  The spatial profile of the magnetization of Mn12 crystals in a swept magnetic
field applied along the easy axis is determined from measurements of the local
magnetic induction along the sample surface using an array of Hall sensors. We
find that the magnetization is not uniform inside the sample, but rather shows
some spatial oscillations which become more prominent around the resonance
field values. Moreover, it appears that different regions of the sample are at
resonance at different values of the applied field and that the sweep rate of
the internal magnetic induction is spatially non-uniform. We present a model
which describes the evolution of the non-uniformities as a function of the
applied field. Finally we show that the degree of non-uniformity can be
manipulated by sweeping the magnetic field back and forth through part of the
resonance.
