  Inhomogeneities of the field in the superconducting magnet of a
superconducting quantum intereference device (SQUID) magnetometer can create
serious artefacts in the d.c. magnetization measurements of superconducting
samples. We discuss the problem focusing on the procedure of calculating the
magnetic moment of a sample from the measured SQUID output signal. The (weakly)
ferromagnetic superconductor RuSr2GdCu2O8 has been reported sensitive to
inhomogeneities of the SQUID's superconducting magnet, which create artefacts
in its measured magnetization. Indeed, we show that, because of the small
values of the magnetic hysteresis width, field changes less than 100 microTesla
over the scaning length inside the magnet are enough to create spurious signals
in the measured magnetic moment of this compound.
