  Magnetic nanoparticle (MNP) has attracted interest in various research fields
due to its special superparamagnetic and strong magneto-optical effects,
especially as contrast agents to enhance the contrast of medical imaging. By
introducing the interaction coefficient, we propose a model of the nonlinear
Faraday rotation of MNP under the excitation of an external alternating
magnetic field. In our homemade device (which can detect the rotation angle as
low as about 2e-7 rad), it has been verified that the higher harmonics of the
Faraday rotation can avoid the interference of paramagnetic and diamagnetic
background at lower concentrations. What's more, the higher harmonics of the
Faraday rotation of MNP can be detected in real time and they have a linear
relationship with concentration. In the future, it is expected to use MNP as a
magneto-optical contrast agent to achieve high-resolution imaging in vivo.
