Article Cite This: Chem. Mater. 2018, 30, 1146−1154
pubs.acs.org/cm
Chemical Pressure Stabilization of the Cubic B20 Structure in Skyrmion Hosting Fe1−xCoxGe Alloys Matthew J. Stolt, Xavier Sigelko, Nitish Mathur, and Song Jin* Department of Chemistry, University of Wisconsin-Madison, 1101 University Avenue, Madison, Wisconsin 53706, United States S Supporting Information *
ABSTRACT: Iron monogermanide (FeGe) with the noncentrosymmetric cubic B20 structure is a well-known helimagnet and a magnetic skyrmion host with a relatively high ordering temperature (∼280 K). FeGe and related metal monogermanide compounds, such as CoGe and MnGe, have several structural polymorphs and typically require high pressure (∼4 GPa) and high temperature (∼1000 °C) to synthesize in the cubic B20 structure. Here, we report that the cubic B20 phase of both FeGe and alloys of Fe1−xCoxGe could in fact be formed without the application of high pressure by simply reacting elemental powders at modest temperatures (550 °C). Furthermore, the incorporation of Co into Fe1−xCoxGe (0.05 ≤ x ≤ 0.1) stabilizes the cubic B20 structure up to 650 °C, which we propose is caused by chemical pressure induced by the incorporation of Co into the lattice. Interestingly, chemical vapor transport reactions using the Fe1−xCoxGe alloys as precursors yield plentiful growth of large (0.1 to 1 mm) single crystals of pure FeGe. Magnetic susceptibility measurements of the Fe0.95Co0.05Ge alloy show evidence of a skyrmion phase not previously reported in the Fe1−xCoxGe system.
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INTRODUCTION Since their discovery in 2009,1 magnetic skyrmions and the materials that can host them have received significant attention from the physics and materials research community. Magnetic skyrmions are exotic magnetic domains that have a vortex-like spin structure and have potential as carriers in next generation information storage and processing.2,3 Noncollinear spin structures like skyrmions and helimagnetic phases are most commonly formed from a competition between the ferromagnetic exchange and the Dzyaloshinskii−Moriya (DM) exchange. The DM exchange interaction exists only in material systems that lack inversion symmetry, including noncentrosymmetric crystal systems, such as the cubic B20 structure, or asymmetrically layered thin films.3 Many of the most well studied skyrmion host materials (i.e., MnSi,1,4−6 Fe1−xCoxSi,7,8 and FeGe9−11) have this noncentrosymmetric cubic B20 crystal structure. Recently, there have been increasing efforts in exploring new skyrmion hosting materials beyond the B20 structure resulting in the discovery of several new skyrmion hosting materials, such as Fe3Sn2,12 GaV4S8,13 Co8Zn8Mn4,14 and Mn1.4Pt0.9Pd0.1.15 Some of these materials, such as Co8Zn8Mn4, can host skyrmions in a wide range of temperatures including above room temperature. However, the solid state materials chemistry of these new skyrmion hosting materials, as well as the more classical skyrmion materials, such as cubic B20 FeGe, needs to be further developed. Fully understanding the solid state materials chemistry of both the classical and the new skyrmion hosting materials may lead to © 2018 American Chemical Society
the discovery of more skyrmion materials as well as improvement of the skyrmionic properties of the existent skyrmion hosting materials. Cubic B20 FeGe has been known for decades as a helimagnet16,17 and currently is one of the most well studied skyrmion hosting materials2,9,10,18−24 due to its relatively high helical transition temperature (Tc) of ∼280 K. However, FeGe has three structural polymorphs with varying stability: the cubic B20 (space group P213, Figure 1a), the hexagonal (space group P6/mmm, Figure 1b), and the monoclinic (space group C2/m, Figure 1c) structures. Currently, to produce bulk powder samples of cubic B20 FeGe in large quantities, simultaneous high pressure (∼4 GPa) and high temperature (∼1000 °C) reaction conditions are required.9 In fact, most transition metal
Figure 1. Unit cell representations of the three structural polymorphs of FeGe: (a) the noncentrosymmetric cubic B20 phase, (b) the hexagonal structure unique to FeGe as a monogermanide, and (c) the monoclinic structure shared by both FeGe and CoGe. Received: December 19, 2017 Revised: January 18, 2018 Published: January 22, 2018 1146
DOI: 10.1021/acs.chemmater.7b05261 Chem. Mater. 2018, 30, 1146−1154
Article
Chemistry of Materials
Figure 2. X-ray diffraction patterns for the as-synthesized Fe1−xCoxGe samples. (a) Samples reacted at 550 °C in comparison with standard patterns for both the cubic B20 FeGe and monoclinic CoGe structures simulated from ICSD CIF nos. 43054 and 43677, respectively. (b) Excerpts from the XRD patterns in (a) highlighting the peak shifting of the most intense peak for the cubic B20 phase as more Co is incorporated into the structure. (c) Samples reacted at 650 °C in comparison with both previously mentioned standard patterns as well as the standard pattern for the hexagonal FeGe structure simulated from ICSD CIF no. 53459. (d) Excerpts from the XRD patterns in (c) highlighting the peak shifting of the most intense peak for the cubic B20 phase as more Co is incorporated into the structure.
temperatures (up to 650 °C) that we propose is caused by a positive chemical pressure due to the mixing of Co atoms into the lattice. Finally, we provide AC magnetometry evidence for the existence of the skyrmion phase in pure FeGe and a skyrmion phase in the Fe0.95Co0.05Ge alloy that has not been previously reported.
monogermanides with the cubic B20 structure and their alloys including CoGe,25 MnGe,25 Fe1−xCoxGe,26 and Fe1−xMnxGe27 are also produced under high pressure. The high pressure reactions favor the cubic B20 structure because the cubic B20 structure unit cell has the highest density of any of the polymorphs in all the above cases.25,28 These high pressure reactions were developed in the late 1980s25 and have been the dominant synthetic technique utilized to synthesize B20 structured compounds for many magnetic studies to this date.21,29−34 Another more recent route to powders of cubic B20 FeGe is through mechanical alloying where it has been shown that ball milling a mixture of cubic and hexagonal FeGe resulted in the formation of a pure cubic B20 FeGe phase,35 likely due to the intense local pressure created in the ball mill apparatus. The solid state chemistry of both the FeGe phase stability and the selective bulk synthesis of the cubic B20 FeGe has not been carefully analyzed otherwise. Single crystals of cubic B20 FeGe are traditionally grown using a chemical vapor transport (CVT) method developed by M. Richardson28 in the late 1960s, which is still used to this day.22,36,37 Nanowires of the cubic B20 FeGe phase have also been synthesized recently via a vapor phase reaction, but the reaction depended on a seeding of the cubic B20 FeGe phase using small particles of cubic FeGe dispersed on a Ge(100) substrate as seeds.23 However, for the bulk synthesis of the cubic B20 FeGe powder, it has been practically assumed that it is necessary to use high pressure, which is inconvenient and not accessible to every lab. In this paper, we report the low pressure synthesis of both pure FeGe and Fe1−xCoxGe alloys with the cubic B20 structure via traditional solid state chemistry methods at moderate temperature in sealed silica ampules. We also observed a cubic B20 phase stable in the Fe1−xCoxGe samples at higher reaction
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EXPERIMENTAL SECTION
Materials. All chemicals used in the synthetic experiments are of reagent-grade purity. Elemental powders of iron (99.9%,
