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

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