We developed a detection system for the magnetic nanoparticles utilizing an ac susceptibility measurement method. In this method, an excitation field is applied to the particles, and the resulting signal field from the particles is detected with a pickup coil. The coupling of the excitation field to the pickup coil can be reduced to 1/106. When a room temperature copper pickup coil with radius of 2.2 mm and the number of turns of 100 was used, we detected Fe3O4 particles down to the weight of 7 ng. This value was limited by the noise from a substrate although we can expect to detect the weight of 0.8 ng from the intrinsic noise of the system. It was also shown that the intrinsic noise of the system can be much improved by using a cooled pickup coil coupled to a high-Tc superconducting quantum interference device (SQUID).
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