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Controlling Magnetic Ordering in Ca EuCoAs by Chemical Compression Xiaoyan Tan, Alexander A Yaroslavtsev, Huibo Cao, Andrey Y Geondzhian, Alexey P Menushenkov, Roman V Chernikov, Lucie Nataf, V. Ovidiu Garlea, and Michael Shatruk Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b03184 • Publication Date (Web): 22 Sep 2016 Downloaded from http://pubs.acs.org on September 27, 2016
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Chemistry of Materials
Controlling Magnetic Ordering in Ca1–xEuxCo2As2 by Chemical Compression Xiaoyan Tan,1,# Alexander A. Yaroslavtsev,2,3 Huibo Cao,4 Andrey Y. Geondzhian,2 Alexey P. Menushenkov,2 Roman V. Chernikov,5 Lucie Nataf,6 V. Ovidiu Garlea,4 Michael Shatruk*,1 1
Department of Chemistry and Biochemistry, Florida State University, Tallahassee, Florida 32306, USA National Research Nuclear University “Moscow Engineering Physics Institute”, 115409 Moscow, Russia 3 European XFEL GmbH, 22869 Schenefeld, Germany 4 Quantum Condensed Matter Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA 5 DESY Photon Science, 22603 Hamburg, Germany 6 Synchrotron SOLEIL, L'Orme des Merisiers, 91190 Saint-Aubin, France 2
Corresponding author’s e-mail:
[email protected] Abstract To investigate the interplay between electronic structure and itinerant magnetism, Ca1–xEuxCo2As2 solid solutions (x = 0, 0.1, 0.3, 0.4, 0.5, 0.6, 0.65, 0.7, 0.9, 1.0) were prepared by reactions between constituent elements in molten Bi. All the samples crystalize in the ThCr2Si2 structure type. The crystal structure refinement revealed the formation of Co vacancies, the concentration of which decreases as the Eu content increases. The Eu site exhibits mixed valence in all samples. X-ray absorption near-edge structure (XANES) spectroscopy revealed that the average Eu oxidation state decreases from +2.17 at 0 < x ≤ 0.6 to +2.14 at x ≥ 0.65. The same borderline behavior is observed in magnetic properties. The substitution of Eu for Ca causes the transition from the antiferromagnetic (AFM) ordering of Co moments in CaCo2As2 to ferromagnetic (FM) ordering of Co moments in Ca1–xEuxCo2As2 with 0.1 ≤ x ≤ 0.6. At higher Eu content, AFM ordering of Eu moments is observed, while the Co sublattice exhibits only paramagnetic behavior. Single crystal neutron diffraction studies revealed that both Co and Eu sublattices order FM in Ca0.5Eu0.5Co2As2 with the magnetic moments aligned along the tetragonal c axis. In the AFM phases with x ≥ 0.65, only Eu moments are ordered in a helical spin structure defined by an incommensurate propagation vector k = [00q], with the moment lying in the ab plane. The changes in magnetic behavior are well justified by the analysis of the electronic density of states and crystal orbital Hamilton population.
Introduction ThCr2Si2 structure type accommodates a variety of AT2X2 compositions (A = alkali, alkaline, or rare earth metal, T = transition metal, X = metalloid or nonmetal), as shown by more than 1000 representatives reported for this structural family.1 It was recognized early by Hoffmann and Zheng that the extended [T2X2] slabs afford interesting correlations between the valence electron concentration and structural parameters.2 One of the most fascinating recent advances was the discovery of superconductivity in doped BaFe2As2 and related FeAs-based materials.3-4 The strong electron correlations in the [Fe2As2] layer make the behavior of these materials extremely sensitive to various perturbations, such as chemical doping, physical or chemical pressure, magnetic field, temperature, etc. For example, as relevant to the present work, superconductivity with Tc ~ 30 K was triggered in EuFe2As2 above 2 GPa due to pressureinduced electron doping into Fe 3d states.5-6
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In the same vein, we have recently demonstrated that magnetic properties of EuCo2As2 are very sensitive to physical or chemical pressure. At ambient conditions, this material exhibits antiferromagnetic (AFM) ordering due to localized 4f moments of Eu2+ ions, with the Néel temperature (TN) of 47 K.7-8 Under applied pressure, however, we observed mixed valence and ferromagnetism in the Eu sublattice.9 We suggested that the change in the type of ordering of localized Eu2+ magnetic moments became possible due to the emergence of itinerant (delocalized)
Figure 1. Magnetic structures of CaCo2As2 (left) and
ferromagnetic (FM) ordering of Co 3d moments,
EuCo2As2 (right, only one nuclear unit cell shown).
raising the Curie temperature (TC) to 125 K.9 This assumption was indirectly supported by electronic structure calculations, but we were unable to observe directly the FM signal from the Co moments in the X-ray magnetic circular dichroism (XMCD) spectra. To probe the mutual influence of Eu mixed valence and localized (Eu) and itinerant (Co) magnetism in this type of structures, we turned to the method of chemical compression. CaCo2As2, which is isostructural to EuCo2As2 but has a significantly smaller unit cell volume, exhibits itinerant AFM ordering of Co moments at TN = 74 K (Fig. 1). The substitution of 10% of Eu for Ca induces mixed valence of Eu and FM ordering of both Co and Eu moments in Ca0.9Eu0.1Co2As2. This effect is quite remarkable, considering that both CaCo2As2 and EuCo2As2 behave as antiferromagnets under ambient pressure. We related this change to the modification of electronic band structure in the vicinity of the Fermi level.9 The drastically different magnetic behavior of Ca0.9Eu0.1Co2As2 as compared to the parent ternary compounds incited us to extend this investigation to the entire range of solid solutions, Ca1–xEuxCo2As2. Our goal was to explore the evolution of magnetism from the AFM order of itinerant Co 3d moments in CaCo2As2 to the AFM order of localized Eu 4f moments in EuCo2As2. Reported below is a detailed study of the CaCo2As2–EuCo2As2 phase diagram, which reveals the evolution of the Co magnetic behavior from AFM ordering to FM ordering to paramagnetism. Using a comprehensive set of tools, which include neutron diffraction, X-ray absorption spectroscopy, and magnetic measurements, aided by electronic structure calculations, we conclusively show that the transition from the AFM to FM behavior in the Eu sublattice becomes possible only with the emergence of itinerant FM ordering in the Co sublattice due to electron doping into the 3d states caused by chemical compression. The three strikingly different regimes of magnetic behavior span relatively similar crystal structures, thus proving that chemistry can be used as a powerful “handle” for controlling magnetic properties of itinerant magnets.
Materials and Methods Starting Materials. Calcium dendritic pieces (99.98%), finely dispersed powders of arsenic (99.99%), and bismuth granules (99.997%) were purchased from Alfa Aesar and used as received. Europium metal chunks (>99%) were obtained from the Materials Preparation Center at Ames Laboratory, which is supported by the US DOE Basic Energy Sciences. Chunks of calcium and europium metals were cut into
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Chemistry of Materials
smaller pieces during the sample preparation. Cobalt powder (Alfa Aesar, 99.5%) was additionally purified by heating in a flow of H2 gas for 5 h at 775 K. All manipulations during sample preparation were carried out in an argon-filled dry box (O2 content
