Anomalous enhancement of magnetism by nonmagnetic doping in the honeycomb-lattice antiferromagnet <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>ErOCl</mml:mi> </mml:math>

Tuning magnetic anisotropy through chemical doping is a powerful strategy for designing functional ma terials with enhanced magnetic properties. Here, we report an enhanced ${\mathrm{Er}}^{3+}$ magnetic moment resulting from nonmagnetic ${\mathrm{Lu}}^{3+}$ 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 ${\mathrm{Er}}^{3+}$ ion under high magnetic fields, suggesting an unconventional mechanism. Structural analysis reveals that ${\mathrm{Lu}}^{3+}$ 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 $R\overline{3}m$. 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.