Magnetic Amplification at Yb 3+ "Designer Defects" in the van der Waals Ferromagnet CrI 3 .

The two-dimensional (2D) van der Waals ferromagnet CrI 3 has been doped with the magnetic optical impurity Yb 3+ to yield materials that display sharp multiline Yb 3+ photoluminescence (PL) controlled by the magnetism of CrI 3 . Magneto-PL shows that Yb 3+ magnetization is pinned to the magnetization of CrI 3 . An effective internal field of ∼10 T at Yb 3+ is estimated, attributed to strong in-plane Yb 3+ -Cr 3+ superexchange coupling. The anomalously low energy of Yb 3+ PL in CrI 3 reflects relatively high Yb 3+ -I - covalency, contributing to Yb 3+ -Cr 3+ superexchange coupling. The Yb 3+ PL energy and line width both reveal the effects of spontaneous zero-field CrI 3 magnetic ordering within 2D layers below T C , despite the absence of net magnetization in multilayer samples. These results illustrate the use of optical impurities as "designer defects" to introduce unique functionality to 2D magnets.