Pressure-induced modifications of magnetic and optical properties in Yb3+-doped CrBr3

The optical properties of Yb³⁺-doped CrBr₃ have been measured at various pressures, using Yb³⁺ as a spin-bearing luminescent probe of short-range magnetic order. The trends in Raman and optical absorption with pressure indicate compression of [CrBr₆]^3- octahedra with a uniform Cr³⁺-Br^- bond-length contraction of ∼0.005 Å/GPa, despite the soft van der Waals gap. The f-f photoluminescence of Yb³⁺ redshifts with increasing pressure, attributed to enhanced Yb³⁺ f-orbital covalency leading to an increased sensitivity to Br^- spin-orbit coupling at high pressure. An increase in the energy difference between Yb³⁺ up and down (pseudo)spin ground states is observed with increasing pressure, reflecting enhanced Yb³⁺-Cr³⁺ magnetic exchange coupling because of the shortened metal-halide bond lengths. A decrease in T_C of CrBr₃ with increasing pressure counteracts this effect. Exchange splitting of the Yb³⁺ ground state is still observed even above the magnetic ordering temperature of CrBr₃ due to non-cancelling short-range Yb³⁺-Cr³⁺ exchange interactions. Analysis reveals a discontinuity in the pressure dependence of the Yb³⁺-Cr³⁺ magnetic exchange coupling strength (J_Yb-Cr) at ∼3.5 GPa, attributed to a loss of magnetic correlation and a reduction of T_C under pressure. These results highlight the versatility of Yb³⁺ as an optical probe of bonding and magnetism in CrBr₃, and more generally as a designer defect for spin-photonic functionalization in this class of 2D van der Waals magnets.