Studies of Magnetism and CDW order in HoNiC2

Abstract

We present single crystal studies of HoNiC2 by means of crystal diffraction, heat capacity, as well as orientation dependent thermal expansion, magnetisation, magnetic susceptibility and transport studies. HoNiC2 belongs to the family of non-centrosymmetric orthorhombic rare earth nickel dicarbides [1,2], which attracted attention already for decades, initially by their magnetic properties and more recently by the observation of charge density wave (CDW) formation related to their quasi-one-dimensional electronic features. The magnetic ground state of HoNiC2 was characterized by powder neutron diffraction [3] as a two-component antiferromagnetic (AF) one with an incommensurate, non-collinear component appearing below 4 K and a commensurate component appearing below 3 K. Subsequent heat capacity studies of polycrystalline HoNiC2 revealed a thermodynamic phase transition only near 3 K [4,5], while magnetic diffraction intensities observed above 3 K may relate to short range order effects.

The present single crystal heat capacity study reveals a sharp and much enhanced specific heat jump of the AF phase transition at 2.8 K and an additional first order spin-reorientation transition at 2.2 K. The latter remained undetected in earlier studies of polycrystalline samples [4,5]. We will analyse the spin-reorientation transition at 2.2 K in context with a model of the CEF ground state and our data of single crystal ac susceptibility studies.

Our investigation by means of single crystal x-ray diffraction studies of CDW order in HoNiC2 reveals a pair of second order and first order phase transitions. A second order phase transition from the orthorhombic parent structure to an incommensurately modulated CDW takes place at around 321 K, which is followed by a first order transition from the incommensurate to a commensurate CDW at 298 K. The latter reduces the point group symmetry from orthorhombic to monoclinic. The commensurate CDW state has been revealed by earlier diffraction studies [7], while the intermediate incommensurate CDW state remained unresolved.

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