Magnetic Semiconductors

Controlling Spins on fast Timescales

Our work explores the fundamental interplay between charge, spin, and lattice degrees of freedom in materials that combine the hallmark characteristics of semiconductors— electronic bands, excitons and optical band gaps— with magnetic moments.  In such systems the same electrons that participate in optical transitions also carry a magnetic moment that can be aligned through exchange interactions and that can act on existing magnetic moments in the materials.  Understanding how these intertwined properties evolve on the natural time‑scales of the solid—tens to hundreds of femtoseconds—offers a route to both novel material physics insight and the foundation for future spin‑based technologies.

Scientific Goals

The central questions that guide our work concern the possibility of using light not only to read the magnetic state of a material but also to write it, and to determine under what conditions the generated spin-polarisation can survive long enough to be useful. Specifically, we aim to:

Create and control spin‑polarised carrier populations by exploiting the spin‑orbit coupling that is intrinsic to many semiconductor band structures. Circularly polarised, ultrashort optical pulses can selectively excite electrons with a defined angular momentum, thereby imprinting a net spin orientation on the system.

Identify relaxation pathways that preserve spin information.  After excitation, carriers typically lose energy through phonon emission and recombine with opposite‑spin partners, processes that scramble the initial spin alignment.  By mapping the temporal evolution of the excited states we can pinpoint material compositions, crystal symmetries, and excitation conditions that suppress spin‑flip scattering and extend the spin coherence time beyond the carrier lifetime.

Develop purely optical readout schemes.  The same light‑matter interaction that creates a spin population also modifies the material’s optical response.  Changes in photoluminescence intensity, spectral shape, or the rotation of the polarisation plane of a light (Faraday or Kerr rotation) are directly linked to the instantaneous magnetisation.  By correlating these signals with the underlying spin dynamics we establish a contact‑free method for monitoring magnetic order on ultrafast time‑scales.

Elucidate the coupling between spins, charges, and lattice vibrations.  The exchange interaction that orders the spins is mediated by the electronic structure, which in turn is sensitive to lattice distortions.  Coherent phonons generated by the pump pulse can transiently modulate the exchange splitting, providing a direct probe of spin‑phonon coupling.  Understanding this three‑way interaction is essential for any effort to control magnetism with light.

Materials Portfolio

Our investigations now encompass two complementary families of magnetic semiconductors.

The first material focus of our program is transition‑metal‑doped hybrid perovskites (e.g., MAPbI₃ doped with Mn²⁺, Fe³⁺ or Co²⁺).  The perovskite lattice furnishes a soft, polar environment that supports strong spin‑orbit coupling and dynamic Rashba fields, while the embedded transition‑metal ions supply localized magnetic moments.  The hybrid nature of these materials—organic cations, inorganic halide framework, and transition‑metal dopants—creates a rich landscape of spin‑charge‑lattice interactions.  Time‑resolved spectroscopy has shown that the photo‑excited carriers can mediate long‑range ferromagnetic coupling between the dopants on sub‑picosecond time‑scales, a phenomenon that disappears when the lattice is frozen at low temperature, thereby exposing the crucial role of lattice fluctuations in stabilising magnetic order.

A second direction is the rapidly expanding space of organic molecule diradicals.  These are conjugated π‑systems that host two unpaired electrons in spatially separated orbitals, giving rise to intrinsic spin‑½ moments that coexist with a semiconducting frontier‑orbital gap.  Because the radicals are covalently bound within a molecular crystal or a thin‑film stack, the spin density can be tuned through chemical substitution, while the electronic band structure remains responds to optical excitation in the visible or near‑infrared.  Ultrafast pump‑probe experiments on diradical crystals have revealed that the photo‑generated excitons inherit the spin polarisation of the ground‑state radicals, and that the subsequent spin‑flip processes are governed by intermolecular exchange pathways rather than by phonon‑mediated scattering.

By studying inorganic, organic, and hybrid systems side by side we obtain a comparative view of how electronic delocalisation, molecular rigidity, and lattice softness each influence spin preservation and optical control.

Experimental Approach

Our investigations rely on a suite of time‑resolved spectroscopies that together provide a complete picture of the non‑equilibrium dynamics.  An ultrafast pump pulse—typically 30–100 fs in duration and tunable across the band edge—creates a non‑thermal distribution of carriers with a well‑defined spin orientation.  A delayed probe pulse interrogates the system through several complementary channels.

Transient absorption monitors the pump‑induced change in the sample’s absorption spectrum. By tracking the evolution of state‑filling, band‑renormalisation, and spin‑dependent bleaching we obtain quantitative information on carrier relaxation and the degree of spin polarisation as a function of delay time.

Time‑resolved photoluminescence records the emitted photons from recombining carriers with sub‑picosecond temporal resolution.  The circular polarisation degree of the luminescence directly reflects the residual spin orientation of the recombining excitons, allowing us to measure how long the spin information survives after excitation.

Faraday and Kerr rotation measurements detect the rotation of the probe’s linear polarisation caused by the transient magnetisation of the sample.  Because the rotation angle is proportional to the net spin component along the probe direction, these experiments provide a sensitive, model‑independent readout of the magnetic state on the same femtosecond time‑scale as the other techniques.

All measurements are performed in a cryogenic environment (1.5 K – 300 K) and, when required, under a static magnetic field up to 7 T.  The combination of temperature, field, and pump‑photon‑energy control enables us to disentangle the various relaxation mechanisms that compete with spin preservation.