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Abstract:
Resonant Ultrasound Spectroscopy has been used to characterize elastic
and anelastic anomalies in a polycrystalline sample of multiferroic
Pb(Fe0.5Nb0.5) O-3 (PFN). Elastic softening begins at similar to 550 K,
which is close to the Burns temperature marking the development of
dynamical polar nanoregions. A small increase in acoustic loss at
similar to 425 K coincides with the value of T* reported for polar
nanoregions starting to acquire a static or quasi-static component.
Softening of the shear modulus by similar to 30-35% through similar to
395-320 K, together with a peak in acoustic loss, is due to classical
strain/order parameter coupling through the cubic. tetragonal.
monoclinic transition sequence of ferroelectric/ferroelastic
transitions. A plateau of high acoustic loss below similar to 320 K is
due to the mobility under stress of a ferroelastic microstructure but,
instead of the typical effects of freezing of twin wall motion at some
low temperature, there is a steady decrease in loss and increase in
elastic stiffness below similar to 85 K. This is attributed to freezing
of a succession of strain-coupled defects with a range of relaxation
times and is consistent with a report in the literature that PFN
develops a tweed microstructure over a wide temperature interval. No
overt anomaly was observed near the expected Neel point, similar to 145
K, consistent with weak/absent spin/lattice coupling but heat capacity
measurements showed that the antiferromagnetic transition is actually
smeared out or suppressed. Instead, the sample is weakly ferromagnetic
up to similar to 560 K, though it has not been possible to exclude
definitively the possibility that this could be due to some magnetic
impurity. Overall, evidence from the RUS data is of a permeating
influence of static and dynamic strain relaxation effects which are
attributed to local strain heterogeneity on a mesoscopic length scale.
These, in turn, must have a role in determining the magnetic properties
and multiferroic character of PFN.
