Strain relaxation dynamics of multiferroic orthorhombic manganites

Abstract

Resonant Ultrasound Spectroscopy has been used to characterise strain coupling and relaxation behaviour associated with magnetic/magnetoelectric phase transitions in GdMnO3, TbMnO3 and TbMn0.98Fe0.02O3 through their influence on elastic/anelastic properties. Acoustic attenuation ahead of the paramagnetic → colinear-sinusoidal/incommensurate/antiferromagnetic transition at ~41 K correlates with anomalies in dielectric properties and is interpreted in terms of Debye-like freezing processes. A loss peak at ~150 K is related to a steep increase in electrical conductivity with a polaron mechanism. The activation energy, Ea, of ≥ ~0.04 eV from a loss peak at ~80 K is consistent with the existence of a well-defined temperature interval in which the paramagnetic structure is stabilised by local, dynamic correlations of electric and magnetic polarisation that couple with strain and have relaxation times in the vicinity of ~10 -6 s. Comparison with previously published data for Sm0.6Y0.4MnO3 confirms that this pattern may be typical for multiferroic orthorhombic RMnO3 perovskites (R = Gd, Tb, Dy). A frequencydependent loss peak near 10 K observed for TbMnO3 and TbMn0.98Fe0.02O3, but not for GdMnO3, yielded Ea ≥ ~0.002 eV and is interpreted as freezing of some magnetoelastic component of the cycloid structure. Small anomalies in elastic properties associated with the incommensurate and cycloidal magnetic transitions confirm results from thermal expansion data that the magnetic order parameters have weak but significant coupling with strain. Even at strain magnitudes of ~0.1-1 ‰, polaron-like strain effects are clearly important in defining the development and evolution of magnetoelectric properties in these materials. Strains associated with the cubicorthorhombic transition due to the combined Jahn-Teller/octahedral tilting transition in the vicinity of 1500 K are 2-3 orders of magnitude greater. It is inevitable that ferroelastic twin walls due to this transition would have significantly different magnetoelectric properties from homogeneous domains due to magnetoelastic coupling with steep strain gradients.

Magnetoelastic properties of multiferroic hexagonal ErMnO3

“…The present study follows from recent work on strain relaxation in multiferroic orthorhombic manganites, where the initial structural transition is ferroelastic [42]. Coupling of strain with the ferroelectric and magnetic order parameters in orthorhombic GdMnO3, TbMnO3 and Sm0.6Y0.4MnO3 is weak but the existence of acoustic loss peaks provides evidence of Debye-like freezing processes involving local, dynamic correlations of electric and magnetic polarisation ahead of the stability fields of the multiferroic structures.…”

“…Instead, the peak is rounded, has its maximum ~5 K below TN and more nearly resembles a typical Debye-like loss peak with Ea ≥ ~0.05 eV. A peak seen in Q -1 data from orthorhombic GdMnO3 at ~80 K yielded a similar activation energy, ≥ ~0.04 eV, and was attributed to freezing of polaron-like strain clouds around local dynamical magnetic/electric dipoles [42]. The most obvious loss mechanism in ErMnO3 involves freezing of moments of Er 3+ at 4b sites, driven by favourable coupling of strain fields arising from these moments with strain fields due to the long range ordering of moments at Mn 3+ sites.…”

Understanding the magnetic dynamics and magnetostructural coupling in the paramagnetic phase ofTbMnO3by muon-spin spectroscopy

“…Furthermore, intriguing abnormal features in the temperature dependence of structural, magnetic, and thermal properties of TbMnO 3 have been reported in the paramagnetic phase, well above T N . These include the deviation from the Curie-Weiss law of the magnetic susceptibility along the b axis below 200 K [7,8], and the anomalous temperature behavior of optical phonons [8], thermal conductivity [9], lattice parameters [10], and elastic moduli [11] in the 90 to 200 K range. Moreover, the E ω a-polarized THz absorption spectra exhibit a broad and weak band at around 20 cm −1 , at room temperature, whose intensity gradually increases on cooling from 150 K, while the oscillator strength of the lowest-lying optical polar phonon concomitantly decreases [12].…”