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(Fe0.957Co0.043)2As2 by resonant ultrasound spectroscopy as a function of temperature and externally applied magnetic field. Non-linear softening and stiffening of C66 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 (TS ≈ 69 K) ~10 K above the temperature at which C66 would extrapolate to zero ( ≈ 59 K). An absence of anomalies associated with antiferromagnetic ordering below TN ≈ 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 C66 by up to ~6% below the normal—superconducting transition at Tc ≈ 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(Fe0.957Co0.043)2As2 is representative of 122 pnictides as forming a class of multiferroic superconductors in which elastic strain relaxations underpin almost all aspects of coupling between the structural, magnetic and superconducting order parameters and of dynamic properties of the transformation microstructures they contain.
Highlights
Materials with multiple instabilities are of topical interest both for the complex physics they display and for opportunities they provide in relation to the tuning of physical properties in potential device applications
The most general conclusion from this comprehensive investigation of elasticity, heat capacity and magnetism is that strain does permeate every aspect of the overall behavior and properties of a Co-doped pnictide, down to the finest details of how elastic and anelastic properties respond to changes in temperature and magnetic field
The experimental data relate only to Ba(Fe1−xCox)2As2, the pnictides define a distinct class of multiferroic superconductors with some diversity
Summary
Materials with multiple instabilities are of topical interest both for the complex physics they display and for opportunities they provide in relation to the tuning of physical properties in potential device applications. The primary focus tends to be on ranges of chemistry, structure and parameter space where two or more phase boundaries converge but, in any system with multiple instabilities, an important mechanism for coupling between the different order parameters is via common strains This has consequences for the elastic properties even though they may not be the main properties of technological interest. It appears that the ferroelastic microstructure might be important in determining the bulk elastic properties in a 10 K interval below the first transition and that an anelestic loss peak is indicative of the contribution of mobile twin walls, which become pinned below ~50–55 K. A more complete description of the strain relaxation behaviour of the superconducting phase, including the contribution of vortices, is presented elsewhere [20]
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