Abstract
In this paper, we present a new setup for the measurement of element-specific ultrafast magnetization dynamics in ferromagnetic thin films with a sub-15-fs time resolution. Our experiment relies on a split and delay approach which allows us to fully exploit the shortest X-rays pulses delivered by X-ray Free Electrons Lasers (close to the attosecond range), in an X-ray pump - X-ray probe geometry. The setup performance is demonstrated by measuring the ultrafast elemental response of Ni and Fe during demagnetization of ferromagnetic Ni and Ni80Fe20 (Permalloy) samples upon resonant excitation at the corresponding absorption edges. The transient demagnetization process is measured in both reflection and transmission geometry using, respectively, the transverse magneto-optical Kerr effect (T-MOKE) and the Faraday effect as probing mechanisms.
Highlights
A few years ago, the question arose whether nickel and iron would display different ultrafast demagnetization dynamics in Py (Ni80Fe20) upon excitation with femtosecond laser pulses and whether this magnetization quenching would differ from the one observed in the pure materials
Mathias et al [1].and Jana et al [2]. found quite similar demagnetization dynamics for Ni and Fe in Py by using a near infrared (NIR) pump and an element-specific high order harmonic (HHG) probe in a transverse magneto-optic Kerr effect (T-MOKE) geometry : they claimed that Fe is demagnetizing faster than Ni with a small delay (∼10 fs) between the onset of their two dynamics
Using a NIR pump and a soft X-ray probe, exploiting the X-ray circular dichroism (XMCD) at the L absorption edge at the femto-slicing beamline of the BESSY II synchrotron of Helmholtz Zentrum Berlin, Radu et al [3] observed that Ni and Fe have very different magnetization dynamics in a Py sample, and that Ni is faster than Fe
Summary
A few years ago, the question arose whether nickel and iron would display different ultrafast demagnetization dynamics in Py (Ni80Fe20) upon excitation with femtosecond laser pulses and whether this magnetization quenching would differ from the one observed in the pure materials. These approaches have the advantages of getting rid of the jitter and of being theoretically only limited by the duration of the X-ray pulses They offer the powerful possibility to excite core electrons and can trigger dynamics that differ from those generated by NIR lasers. Those methods rely on XFEL special operation modes and are not widely available for users. Our goal is to fully exploit the very short X-ray pulses delivered at XFEL in an X-ray pump X-ray probe geometry to obtain a sub-15-fs time resolution with an easy-to-implement user-side setup This is realized by splitting the X-ray beam into two parts: the first part is used to excite the sample and the second part is used to probe the sample state. We show the results obtained on the Ni and Py systems in two different experimental geometries, reflection, and transmission respectively, using T-MOKE and Faraday effect as magnetization probe
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