Researchers from Freie Universität Berlin, Uppsala University and Helmholtz-Zentrum Berlin für Materialien und Energie (HZB) have directly tracked how magnetic order in a coupled Fe/CoO bilayer collapses within a few hundred femtoseconds after an ultrashort laser pulse, and identified interfacial energy transfer from Fe to CoO as the key channel for quenching the antiferromagnetic order.
The sample consists of an epitaxial 9 (±0.5) monolayer CoO film on Ag(001), capped by 9 (±1) monolayers of Fe. The CoO antiferromagnetic moments are collinear in the film plane and aligned with the Fe magnetization along an Fe ⟨100⟩ easy axis due to strong interfacial coupling; an external magnetic field can rotate this AFM spin axis by 90° in the plane via the Fe layer. Time-resolved measurements were carried out at BESSY II using 60 fs p‑polarized pump pulses at 800 or 400 nm and 100 fs polarized soft x‑ray probe pulses, providing 120 fs temporal resolution in a pump–probe reflection geometry under ultrahigh vacuum at 200 K and a 120 mT in‑plane field.
The team used resonant soft-x-ray reflectivity: x-ray magnetic linear dichroism in reflection (R‑XMLD) at the Co L₂ edge (793.3 eV) to probe the antiferromagnetic CoO sublattice magnetization, and x‑ray magnetic circular dichroism in reflection (R‑XMCD) at the Fe L₃ edge (709.8 eV) to probe the ferromagnetic Fe magnetization. R‑XMLD measures the difference in reflectivity for configurations where the CoO AFM axis is parallel or perpendicular to the linear x‑ray polarization, giving a signal proportional to M2 of the AFM sublattices, while R‑XMCD compares reflectivities for opposite Fe magnetization directions relative to the circular x‑ray helicity. Careful control of polarization and magnetic-field direction suppresses structural dichroism and isolates the magnetic contributions in both layers.
Both the AFM order in CoO and the FM order in Fe are strongly reduced on comparable ultrafast timescales of about 200–400 fs after excitation by 60 fs pulses at 800 or 400 nm, as seen in the element‑resolved reflectivity contrast. At 800 nm, the CoO layer does not absorb directly, so the loss of AFM order in CoO must be driven entirely by energy transferred from the laser‑excited electrons in Fe across the interface. Atomistic spin‑dynamics simulations using a stochastic Landau–Lifshitz–Gilbert model with a temperature description for each layer confirm that direct energy transfer from hot Fe electrons to CoO spins is the primary mechanism, leading to a quenching of CoO magnetic moments on the order of 300 fs and a propagation of the loss of AFM order through the 9‑ML CoO film with a time constant of 276 fs.
When the sample is pumped at 400 nm, the CoO layer is directly excited in addition to the Fe layer. In this case, the magnetic order in CoO decreases much more strongly than in Fe, which cannot be explained by interfacial transfer alone. Including direct excitation of CoO electrons at 400 nm in the atomistic simulations reproduces the larger demagnetization amplitude observed in CoO, showing that both interface‑mediated and direct pathways contribute.
Spintronic devices rely on coupled ferromagnetic–antiferromagnetic stacks for energy‑efficient, ultrafast data processing, but AFM layers are challenging to probe due to their vanishing net magnetization. By combining R‑XMLD and R‑XMCD in a femtosecond pump-probe reflectivity experiment, this work provides a clear, element‑resolved picture of how AFM and FM orders in a realistic Fe/CoO bilayer respond to an ultrashort optical pulse. The results demonstrate that interface‑mediated energy transfer from hot ferromagnetic electrons to antiferromagnetic spins can drive ultrafast loss of AFM order on ~300 fs timescales, and establish time‑resolved R‑XMLD as a powerful tool for studying ultrafast magnetization dynamics in antiferromagnets and their heterostructures.