Bulk magnetic properties of distorted square lattice compounds M′-LnTaO4 (Ln = Tb, Dy, Ho, Er)

We report bulk magnetic properties of the monoclinic lanthanide tantalates, M′-LnTaO4 (Ln= Tb, Dy, Ho, Er), where the magnetic Ln3+ ions are arranged on a distorted 2D square lattice. The heavier analogue M′-YbTaO4 has been investigated as a spin-orbit-coupled, quasi-two-dimensional frustrated magnet, and the properties of the other M′-LnTaO4 are expected to vary depending on the electronic configuration of the Ln ion, namely Kramers vs non-Kramers behaviour and different crystal electric field parameters. In this work, powder neutron diffraction is used to confirm the crystal structure for Ln = Tb, Ho, Er, and to determine the magnetic structure of M′-TbTaO4, which displays long-range antiferromagnetic (AFM) order below TN = 2.1 K. The Tb3+ moments are aligned primarily along the c-axis with AFM nearest-neighbour interactions. Susceptibility data suggest that M′-DyTaO4 may display short-range ordering around 2.7 K, while M′-HoTaO4 and M′-ErTaO4 show AFM correlations but do not order above 1.8 K. Measurements of the magnetic specific heat provide evidence for a Kramers doublet ground state in M′-ErTaO4, similar to its heavier analogue M′-YbTaO4.