From ordered antiferromagnet to spin glass: A new phase Mn4Ir7−xMnxGe6
Graphical abstract
The ideal cubic network of Mn atoms in the Mn4Ir7Ge6 crystal structure gives rise to geometric frustration of nearest and next nearest neighbor antiferromagnetic interactions.
Introduction
The ternary alloy phase diagrams of the Ir–Mn–Si and Ir–Mn–Ge systems have not been fully established. In recent articles we have reported the existence of the ordered phases Mn3IrSi [1], Mn3IrGe [2] and IrMnSi [3], all with interesting magnetic properties. Mn3IrSi and Mn3IrGe crystallize with the AlAu4-type structure [4] and form similar commensurate non-collinear antiferromagnetic structures. The magnetic moments are found on a network of triangles of near neighbor Mn atoms, and as a result of geometric frustration the projections of the moments on the triangle planes form 120° angles [1]. IrMnSi crystallizes in the TiNiSi-type structure (ordered Co2P) [5] and forms an incommensurate cycloidal spiral magnetic structure below 460 K, with the propagation vector along the c-axis and spins rotating around an axis in the ab-plane [3]. Thus its magnetic structure is different from that of the isostructural and isoelectronic compound CoMnSi, which orders ferromagnetically above 390 K and at lower temperatures forms an incommensurate double screw spiral structure. In CoMnSi the spins rotate around the propagation vector, which is parallel to the c-axis [6], [7].
In the course of studying the interplay between crystal structure and magnetic properties in ternary transition metal manganese silicides and germanides we have attempted to prepare the corresponding IrMnGe phase. However, IrMnGe turned out not no be stable in the equiatomic composition at 800 °C, as annealed samples were found to contain two phases: a majority phase of the U4Re7Si6 type [8], corresponding to Mn4Ir7−xMnxGe6 (), and Mn3IrGe as minority phase. Since the existence of the Mn4Ir7−xMnxGe6 phase has not previously been reported, we have investigated the crystal structure and magnetic properties of Mn4Ir7−xMnxGe6 () using X-ray and neutron powder diffraction and SQUID magnetometry.
Section snippets
Sample preparation and phase analysis
Two master samples were prepared by the drop synthesis method [9]: one of composition Mn4Ir7Ge6 (sample A) and one of composition IrMnGe (sample D). Starting materials were single crystal pieces of germanium (Highways International, purity 6 N), pressed pellets of iridium powder (Alfa Aesar, purity 99.95%) and pieces of manganese metal (Cerac, claimed purity 99.99%, purified from manganese oxides by sublimation). A high-frequency induction furnace, with 300 mbar argon atmosphere and the sample
Phase analysis
After annealing at 800 °C the majority of the lines in the X-ray powder diffraction films could for all samples be indexed by a body centered cubic unit cell with a concentration dependent lattice parameter, see Table 1. The estimated Mn substitution (x) of Mn4Ir7−xMnxGe6 in samples B and C, given in Table 1, was calculated assuming that the lattice parameter follows Vegard's law and that samples A and D correspond to and 1.3, respectively, as obtained from crystal structure refinements
Discussion and conclusions
We have identified a new phase, Mn4Ir7−xMnxGe6, which crystallizes with the U4Re7Si6-type structure [8] and shows a solid solubility range of about 8 at% Mn. An orthorhombic IrMnGe phase, crystallizing in the TiNiSi-type structure [5], is only obtained in samples of the equiatomic composition rapidly cooled from the melt.
Numerous rare earth transition metal germanides and a few silicides with the U4Re7Si6-type structure are reported, see, e.g., [8], [12], [13], [14], [15], [16], [17]. For these,
Acknowledgments
Håkan Rundlöf is acknowledged for skilful assistance in neutron powder diffraction data collection. We are grateful to Juan Rodríguez-Carvajal for helpful comments on the use of the FULLPROF program for magnetic structure refinements. Financial support from the Swedish Research Council (VR) and the Swedish Foundation for Strategic Research (SSF) is acknowledged.
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