Fig. 1. (Color online) Crystal and electronic structure characterizations. High-resolution TEM images of CsPb_1–xNi_xBr₃ nanocrystals with (a) x = 0 and (b) x = 0.31%. The insets show corresponding low-resolution TEM images. (c) XRD patterns of CsPb_1–xNi_xBr₃ nanocrystals with variant x values. The bottom blue vertical lines index the XRD patterns of CsPbBr₃ with a cubic-phase structure (PDF#54-0752). The enlarged view of the XRD spectra near 2θ = 21.5º is also shown. (d) X-ray photoelectron spectroscopy of the Ni 2p level in CsPb_1–xNi_xBr₃ with x = 0.31%. (e) Photoluminescence spectra of RT-synthesized CsPb_1–xNi_xBr₃ nanocrystals with different x, measured at 300 K. (f) The ESR spectra of CsPb_1–xNi_xBr₃ nanocrystals with x = 0 and 0.31%, measured at 300 K. 1 Oe = 0.1 mT. All samples used here were RT-synthesized.

Fig. 2. (Color online) First-principle calculations. (a) The 3 × 3 × 3 CsPbBr₃ supercells used in DFT calculations. Left: perfect lattice, where the Br atom indicated by the blue arrow will be removed to create a V_Br. Right: relaxed lattice with the presence of a V_Br. (b) DCD of the perfect (left) and defective (right) CsPbBr₃. High (low) charge density corresponds to charge accumulation (depletion) regimes. (c) Spin-resolved total and partial DOSs of the perfect and defective CsPbBr₃ supercells.

Fig. 3. (Color online) Magnetic properties of RT-synthesized LHPs. (a) Magnetization versus H_ext curves of RT-synthesized pure CsPbBr₃ nanocrystals measured at several representative temperatures. The diamagnetic signals have already been subtracted. Inset shows the enlarged view of low field regimes between ±0.5 kOe. (b) Temperature dependence of M_s of pure CsPbBr₃ nanocrystals. Effects of (c) OAmBr passivation, (d) OAmCl passivation, (e) OAmI passivation, and (f) OAmBr passivation on the ferromagnetism of CsPbBr₃, CsPbCl₃, CsPbI₃, and CH₃NH₃PbBr₃, respectively, measured at 300 K.

Fig. 4. (Color online) 3d dopant effects on the magnetic properties of CsPbBr₃. (a) Magnetization versus H_ext curves of RT-synthesized CsPbBr₃ nanocrystals doped with 1.7% Mn, 0.54% Fe, 0.87% Co, 0.31% Ni, 0.35% Cu, and 0.86% Zn, measured at 300 K. (b) M_s of the magnetization curves presented in (a). (c) Magnetization versus H_ext curves of RT-synthesized CsPb_1–xNi_xBr₃ nanocrystals with x = 0, 0.12%, 0.31%, and 0.46%, measured at 300 K. Inset shows the x dependence of M_s. (d) Magnetization versus H_ext curves of RT-synthesized CsPb_1–xNi_xBr₃ nanocrystals with x = 0.31% at measuring temperatures of 4, 100, 200, 300, and 400 K. The diamagnetic signals have already been subtracted.

Fig. 5. (Color online) Schematic of vacancy-induced ferromagnetism. A donor electron associated with a V_Br polarizes the surrounding lattices within its hydrogenic orbital, leading to the formation of a magnetic polaron (gray circles). Due to the shallow nature of the V_Br, the magnetic polarons have extended wave functions. As a result, the overlap of the magnetic polarons aligns the spins of the V_Br (red arrows) via exchange coupling, producing long-range spin ordering (i.e., ferromagnetism). Regarding a Ni²⁺ ion with a 3d⁸ subshell, the only unoccupied 3d orbitals available are in the e_g level. Therefore, the exchange coupling between the Ni²⁺ spins (blue arrows) and the V_Br spins is antiferromagnetic. Nonetheless, the exchange coupling between two V_Br mediated by a same Ni²⁺ ion is ferromagnetic. Note here that the Cs ions are not shown, and that the sizes of the magnetic polarons are not scaled to their calculated values (see the Discussion section of the main text).