Crystal and magnetic structure of CrAs under extreme conditions

• Kristall- und magnetische Struktur von CrAs unter extremen Bedingungen

Eich, Andreas; Friese, Karen (Thesis advisor); Roth, Georg (Thesis advisor)

Jülich : Forschungszentrum Jülich GmbH, Zentralbibliothek, Verlag (2022)
Book, Dissertation / PhD Thesis

In: Schriften des Forschungszentrums Jülich. Reihe Schlüsseltechnologien = Key technologies 260
Page(s)/Article-Nr.: viii, 235 Seiten : Illustrationen, Diagramme

Dissertation, RWTH Aachen University, 2022

Abstract

In the ongoing and wide-ranging efforts to understand superconductivity and its interplay with magnetism, chromium arsenide (CrAs) is considered a model system for the coexistence of superconductivity and helimagnetism. The superconductivity in CrAs is induced by pressure and forms a dome-like phase region with a maximum T_c = 2.2 K at about 1 GPa in the vicinity of the magnetic phase region where the magnetic structure of CrAs is described as a double helix. With most of the research into CrAs so far focused on macroscopic properties, the principal aim of this work was the investigation of the crystal and magnetic structure of CrAs at low temperatures and high pressures on the basis of X-ray and neutron diffraction measurements. The crystal structure of CrAs was investigated at ambient pressure in the temperature range 35...400 K and at room temperature in the pressure range up to 9.46 GPa with synchrotron and laboratory X-ray single-crystal measurements. For the lattice parameters, our results confirm the previously reported abrupt changes that are coupled to the magnetostructural transition from the paramagnetic to the magnetically ordered phase at T_N = 267 K. The associated large change in the unit cell volume and the resulting formation of microcracks is indirectly also seen in supporting measurements of the macroscopic resistance over several cooling/heating cycles. Beyond this previously known effect of the phase transition, our results show a second effect in that the microstructure exhibits severe twinning below the transition temperature, which is attributed to the speciőc behavior of the lattice parameters. In contrast to the low-temperature behavior, no anomalous behavior is observed in the crystal structure as function of pressure up to 9.46 GPa. In the detailed study of the interatomic distances in the crystal structure of CrAs, the Cr--Cr distances are of primary interest, since the Cr atoms carry the magnetic moment in the structure. It reveals that in particular one distance shows anomalous behavior in that within the paramagnetic region of CrAs -- above the transition temperature and at high pressures -- it is remarkably unaffected by either temperature or pressure within the experimental range, i.e. it remains approximately constant.The magnetic structure of CrAs was investigated in this work for the first time by means of neutron single-crystal diffraction. The result clearly shows that the model of the magnetic structure of CrAs that has been used in the literature so far has to be revised. Based on our single-crystal data, the established double-helical model can definitely be discarded. While our data do not allow an unambiguous determination of the correct model, we identify four candidate models based on the agreement with the data, for which the lower-symmetrical magnetic superspace groups and magnetic moment restraints are taken into account: P2_1[010]|eq.+opt., Pa|eq.+opt., P-1|eq.+opt. and P1|eq.+opt.. None of the characteristic features of the reported double-helical model (Cr spins restrained to a crystallographic plane, Cr magnetic moment value, symmetry dependence of Cr sites, and local antiferromagnetic order) is replicated in any of these models. In addition to the single-crystal measurements, supporting neutron powder diffraction measurements provide the temperature-dependence of the propagation vector of the incommensurate magnetic structure of CrAs.Aside from the scientific investigation of the crystal and magnetic structure of CrAs, part of this thesis is devoted to the development of high-pressure devices suitable for neutron diffraction experiments under extreme conditions, i.e. simultaneous high pressure, low temperature and high magnetic őelds. Several such devices, based on the general design of a clamp cell, are specifically designed to fit into the existing experimental setups of selected instruments at the Heinz Maier-Leibnitz Zentrum (MLZ), where they are intended for future use as generally available sample environment. Within this work, various tests on these clamp cells were performed regarding characterization and calibration. This includes pressure/load curves, experimental neutron transmission and measurement of magnetic reŕections using neutron radiation. Independent of neutrons, the thermal response in a cryostat and loading calibration curves based on ruby luminescence are measured. This work thus presents a new experimental option available for instruments and users at the MLZ, openingnew possibilities for science under extreme conditions.

Institutions

• Division of Earth Sciences and Geography [530000]
• Chair and Institute of Crystallography [542110]