Abstract

We experimentally investigated magnetization relaxation dynamics in the largely unexplored time window extending from few picoseconds up to two nanoseconds following femtosecond laser pulse excitation. We triggered magnetization dynamics in ${[\mathrm{Co}(0.4\phantom{\rule{0.16em}{0ex}}\mathrm{nm})/\mathrm{Pt}(0.7\phantom{\rule{0.16em}{0ex}}\mathrm{nm})]}_{3}$ multilayers and measured the resulting magneto-optic response by recording both transient hysteresis loops as well as transients of magnetization dynamics. We observe that the coercive field of the sample is still strongly suppressed even \ensuremath{\sim}1 ms after the laser excitation, which is three orders of magnitude longer than the recovery time of the magnetization amplitude. In addition, we succeeded to fit the magnetization relaxation data in the entire experimentally observed time window by considering two phenomenological time constants ${\ensuremath{\tau}}_{f}^{*}$ and ${\ensuremath{\tau}}_{s}^{*}$ describing fast (ps) and slow (ns) magnetization relaxation processes, respectively. The fits of the data suggest a magnetic field dependent relaxation slowdown beyond 100 ps after excitation. We observe an explosion of the ${\ensuremath{\tau}}_{f}^{*}$ and ${\ensuremath{\tau}}_{s}^{*}$ values when the magnetization is completely quenched and relaxes intrinsically in the absence of an external magnetic field. We interpret the phenomenological time constants ${\ensuremath{\tau}}_{f}^{*}$ and ${\ensuremath{\tau}}_{s}^{*}$ using an intuitive physical picture based on the Landau-Lifshitz-Bloch model and numerical solutions of the extended three-temperature model [Shim et al., Sci. Rep. 10, 6355 (2020)].

Highlights

  • Ultrafast control of the magnetization orientation in ferromagnetic thin films on femto- and picosecond timescales currently attracts strong interest, because of its scientific allure, and for its vast potential to impact future magnetic storage technologies

  • We present an experimental study on magnetization relaxation processes following femtosecond optical excitation of [Co/Pt]3 multilayers examined by the timeresolved polar magneto-optic Kerr effect (TR-P-MOKE)

  • 2-nm-thin Pt capping layer were added in order to achieve perpendicular magnetic anisotropy (PMA) and to prevent oxidation, respectively

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Summary

INTRODUCTION

Ultrafast control of the magnetization orientation in ferromagnetic thin films on femto- and picosecond timescales currently attracts strong interest, because of its scientific allure, and for its vast potential to impact future magnetic storage technologies. The LLB model suggests [33] that longitudinal and transverse relaxation can be treated as independent processes and contribute to the magnetization dynamics at different timescales. Experiments using element-selective soft x-ray probing [39] suggest that the magnetization relaxation processes after optical excitation may be associated with phonon [40] or magnon generation [41] due to the presence of hot electrons. Each of these processes may affect the magnetization dynamics on different timescales [30]. ∗ f on the LLB model and numerical solutions of the E3TM, which fits our data very well for timescales

EXPERIMENTAL PROCEDURES
Pump-probe experiments
Constant and pulsed field initialization of the magnetic state
Measurement of transient hysteresis loops
Phenomenological response function
TRANSIENT MAGNETIZATION HYSTERESES M VS H
IMPACT OF THE EXTERNAL FIELD
IMPACT OF LASER FLUENCE
Findings
VIII. CONCLUSIONS
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