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

Journal name:
Nature
Volume:
472,
Pages:
205–208
Date published:
DOI:
doi:10.1038/nature09901
Received
Accepted
Published online

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 (10–100fs), 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.

At a glance

Figures

  1. Ferrimagnetic alignment of the Fe and Gd magnetic moments as measured by element-specific XMCD hysteresis.
    Figure 1: Ferrimagnetic alignment of the Fe and Gd magnetic moments as measured by element-specific XMCD hysteresis.

    a, b, Top, XMCD signals measured at the Fe and Gd absorption edges as a function of applied magnetic field below (a) and above (b) the magnetization compensation temperature (TM), demonstrating the ferrimagnetic alignment of the Fe and Gd magnetic moments. a, b, Bottom, a generic ferrimagnet, showing the alignment of the magnetic moments of the constituent sub-lattices with respect to the external magnetic field, H.

  2. Element-resolved dynamics of the Fe and Gd magnetic moments measured by time-resolved XMCD with femtosecond time-resolution.
    Figure 2: Element-resolved dynamics of the Fe and Gd magnetic moments measured by time-resolved XMCD with femtosecond time-resolution.

    a, Transient dynamics of the Fe (open circles) and Gd (filled circles) magnetic moments measured within the first 3ps. b, As a but on a 12ps timescale. Error bars of the experimental data represent the statistical standard error. The measurements were performed at a sample temperature of 83K for an incident laser fluence of 4.4mJcm−2. Experimental time resolution of 100fs is depicted by the solid Gaussian profile. The solid lines are fits according to a double exponential fit function (Supplementary Information). The dashed line in both panels depicts the magnetization of the Fe sublattice taken with the opposite sign (that is, opposite with respect to the sign of the measured Fe data).

  3. Computed time-resolved dynamics of the Fe and Gd magnetic moments from localized atomistic spin model.
    Figure 3: Computed time-resolved dynamics of the Fe and Gd magnetic moments from localized atomistic spin model.

    a, Cartoon-like illustration of the non-equilibrium dynamics of the Fe and Gd magnetizations with respect to an external magnetic field H. The lengths of the arrows are scaled to the magnitude of the transient XMCD signals shown in Fig. 2. b, c, Simulated dynamics for the first 3ps (b) and the first 12ps (c) after laser excitation. The calculations were performed for a peak electronic temperature of 1,492K with the corresponding transient electronic and phononic temperatures shown in c. The transient magnetization changes are normalized to magnetization values at negative delays, that is, to equilibrium values. As is clearly seen, the demagnetization of the Fe is much faster than that of the Gd (see inset in b; axes same as main panel). For a time of ~0.5ps, we observe a parallel alignment of magnetizations of the sublattices. The agreement with the experimental data in Fig. 2 is qualitatively excellent.

References

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Author information

Affiliations

  1. Radboud University Nijmegen, Institute for Molecules and Materials, Heyendaalseweg 135, 6525 AJ Nijmegen, The Netherlands

    • I. Radu,
    • K. Vahaplar,
    • A. Kirilyuk,
    • Th. Rasing &
    • A. V. Kimel
  2. Helmholtz-Zentrum Berlin für Materialien und Energie, BESSY II, Albert-Einstein-Strasse 15, 12489 Berlin, Germany

    • I. Radu,
    • C. Stamm,
    • T. Kachel,
    • N. Pontius &
    • H. A. Dürr
  3. SLAC National Accelerator Laboratory, Menlo Park, California 94025, USA

    • H. A. Dürr
  4. Department of Physics, University of York, York YO10 5DD, UK

    • T. A. Ostler,
    • J. Barker,
    • R. F. L. Evans &
    • R. W. Chantrell
  5. College of Science and Technology, Nihon University, 7-24-1 Funabashi, Chiba, Japan

    • A. Tsukamoto &
    • A. Itoh
  6. PRESTO, Japan Science and Technology Agency, 4-1-8 Honcho Kawaguchi, Saitama, Japan

    • A. Tsukamoto

Contributions

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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The authors declare no competing financial interests.

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Supplementary information

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  1. Supplementary Information (821 KB)

    The file contains Supplementary Text and Data, Supplementary Figures 1-6 with legends and additional references.

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