Magnetic Amplification at Yb3+ “Designer Defects” in the van der Waals Ferromagnet CrI3

The two-dimensional (2D) van der Waals ferromagnet CrI3 has been doped with the magnetic optical impurity Yb3+ to yield materials that display sharp multiline Yb3+ photoluminescence (PL) controlled by the magnetism of CrI3. Magneto-PL shows that Yb3+ magnetization is pinned to the magnetization of CrI3. An effective internal field of ∼10 T at Yb3+ is estimated, attributed to strong in-plane Yb3+–Cr3+ superexchange coupling. The anomalously low energy of Yb3+ PL in CrI3 reflects relatively high Yb3+–I– covalency, contributing to Yb3+–Cr3+ superexchange coupling. The Yb3+ PL energy and line width both reveal the effects of spontaneous zero-field CrI3 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.