Peer review of: Deterministic all-optical magnetization writing facilitated by non-local transfer of spin angular mo (2020)

Reviewers' Comments: 
 
Reviewer #1: 
Remarks to the Author: 

The manuscript by van Hees et al. aims at demonstrating deterministic all-optical magnetic 
switching for a material, Co/Gd bilayers, where previously mainly so-called toggle switching 
(magnetization reversal for each fs laser pulse) was observed. They achieve this by combining 
toggle switching with spin currents induced by demagnetizing an adjacent CoNi multilayer. The 
results are new and provide interesting insight into the interplay of the two mentioned effects, e.g. 
by varying the spin current strength. The proposed pulse sequences also provide an appealing 
outlook for applications. I would therefore support publication in Nature Communications after 
several inconsistencies are removed: 
1. It is mentioned several times in the manuscript that all-optical switching can be “defined by the 
helicity of many incoming laser pulses, or toggled by a single pulse” (abstract). The former is not 
generally true, see PRL 103, 117201 (2009). This reference also contradicts the authors’ 
subsequent sentence in the abstract. 
2. I find it confusing that the authors call their method ‘single pulse deterministic magnetization 
writing’ (p. 3 last paragraph) but then admit that up to 2 pulses are needed for writing. Surely 
they can find a better description. 
3. A similar experiment (ref. [20]) generating different/complementary results is briefly mentioned 
twice. Since the present paper can be considered a follow up experiment a thorough discussion of 
the present results in light of ref. [20] is required. For instance, why does in one case the 
reference layer switch and in the other case it does not? What are the relative strengths of spin 
currents from and to the free layer? Does the nature of the reference layer or the free layer, e.g. 
alloy vs. bilayer etc., play a role? 
4. In Fig. 3a the greyscales for AP+ and P+ states seem to be somewhat off. At least the legend 
does not match the figure. In this respect: How do the authors diffierentiate their proposed 
switching from that in ref. [20]? How can they differentiate in the microscopy images switching P+ 
to AP+ from, say, P+ to AP-? I understand MOKE hystersis loops were measured in a different 
setup. What is the accuracy of doing this in the microscope? A figure or better description would 
help. 
 
 
 
Reviewer #2: 
Remarks to the Author: 
Toggle switching of GdFeCo alloys by single laser pulses has attracted a lot of interest lately since 
it bears the potential to speed up applications based on magnetic switching orders of magnitude 
with respect to traditional switching of magnetic devices. Previous works have demonstrated 
Gd/Co bilayers to also show the same toogle switching. 
 
For magnetic recording purposes toogle switching is inconvenient and one would prefer a scheme 
where at each instance one could decide to which direction the polarity should switch. 
 
The authors address this question by proposing a scheme based on adding a ferromagnetic 
reference layer to Gd/Co bilayers. The magnetic state of the reference breaks the symmetry of the 
total system under switching of Gd/Co magnetic polarity. 
 
The authors conduct the necessary experiments to demonstrate their idea. They first demonstrate 
that there is a laser fluence window where deterministic switching is possible. Second, they use 
the fact that toogle switching and deterministic switching need of a different laser fluence to 
propose a protocol for magnetic recording. When two consecutive pulses with laser fluence 
necessary for i) deterministic switching and ii) toogle switching are applied, a well localized region 
of the sample can be switched at will. This protocol and its demonstration in new. 
Perhaps, the main shortcoming of this protocol is the necessity to use two pulses, which have to 
be separated by some time delay, probably around nanoseconds, to recover the equilibrium initial 
state. This would slow down the magnetic writing effectively to the nanosecond time scale. The 
authors might consider discussing the possibility to speed up the propose protocol by applying two 
shortly-delayed pulses to their structure. The following works on the effect of applying two 
consecutive shortly-delayed laser pulses could be of interes: 
 
* 'Interplay of heating and helicity in all-optical magnetization switching 
Sabine Alebrand, Alexander Hassdenteufel, Daniel Steil, Mirko Cinchetti, and Martin Aeschlimann, 
Phys. Rev. B 85, 092401 (2012) 
 
* Ultrafast double magnetization switching in GdFeCo with two picosecond-delayed femtosecond 
pump pulses 
Appl. Phys. Lett. 113, 062402 (2018) 
 
The physics behind this symmetry breaking is related to the supposed spin currents coming from 
the reference layer and acting somehow on the Gd/Co magnetization of the free layer during the 
switching process. These physics is not demonstrated in this paper but it can be inferred that this 
could be the case from the data shown in Figure 3. I believe the possibility of controlling AOS with 
ultrafast spin currents is a great idea and this work could be a stepping stone along this line. 
However, I feel that a deeper discussion is necessary about the physics of the spin currents that 
could be optically generated in the reference layer, and how it travels and it is injected into Co/Gd, 
how exactly the scattering between mobile and localized electron occurs. Would be more efficient 
to keep the reference layer magnetization transversal to the free layer magnetization to obtain the 
maximum torque from the scattering of mobile electron's spin and the local Co spins? 
 
To summarize, the work presented in the current manuscript is excellent, well-conducted, the 
manuscript is well-written, the idea is new, the protocol for magnetic recording works, and the 
physics behind this proposal seems to make sense and the authors have somehow (not 
completely) demonstrated that the non-local transfer of spin angular momentum could be the 
origin of observed behavior. I would therefore like to see it published. 
 
Unai Atxitia 
 
 
 
Reviewer #3: 
Remarks to the Author: 
The manuscript by Y. Hees et al discusses the novel possibility to use laser-induced spin current 
pulses for assisting the switching of magnetization. Laser irradiation of a sample consisting of a 
free and reference layers results in the generation of perpendicular spin current operating on the 
femtosecond timescale. The authors analyze various regimes of the spin current-assisted switching 
and conclude that depending on the mutual alignment of the magnetizations in the free and 
reference layers, spin current pulse can facilitate or hinder switching of magnetization in the free 
layer. Based on these findings, they discuss switching protocols allowing deterministic 
magnetization switching. The manuscript is well organized and clearly written. 
 
The experimental results presented in the manuscript are very interesting and definitely are worth 
publishing in one way or another. However, in my view the manuscript has a very serious leaning 
towards applied research and does little to address fundamental questions pertinent to 
magnetization switching. For instance, in the conclusions section the only point relevant for the 
fundamental issues is “the system described here […] can provide a versatile method to further 
enhance the general understanding of optically generated spin currents”. This is, in my opinion, 
not enough to justify the publication just yet. The manuscript could be significantly improved by 
further analysis of the experimental results, in particular, in what concerns the optical generation 
of spin current pulses in such complex multilayer systems as the one used in this work. Should the 
authors imply that deterministic switching of magnetization can be used for probing spin 
polarization of laser-induced current pulses, this could be discussed more closely. 
 
Furthermore, the optical generation of spin current in the Co/Ni multilayer deserves further 
attention. The manuscript would strongly benefit from an analysis of this process, especially 
including the rather complex excitation scheme (Fig. 1). Here, one would expect two competing 
spin current pulses propagating in the opposite directions. Moreover, what is the mechanism of the 
spin current generation? If it is similar to the one discussed in Ref. 25, how does the increased 
number of interfaces influence the spin polarization degree? How well-defined are the interfaces 
and if there is “additional alloying during growth” (page 11), what is its role with regards to the 
spin scattering at interfaces? What is the characteristic depth from which the spin current is 
effectively generated? All these are important questions which should be discussed in the 
manuscript.