  A Hall magnetometer with variable sensitivity is constructed to measure the
magnetic properties of magnetic nanoparticles manufactured by different
methods. This novel magnetometer can also be used to measure bulk materials and
samples in liquids. The magnetometer is constructed with two commercial
Hall-effect sensors in an acrylic structure, which serves as the support for a
micrometer and the circuit board with the sensors. For operation, the
magnetometer it acquires a complete magnetization curve in a few minutes. If
has a magnetic moment sensitivity of 1.3*10-9 Am2 to sensitivity of 493 mV/mT,
the sensitivity can be adjustable in the range of 10 to 493 mV/mT. Its
performance is tested with magnetic nanoparticles. As an application example,
we estimate the mean diameter of the nanoparticles using the magnetic curves.
The results are compared with those obtained by other techniques, such as
transmission electron microscopy (TEM), X-ray diffraction (XRD) and dynamic
light scattering (DLS). The magnetization results are also compared with those
obtained by independent commercial magnetometers, which reveals errors of
approximately 0.31 Am2/kg (i.e., 0.6%) in the saturation region.
