Synthesis and Structural and Magnetic Properties of Polycrystalline GaMo4Se8
Round 1
Reviewer 1 Report
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Comments for author File: 
Comments.pdf
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Author Response
We appreciate the referee’s positive appreciation of our manuscript and the useful comments. As suggested, we have performed an EDS analysis in a few crystalline grains to confirm the sample composition. The average composition is indeed close to that expected for GaMo4Se8. We also improved the picture of the XRD diffractogram and the quality of figures 6 and 8 as suggested.
Author Response File: Author Response.pdf
Reviewer 2 Report
The manuscript is devoted to the conduction “a thorough study of the GaMo4Se8 compound covering a novel synthetic route and structural and magnetic characterisation, to investigate its magnetic properties that put in evidence the existence of a skyrmionic magnetic phase”.
The authors have reached interesting results for obtained by themselves the polycrystalline GaMo4Se8 synthesized in two-steps method.
They proved by means of XRD data that there are no extraneous phases in GaMo4Se8 sample.
They investigated the grain size distribution and the morphology of the obtained GaMo4Se8 sample by SEM and found that the grain sizes are the range 300 nm - 1mm.
They performed the thermomagnetic M(T) measurements on a cryogen free Dynacool Quantum Design 9T PPMS system. The skyrmionic magnetic phase found in these type of compounds occurs close to special temperature TC, And this phase as stated by authors is stabilised under the action of an applied magnetic field.
An existence of skyrmionic magnetic phase is justified by two ways. First, from the Figure 5 with M(H) curves covering the temperature range [2, 28] K and following from it the figure 5b curves. for derivative off magnetization dM(H)/dH . Second. From study of AC susceptibility measurements presented on the Figure 7. From both ways the authors get to conclusion by means of respective figures 6, 8 that the skyrmionic magnetic phase exists within the range [2, 28] K.
However, the paper contains some place to become more clear and exact.
E.g.,
1) On the p. 2, l.37 the authors have written on “DM interaction” whereas its explanation appears only below (l. 44).
The paper contains new interesting within result and the authors shown shown that the skyrmionic magnetic phase is stabilised an applied magnetic field in a large temperature range, from 2 K up to TC, in a polycrystalline GaMo4Se8 sample.
I think the manuscript maybe accepted for publication in the Magnetochemistry with small correction according to the comments above.
Comments for author File: Comments.pdf
The English may be improved, especially in the Introduction.
Author Response
We appreciate your comments. This issue was corrected in the paper.
Author Response File: Author Response.pdf
Reviewer 3 Report
In this paper, José F. Malta et al. presents the study of polycrystalline GaMo4Se8 based on the synthesis, structural, and magnetic analysis. However, I couldn’t find any interesting results with this manuscript. There are many issues with this manuscript.
- The quality of the figures is not very good.
- The manuscript data are not well presented and described.
- Magnetic phase diagram of Polycrystalline GaMo4Se8 looks very similar to that of GaMo4S8 as that reported by the authors in Reference [33].
Hence, I would not recommend to publish this manuscript.
Comments for author File: Comments.pdf
Author Response
We appreciate your comments. It is a fact that GaMo4S8 and GaMo4Se8 have similar magnetic phase diagrams below Tc. However, they are different compounds, with different chalcogenide elements. In contrast to GaMo4Se8, skyrmions are only found at high magnetic fields (> 1T) in GaMo4S8, while in GaMo4Se8skyrmions are stabilised at lower magnetic fields. Moreover, the intermediate spin-glass phase observed in GaMo4S8 is absent in GaMo4Se8. These interesting features already clearly justify, in our opinion, the publication of this study.
In addition to the detailed structural and magnetic data provided in our manuscript, we emphasise that our sample of GaMo4Se8 was prepared by an alternative method than the typical one reported in the literature. Thus, we believe that also because of this our work is of interest to the community working on the hot topic of magnetic skyrmionics compounds. Following the Reviewer suggestions, the overall quality of our manuscript, as well as the quality of the figures, was strongly improved.
Author Response File: Author Response.pdf
Reviewer 4 Report
The authors performed DC (VSM) and AC magnetometry to investigate the magnetic phase diagram to distinguish the presence of cycloidal, skyrmionic, and ferromagnetic phases in the polycrystalline GaMo4Se8. The results obtained are reasonable for consideration for publication in this journal. However, there are several issues that must be clarified before the manuscript can be given further consideration for publication.
1) Introduction is well written, however, for the benefit of the general audience, adding some more recent work on similar compounds will be useful.
2) One of the main points of the present manuscript is the synthesis and structural properties therefore It’s a good idea to show fitting parameters (R factors and Chi2 etc) inside fitted XRD spectra in Fig2. Further, I am not able to see the different colors in this figure as looks black and white to me. Please modify this accordingly otherwise it's difficult for the reader.
3) Also, provide a table with all the crystallographic information, for example, space group information, site occupancies, lattice parameters etc.
4) Authors have shown the SEM image which is not very clear, if possible change it with another one. Moreover, having the EDS data will further confirm the purity of the sample as I can see XRD fitting has a really reasonable difference curve.
5) Fig. 4 (b), only ZFC is shown but it's written ZFC and FC in the figure caption so please modify accordingly.
6) In the conclusion, “In this work we have accomplished the synthesis of high purity GaMo4Se8 …..”, for this statement, one has to show a complete XRD fitted pattern, a table, and EDS data (it is basically repeated from me).
7) In general, all the figures should be modified as the font size is smaller. it can be improved.
There are some typos and errors in the manuscript that must be carefully checked. Authors can further improve the quality of English in the manuscript.
Author Response
1) We appreciate your comments. Most of the reported studies of skyrmions in lacunar spinels are very recent. In the case of GaMo4Se8, the first study was published in 2020 and there are only two papers that put in evidence the skyrmion phase (K. Routlege et al., Chem. Mater. 2021, 33, 14, 5718–5729 and Schuller et al. Phys. Rev. Materials 4, 064402). Skyrmions were only found in GaMo4Se8 in 2022 (A Butykai, et al.,npj 287 Quantum Materials 2022, 7, 26) and our previous work (J. Malta et al, J. Mater. Chem. C, 2022,10, 12043-12053) prove the evidence of such phase with the addition of a cluster spin glass close TC. References to these works are provided in our manuscript.
2) The figure was improved according to your suggestions.
3) The table was provided in the manuscript with the crystallographic information obtained from the Rietveld refinement.
4) We have improved the SEM image, and also performed the EDS analysis to confirm the sample stoichiometry. The obtained spectra confirmed that the sample composition is close to that expected for GaMo4Se8.
5) The figure caption was modified.
6) We have performed the EDS analysis and we have completed the XRD fitted pattern. Both prove the purity of the GaMo4Se8 sample.
7) We have improved the figures according to your suggestions.
Author Response File: Author Response.pdf
Reviewer 5 Report
The authors reported polycrystalline GaMo4Se8 synthesized by two-step solid state reaction approach, which has been proposed to obtain other lacunar spinels like GaMo4S8. The details in the discussion of the thermomagnetic analysis are informative in this manuscript, and the most valuable accomplishment of this work is the innovated two-step synthetic route of manufacturing high purity lacunar spinel compound. While the paper is quite clear, it could be further improved by the following suggestions and discussions.
1) In this manuscript, authors synthesized GaMo4Se8 with a two-step solid state reaction. In most of literature, to prepare high purity GaM4X8 family samples, quenched or rapid cooling process are used, which is different from this paper slowly cooling down method. I wonder why authors select this cooling process.
And authors should state the benefit (like prevent any impurity phases or enhanced densification...) of using a two-steps method rather than conventional single time reaction. Also I am curious if that is the reason the prepared samples have high impurity as authors stated in the XRD analysis section.
2) From the SEM image, the prepared samples have a large size distribution, some of small grains are even smaller than 200 um. Is there any additional technique was applied to prepare powder samples (like ball mill crush or grinding?) after second sintering reaction. I am curious the reason of this nonuniformity and if there is any phase/chemical difference between those small particles and large grains.
3) For the AC susceptibility measurements, what the frequency of the ACMS II were used in this experiment. And I wonder if the phase diagram is dependent with any frequency variation since I noticed that phase evolutions are slightly different in the low field (H) skyrmion/cycloidal interface region if comparing AC and DC phase diagram although temperature transition is similar.
4) Another thing I am interested is, the prepared sample in this manuscript is polycrystalline, I wonder if authors could prepare single crystal based on two-step reactions by tunneling the dwelling time, reaction temperature, or ramping up/down rate. If could prepare pure single crystal samples by similar method, it would be helpful to determine the crystalline effect of skyrmion on GaM4B8 materials.
5) I suggest authors should add more potential applications introduction of lacunar spinels (AB4X8) to the first part of manuscript like resistive based memories, skyrmions vertex spintronic devices, superconductive related applications.
Author Response
1) We appreciate your comments. We already performed such cooling process, with good results, for the synthesis of GaMo4S8 (J. Malta et al, J. Mater. Chem. C, 2022,10, 12043-12053. GaMo4S8 was obtained by reacting Ga and MoS2. The final product did not present any extraneous phases. We implemented a similar synthetic route for GaMo4Se8, by reacting Ga with MoSe2. The benefit of this synthesis relies on avoiding the flux method described in the literature, which mandates the use of molybdenum tubes under higher temperatures (1500 oC). Moreover, the quenching process that is needed to prevent the formation of extraneous phases when the synthesis is performed by the conventional flux method, starting from the elements, is no longer needed (see below)
The using of the MoSe2 precursor avoid the formation of Mo3Ga. It was proved and stated in our previous work (J. Malta et al, J. Mater. Chem. C, 2022,10, 12043-12053) that high purity GaMo4S8 can be obtained by reacting Ga and MoS2, and using a slow cooling process (no need for quenching). The reason is that by reacting Ga with the MoSe2 precursor, we avoid the formation of Mo3Ga contaminant. This impurity tends to be formed when we react directly stoichiometric amounts of Ga, Mo and Se, and in this case, it is mandatory to perform a quenching to avoid this problem. This explanation was added to the paper.
2) The sample represented in SEM image was indeed ground and we have changed the SEM image for one of a non-ground sample.
3) For VSM measurements we have used powder packaged in a Teflon sample holder, while for AC susceptibility we have used a small pellet glued in a quartz tube, and an AC frequency of 1 KHz. There may be some frequency dependence of the magnetic phase boundaries, but we have not performed a detailed study of the AC susceptibility as function of frequency.
4) The grains obtained through such method are not large enough to be considered “single crystals”, in the sense of being useful for e.g. measurements of magnetic/electric transport properties as function of crystallographic direction. Nevertheless, we can prepare single crystals of GaM4B8 through a chemical vapour transport reaction, by using the polycrystalline powder and a transport agent (iodine, p.e.). Tweaking the dwelling time and the ramping rate in our method may not be enough to allow for single crystal growth, but indeed it would be certainly be worthwhile trying it using a much slower cooling rate. Thank you for the suggestion.
5) We appreciate your suggestion. In fact, lacunar spinels have more potential applications. Our introduction is more focused on the lacunar spinels from the GaM4X8 family (M = V, Mo; X = S, Se). Of course, other properties such as superconductivity can be found in lacunar spinels, namely in GaTa4Se8 under pressure.
Author Response File: Author Response.pdf
Round 2
Reviewer 3 Report
The authors have improved the whole manuscript. Now, the manuscript is in a good shape and can be accepted.
