Peer review of: Resonant thermal energy transfer to magnons in a ferromagnetic nanolayer (2020)

Reviewers' comments: 

Reviewer #1 (Remarks to the Author): 

This manuscript describes an interesting concept and experiment on resonantly harvesting thermal 
energies (temperature oscillations) by using the magnon mode excitations. Experimental verification 
for the magnon excitation is demonstrated via the time-resolved Kerr effect. Data interpretation is 
given by using the calculated temperature variation in the context of the micromagnetic LLG theorem. 
The result is overall interesting and may present a broad impact on the field of ultrafast dynamics and 
spintronics. The work may be ultimately considered publishing in Nature Communications. However, 
there are some unconvincing aspects related to the experiment. The theoretical description is also 
quite preliminary. These aspects should be improved before the manuscript can be accepted. 

My comments to the authors are: 

(1) The Temperature calibration is rather unsatisfactory. It is basically estimated from a calculation 
without any additional validation. As far as I understand, this experiment was performed at room 
temperature. Was the experiment performed in a vacuum chamber or just ambient? The authors claim 
a global temperature rise of ~10 – 42 K and an oscillation of ~1K. This sounds rather challenging to 
control and also to be able to excite coherent magnons. The authors should discuss the stability and 
temperature background. The values for the temperature estimates should also have errorbars with 
them. 

(2) As far as I know, the experiment is performed at a single frequency, making it less convincing. 
What technical limitation prevents the author to try other frequencies? If not, I suggest the authors 
scan a small range of frequency for their laser heating drive (and subsequently detect Kerr dynamics 
and then FFT). This is probably not critical for the thermal part, but will be very helpful in 
understanding the magnon part. To validate that the spin dynamics are coherent, a dispersion (e.g. 
Kittel) relation should be obtained, including discussion on the linear and nonlinear regimes based on 
the power dependence. 

(3) The authors claim that their technique can work in the GHz and even sub-THz regime. They should 
provide a more clearly bandwidth with good justification. This will involve the discussions of their 
setup, materials systems, and other potential limiting factors against extending the bandwidth both 
ends (lower GHz or upper THz). 

(4) It seems that the underlying mechanism for such temperature-driven spin dynamics lies in the 
strong modulation effect of the magnetic anisotropy, which behaves like an effective field. The authors 
should add quantitative discussions on the magnitude and effectiveness of such modulation, using 
their material therein. In addition, is FeGa alloy chosen for some unique material concerns? Can other 
ferromagnets work as good as FeGa? If not, why. 

(5) In addition to the temperature-induced anisotropy modulation, thermal gradient torques (via spin-
Seebeck effect for example), if excited, may also contribute to the magnon excitation. This is a 
different mechanism where true spin torques are actually generated owing to the spin-orbit coupling 
(of FeGa, for example). There seem no relevant discussions on ruling out such in the present 
manuscript. 


Reviewer #2 (Remarks to the Author): 

This paper experimentally studied the magnon generation using pulsed laser heating on a 
Fe0.81Ga0.19 film grown on a GaAs substrate. The authors used a pump-probe method, where a 
periodic laser pulse is used to heat up the sample at a frequency of 10 GHz and used another laser at 
1 GHz to probe the Kerr rotation single to infer the magnon information. They then used a 
phenomenological to link the magnetization precession amplitude to temperature variation excited by 
the pump laser. The paper is written in a way that is very difficult to understand and may appeal only 
domain experts instead of broader audience, which Nat. Comm. emphasizes. This is my feeling, but I 
am not using it as a criterion to judge the quality of the paper. It is the job of the editor. 

Technically, I think the results are very solid and it is an interesting observation, but the physics is not 
clearly elucidated. Below are my major comments: 

1. Most of the results show tuning the B field can lead to peaks in the Kerr rotation signal. If the B 
field is with a value that is off-resonance, there is little signal, but if it is on-resonance, there are 
peaks. I am almost lost on why you need a pulsed heating source at a specific frequency. What if you 
match your frequency to 20 GHz, which seems to have the largest amplitude in Fig. 1a? I actually only 
see a peak at 20 GHz there. To criticize lightly, the paper does not explain the rationales well. 
2. I get that the results show heating can lead to larger magnetization precession amplitude, which 
presumably relates to the population of magnons. But how did this happen in a mechanistic level? 
Which phonon modes couple to which magnon modes? If electron is playing a rule here? Seems that 
there is phonon-electron coupling in the picture as well. The phenomenological observation of heating 
lead to magnetization enhancement has been studied extensively. In my opinion, the use of a pulsed 
light is just another way of showing the same phenomena. 
3. Magnon has a dispersion, meaning there is a spectrum of frequency with different wavevectors. 
How is this playing a role in the picture? 
4. The energy conversion efficiency is extremely low. It is fine, but the authors started the paper with 
discussing energy conversion. 



Reviewer #3 (Remarks to the Author): 

This manuscript develops the idea of thermal energy harvesting by channeling it into magnon 
excitations in ferromagnetic metal thin films. The idea is novel and the paper deserves to get proper 
exposure through publication in Nature Communications. However, improvements are needed before it 
becomes suitable for publication. A major one is a discussion of (quantum) efficiencies in channeling 
resonant energy through modulated optical heat injection as compared to microwaves, a method that 
that been well known for decades. In other words, under what conditions can the authors imagine 
resonant optical heating having advantages and possibly becoming a good candidate for device 
applications? 

A clarification is needed with regard to the feasibility of exciting magnon resonances which are based 
on the Lifshitz-Gilbert equation (1) so that the resonance condition can be attained. Typically fs lasers 
come with fixed rep rates so the generated temperature field can match the natural magnon 
frequencies only under some tenability conditions. This should be made clear in the text. As a follow-
up question, does thermal field resonant attainment have to excite magnon resonances only at the 
peak, or a sufficient energy transfer can occur at off-peak magnetic field values? 
The occluded temperature field of Eq. (2) involves a stationary background DT. This will grow with 
successive pulsing of the laser as more thermal energy is pumped into the system, including the 
ferromagnetic thin film. The authors cite 42K background heating which sounds quite high. What 
(counter-efficiency) role does the growing background play in the operation of the magnon resonance 
system and how would performance improve with efficiency of removal of that heat? It was already 
mentioned that increasing DT shifts the resonance Bn field (and no doubt broadens the peak due to 
increased random magnetic motion). Are there interface conditions between thin film and surrounding 
space to be taken into account for optimization? Also, following resonant magnon motion (i.e. 
absorption of the thermal energy by the magnon system resulting in resonant motion), does there 
follow further conversion to heat, likely at the surfaces of the thin layer where collective magnon 
motion may be degraded due to their spatial discontinuity? This is an issue of adiabaticity, I reckon. 
The authors anticipate this energy transfer concept “to generate magnons in ferromagnetic layers 
deposited on processors and other microchips operating at GHz and sub-THz clock frequencies.” What 
lossy mechanisms can they propose that might compromise the efficiency of resonant amplitude 
through further thermal dissipation? Wouldn’t additional dissipation terms have to be added to Eq. (1) 
to predict the inevitable further heat diffusion losses? 

Minor issues: In Fig. 3b the 15-fold increase of AB is claimed against the baseline, horizontal dashed 
line, which, however, looks artificially drawn and not based on data. It can be noted that small 
variations in the exact position of the baseline can change the 15-fold figure drastically: for example, 
a slight rigid upward translation to meet the tail end of the peaked distribution would reduce that 
figure anywhere down to between 8-fold (left boundary) and 4-fold (right boundary). 
On line 110 does KR signal stand for Kerr? On line 136, t0 should be changed to τ0. Please update.