Persistent spectral hole burning has been observed in CaF2:Sm 2+ and originates from photoionization of the Sm2+ ion. This represents a new class of persistent hole-burning materials. We have used hole burning to measure the nonlinear Zeeman effect of the 7FoA1g → Alu transition at 695.8 nm with a resolution of ~100 MHz. We find a nonlinear coefficient of 0.93 Hz/G2, which is close to the value of 1.0 Hz/G2 calculated for the 7F0-7F1 interaction. This shows that the dominant contribution to the nonlinear Zeeman effect comes from the ground state. © 1984 Optical Society of America.