Magnetic ordering in Eu₂In and Eu₂Sn

Eu2In and Eu2Sn crystallize in the orthorhombic Co2Si-type structure (oP12, Pnma, No. 62) with In and Sn atoms occupying one 4c site and the Eu atoms filling two other 4c sites. Eu2In has a nearly ideal first-order magnetostructural transition (FOMT) at 55 K with a hysteresis of less than 0.1 K, a large entropy change and an adiabatic temperature change of 5.0 K in a field of 2 T. The anhysteretic nature of the FOMT is likely due to there being no change in cell symmetry and relatively small changes in the lattice parameters. There is no magnetostructural transition in Eu2Sn. Here we present the results of powder neutron diffraction, magnetization, and Eu M & ouml;ssbauer spectroscopy aimed to investigate the nature of magnetic order for both Eu2In and Eu2Sn. The Eu M & ouml;ssbauer spectrum of Eu2In at 5 K shows two equal area components, consistent with Eu occupying two equal multiplicity crystallographic sites. However, the different hyperfine fields (B) of 27 T and 17 T suggest that the magnetic environments of the Eu moments on the two 4c sites are different. Neutron diffraction data at 2.5 K show that in Eu2In the order is ferromagnetic, with Eu moments on both Eu sites oriented parallel to the a-axis; moment values of 6.8 mu(B) and 6.5 mu(B) were found. For Eu2Sn measurements find two antiferromagnetic transitions, which are corroborated by neutron diffraction. Analysis of density-functional theory calculations shows negligible energy difference between differing magnetic configurations, indirectly supporting stability of multiple magnetic structures observed experimentally. While the transition at T-N1 = 30 K corresponds to the formation of a simple k(1) = 0 antiferromagnetic structure with Eu-moments pointing along the b-axis, at T-N2 = 13 K a coexisting second magnetic order with k(2) = [0, 1/2, 1/2] appears.