Enhanced Neel temperature and unusual thermal expansion in flux-grown FeCrAs crystals

TL;DR

FeCrAs, a ZrNiAl-type distorted kagome intermetallic with itinerant magnetism, is studied via Sn-flux crystal growth to probe how stoichiometry and lattice coupling influence its properties. The authors combine single-crystal XRD, magnetization, resistivity, and heat capacity measurements to reveal a Neel temperature of $T_N = 150$ K and a Sommerfeld coefficient of $\gamma \approx 18$ mJ/K^2/mol, with values lower than some previous reports. They also observe an anomalous thermal expansion where the c-axis remains nearly constant above $T_N$, indicating strong spin-lattice coupling that may connect to the material’s unusual transport behavior. The findings underscore the sensitivity of FeCrAs to Cr/Fe stoichiometry (Cr-rich Fe0.9Cr1.1As) and demonstrate flux growth as an effective route to tune magnetic and lattice properties in this system, with implications for understanding altermagnetism and related phenomena.

Abstract

We report results from our experimental investigation of the distorted-kagome compound FeCrAs. For this work, we developed a procedure using tin metal as a flux to produce needlelike crystals. The crystals were characterized by single crystal x-ray diffraction as well as measurements of magnetization, electrical transport, and heat capacity. The physical behaviors are generally similar to published results on crystals grown from a stoichiometric melt with two notable exceptions. The Sommerfeld coefficient is found to be 18 mJ/K2/mol, a little more than half of the previously reported value, and the Neel temperature is found to be 150 K, about 25K higher than in previous reports. The reason for these discrepancies are uncertain, but they may be related to differences in stoichiometry or disorder; it is expected that the Cr/Fe ratio has some variability in this compound. In addition, we find unusual thermal expansion behavior, with an anomaly at the Neel temperature and nearly temperature independent thermal expansion along the hexagonal c-axis above this transition. This suggests significant spin-lattice coupling, which may provide insight into non-metallic transport properties that have been associated with anomalous charge carrier scattering.

Figures (5)

• Figure 1: The crystal structure of FeCrAs. (a) The full unit cell. (b) The triangular arrangement of Fe atoms in the z = 1/2 plane. (c) The distorted kagome net of Cr atoms in the z = 0 plane. (d) A selection of FeCrAs crystals grown from a Sn flux. (e-g) Lattice parameters and unit cell volume determined from single crystal x-ray diffraction on three FeCrAs crystals. Error bars on data in panels (e) and (g) are smaller than the data markers. Dashed vertical lines mark the antiferromagnetic ordering temperature for the crystals.
• Figure 2: Three different resistivity behaviors observed in different crystals. The low temperature behaviors are shown in the upper inset. Metallic temperature dependence is correlated with a stronger superconducting character below about 4 K (the superconducting critical temperature of Sn). The lower inset shows a scanning electron microscope image of a cross section of a broken crystal with metallic conductivity revealing a core of Sn that appears brighter in this backscatter image. In the remainder of this paper, data are only shown for crystals with resistivities that rise upon cooling and that have no anomaly associated with Sn superconductivity, like the dark gray diamonds shown here (crystal C1).
• Figure 3: Results of electrical transport measurements. (a) The temperature dependence of the electrical resistance measured on ten samples with the current flowing along the c-axis and normalized to their values at 300 K. These include crystals C1, R28, R29, and R30 that were used to collect data shown in subsequent figures. (b) Data near the phase transition measured on heating and cooling at a rate of 2 K per minute (crystal R28). (c) Longitudinal (crystal R29) and transverse (crystal C1) magnetoresistance measured at 2 K, with a second order polynomial fit (black line) shown for the longitudinal data.
• Figure 4: Results of heat capacity measurements. (a) The temperature dependence of the heat capacity measured in zero magnetic field (crystals R28, R29, R30). The inset shows low temperature data (1.9$-$9 K) plotted as c$_P$/T vs T$^2$ along with a linear fit. (b) Data from panel (a) near the magnetic phase transition. (c) Data from another crystal (C1) near the phase transition measured in zero field and 50 kOe applied perpendicular to the c-axis.
• Figure 5: Results of magnetization measurements for two different samples (crystal C1 in panels a, b, and c, and crystals R28, R29, and R30 measured together in panels d, e, and f). Isothermal magnetization curves measured at 2 K are shown in panels (a) and (d). Temperature dependent data measured at 10 kOe are shown in panels (b) and (e). Temperature dependent data measured at 1 kOe are shown in panels (c) and (f), with both field-cooled-cooling and zero-field-cooled-warming (ZFC) data shown for $H \perp c$.