Ferroelasticity, anelasticity and magnetoelastic relaxation in Co-doped iron pnictide: Ba(Fe_0.957Co_0.043)₂As₂

Abstract

The hypothesis that strain has a permeating influence on ferroelastic, magnetic and superconducting transitions in 122 iron pnictides has been tested by investigating variations of the elastic and anelastic properties of a single crystal of Ba(Fe 0.957 Co 0.043 ) 2 As 2 by resonant ultrasound spectroscopy as a function of temperature and externally applied magnetic field. Non-linear softening and stiffening of C 66 in the stability fields of both the tetragonal and orthorhombic structures has been found to conform quantitatively to the Landau expansion for a pseudoproper ferroelastic transition which is second order in character. The only exception is that the transition occurs at a temperature (T S ≈ 69 K) ~10 K above the temperature at which C 66 would extrapolate to zero (T * cE ≈ 59 K). An absence of anomalies associated with antiferromagnetic ordering below T N ≈ 60 K implies that coupling of the magnetic order parameter with shear strain is weak. It is concluded that linear-quadratic coupling between the structural/electronic and antiferromagnetic order parameters is suppressed due to the effects of local heterogeneous strain fields arising from the substitution of Fe by Co. An acoustic loss peak at ~50-55 K is attributed to the influence of mobile ferroelastic twin walls that become pinned by a thermally activated process involving polaronic defects. Softening of C 66 by up to ~6% below the normal-superconducting transition at T c ≈ 13 K demonstrates an effective coupling of the shear strain with the order parameter for the superconducting transition which arises indirectly as a consequence of unfavourable coupling of the superconducting order parameter with the ferroelastic order parameter. Ba(Fe 0.957 Co 0.043 ) 2 As 2 is representative of 122 pnictides as forming a class of multiferroic superconductors in which elastic strain relaxations Original content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence.

Signatures of Anelastic Domain Relaxation in Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$ Investigated by Mechanical Modulation of Resistivity

“…Measurement for an x = 0.07 sample, which remains tetragonal at all temperatures, shows no such peak, which excludes the possibility that the measurements in x = 0.025 and x = 0.052 are merely artifacts from sample preparation. Instead, we proceed to demonstrate that the temperature, amplitude, and frequency dependence of tan(φ) is qualitatively and quantitatively consistent with anelastic relaxation of domain walls with an activation energy of approximately E a = 9.09±0.74× 10 −3 eV in the zero strain limit, which is on a similar energy scale to previous measurements attributed to polaronic defects 14 . As the amplitude of mechanical perturbation is increased, we find which is shown in panel (c) of fig.…”

“…At present, the energies extracted for the activation energy of the domain wall motion at vanishing strain from these measurements, E a (ε → 0) = 9.09 ± 0.74 × 10 −3 eV, are lower than what is reported from resonant ultrasound (E a ∈ (0.03eV, 0.06eV) ) by Carpenter, et al 14 However, there are a number of experimental factors that could account for this discrepancy, which is about 4 times larger than what is found here from measurements of the resistivity response to mechanical perturbation.…”

Strain relaxation behaviour of vortices in a multiferroic superconductor

“…The two single crystals of Ba(Fe 0.957 Co 0.043 ) 2 As 2 investigated in the present study were the same as used by Carpenter et al [20] for determinations of magnetic and elastic properties through the ferroelastic and magnetic phase transitions. They were chosen from a batch of self-flux grown crystals (TWOX1128) listed in Böhmer [21].…”

High-pressure phase transition and equation of state of hydrous Al-bearing silica

“…Despite never being observed in stishovite samples before, a plateau of the vibrational frequencies in ferroic materials is in fact not unusual. For instance, recent resonant ultrasound spectroscopy (RUS) measurements of Co-doped Fe-pnictide at low temperature highlighted similar features, which were explained as the effect of interacting parameters driving different structural transition in a narrow temperature interval (Carpenter et al 2019). In the light of all the above-mentioned considerations, the post-stishovite transition in H-rich Al-bearing silica cannot be regarded as a pure second-order ferroelastic transformation and further measurements of its elasticity and Hbond evolution at high pressure are required to quantitatively constrain the extent of its elastic shear softening and the involved transformation mechanism.…”