In this work we present a system to measure weak magnetic field, based
on measurements of anhysteretic states of a ferromagnetic nucleus. In
order to improve this magnetometric sensor, cores of amorphous ribbon
s with different characteristics have been tested. An electronic set-u
p has been built to perform the measurements and to avoid the sensor n
oise. The sensitivity of this device exhibits a large dependence on th
e magnetic anisotropy distribution of the core. A theoretical model ha
s been deduced to explain the obtained results.