A magnetic-field-induced diffraction pattern was studied by a new tech
nique: thermal lens coupled magneto-optical effect. In low fields, the
diffraction angle has an almost quadratic dependence: thetais-proport
ional-toH2. This effect can be used as an all optical magnetic field s
ensor with a resolution of 0.5 Oe, with a potential for further improv
ement. The method can also lead to the measurement of the onset of agg
lomeration in the sample. (C) 1994 American Institute of Physics.