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DESCRIPTION:Jeff Lynn\, National Institute of Standards and Technology\n\nA
few results from recent neutron and x-ray scattering results will be pres
ented on both CMR and multiferroic systems. For the multiferroics\, we ha
ve been investigating the magnetic structure of hexagonal HoMnO 3
sub> as a function of temperature and field [1]\, which is a commensurate
antiferromagnetic (T N =72 K) ferroelectric (T C
=875 K). Three different chiral symmetries describe the zero field magne
tic phases\, with strong dielectric anomalies associated with the phase tr
ansitions. The spin dynamics are well described by a Heisenberg model in
two dimensions. Orthorhombic TbMnO 3 develops a longitudinal
ly polarized spin density wave state below 41 K\, with a change in magneti
c structure at 28 K that permits the development of ferroelectricity\, whi
le the magnetic structure remains incommensurate [2]. The magnetism is pa
rticularly sensitive to Na doping [3]. For the RMn 2 O
5 system (R=Tb\, Dy\, Ho) strong anomalies in the specific heat\,
thermal expansion\, and dielectric constant are a manifestation of the mag
netic coupling to the ferroelectricity [4]. Strong magnetoelastic couplin
g is also found in the triangular antiferromagnetic multiferroic CuFeO 2 [5]. For the Kagome staircase system Co 3 V 2 O 8 the rich variety of magnetic phases and lock-i
n transitions is a signature of competing interactions [6]\, and is quite
different from ferroelectric Ni 3 V 2 O 8
. For the CMR systems\, we will review recent results for the pola
ron dynamics in optimally doped La-BaMnO 3 and La-SrMnO
3 \, and compare these results with La-CaMnO 3 [7] a
nd the bilayer system [8]. The overall behavior observed in the CMR regim
e of the manganites is quite similar to that observed in the relaxor ferro
electrics as well as the spin and charge stripes found cuprate oxides\, de
monstrating a commonality of many of the underlying physical concepts of t
hese perovskite oxides.
DTSTART:20070927T200000Z
LOCATION:Physics Building\, Room 204
SUMMARY:Recent Results on CMR and Multiferroic Manganese Oxides
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