scale magnetic grains inside the particle and discrete switching of the largest grains at a coercive field. Here we investigate the role of thermally excited switching of the magnetization of all grain sizes on the overall particle magnetization and torque. We assume that the magnetic grains inside the particle have a uniaxial anisotropy and calculate the magnetisation using the Stoner–Wohlfarth theory. The model is able to predict a maximum torque at a phase lag between the applied field and the remanent moment less than 90 degrees, as has been observed in recent experiments. [1] A. v. Reenen, F. Gutierrez Mejia, L.J. van IJzendoorn and M.W.J. Prins, Biophys. J. 104 (2013) 1073-1080