[Paper Review] Ultrafast element-resolved magneto-optics using a fiber-laser-driven extreme ultraviolet light source

Christina Möller, Henrike Probst+8 more|arXiv (Cornell University)|Mar 23, 2021

Laser-Matter Interactions and Applications51 references20 citations

TL;DR

This paper presents a high-repetition-rate fiber-laser-driven extreme ultraviolet (EUV) source for ultrafast, element-resolved magneto-optical Kerr effect (T-MOKE) measurements. By using a 300 kHz Yb-fiber laser to drive high-harmonic generation, the setup achieves high signal-to-noise ratio and stable EUV flux, enabling precise, time- and energy-resolved demagnetization dynamics of Fe and Ni in thin films with 10 fs time resolution and 58 fs temporal resolution. The method resolves distinct element-specific demagnetization delays and amplitudes in Fe19Ni81 permalloy, demonstrating sub-10 fs accuracy in relative dynamics within a 10-ps window.

ABSTRACT

We present a novel setup to measure the transverse magneto-optical Kerr effect in the extreme ultraviolet spectral range at exceptionally high repetition rates based on a fiber laser amplifier system. This affords a very high and stable flux of extreme ultraviolet light, which we use to measure element-resolved demagnetization dynamics with unprecedented depth of information. Furthermore, the setup is equipped with a strong electromagnet and a cryostat, allowing measurements between 10 and 420 K using magnetic fields up to 0.86 T. The performance of our setup is demonstrated by a set of temperature- and time-dependent magnetization measurements showing distinct element-dependent behavior.

Motivation & Objective

• To develop a high-repetition-rate, high-signal EUV source for ultrafast magneto-optics to overcome limitations of low-repetition-rate Ti:Sapphire lasers.
• To enable element-resolved, time- and energy-resolved T-MOKE measurements in the extreme ultraviolet (EUV) range using high-harmonic generation (HHG).
• To achieve high signal-to-noise ratio and stable data acquisition by leveraging a high-power Yb-fiber laser system at 100–300 kHz repetition rates.
• To measure ultrafast demagnetization dynamics in magnetic materials with high temporal and spectral resolution, including temperature and magnetic field dependence.
• To demonstrate the capability of resolving element-specific magnetic dynamics in complex systems such as Fe19Ni81 permalloy and double perovskite LNMO.

Proposed method

• A Yb-fiber laser system generates 1030 nm, 300 kHz, sub-40 fs pulses with up to 150 µJ pulse energy via self-phase modulation and chirped mirror compression.
• High harmonics are generated in an argon gas jet using intense IR pulses, producing EUV light in the 30–72 eV range covering M2,3 edges of Fe, Co, Ni, and Mn.
• The EUV beam is isolated from the fundamental IR beam using grazing-incidence plates (GIPs) and an Al filter, with a B4C-coated toroidal mirror focusing the beam onto the sample.
• Transverse magneto-optical Kerr effect (T-MOKE) is measured via intensity difference between opposite magnetization states, with magnetic asymmetry A = (I↑ - I↓)/(I↑ + I↓) calculated from reflected intensities.
• A custom spectrometer with a toroidal mirror, grating, and CCD camera disperses the reflected EUV light for energy-resolved detection.
• The setup integrates a cryostat (10–420 K) and a strong electromagnet (up to 0.86 T), enabling temperature- and field-dependent measurements.

Experimental results

Research questions

• RQ1 Can a high-repetition-rate fiber-laser-driven EUV source enable element-resolved, time-resolved T-MOKE with high signal-to-noise ratio?
• RQ2 What is the relative demagnetization delay between Fe and Ni in Fe19Ni81 permalloy films at room temperature?
• RQ3 How stable and reproducible are the ultrafast demagnetization dynamics over extended measurement times?
• RQ4 To what extent can the magnetic asymmetry signal resolve element-specific contributions in complex magnetic systems?
• RQ5 Can the system resolve sub-10 fs dynamics in the demagnetization process with high temporal resolution?

Key findings

• The fiber-laser-driven EUV source achieves a repetition rate of up to 300 kHz, enabling high-signal, low-noise data acquisition in just one hour.
• The setup resolves a 10 fs delay in demagnetization between Ni (137±10 fs onset) and Fe (185±21 fs onset) in Fe19Ni81 permalloy, with a 10 fs accuracy in relative timing.
• The magnetic asymmetry signal shows a 27.6% fractional demagnetization (ΔAm) and 19.9% remagnetization (ΔAr) for Ni at the M-edge, with τm = 147±4 fs and τr = 1.67±0.04 ps.
• For Fe, the fit yields ΔAm = 25.3% and ΔAr = 18.2%, with τm = 185±21 fs and τr = 1.96±0.24 ps, indicating slower demagnetization and remagnetization than in Ni.
• The 12-hour measurement data show excellent reproducibility, with fit results differing by less than 10% from the 1-hour data, confirming stability and low noise.
• The 1-hour data set yields results consistent with the full 12-hour data, demonstrating that high-quality ultrafast dynamics can be captured in minimal time.