Imaging of Magnetic Nanoparticles Using a Second Harmonic of Magnetization With DC Bias Field

We have developed a method to improve the detection sensitivity for the magnetization M of magnetic nanoparticles (MNPs) using a high T_c superconducting quantum interference device magnetometer. The M response of MNP to an applied magnetic field H(M-H characteristics) can be divided into a linear region and a saturation region, which are separated at a transition point H_k. When applying an excitation ac magnetic field (H_ac) and an additional dc bias field H_dc = H_k, the second harmonic of M reaches a maximum due to the nonlinearity of its M-H characteristics. This harmonic is stronger than any other harmonics, including a third harmonic. The advantage of using the second harmonic response is that the response can be measured even in a small field H_ac. The M response of MNP was systematically analyzed and experimentally demonstrated. For conventional detection using a third harmonic, the amplitude of the H_ac must be larger than the threshold level, which is almost the same as H_k. Detection methods using a second harmonic can be applied to magnetic particle imaging. We finally demonstrate the construction of a 1-D image of two separated bottle-shaped MNP samples using the method with a lock-in amplifier.