Néel vs Brownian
The ability to detect nanoparticles only when they are bound to their target cells relies on large differences in both the size and timing of the magnetic field they emit, depending upon whether they are bound to their target cells.
Free particles release a low magnetic field in less than a millisecond after they are magnetized (Brownian relaxation), while bound particles release a much larger magnetic field over 0.1 to 1 second (Néel relaxation).
We can specifically measure particles bound to cells