Controlling coherent and incoherent spin dynamics by steering the photoinduced energy flow

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Controlling coherent and incoherent spin dynamics by steering the photoinduced energy flow

D. Bossini, A. M. Kalashnikova, R. V. Pisarev, Th. Rasing, and A. V. Kimel

Phys. Rev. B 89, 060405(R) – Published 19 February 2014

Abstract

We present a femtosecond spectroscopic magneto-optical investigation of the coherent and incoherent spin dynamics in the antiferromagnetic dielectric ${KNiF}_{3}$ . The pathways of the photoinduced energy flow to spins were controlled by tuning the pump photon energy. In particular, we demonstrate that laser pulses, with photon energy tuned to a nearly-zero-absorption region, excite the spin system without any signatures of heating of electrons or phonons. In this regime the ultrafast excitation of coherent spin waves is followed by a gradual increase of the spin temperature solely due to decoherence of the laser-generated magnons, as revealed by our simultaneous measurement of both the transversal and the longitudinal component of the spin dynamics.

Received 14 November 2013
Revised 28 January 2014

DOI:https://doi.org/10.1103/PhysRevB.89.060405

Authors & Affiliations

D. Bossini^1,*, A. M. Kalashnikova², R. V. Pisarev², Th. Rasing¹, and A. V. Kimel¹

¹Institute for Molecules and Materials, Radboud University Nijmegen, Heyendaalseweg 135, Nijmegen, The Netherlands
²A. F. Ioffe Physical-Technical Institute, Russian Academy of Sciences, 194021 St. Petersburg, Russia

^*d.bossini@science.ru.nl

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Vol. 89, Iss. 6 — 1 February 2014

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Images

Figure 1
Spectral dependence of the absorption coefficient in ${KNiF}_{3}$ at room temperature. The levels corresponding to the absorption bands and the ground state $^{3} A_{2 g}$ are also shown. In the range 2–2.4 eV a nearly-zero-absorption region is observed. The excitation photon energies used in the pump-probe experiment are indicated. In the inset, we report schematically the experimental geometry. We measured variations in the ellipticity of the probe polarization, due to magnetic linear birefringence.
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Figure 2
(a) Photoinduced changes in the ellipticity of the probe polarization as a function of the pump-probe delay. The data are normalized on the pump fluence, which was approximately 12 mJ/cm $^{2}$ . The central photon energies of the pump and probe pulses were 1.5 and 1.9 eV, respectively. The temperature was 10 K. The fit to the data was performed with the function reported in Eq. (1). The absorption coefficient corresponding to the pump photon energy is shown. The increase of the background is characterized by the parameter $B$ [see Eq. (1)]. In the inset we report the temperature dependence of the incoherent dynamics, measured with pump and probe photon energies set to 1.5 eV. The dashed line is a guide to the eye. (b) Diagram showing the energy flow to the spin system in the case of excitation resonant with an absorption band (see the main text).
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Figure 3
(a) Photoinduced changes in the ellipticity of the probe polarization as a function of the pump-probe delay obtained with the pump photon energy tuned to 3.0 eV (blue dots) and 1.9 eV (red dots). The probe photon energy was always 1.5 eV. The values of the absorption coefficient of the sample corresponding to these pump photon energies are reported. Solid lines are fits to the data using Eq. (1). (b) Amplitude $B$ of the incoherent dynamics obtained from the fit as a function of the absorption. A sharp decrease is observed for $α \approx 0$ . The dashed line is a guide to the eye. (c) Characteristic rise time of the incoherent dynamics (open dots) and decoherence time (solid dots) obtained from the fit as a function of the absorption. Note the abrupt decrease of $τ_{r}$ at $α \approx 0$ . The dashed lines are guides to the eye. The error bars in (b) and (c) account for the standard deviation of the fit.
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Figure 4
(a) Photoinduced changes in the ellipticity of the probe polarization as a function of the pump-probe delay. The fit to the data is performed using Eq. (1) (see the main text). In the inset we report the temperature dependence of the amplitude of the incoherent dynamics. Two experimental curves are reported: measurement performed with a pump photon energy of 1.5 eV (open magenta dots) and 2.2 eV (solid green dots, multiplied by a factor of 10 for the sake of comparison). The probe photon energy was 1.5 eV in both cases. The two trends are fundamentally different, since the value of $B$ for the 2.2 eV photon energy vanishes significantly faster than in the case of the 1.5 eV stimulus. Data are normalized on the pump fluence. The error bars account for the standard deviation of the fit. The orange curve is a plot of the $T^{- 3}$ law. (b) Diagram showing the energy flow to the spin system in case of excitation energy tuned to the nearly-zero-absorption regime (see the main text).
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