  We demonstrate that the volume of the Fermi surface, measured very precisely
using de Haas-van Alphen oscillations, can be used to probe changes in the
nature and occupancy of localized electronic states. In systems with
unconventional ordered states, this allows an underlying electronic order
parameter to be followed to very low temperatures. We describe this effect in
the field-induced antiferroquadrupolar (AFQ) ordered phase of PrOs4Sb12, a
heavy fermion intermetallic compound. We find that the phase of de Haas-van
Alphen oscillations is sensitively coupled, through the Fermi volume, to the
configuration of the Pr f-electron states that are responsible for AFQ order.
In particular, the \beta-sheet of the Fermi surface expands or shrinks as the
occupancy of two competing localized Pr crystal field states changes. Our
results are in good agreement with previous measurements, above 300 mK, of the
AFQ order parameter by other methods. In addition, the low temperature
sensitivity of our measurement technique reveals a strong and previously
unrecognized influence of hyperfine coupling on the order parameter below 300
mK within the AFQ phase. Such hyperfine couplings could provide insight into
the nature of hidden order states in other systems.
