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Transient ferromagnetic-like state mediating ultrafast reversal of antiferromagnetically coupled spins

Abstract

Ferromagnetic or antiferromagnetic spin ordering is governed by the exchange interaction, the strongest force in magnetism1,2,3,4. Understanding spin dynamics in magnetic materials is an issue of crucial importance for progress in information processing and recording technology. Usually the dynamics are studied by observing the collective response of exchange-coupled spins, that is, spin resonances, after an external perturbation by a pulse of magnetic field, current or light. The periods of the corresponding resonances range from one nanosecond for ferromagnets down to one picosecond for antiferromagnets. However, virtually nothing is known about the behaviour of spins in a magnetic material after being excited on a timescale faster than that corresponding to the exchange interaction (10–100 fs), that is, in a non-adiabatic way. Here we use the element-specific technique X-ray magnetic circular dichroism to study spin reversal in GdFeCo that is optically excited on a timescale pertinent to the characteristic time of the exchange interaction between Gd and Fe spins. We unexpectedly find that the ultrafast spin reversal in this material, where spins are coupled antiferromagnetically, occurs by way of a transient ferromagnetic-like state. Following the optical excitation, the net magnetizations of the Gd and Fe sublattices rapidly collapse, switch their direction and rebuild their net magnetic moments at substantially different timescales; the net magnetic moment of the Gd sublattice is found to reverse within 1.5 picoseconds, which is substantially slower than the Fe reversal time of 300 femtoseconds. Consequently, a transient state characterized by a temporary parallel alignment of the net Gd and Fe moments emerges, despite their ground-state antiferromagnetic coupling. These surprising observations, supported by atomistic simulations, provide a concept for the possibility of manipulating magnetic order on the timescale of the exchange interaction.

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Figure 1: Ferrimagnetic alignment of the Fe and Gd magnetic moments as measured by element-specific XMCD hysteresis.
Figure 2: Element-resolved dynamics of the Fe and Gd magnetic moments measured by time-resolved XMCD with femtosecond time-resolution.
Figure 3: Computed time-resolved dynamics of the Fe and Gd magnetic moments from localized atomistic spin model.

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Acknowledgements

We acknowledge technical support provided by R. Mitzner, K. Holldack and T. Quast during the slicing measurements. We thank F. Radu and R. Abrudan for their support and help with the static XMCD measurements as well as for discussions. This work was supported by the European Community's Seventh Framework Programme FP7/2007-2013 (grants NMP3-SL-2008-214469 (UltraMagnetron) and 214810 (FANTOMAS) as well as grant no. 226716), the Foundation for Fundamental Research on Matter (FOM), the Netherlands Organization for Scientific Research (NWO), the Nihon University Strategic Projects for Academic Research and the US Department of Energy under contract DE-AC02-76SF00515.

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I.R., A.V.K., A.K. and Th.R. designed and coordinated the project; I.R., K.V., C.S. and T.K. performed the measurements; C.S., T.K., N.P. and H.A.D. developed the femtoslicing facility at BESSY II Berlin; I.R. and K.V. performed the data analysis; T.A.O., J.B., R.F.L.E. and R.W.C. developed the atomistic model and performed the calculations; A.T. and A.I. grew and optimized the samples; and I.R. and A.V.K. coordinated the work on the paper. All the authors contributed to the writing of the manuscript.

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Correspondence to I. Radu or A. V. Kimel.

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The authors declare no competing financial interests.

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Radu, I., Vahaplar, K., Stamm, C. et al. Transient ferromagnetic-like state mediating ultrafast reversal of antiferromagnetically coupled spins. Nature 472, 205–208 (2011). https://doi.org/10.1038/nature09901

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