Lead-free halide double perovskites (A₂BB’X₆) are promising non-toxic alternatives to lead-based perovskites (ABX₃), widely studied for optoelectronic, light-emitting, and photocatalytic applications. While their optical properties have been extensively investigated, their magnetic behavior remains unexplored. Doping of double perovskites with transition metal ions, such as Fe³⁺, may introduce magnetic functionalities and expand their potential range of applications.

In this study, Cs₂AgBiBr₆ and Cs₂AgBiCl₆ single crystals doped with varying initial Fe³⁺ concentrations (5%, 10%, 15%, with respect to Bi) were grown using controlled cooling crystallization technique. Powder X-ray diffraction showed no significant structural transitions or secondary phases. This is understandable since inductively coupled plasma optical emission spectroscopy showed actual Fe content below 0.1%, confirming the low doping tolerance of Cs₂AgBiBr₆ lattice for iron.

UV-Vis absorption measurements revealed a pronounced smearing of the initially sharp absorption edge, appearing as if redshifted, upon increasing the doping levels. Accordingly, photoluminescence (PL) spectra showed both a redshift of emission peaks and reduction of PL intensity upon doping due to photon reabsorption. Thermal annealing of Fe-doped Cs₂AgBiBr₆ at 300 °C partially restored optical characteristics, suggesting that the observed changes of optical parameters are defect-related and can be reversed by thermal treatment.

Electron paramagnetic resonance (EPR) spectroscopy of Fe-doped Cs₂AgBiBr₆ crystals exhibited a distinct paramagnetic behavior with complex spin interactions depending on temperature and crystal orientation. The absence of the EPR signal in undoped Cs₂AgBiBr₆ crystals suggests that the observed spectra originate from iron-related paramagnetic species with an effective spin state of S = 5/2. Following thermal treatment of the crystal, however, the spectrum vanishes, indicating that the initial EPR signal arose from Fe-related complexes, possibly involving vacancies. Cs₂AgBiCl₆ crystals, in contrast, exhibited intrinsic paramagnetic species even in undoped state, while doping led to the emergence of additional paramagnetic centers, likely associated with structural defects induced by Fe³⁺.

Superconducting quantum interference device (SQUID) magnetometry of Fe-doped Cs₂AgBiBr₆ crystals revealed clear ferromagnetic-type hysteresis loops at room temperature, compared to negligible response of undoped samples. Temperature-dependent measurements showed rising spontaneous magnetization below 20-30K. Measurements under zero-field cooling and field-cooling regimes exhibited magnetization irreversibility, indicating inhomogeneous ferromagnetism, likely caused by Fe-related defect clusters or nanoscale ferromagnetic clusters.   

Our findings demonstrate that even small amounts of Fe³⁺ can significantly alter the optical and magnetic properties of lead-free double perovskites, highlighting the general importance of carefully controlled doping and defect engineering in crystalline semiconducting perovskites, hereby enabling applications beyond photovoltaics, not to mention analyzing their magnetic properties using other transition metals.