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

Abstract

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 T_N = 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.

Figures (21)

• Figure 1: Crystal structure of $M'$-LnTaO4, space group $P2/c$ (No. 13), viewed along different crystallographic axes ($b$ and $a$ respectively). Blue atoms are $Ln^{3+}$, purple are Ta^5+ and red are O^2-. In (b), dashed and dotted lines highlight the distorted square lattice of $Ln^{3+}$ ions.
• Figure 2: Rietveld refinement against room-temperature TOF PND data for $M'$-TbTaO4.
• Figure 3: Unit cell volume of $M'$-LnTaO4 as a function of 8-coordinate $Ln^{3+}$ ionic radius Shannon1976 from Rietveld refinements against both high-resolution X-ray and neutron diffraction data. Error bars are smaller than the datapoints. Literature data taken from references Brixner1983Mullens2023Wang2017Markiv2002Kumar2024.
• Figure 4: Magnetic susceptibility $\chi$ and inverse susceptibility $(\chi-\chi_0)^{-1}$, fitted to the Curie-Weiss law in the range 50--300 K (black lines), for the four $M'$-LnTaO4 compounds in this study. The black dashed lines show the extrapolation of the fit below 50 K. Insets show the low-temperature region.
• Figure 5: Magnetic isotherms for $M'$-LnTaO4. The dashed lines show the linear fits in the region 5-7 T, extrapolated back to 0 T.