Magnetocaloric behavior and magnetic ordering in MnPdGa

MnPdGa, a compound crystallizing in the ${\mathrm{Ni}}_2\mathrm{In}$ structure, is a material displaying magnetic ordering near room temperature and is a potential ambient-temperature magnetocaloric. Screening based on electronic structure calculations suggest that MnPdGa may exhibit a high magnetocaloric figure of merit due to its strong magnetostructural coupling. Here we report the preparation of MnPdGa and employ high–resolution synchrotron x-ray diffraction to confirm its hexagonal ${\mathrm{Ni}}_2\mathrm{In}$-type structure. The zero-field ground state is shown to be a conical spin-wave state, defined by a long-range modulation of the conventional conical antiferromagnet structure. Near the Curie temperature, the measurements carried out here coupled with electronic structure calculations suggest that a fully ferromagnetic state can form at elevated temperatures under an applied field. A peak magnetocaloric entropy change $\mathrm{\Delta }{S}_M=-3.54\mathrm{J}{\mathrm{kg}}^{-1}{\mathrm{K}}^{-1}$ ($30.1\mathrm{mJ}{\mathrm{cm}}^{-3}{\mathrm{K}}^{-1}$) is measured at $T_C=315\mathrm{K}$ at an applied field $H=5\mathrm{T}$. The exchange-driven, nontrivial magnetic structure found in MnPdGa is compared with the somewhat better-studied MnPtGa on the basis of electronic structure calculations.

Figure 1

MnPdGa crystallizes in the hexagonal $P{6}_3/mmc$ (no. 194) space group. Looking down the $c$ axis, the Pd and Ga make honeycomb lattice layers in the $ab$ plane, with Mn atoms centered between the honeycomb layers. As seen looking down the $a$ axis, the honeycomb layers alternate with layers of Mn on a triangular lattice. The atoms in this view are represented with displacement ellipsoids refined from the synchrotron diffraction data shown at 95% probability.

Figure 2

High-resolution synchrotron x-ray diffraction data acquired at $T=295\mathrm{K}$ fit using Rietveld refinement between $Q=1.25{\AA }^{-1}$ to $11.4{\AA }^{-1}$. MnPdGa crystallizes in the hexagonal $P{6}_3/mmc$ (no. 194) space group with cell parameters $a=b=4.34263\left(3\right)\AA$ and $c=5.51951\left(5\right)\AA$, and a volume of 90.147(1) ${\AA }^3$. The inset shows $hkl$ positions calculated from $P{6}_3/mmc$, which line up with all observed peaks.

Figure 3

(a) ZFC and FC temperature-dependent magnetization of MnPdGa taken under a constant field of $H=0.1\mathrm{T}$. The sample ferromagnetically orders at $T_C=315\mathrm{K}$ and there is an AFM component at around $T_N=160\mathrm{K}$. (b) Field-dependent magnetization taken at $T=10\mathrm{K}$ between $H=-5\mathrm{T}$ and 5 T. (c) Temperature-dependent magnetization around 150 K to 300 K is shown to highlight the AFM component.

Figure 4

(a) Temperature-dependent magnetization data were taken at various fields. (b) Derivatives of each curve were taken and integrated to yield $\mathrm{\Delta }{S}_M$. The maximum $-\mathrm{\Delta }{S}_M$ is 3.54 J ${\mathrm{kg}}^{-1}{\mathrm{K}}^{-1}$.

Figure 5

Computational comparison of the short- and long-range magnetic structures in MnPdGa and MnPtGa. (a, b) Energy of the canted AFM phase as a function of canting angle $\theta$, where the spins alternate canting directions along (a) the $c$-direction or (b) the $a$-direction. (c) Energy of conical spin waves defined by a rotation of the locally optimal (a) canted AFM phase around the $c$-axis, with propagation wave vector $q$.

Figure 6

Electronic density of states (DOS) for MnPdGa with different magnetic structures. (a) The DOS with no spin-polarization. (b) Collinear FM state. States are split into majority spin states (shown as positive DOS) and minority spin states (shown as negative DOS). (c) The lowest energy calculated canted magnetic state (see Fig. 5 for a depiction of the structure). All states are shown in the same spin channel.