Elastic and magnetoelastic relaxation behaviour of multiferroic (ferromagnetic + ferroelectric + ferroelastic) Pb(Fe_0.5Nb_0.5)O₃perovskite

Abstract

Resonant Ultrasound Spectroscopy has been used to characterize elastic and anelastic anomalies in a polycrystalline sample of multiferroic Pb(Fe(0.5)Nb(0.5))O(3) (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.

Elastic and anelastic relaxation behaviour of perovskite multiferroics I: PbZr0.53Ti0.47O3 (PZT)–PbFe0.5Nb0.5O3 (PFN)

“…A closely similar pattern is seen with increasing PZ content along the PZ-PFN join in Lidoped samples [42]. There is, however, great variability between samples, and weak ferromagnetism has been reported both at room temperature and at temperatures up to *530-580 K [28,[86][87][88][89] [47]. It remains possible that the weak room temperature ferromagnetism in PFN and PZT-PFN is due to the presence of some minor impurity phase, but Kuzian et al [90,91] have shown that ferrimagnetic ordering might occur in PFN, while Glinchuk et al [4] have shown how a nanodomain structure of local chemical ordering could lead to ferromagnetism in PZT-PFN.…”

“…At least in some samples, the low temperature structure may be rhombohedral (R3m) [21]. The first transition is close to tricritical in character and typically occurs at *380-390 K. The second, 20-40 K lower, is generally considered to be first order in character [21][22][23][24][25][26][27][28][29][30][31]. With increasing PT content along the PFN-PT binary join, the stability field of the monoclinic structure diminishes such that the P4mm-Cm transition point extrapolates to 0 K at *8-12 % PT [32,33].…”

“…From the same work [33], it appears that a magnetic anomaly attributed to a spin glass Locations of the Pm 3m À P4mm and P4mm-Cm transitions in PZT are taken from the phase diagram of Jaffe et al [38] and for the Cm-Cc transition from Cordero et al [44]. Transition temperatures shown for PFN are based on a review of the literature in Carpenter et al [28]. Values of T m for Pm 3m À P4mm at intermediate temperatures from the literature for nearby compositions [48,70] fall on the straight line drawn between T c values of the pure end members.…”

“…It can be weakly dispersive in some samples [101][102][103], however, with variability of the local order responsible for relaxor as opposed to classical ferroelectric behaviour. Evidence for a temperature below which polar nanoregions (PNR's) might be present is provided by acoustic emission (564-603 K [30]), thermal expansion data (*690 K [104]) and the onset of elastic softening (*550 K [28]). No information is yet available for local ordering in ternary PZT-PFN, but ceramics near the MPB have dielectric maxima related to the ferroelectric transition which are reported to be generally broad and independent of frequency [65,67,68,70].…”

“…They are referred to below as PZTFN6, PZTFN4 and PZTFN3, respectively. As described in Sanchez et al [3] and in more detail by Carpenter et al [28] for PFN, they were prepared from oxide starting materials by a conventional solid-state reaction route. The final stage was heating of pressed pellets to 1250°C (PZTFN4, PZTFN3) or 1100°C (PZTFN6) at 10°C min -1 , annealing at this temperature for 4 h and cooling back to room temperature, again at 10°C min -1 .…”