Abstract
Resonant Ultrasound Spectroscopy has been used to characterize elastic and anelastic anomalies in a polycrystalline sample of multiferroic Pb(Fe0.5Nb0.5)O3 (PFN). Elastic softening begins at ~550 K, which is close to the Burns temperature marking the development of dynamical polar nanoregions. A small increase in acoustic loss at ~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 ~30–35% through ~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 ~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 ~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 Néel point, ~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 ~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.
Highlights
Phases with the perovskite structure in the solid solutions Pb(Zr0.53Ti0.47)O3 (PZT)–Pb(Fe0.5Nb0.5)O3 (PFN) and Pb(Zr0.53Ti0.47)O3–Pb(Fe0.5Ta0.5)O3 (PFT) have recently been shown to display simultaneous ferromagnetic and ferroelectric properties at room temperature [1,2,3,4]
A plateau of high acoustic loss below ~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 ~85 K
No overt anomaly was observed near the expected Néel point, ~145 K, consistent with weak/absent spin/lattice coupling but heat capacity measurements showed that the antiferromagnetic transition is smeared out or suppressed
Summary
Phases with the perovskite structure in the solid solutions Pb(Zr0.53Ti0.47)O3 (PZT)–Pb(Fe0.5Nb0.5)O3 (PFN) and Pb(Zr0.53Ti0.47)O3–Pb(Fe0.5Ta0.5)O3 (PFT) have recently been shown to display simultaneous ferromagnetic and ferroelectric properties at room temperature [1,2,3,4]. Control of ferroelectric domain patterns by application of a magnetic field has been demonstrated [5]. These materials are the lowest-loss room-temperature multiferroics currently known [1]. They combine aspects of the properties of the pure, end member phases to produce ferromagnetism, antiferromagnetism, ferroelectricity and ferroelasticity. As part of a wider study of the elastic and anelastic properties of selected phases from the PFN–PZT and PFN–PZT solid solutions, aspects of the ferroelastic behaviour of a ceramic sample of end-member PFN which displays ferromagnetism at room temperature have been investigated by resonant ultrasound spectroscopy (RUS). We present results which draw attention to the permeating presence of static and dynamic strain coupling effects
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