We describe a modified version of a magneto-optical Kerr magnetometer which
allows measuring the hysteresis loop of a specific portion of the sample s
urface. The diameter of the sampled area can be optically tuned from severa
l mm down to 15 mu m. High quality hysteresis loops can be measured in a fe
w seconds. Strong fluctuations are observed when the spot diameter is reduc
ed to a few hundred microns or less. These fluctuations are related to the
well-known Barkhausen noise here investigated for the first time in a syste
matic way using optical techniques. Preliminary data for epitaxial Fe grown
on MgO are presented. (C) 2000 American Institute of Physics. [S0034-6748(
00)01904-3].