Strong Spin-Dependent Interactions of Photoexcited Charge Carriers with Magnetic Transition Metal Dopants in MAPbBr₃

Combining the advantageous electrical and optical properties of semiconductors with magnetic characteristics gives access to extraordinary phenomena and applications. Fully inorganic dilute magnetic semiconductors (DMS) have been known for decades, which show diverse functionalities like control of magnetism by electrical fields. Typically, the material class of DMS is obtained by introducing a substantial number of magnetic ions to an otherwise non-magnetic host semiconductor.<br/>In this work we investigate the magneto-optical properties of the heavily doped hybrid semiconductor MAPbBr₃ (CH₃NH₃PbBr₃) with circularly polarized broadband femtosecond transient absorption (CTA) and pump-probe Faraday rotation, from which we find strong interactions between the excited charge carriers’ angular momentum and the dopants’ spin.<br/>Due to their outstanding optoelectronic properties and high defect tolerance, organo-metal halide perovskites form an ideal system for efficient magnetic doping. We prepared a selection of magnetic perovskites using simple solution processing techniques by doping of the host MAPbBr₃ perovskite with three different magnetic ions Mn²⁺, Co²⁺, Ni²⁺. We investigated the fundamental optical, structural and magnetic properties of the perovskites with absorption and photoluminescence spectroscopy, XRD and SQUID measurements. Doping of the host lattice leads in all cases to structural changes and an increase or decrease of the bandgap of several 10 meV for Co²⁺ and Mn²⁺/Ni²⁺, respectively. We observe paramagnetic properties for all dopants and different doping concentrations.<br/>To reveal the impact of doping on excitation dynamics, we employ the technique of CTA at cryogenic temperatures down to 4 K and magnetic fields up to 700 mT to gain a better understanding of possible ultrafast processes and coupling mechanisms between the angular momentum of photoexcited charge carriers and the d-electron spin of the dopants. Measuring CTA with different combinations of right- and left-handed circularly polarized pump (<i>λ</i>_ex = 515 nm) and probe (super-continuum white light) at variable temperatures and magnetic fields with an exceptional spectral resolution of 0.1 nm enabled us to precisely compare the energetic shifts and ultrafast dynamics of the charge carriers. For all investigated samples we observed an energy splitting of the CTA spectra for two different circular polarizations of the pump pulse even at zero magnetic field, which is substantially larger for the doped samples. The lifetime of this splitting is temperature-dependent and exceeds our investigable time span of 2 ns for the doped samples at 4 K. Pump-probe Faraday rotation measurements at 4 K underpin the different spin lifetime of undoped and doped samples. Both, lattice deformations and remanent magnetism due to high doping concentrations could lead to an energetically preferred spin alignment of the photoexcited charge carriers mediated via spin-orbit coupling. We plan to employ transient circularly polarized photoluminescence spectroscopy (CPL) to show if this spin polarization is long-lived and if it will lead to a significant degree of circularly polarized PL, which would be of high interest for future light emitting diode applications.