Magnetic, thermal, and electronic-transport properties of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi>EuMg</mml:mi><mml:mn>2</mml:mn></mml:msub><mml:msub><mml:mi>Bi</mml:mi><mml:mn>2</mml:mn></mml:msub></mml:mrow></mml:math> single crystals
Santanu Pakhira, M. A. Tanatar, D. C. Johnston
Abstract
The trigonal compound ${\mathrm{EuMg}}_{2}{\mathrm{Bi}}_{2}$ has recently been discussed in terms of its topological band properties. These are intertwined with its magnetic properties. Here detailed studies of the magnetic, thermal, and electronic-transport properties of ${\mathrm{EuMg}}_{2}{\mathrm{Bi}}_{2}$ single crystals are presented. The ${\mathrm{Eu}}^{+2}$ spins 7/2 in ${\mathrm{EuMg}}_{2}{\mathrm{Bi}}_{2}$ exhibit an antiferromagnetic (AFM) transition at a temperature ${T}_{\mathrm{N}}=6.7$ K, as previously reported. By analyzing the anisotropic magnetic susceptibility $\ensuremath{\chi}$ data below ${T}_{\mathrm{N}}$ in terms of molecular-field theory (MFT), the AFM structure is inferred to be a $c$-axis helix, where the ordered moments in the hexagonal $ab$-plane layers are aligned ferromagnetically in the $ab$ plane with a turn angle between the moments in adjacent moment planes along the $c$ axis of $\ensuremath{\approx}{120}^{\ensuremath{\circ}}$. An alternate but less likely magnetic structure is a planar structure with nearest-neighbor Eu spins aligned at $\ensuremath{\approx}{120}^{\ensuremath{\circ}}$ with respect to each other, where these ordered-moment layers are stacked along the $c$ axis. The magnetic heat capacity exhibits a $\ensuremath{\lambda}$ anomaly at ${T}_{\mathrm{N}}$ with evidence of dynamic short-range magnetic fluctuations both above and below ${T}_{\mathrm{N}}$. The high-$T$ limit of the magnetic entropy is close to the theoretical value for spins 7/2. The in-plane electrical resistivity $\ensuremath{\rho}(T)$ data indicate metallic character with a mild and disorder-sensitive upturn below ${T}_{\mathrm{min}}=23$ K. An anomalous rapid drop in $\ensuremath{\rho}(T)$ on cooling below ${T}_{\mathrm{N}}$ as found in zero field is replaced by a two-step decrease in magnetic fields. The $\ensuremath{\rho}(T)$ measurements also reveal an additional transition below ${T}_{\mathrm{N}}$ in applied fields of unknown origin that is not observed in the other measurements and may be associated with an incommensurate to commensurate AFM transition. The dependence of ${T}_{\mathrm{N}}$ on the $c$-axis magnetic field ${H}_{\ensuremath{\perp}}$ was derived from the field-dependent $\ensuremath{\chi}(T)$, ${C}_{\mathrm{p}}(T)$, and $\ensuremath{\rho}(T)$ measurements. This ${T}_{\mathrm{N}}({H}_{\ensuremath{\perp}})$ was found to be consistent with the prediction of MFT for a $c$-axis helix with $S=7/2$ and was used to generate a phase diagram in the ${H}_{\ensuremath{\perp}}\text{\ensuremath{-}}T$ plane.