Ultrafast Electron Dynamics in Magnetic Thin Films

Round 1

Reviewer 1 Report

The manuscript titled by "Ultrafast Electron Dynamics in Magnetic Thin Films", reports ultrafast spin dynamics in magnetic systems using a long wavelength terahertz (THz) pump. The authors carried out THz pump/800nm probe measurement in a nickel film. Both THz-induced MOKE and  birefringence was measured. The data is analyzed in term of the charge, spin and phonon dynamics. The authors concluded that the ps-demagnetization is induced by THz pulse heating within the sample that is comparable to the Curie temperature of nickel. Although the data looks impressive, but the experiment is routine and does not bring out new physics to the community. In addition, there is no detail analysis and modeling of the data, therefore, the discussion and conclusion are not substantial and convincible. Overall, the presentation of the paper is poor.

There are also several critical issues in the paper?

1. The authors measured both MOKE and birefringence data. What is the relation between them? How do the authors calibration MOKE signal with real magnetization data?
2. In line 143, the "Figure.1.c" should be "Figure.1.d",   the "Figure.1.d" should be "Figure.1.e". In line 146, the "Figure.1.e" should be "Figure.1.f".
3. In Fig. 1e, why do the signals not start at 0 ps?
4. In Fig. 1b, and c, what are the spectra with 6THz filter? 
5. What is the 5ps oscillation in Fig. 2? Why do the authors mention coherent acoustic phonons?
6. About Fig. 3, Why does electron responds quicker and relaxes in a couple ps? Why does magnetization responds slower and relaxed very slowly?

Author Response

We humbly thank the referee for the comments, we will respond to the issues raised point by point:

 

Comments and Suggestions for Authors:

We thank the referee for the suggestions, in this work, we have observed complete demagnetisation of the sample in the absence of sample damage (as the sample regain complete magnetisation). This has not been observed previously for nickel. Moreover, we separated charge, magnetisation and phononic dynamics of the system. This is also novel for nickel. Lastly, from the comments of the other referees: "The manuscript ... is of interest to researchers and THz community", and "The paper is interesting for researchers working in the field of ultrafast spectroscopy and ferromagnetic thin films."

 

1. The referee is correct, more details of the relationship between the MOKE and birefringence could be given. We have added the following paragraph to the manuscript: "The magnetic field modulation is locked to the laser clock frequency, and its magnitude is larger than the saturation magnetic field of the sample. Therefore, the measured birefringence angle without the THz pump can be characterised as complete magnetisation of the sample. When the sample is pumped, the birefringence changes linearly with the sample magnetisation, which can then be extracted accordingly. This experimental setup was also characterised in-depth in a previous work."
2.  We thank the referee for pointing out this typo, and have corrected it.
3. The signals of figure 1.d. and 1.e. are shifted such that the maximum demagnetisation is observed at time 0. This allows for comparisons between signals from different fluences. we thank the referee for this chance to enrich the text, and have added the following sentence: "... as shown in Figure.1.d, where the time axis of the signals were shifted such that maximum demagnetisation is observed at 0, and the timescale of remagnetisation can be compared for all pulse fluences." 
4. The referee is correct. Unfortunately, the 6 THz filtered signal is not available to us. However, we have the transmission spectra of the filter (), as given by the manufacturer, and can submit it in the supplementary material.
5. We thank the referee for the query, in our view the sample responds to the excitation pump in three ways. The excitation can be reflected as an electronic response (1), this electronic response is also present in the 3d orbitals of the nickel, inducing a magnetic response (2), and lastly the remaining excitation is reflected in electron-phonon coupling (3) as described in references 8,11,12 and 13. We observe the ~ps oscillations in figure 2.d and therefore attribute these oscillations as phononic. To elucidate this in the text, we have slightly modified one of the sentences describing figure 2: "Here the oscillations represent the coherent acoustic phonons and are independently extracted in Figure.2.d, where ~ps to sub ps oscillations are observed."
6. The referee is correct, the time scales of the electronic and magnetic responses are different. In our view this is due to the electron screening of the magnetic orbitals of the system. Another way of viewing this difference in response time: the metallic bonds of the sample is formed by delocalised electrons, the electronic response of the system therefore has an electronic bandwidth of ~eV, where as the energies involved in magnetic response corresponds to the super exchange variable, with a magnitude of ~1/10 eV. Therefore, the electrons can thermalise faster than the magnetic relaxation.

Reviewer 2 Report

The manuscript "Ultrafast Electron Dynamics in Magnetic Thin Films" by Lee et. al is of interest to researchers and THz community. topic in within the scope of journal. I find this manuscript fit to be publishes as it is.

Author Response

We wish to thank the referee for taking the time to read and review our work.

Reviewer 3 Report

In the present paper the authors have investigated the mechanisms of demagnetisation in nickel thin film under femtosecond laser pulses irradiation. They employ pump probe time resolved spectroscopy with THz pump and optical probe. The main claim of the paper is that the demagnetization does not depend directly on the frequency since the THz pump pulse leads to similar effect due to optical counterpart. The heating of the sample is found as the main cause of the demagnetization process.

 

The paper is interesting for researchers working in the field of ultrafast spectroscopy and ferromagnetic thin films.

I suggest adding a few revisions before publications.

• Add more experimental details on the setup, i.e modulation details on the detection scheme.
• A more detailed explanation of the data analysis to retrieve the magnetic and electronic contribution from the time resolved analysis
• What is the origin of the oscillation of 5 ps in figure three? And how the period is related to the laser pump frequency if it is?
• The main claim of the paper is that the demagnetization does not depend on the photon energy but is just due to thermal effect. Can the author show the time resolved response of the same material to an optical probe?

Author Response

We thank the referee for reviewing our work, and lay out our replies to the comments raised point by point below:

 

1. We agree with the referee, and have added more details particularly on the modulation of the setup: "The THz is generated by optical rectification of near infrared pulses (1550 nm central frequency, 100 Hz, 50 fs, 3.5 mJ, Light Conversion OPA system) and an organic crystal DAST (Swiss Terahertz GmbH), 350 um thick, and is modulated via a mechanical chopper at 37 Hz." and "The magnetic field modulation is locked to the laser clock frequency, and its magnitude is larger than the saturation magnetic field of the sample. Therefore, the measured birefringence angle without the THz pump can be characterised as complete magnetisation of the sample. When the sample is pumped, the birefringence changes linearly with the sample magnetisation, which can then be extracted accordingly. This experimental setup was also characterised in-depth in a previous work."
2. We wish to thank the referee for this opportunity to enrich the manuscript, we have added the following to the manuscript: "If the signals under opposite magnetic biases are summed, we receive a bias-averaged total signal with features that do not depend on (the initial magnetisation state, and hence) the magnetic response of the sample. Consequently, the difference of opposite magnetic bias signals would give only the magnetic response of the material. Hence, the magnetic spin dynamics response (Figure.2.b) could be separated from the charge and lattice degrees of freedom (shown in Figure.2.c)."
3. The referee is correct, the ~ps oscillations were not addressed in the manuscript. We observe these oscillations in Figure.2.a,c and d, in our view these oscillations are due to electron-phonon coupling of the system, and we therefore attribute these as phononic oscillations. To clarify this in the text, we have added: "Here the oscillations represent the coherent acoustic phonons and are independently extracted in Figure.2.d, where ~ps to sub ps oscillations are observed."
4. Unfortunately, our experiment is based on a 800 nm probe. To address this point made by the referee, we have added to the conclusion "To further investigate this system, an optical probe can be used in the set up."

Round 2

Reviewer 1 Report

The authors explained most my comments. However, I still have trouble to understand the experimental setup about probe light. Note the distinction between the Faraday effect and the Kerr effect. Magnetic circular birefringence is the cause of Faraday rotation and Kerr ellipticity, whereas magnetic circular dichroism is the mechanism behind Kerr rotation and Faraday ellipticity. Magnetic circular dichroism: different damping of both light’s eigen-modes when a field is applied. Magnetic circular birefringence: different propagation speed of both light’s eigen-modes when a field is applied. What do  the authors measure exactly? Kerr rotation or Kerr ellipticity? 

Author Response

We thank the referee for this opportunity to further clarify.

Experimentally we measure the Kerr ellipticity of the system. However, we have added a quarter wave plate in the path of the reflected beam to convert the ellipticity into rotation. So, on our instruments, the observation is actually a rotation. The ellipticity and rotation are linearly related.