  Magnetite (Fe3O4) is of fundamental importance as the original magnetic
material and also for the Verwey transition near T_V = 125 K, below which a
complex lattice distortion and electron orders occur. The Verwey transition is
suppressed by strain or chemical doping effects giving rise to well-documented
first and second-order regimes, but the origin of the order change is unclear.
Here, we show that slow oxidation of monodisperse Fe3O4 nanoparticles leads to
an intriguing variation of the Verwey transition that elucidates the doping
effects. Exposure to various fixed oxygen pressures at ambient temperature
leads to an initial drop to TV minima as low as 70 K after 45-75 days, followed
by recovery to a constant value of 95 K after 160 days that persists in all
experiments for aging times up to 1070 days. A physical model based on both
doping and doping-gradient effects accounts quantitatively for this evolution
and demonstrates that the persistent 95 K value corresponds to the lower limit
for homogenously doped magnetite and hence for the first order regime. In
comparison, further suppression down to 70 K results from inhomogeneous strains
that characterize the second-order region. This work demonstrates that slow
reactions of nanoparticles can give exquisite control and separation of
homogenous and inhomogeneous doping or strain effects on an nm scale and offers
opportunities for similar insights into complex electronic and magnetic phase
transitions in other materials.
