  The present article reports a method for the average grain size evaluation of
superconducting nano-particles through their magnetic properties. The use of
SQUID magnetometry to determine the average MgB2 particle size was investigated
and the results compared with those achieved through other techniques. In
particular the data obtained from zero field cooled magnetization measurement
as function of the temperature were compared with the results obtained by
scanning electron microscopy and Brunauer-Emmett-Teller techniques. The
particle magnetization was measured by a commercial SQUID magnetometer in
magnetic field (1 mT) and temperatures ranging from 5 to 50 K dispersing the
powders in a grease medium. The grain size is obtained by fitting the data
taking into account the Ginzburg-Landau temperature dependence of the London
penetration depth. Variations on typical modeling parameters were explored in
order to gain a better picture of the average grain size and the effectiveness
of various measurement techniques. We find that it is possible to use the
magnetization measurements to determine the average grain size even if the SEM
image analysis allows extracting more information about the grain size
distribution. Furthermore a Matlab routine has been developed in order to get
automatic analysis of SEM images.
