The magnetic structures of the Ho-based i-MAX phase (Mo2/3Ho1/3)2GaC were studied with neutron powder diffraction at low temperature. (Mo2/3Ho1/3)2GaC crystallizes in the orthorhombic space group Cmcm. The material undergoes two successive antiferromagnetic transitions at TN1 = 10 K and TN2 = 7.2 K. The magnetic structure below TN1 is incommensurate with the propagation vector k1 = (0, ky, 0) with ky = 0.696 (1) at 9 K. For the analysis of the magnetic structure, a group-theoretical approach based on the space group of the nuclear structure and its subgroups was employed. A model in the (3+1)D superspace group Cmcm.1'(0β0)s0ss yielded the most accurate results in neutron powder diffraction refinements. The determined structure was found to be an incommensurate longitudinal amplitude-modulated magnetic structure. Below TN2, additional magnetic satellites develop. They could be indexed by a propagation vector k2 = (τx, 0, 0) with the τx value increasing below TN2 until it stabilizes at approximately 3 K at 0.075. A magnetic structure determination considering two propagation vectors k1 and k2 was carried out using the superspace formalism by building the corresponding (3+2)D model. The determination was based on the observation that the additional magnetic peaks emerge exclusively in the vicinity of the incommensurate magnetic peaks with propagation vector k1, and not in the vicinity of nuclear peaks. This indicates that only mixed-index reflections were observed, and not reflections purely related to k2. The magnetic superspace group (MSSG) that was determined is Amma.1' (0,β,0)00s0 (0,0,γ)ss0s. The structure can be described as a longitudinal amplitude-modulated structure, which itself is amplitude-modulated in a perpendicular direction. This represents a very unusual case of a 2-k magnetic structure with no symmetry relation between the propagation vectors.

Department of Physics Chemistry and Biology Linkopings Universitet Linköping SE 58183 Sweden

Institut Néel Université Grenoble Alpes and CNRS Grenoble 38042 France

Institute of Physics of the Czech Academy of Sciences Na Slovance 1999 2 18200 Praha 8 Czechia

Université Grenoble Alpes Centre national de la recherche scientifique Grenoble 38000 France

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