Dynamic control of magnetic nanowires by light-induced domain-wall kickoffs

Journal name:
Nature Materials
Volume:
12,
Pages:
202–206
Year published:
DOI:
doi:10.1038/nmat3498
Received
Accepted
Published online

Controlling the speed at which systems evolve is a challenge shared by all disciplines, and otherwise unrelated areas use common theoretical frameworks towards this goal. A particularly widespread model is Glauber dynamics1, which describes the time evolution of the Ising model and can be applied to any binary system2, 3, 4, 5, 6, 7. Here we show, using molecular nanowires under irradiation, that Glauber dynamics can be controlled by a novel domain-wall kickoff mechanism. In contrast to known processes, the kickoff has unambiguous fingerprints, slowing down the spin-flip attempt rate by several orders of magnitude, and following a scaling law. The required irradiance is very low, a substantial improvement over present methods of magneto-optical switching8, 9. These results provide a new way to control and study stochastic dynamic processes. Being general for Glauber dynamics, they can be extended to different kinds of magnetic nanowires and to numerous fields, ranging from social evolution2 to neural networks5 and chemical reactivity3, 4.

At a glance

Figures

  1. Glauber dynamics and domain-wall kickoff.
    Figure 1: Glauber dynamics and domain-wall kickoff.

    a, Standard Glauber dynamics. At the beginning the system is completely polarized. The magnetic excitation nucleates at an energy cost and the domain wall (green) can then move by a random walk process. Nucleation, governed by the Arrhenius law τ  =  τ0eΔE/kBT, costs ΔE  =  4JS2 inside the chains or ΔE  =  2JS2at the chain border, and the prefactor τ0 is connected with the flipping rate of an isolated spin. b, Kickoff mechanism. A photon is absorbed at one site of the chain, creating a Frenkel exciton (first row). The intrachain exchange coupling with a spin at the exciton site (green spin), Jexc, becomes lower than J and, for the duration of the exciton, the nucleation energy is thus reduced. The so-created domain wall (third row) can propagate only after the exciton has decayed (fourth row). Consequently the prefactor is now τ0exc, linked to the exciton lifetime and much longer than τ0.

  2. Torque magnetometry.
    Figure 2: Torque magnetometry.

    a, Crystal and magnetic structure of [Co(hfac)2(NIT-PhOMe)], with the non-collinear anisotropy axes of Co(II). b, Scheme of the photon excited torque magnetometer. θ represents the angle between the magnetic field Hand the crystallographic axis c. The zoom-in shows the crystal orientation on the cantilever. c, Angular dependence of the torque at H  =  20 kOe, at temperature T above and below the slow relaxing regime (red and blue lines, respectively). The dashed line highlights the direction . d, Angle and field dependence of the torque in the slow-relaxing regime (T  =  1.5 K). The red line and the semitransparent plane represent zero torque, the blue line corresponds to .

  3. Irradiation effect.
    Figure 3: Irradiation effect.

    a, Switching of the magnetization dynamics at 5 K. Grey and black circles are acquired without irradiation. Red circles are acquired with the laser on. Without the sample (blue and red circles), there is no observable effect of irradiation. b, Magnetization decay curves recorded under continuous irradiation every 50 mK between 4.7 and 5.3 K (top) and at different irradiance (bottom, see d for I values). c, Arrhenius law for the slow relaxing component of the decay under no irradiation and on varying the irradiance. The line is the Arrhenius law extracted from SQUID and a.c. susceptibility data. d, Arrhenius law without (grey and black symbols) and with irradiation (coloured). Full circles are a.c. susceptibility data; open circles are SQUID decay times; grey spheres represent torque without irradiation; coloured spheres represent torque decay times for different irradiance. The black line represents fitting of the a.c. and SQUID data and coloured lines are obtained using the theoretical kickoff model.

  4. Domain-wall kickoff.
    Figure 4: Domain-wall kickoff.

    a, Temperature dependence of the distribution of relaxation times in presence of the kickoff mechanism. The probability P(τ) of having a certain relaxation time (solid surface) is calculated using the [Co(hfac)2(NIT-PhOMe)] parameters. Dots indicate experimental data at I  =  1.05 μW cm−2: black spheres refer to the chains that follow standard Glauber dynamics even during irradiation, red ones to the chains relaxing via the kickoff mechanism. Grey spheres represent a.c. and SQUID measurements. b, Scaling plot of the Arrhenius law for different irradiance values I (colour scale).

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Author information

Affiliations

  1. 1. Physikalisches Institut, Universität Stuttgart, Pfaffenwaldring 57, D-70550 Stuttgart, Germany

    • Eric Heintze,
    • Fadi El Hallak,
    • Conrad Clauß,
    • Martin Dressel &
    • Lapo Bogani
  2. Department of Physics and Astronomy, University of Florence, Via G. Sansone 1, I-50019 Sesto Fiorentino (FI), Italy

    • Angelo Rettori
  3. S3 Centre, Institute Nanoscience, CNR, Via G. Campi 213/a, I-41125 Modena, Italy

    • Angelo Rettori
  4. Institute for Complex Systems, ISC-CNR, Via Madonna del Piano 10, I-50019 Sesto Fiorentino (FI), Italy

    • Maria Gloria Pini
  5. Department of Chemistry, University of Florence, Via Lastruccia 3, I-50019 Sesto Fiorentino (FI), Italy

    • Federico Totti

Contributions

F.E.H., E.H. and L.B. developed the experimental set-ups and performed the torque measurements. L.B., M.G.P. and A.R. developed the kickoff model. F.T. performed the ab initio calculations. L.B. devised the experiment, synthetized the samples and wrote the paper. All authors discussed the results and contributed to the manuscript.

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The authors declare no competing financial interests.

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