Complex magnetic interactions in geometrically frustrated TbOF

Abstract

We have identified TbOF as a unique frustrated and mixed-anion lattice, hosting unconventional magnetism. By means of magnetization, specific heat and neutron diffraction measurements down to 90 mK, as well as DFT calculations, we present a comprehensive study of the magnetic and structural properties of TbOF. We show that at 9.7 K, TbOF undergoes a structural phase transition accompanied by short-range magnetic correlations, in contrast to previously proposed long-range antiferromagnetic order. At lower temperatures, we observe two magnetic ordering transitions, consisting of incommensurate spin density waves and antiferromagnetic and ferromagnetic correlations. Furthermore, we observe metastable and hysteresis behavior below 2.0 K, highlighting the richness of complex magnetic interactions in TbOF. These results uniquely clarify the magnetic phase diagram of TbOF and highlight the intricate interplay between structure and magnetism in rare-earth oxyfluorides.

Figures (7)

• Figure 1: Crystal structure of TbOF at room temperature, depicted along the a-axis (top) and c-axis (bottom). Relevant bond lenghts are indicated.
• Figure 2: Temperature-dependence of the magnetization ($M/H$), blue circles, left axis) and specific heat (C, red squares, right axis) for TbOF. The plotted magnetization is a compilation of measurements above and below 1.8 K, measured under applied magnetic fields of 0.1 and 0.01 T, respectively. The specific heat was measured in the absence of an applied magnetic field. $T_1$, $T_2$ and $T_{\text{RSO}}$, represent the two magnetic order and short-range-order transitions, respectively. The insert displays the inverse of the magnetization with a Curie-Weiss fit plotted on top.
• Figure 3: Raw powder neutron diffraction data of TbOF ($\lambda = 1.886 \ \text{\AA}$), collected at 300 K, 8.0 K, and 1.6 K. The 300 K pattern (yellow) shows only nuclear reflections consistent with the $\text{R}\bar{3}m$ structure. The 8.0 K dataset (red), recorded below the 9.7 K phase transition shows a reduction in background compared to 300 K, with a concentration of increased scattering in the form of a very broad diffuse maximum centered around 1.0 $\text{Å}^{-1}$, indicative of short-range magnetic correlations. At 1.55 K (purple), below the 3.5 K phase transition, this diffuse feature disappears and magnetic Bragg reflections emerge, consistent with long-range magnetic order.
• Figure 4: Rietveld refinement of the powder neutron diffraction data for TbOF at 2.20 K ($\lambda = 2.449\;\text{Å}$). The nuclear and magnetic Bragg reflections are depicted by the green and blue tickmarks, respectively. The copper sample can gives rise to peaks indicated by the orange tickmarks and included in the refinement. The refinement was performed by fitting this data and that measured with $\lambda = 1.494 \;\text{Å}$ simultaneously using the same structural model (see Figure S6 in SI).
• Figure 5: Visualization of the magnetic structure of TbOF at 2.2 K, presented as a supercell. O and F are omitted for clarity. Top: a single Tb layer viewed perpendicular to the ab-plane. The Tb–Tb distances and Tb–Tb–Tb angles are indicated, showing the isosceles nature of the triangles in the plane. Bottom: view along the a-axis. The dashed lines are included as a guide to the eye, connecting them to the top view.