  We show that a single atomic magnetometer in a magnetically unshielded
environment can be used to perform magnetic particle spectroscopy (MPS) and AC
susceptometry (ACS) on liquid-suspended magnetic nanoparticles. We demonstrate
methods allowing a simultaneous recording of M(H) and dM/dH(H) dependences of
samples containing down to 1 $\mu$g of iron. Our results pave the way towards
an atomic magnetometer based MPI scanner.
