Anomalous enhancement of magnetism by nonmagnetic doping in the honeycomb-lattice antiferromagnet ErOCl

TL;DR

This study demonstrates that nonmagnetic Lu3+ doping in the honeycomb-lattice antiferromagnet ErOCl enhances magnetization per Er3+ at low temperature under high magnetic fields, contrary to conventional dilution effects. The enhancement arises from chemical pressure: Lu3+ substitution contracts the c-axis, increasing the axial CEF parameter $B_{2}^{0}$ and strengthening magnetic anisotropy, as evidenced by XRD, Raman shifts of CEF levels, and magnetization measurements. The authors combine CEF modeling with a mean-field XXZ framework to connect structural distortions, CEF excitations, and magnetic responses across Lu doping levels in Lu$_x$Er$_{1-x}$OCl. The work provides a general pathway to tailor magnetic anisotropy in layered rare-earth materials via targeted lattice distortions engineered through chemical substitution, with potential implications for designing anisotropic quantum magnets.

Abstract

Tuning magnetic anisotropy through chemical doping is a powerful strategy for designing functional materials with enhanced magnetic properties. Here, we report an enhanced Er^3+ magnetic moment resulting from nonmagnetic Lu^3+ substitution in the honeycomb-lattice antiferromagnet ErOCl. Unlike the Curie-Weiss type divergence typically observed in diluted magnetic systems, our findings reveal a distinct enhancement of magnetization per Er^3+ ion under high magnetic fields, suggesting an unconventional mechanism. Structural analysis reveals that Lu^3+ doping leads to a pronounced contraction of the c axis, which is attributed to chemical pressure effects, while preserving the layered SmSI-type crystal structure with space group R-3m. High-resolution Raman spectroscopy reveals a systematic blueshift of the first and seventh crystalline electric field (CEF) excitations, indicating an increase in the axial CEF parameter B_2^0. This modification enhances the magnetic anisotropy along the c axis, leading to a significant increase in magnetization at low temperatures and under high magnetic fields, contrary to conventional expectations for magnetic dilution. Our work not only clarifies the intimate connection between magnetism and CEF in rare-earth compounds, but more importantly, it reveals a physical pathway to effectively tune magnetic anisotropy via anisotropic lattice distortion induced by chemical pressure.

Figures (4)

• Figure 1: Crystal structure of ErOCl, polycrystalline XRD pattern, and schematic diagram of CEF energy levels. (a) Crystal structure of ErOCl (space group $R\overline{3}m$), showing the layered structure formed by ErO$_4$Cl$_3$ polyhedra. (b) The sevenfold coordination of Er$^3+$ by four O$^2-$ and three Cl$^-$. (c) The bond lengths and bond angles of both NN and NNN magnetic ions are labeled. (d) Powder XRD pattern at 300 K showing observed data (blue open circles), Rietveld refinement result (red solid line), and difference (green line). Light red tick marks indicate expected Bragg peak position. The quality of the Rietveld refinement is indicated by $R_{wp} = 5.01\%$. (e) Free Er$^{3+}$ ions first experience splitting of the spectral terms due to SOC, followed by further splitting of the CEF levels in the CEF environment. For ErOCl, the energy span between the CEF ground state and the highest excited state of the $^{4}I_{15/2}$ term is approximately 50 meV.
• Figure 2: Raman scattering spectra and CEF excitations in ErOCl. (a) The Raman scattering spectra of ErOCl at different temperatures. Six Raman-active phonon modes are clearly observed and are labeled in black near the corresponding peaks. Seven CEF excitation peaks from the ground state to various excited states are marked with red dashed lines. Three excitation peaks between CEF excited states are observed and marked with blue dashed lines. (b) Temperature dependence of CEF excitations from the ground state to various excited states. For the Stokes process, Raman intensity increases as the ground state occupation number rises with decreasing temperature, consistent with the enhancement seen in (a) 52. (c) Temperature dependence of excitations between CEF excited states. The blue open circles are experimental data. The blue solid lines represent the calculated results based on the CEF eigen excitations in (b). The disappearance of those peaks at low temperatures in (a) corresponds to a decrease in the occupation number of excited states. (d)-(e) The Raman scattering spectra of ErOCl at 1.8 K. RL and RR represent cross-circular and parallel-circular polarization configurations, respectively.
• Figure 3: Magnetization of ErOCl and Lu$_{0.78}$Er$_{0.22}$OCl. (a)-(f) Temperature dependent magnetization ($M/H$-$T$) measured under different magnetic fields along the $a$-axis and $c$-axis for ErOCl (blue open squares) and Lu$_{0.78}$Er$_{0.22}$OCl (red open circles). (g) and (h) Magnetic field dependent magnetization ($M$-$H$) at $T$ = 1.8 K up to 14 T along the $a$-axis and $c$-axis for ErOCl (blue open squares) and Lu$_{0.78}$Er$_{0.22}$OCl (red open circles). Blue solid lines represent the MF calculation results, while the light blue solid lines represent the CEF calculation results.
• Figure 4: Effect of Lu$^{3+}$ doping on the structural, vibrational, and magnetic properties of Lu$_x$Er$_{1-x}$OCl single crystals. (a)–(e) XRD $\left(00l\right)$ Bragg peaks, open squares represent experimental data, and solid lines are fitting results. Insets: optical images of samples with different Lu$^{3+}$ concentrations. (f) Evolution of the (003) Bragg peak with increasing Lu$^{3+}$ content, showing a shift toward higher $2\theta$. (g) Lattice volume as a function of Lu$^{3+}$ content; purple squares denote values from four-circle diffraction, and the solid line corresponds to Vegard’s law. (h) Relative change in lattice parameters with Lu$^{3+}$ doping. (i), (j) Raman spectra of the first and seventh CEF excitations at 1.8 K. (k) $M/H$–$T$ and (l) $M$–$H$ curves along the $c$-axis for different Lu$^{3+}$ concentrations. Open symbols: experimental data; solid lines: calculations based on CEF theory.