32 research outputs found
Electrically-driven phase transition in magnetite nanostructures
Magnetite (FeO), an archetypal transition metal oxide, has been
used for thousands of years, from lodestones in primitive compasses[1] to a
candidate material for magnetoelectronic devices.[2] In 1939 Verwey[3] found
that bulk magnetite undergoes a transition at T 120 K from a
high temperature "bad metal" conducting phase to a low-temperature insulating
phase. He suggested[4] that high temperature conduction is via the fluctuating
and correlated valences of the octahedral iron atoms, and that the transition
is the onset of charge ordering upon cooling. The Verwey transition mechanism
and the question of charge ordering remain highly controversial.[5-11] Here we
show that magnetite nanocrystals and single-crystal thin films exhibit an
electrically driven phase transition below the Verwey temperature. The
signature of this transition is the onset of sharp conductance switching in
high electric fields, hysteretic in voltage. We demonstrate that this
transition is not due to local heating, but instead is due to the breakdown of
the correlated insulating state when driven out of equilibrium by electrical
bias. We anticipate that further studies of this newly observed transition and
its low-temperature conducting phase will shed light on how charge ordering and
vibrational degrees of freedom determine the ground state of this important
compound.Comment: 17 pages, 4 figure
A compensating monochromator crystal bender at the HMI multipole wiggler beamline MAGS
A compensating watercooled crystal bender for high heat loads has been built and successfully commissioned at the new multipole wiggler beamline MAGS of the Hahn Meitner Institute at the synchrotron radiation source BESSY. The beamline takes a 3 x 0.3 mrad fan of the wiggler beam, corresponding to a heat load of up to 2000 W. Although the crystal bender was originally designed for maximum heat loads of 600 W, it was found to work with heat loads of up to 800 W, reducing the Si 111 rocking curve width from 22 to 11 arcsec at the Cu K edge 8.9 keV . In addition, the good mechanical reproducibility of the device is illustrate