Antiferromagnets have magnetic long-range order but no net macroscopic magnetization. They have certain advantages over ferromagnets such as faster dynamics, no magnetic stray fields, and a wider choice of materials. On the downside, they are more difficult to study because of the absence of a macroscopic magnetic moment. Although they are widely used to influence the properties of adjacent ferromagnetic layers in devices, little was known about how such bilayers respond on ultrafast timescales to an ultrashort laser pulse.

Physicists in the CRC/TRR 227 from Freie Universität Berlin, Helmholtz-Zentrum Berlin, and Uppsala University (projects A03, A07, and Mercator fellow) have now achieved to track the ultrafast loss of magnetic order in both layers of an antiferromagnetic–ferromagnetic bilayer system. They chose a nine-atom-thick iron (Fe) film as the ferromagnet and a nine-atom-thick cobalt oxide (CoO) film as the antiferromagnet, stacked on a silver single-crystal surface. Using time-sliced ultrashort soft-x-ray pulses from the synchrotron radiation source BESSY II of the Helmholtz-Zentrum Berlin, synchronized with ultrashort laser pulses, they performed a stroboscopic pump–probe experiment that takes rapid “snapshots” of the magnetic state after excitation. Element-specific magnetic contrast was obtained via the so-called x-ray magnetic circular dichroism for Fe and the x-ray magnetic linear dichroism for CoO in an experiment detecting the reflected intensity of circularly or linearly polarized soft x-rays, respectively. Since these effects occur only if the wavelength of the soft x rays is tuned to the elemental absorption energies of Fe and Co, which are different, the dynamics of the antiferromagnetic order in CoO and the ferromagnetic order in Fe can be determined separately. Both orders were found to collapse on a timescale of about 300 fs (= 3 × 10^–13 s) after the sample gets hit by a laser pulse of 800 nm wavelength.

This is a surprise, since CoO is transparent at this wavelength and therefore does not absorb the laser pulse directly. A transfer of excitation from the Fe layer into the CoO layer is thus the dominating mechanism for the ultrafast loss of CoO antiferromagnetic order. Theoretical calculations reveal that on the experimentally observed ultrafast timescale, only a direct energy transfer from the excited electrons in Fe to the spin system of CoO across the interface between both layers can explain the experimental results. This is important for designing antiferromagnetic–ferromagnetic layered systems for the fastest spintronic applications.

---

Figure caption: 

Left: Sketch of the sample – a ferromagnetic Fe layer and an antiferromagnetic CoO layer, each nine atoms thick, on a Ag single crystal – and the pump–probe setup (pump: infrared laser pulse; probe: soft-x-ray reflectivity). Right: Temporal evolution of the magnetic order in CoO (orange) and Fe (blue) after excitation with an 800 nm pump, measured by x-ray magnetic linear and circular dichroism, respectively. Symbols: experimental data; dashed lines: theoretical simulations.

Article reference:

Element-Selective Probing of Ultrafast Ferromagnetic-Antiferromagnetic Order Dynamics in Fe/CoO Bilayers
C. S. Awsaf, S. Thakur, M. Weißenhofer, J. Gördes, M. Walter, M.-A. Mawass, N. Pontius, C. Schüßler-Langeheine, P. M. Oppeneer, and W. Kuch
Phys. Rev. Lett 136, 126705 (2026) - DOI: 10.1103/gcwk-tsj5

Contact:

Wolfgang Kuch, e-mail: kuch@physik.fu-berlin.de