Abstract
Detwinning of magnetic (nematic) domains in Fe-based superconductors has so far only been obtained through mechanical straining, which considerably perturbs the ground state of these materials. The recently discovered nonmechanical detwinning in by ultralow magnetic fields offers an entirely different, nonperturbing way to achieve the same goal. However, this way seemed risky due to the lack of a microscopic understanding of the magnetically driven detwinning. Specifically, the following issues remained unexplained: (i) ultralow value of the first detwinning field of approximately 0.1 T, two orders of magnitude below that of , and (ii) reversal of the preferential domain orientation at approximately 1 T and restoration of the low-field orientation above 10–15 T. In this paper, we present, using published as well as newly measured data, a full theory that quantitatively explains all the observations. The key ingredient of this theory is a biquadratic coupling between Fe and Eu spins, analogous to the Fe-Fe biquadratic coupling that drives the nematic transition in this family of materials.
- Received 10 June 2017
- Revised 8 December 2017
DOI:https://doi.org/10.1103/PhysRevX.8.011011
Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.
Published by the American Physical Society
Physics Subject Headings (PhySH)
Popular Summary
In the last 10 years, iron-based high-temperature superconductors (FeBS) have gained considerable attention. To study the intriguing interplay between magnetism and superconductivity in many FeBS, researchers need to eliminate one of the two kinds of magnetic domains in the crystal, a process called detwinning. This is usually done by applying an external stress, a complicated and poorly controlled process, or by a very high external magnetic field. However, in one compound, (Eu122), the necessary field is 2 orders of magnitude weaker than for similar compounds, and as the magnetic field increases, the domains spontaneously rotate by 90°, not once but twice. We develop a theoretical model that explains all the observations and reveals the underlying physics.
One of the puzzling aspects of the Eu122 paradox is that, while the Eu ion carries a magnetic moment, it does not couple with the crystal lattice, so it should be able to rotate freely in an external magnetic field without triggering any crystallographic change. We resolve this discrepancy by introducing a minuscule biquadratic coupling between the magnetic moments of Eu and Fe, which tries to make the two as parallel as possible. This model fully explains all reorientation transitions, despite the fact that the interaction in question has energy equivalent to 0.1 K, nearly vanishing by all electronic measures.
Our results open new avenues for exploring the physics of not just Eu-based iron superconductors but for a range of similar compounds as well.