I. Introduction
Many magnetic separation systems with a superconducting magnet have been developed [1]–[3]. The superconducting magnet can generate a high magnetic field, and ferromagnets magnetized in the high magnetic field create strong magnetic field gradients. The strong magnetic field gradients attract magnetic particles in fluid to the ferromagnets. The magnetic separation technique using the magnetic field gradients is, however, unsuitable for separating magnetic particles with a very small radius (below 100 nm). The reason is that the magnetic force on the particles is relatively smaller than their diffusion force. On the other hand, the magnetic chromatography system is a very useful device. It utilizes the strong magnetic field gradients to separate the fine magnetic particles with different magnetic susceptibilities in a colloidal mixture [4]–[6].