Superparamagnetic nanoparticle detection using second harmonic of magnetization response

We introduce a method to improve the detection sensitivity for the magnetization M of superparamagnetic nanoparticles (MNP). The M response of MNP to an applied magnetic field H (M-H characteristics) could be divided into a linear region and a saturation region, which are separated at a transition point H(k). When applying an excitation magnetic field (H(ac)) with a frequency ω0 and an additional dc bias field H(dc) = H(k), the second harmonic of M reaches the maximum due to the nonlinearity of the M-H characteristics. It is stronger than any other harmonics and responsible for small H(ac) without a threshold. The second harmonic selected as the readout criterion for M response of MNP is systematically analyzed and experimentally proven.