Nanogranular Strontium Ferromolybdate/Strontium Molybdate Ceramics—A Magnetic Material Possessing a Natural Core-Shell Structure

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

 

In this work, the authors reported a combination of experimental work to synthesize a nanogranular, strontium ferromolybdate/strontium molybdate core‐shell ceramics and computational work with sufficient modeling to understand their tunnel magnetoresistance behaviors. The work presented in the manuscript is of good quality. I would like to recommend it for publication after the authors addressed the following comments:

1) I recommend adding subsections and separate the synthesis and equation derivations into different subsections in the "Materials and Methods" section. Currently the materials synthesis methods and the modeling equations derivations are written together. The equation derivations are important for understanding the results, but the current format made this part looks too redundant. I would recommend writing a summarized modeling method for the "Materials and Methods" and write a separate section to discuss more details about the equation derivations later.

2) Since the authors have synthesized the SFMO/SMO core-shell ceramics, why do the authors use experimental data from cited work [11] for their modeling instead of obtaining data from their own synthesized materials for Figure 1 and 4. It seems like that only the experimental data in Figure 2 was measured on the materials synthesized in this work. How much difference are the TMR values measured on materials obtained in this work compared with the previous published values?

3) Could the authors provide any materials characterization data to prove that the structure, size of the core-shell ceramics reported in the study are close to the cited work whose experimental data that the authors used for modeling?

4) I would recommend highlighting in the figure that whether the experimental data is from other published work or measured from the materials reported in this study

5) The assumed values for the modeling, such as "rSFMO = 5.68 kg m‐3 and μ = 4 μB f.u.‐1", could you provide more experimental values from your synthesized materials to validate the values used in the modeling?

 

6) Could you provide more information on the Wheatstone bridge configuration in the results and discussion section before mentioning it directly at the end of the conclusions?

Comments on the Quality of English Language

 

The English is in general of high quality. However, I noticed a few sentences that are not in English grammar. Please rephrase them for better readability.

Line 39, " Under specific synthesis conditions, SFMO ceramics consist of SrMoO4 (SMO) intergrain energy barriers between conductive bulks of SFMO grains induced by a small oxygen excess during material fabrication [4]"

Line 87, " with e and e0 the dielectric and vacuum permittivities, respectively."

Author Response

1) I recommend adding subsections and separate the synthesis and equation derivations into different subsections in the "Materials and Methods" section. Currently the materials synthesis methods and the modeling equations derivations are written together. The equation derivations are important for understanding the results, but the current format made this part looks too redundant. I would recommend writing a summarized modeling method for the "Materials and Methods" and write a separate section to discuss more details about the equation derivations later.

done

2) Since the authors have synthesized the SFMO/SMO core-shell ceramics, why do the authors use experimental data from cited work [11] for their modeling instead of obtaining data from their own synthesized materials for Figure 1 and 4. It seems like that only the experimental data in Figure 2 was measured on the materials synthesized in this work. How much difference are the TMR values measured on materials obtained in this work compared with the previous published values?

This paper analyses research on SFMO/SMO core-shell ceramics carried out over several years. With regard to previous publications considering measured material properties, the present work closes a gap by estimating magnetic field sensitivity and temperature coefficient of magnetoresistance which are both relevant for future innovative device application. For validation of the derived model, we calculated the tunnelmagnetoresistance and compared it to experimental results of already published data of our material synthesized by the same procedure. By revising the paper, we took care that all used values were derived even from our material of the same synthesis procedure. 
Our previous publications Ref. 9, 15, 16, 35 and 52 are related to the same material. This work simply make use of unpublished previously data. As a result, all used for modeling data is obtained from our own synthesized material.

3) Could the authors provide any materials characterization data to prove that the structure, size of the core-shell ceramics reported in the study are close to the cited work whose experimental data that the authors used for modeling?

done

4) I would recommend highlighting in the figure that whether the experimental data is from other published work or measured from the materials reported in this study.

Figures 1-3 are newly added results of sample characterization. In figure 4 we give a reference to TMR data of material of the same synthesis procedure previously published in other form in Ref. 15. Figure 5 was derived by a new analysis of our measured data. Figure 6 and 7 are new calculation results.

5) The assumed values for the modeling, such as "rSFMO = 5.68 kg m‐3 and μ = 4 μB f.u.‐1", could you provide more experimental values from your synthesized materials to validate the values used in the modeling?

A theoretical density is now calculated from the unit cell volume of our material while the saturation magnetization was taken from our previous experimental data. Based on these values, also the b₂ values in table 1 were recalculated.

6) Could you provide more information on the Wheatstone bridge configuration in the results and discussion section before mentioning it directly at the end of the conclusions?

We have not made measurements using a Wheatstone bridge. Here, we intended to make an outlook for future work. The Wheatstone bridge configuration is described in many textbook on measurement technology. A short operational principle is added to the manuscript text

Reviewer 2 Report

Comments and Suggestions for Authors

In this work, the authors prepared strontium ferro-molybdate/strontium molybdate core-shell ceramics and investigated their properties including tunnel magnetoresistance, magnetic field sensitivity and temperature coefficient of the tunnel magnetoresistance. However, this work is not innovative enough to be published in this journal.

1.     How do the authors demonstrate that the synthesized ceramics have a core-shell structure? Further material characterization should have been performed, such as XRD, SEM and TEM.

2.     The introduction is weak. The author should give an adequate review of the last achievements in the research area with adequate references. It is recommended to mention novelty (originality) of the work at the end of the introduction. 

3.     The conclusion should be concise and only summarize the most important contribution of the research.

Author Response

1.     How do the authors demonstrate that the synthesized ceramics have a core-shell structure? Further material characterization should have been performed, such as XRD, SEM and TEM.

XRD and SEM were now added.

2.     The introduction is weak. The author should give an adequate review of the last achievements in the research area with adequate references. It is recommended to mention novelty (originality) of the work at the end of the introduction.

All research on tunnel magnetoresistance of granular thin films is devoted to ferromagnetic granules in an insulating matrix. For ferromagnetic metals, the pioneering references ref.1 and ref.2 were cited. We have now added adequate references for ceramic manganite granules (exhibiting a colossal magnetoresistance) to the already reviewed research on SFMO granules, that is the material considered in this work. Ref. 3 is related to the colossal magnetoresistance in manganites, while Ref. 4 is a pioneering work on spin polarized tunneling in manganites. Later work does not report sufficient achievements. Also, we have emphasized the novelty of this work at the end of the introduction.

3.     The conclusion should be concise and only summarize the most important contribution of the research.

Our conclusions are devoted to the implementation of synthesis of SFMO/SMO core-shell ceramics, the appropriate for such structures modeling of magnetoresistance and the resulting from the model application aspects. Correspondingly, conclusions were revised.

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

1.     It is recommended that the authors modify the title, as it is not appropriate to use "Core-shell" twice.

2.     In the Materials characterization section, the author says “FullProf software by the Rietveld refinement on the base of X-ray diffraction (XRD) data”, did the author do the XRD refinement? Why is nothing reflected in the manuscript?

3.     The standard PDF cards should be added to Figure 1. What’s the crystal structure of Sr₂FeMoO₆ and SrMoO₄?

4.     The experimental conditions should not appear too much on the figure captions, such as Figures 1 and 2.

5.     Figure 2, the authors should do the EDS analysis for the core and shell separately to clearly see the difference. Also, it is recommended that the authors label the core and shell in the SEM image.

6.     The Conclusion section needs to be rewritten. There are a lot of inappropriate expressions here. For example, the author says “After that the powder was pressed into tablets with a diameter of 10 mm and a thickness of 3 mm, under a pressure of 4 GPa at 530 °C for 1 min. Dielectric SMO shells were formed in these samples on the surface of the SFMO granules by annealing at 530 °C in an Ar flow with a rate of 11 sccm for 5 h.”, which is not conclusion. The authors should write the conclusion as one paragraph, clearly stating the effects of the core-shell structural ceramics prepared in this work and the physical mechanisms behind them.

7.     Authors should double-check their manuscripts for grammar, spelling, and format. For example, the formatting of figure and table captions should be consistent.

Comments on the Quality of English Language

Extensive editing of English language required.

Author Response

1.     It is recommended that the authors modify the title, as it is not appropriate to use "Core-shell" twice.

now revised: Nanogranular strontium ferromolybdate/strontium molybdate ceramics - a magnetic material possessing a natural core-shell structure

2.     In the Materials characterization section, the author says “FullProf software by the Rietveld refinement on the base of X-ray diffraction (XRD) data”, did the author do the XRD refinement? Why is nothing reflected in the manuscript?

Rietveld refinement was made by Powder Cell in order to determine the site occupation factor and thus the B-site antisite disorder, a sufficient parameter of SFMO ceramics quality because of the saturation magnetization is directly related to antisite disorder. However, since we have a two phase core-shell structure, we had doubt about the proper interpretation of antisite disorder in such materials. Thus, it was not reflected in the manuscript. Now, we have canceled all corresponding text in the revised version.

3.     The standard PDF cards should be added to Figure 1. What’s the crystal structure of Sr2FeMoO6 and SrMoO4?

PDF cards are now given. In the manuscript the following text was added:
X-ray diffraction analysis revealed the presence of reflections of the SFMO (ICSD 99933) and the SMO (ICSD 28025)  phase (Figure 1). At room temperature, SFMO has a structure with tetragonal symmetry and space group I4/mmm, and the lattice constants and cell volume are respectively equal to a=b=5.57(1) Å, c=7.89(2) Å, V = 244.93(6) ų. SMO has a structure with tetragonal symmetry with space group I4₁/a, with lattice and cell volume constants a=b=5.39(4) Å, c= 12.01(7) Å, V=349.63(7) ų.

4.     The experimental conditions should not appear too much on the figure captions, such as Figures 1 and 2.

Experimental conditions in the figure captions were now removed.

5.     Figure 2, the authors should do the EDS analysis for the core and shell separately to clearly see the difference. Also, it is recommended that the authors label the core and shell in the SEM image.

Figure 2 now shows EDS patter of the core and the shell area separately. Core and shell regions are clearly separated in the SEM image by the grey contrast caused by different composition. The difference is described in the manuscript text:
The EDX pattern of point 1 (core) corresponds to the one of a SFMO reference sample while the EDX pattern of point 2 (shell) corresponds nearly to the one of SMO. Some Fe is visible also at point 2 since the size of the excited by the electron beam region exceeds the small thickness of the shell.

6.     The Conclusion section needs to be rewritten. There are a lot of inappropriate expressions here. For example, the author says “After that the powder was pressed into tablets with a diameter of 10 mm and a thickness of 3 mm, under a pressure of 4 GPa at 530 °C for 1 min. Dielectric SMO shells were formed in these samples on the surface of the SFMO granules by annealing at 530 °C in an Ar flow with a rate of 11 sccm for 5 h.”, which is not conclusion. The authors should write the conclusion as one paragraph, clearly stating the effects of the core-shell structural ceramics prepared in this work and the physical mechanisms behind them.

Inappropriate details of SMO/SFMO structure fabrication were now deleted from conclusions. Our conclusions focus on the practical application of the fabricated material. The physical mechanisms behind the obtained effects are disclosed in the model. For instance superparamagnetic granules possess a different to ferrimagnetic granules behaviour which is the origin of the relatively high and constant at low magnetic fluxes sensitivity. For the first time, statements on the temperature coefficient of such core-shell structure were made.

7.     Authors should double-check their manuscripts for grammar, spelling, and format. For example, the formatting of figure and table captions should be consistent.

Gammar was now checked by a native speaker.
Spelling was automatically made by WORD.
The manuscript was formatted following the MDPI template. Now, the correspondence to the template was checked again.

Round 3

Reviewer 2 Report

Comments and Suggestions for Authors

1.     The author should add the PDF cards for both phases to the Figure 1.

2. The crystal structure diagram for both phases still needs to be added and combined with Figure 1.

3.     The conclusion should be a single paragraph.

4.     Authors should check all references carefully. There are too many references in the manuscript that are inconsistent, such as journal abbreviations, punctuation, and so on. 

Author Response

1.     The author should add the PDF cards for both phases to the Figure 1.

Done, but now there is some redundancy to the manuscript text.

2. The crystal structure diagram for both phases still needs to be added and combined with Figure 1.

A "crystal structure diagram" is not a well defined and not generally accepted nomenclature (cf. IUPAC recommendations, encyclopedia Britannica). In a number of publications, it is used to describe the bulk arrangement of atoms of the crystal in three-dimensional space. In this meaning, the nomenclature "crystal structure diagram" is used in Russian scientific literature. However, the crystal structure of the separate phases SFMO and SMO is little instructive for the reader since it can be found in many textbooks and  publications. Moreover, the crystal structure is easily built by means of the ICSD-files given in the paper using, for instance, the computer code VESTA (https://doi.org/10.1107/S0021889811038970). Of scientific interest is the matching of the crystal structures of both phases at the core-shell interface. Although, this requires sophisticated and expensive research beyond the scope of this paper. Current data is not sufficient to curry out this task. On the other hand, too much information takes the reader away from the main ideas that we, the authors, would like to convey to him.

3.     The conclusion should be a single paragraph.

We have not found such a requirement in the MDPI instructions for authors. This section is not mandatory but can be added to the manuscript if the discussion is too complex. We have now revised the conclusions making two paragraphs. Following the comments of the referee in the first round, we have summarized our most important contribution of the research in an application-related manner. All outcome concerning magnetoresistance were included in the first paragraph. The results concerning temperature coefficient were compiled in a separate second paragraph due to their outstanding novelty since this topic for such structures was not previously considered by other researchers in literature. Moreover, the large temperature coefficient is a large drawback. Thus, the first paragraph describes the application peculiarities and the second paragraph focuses on the main drawback.

4.     Authors should check all references carefully. There are too many references in the manuscript that are inconsistent, such as journal abbreviations, punctuation, and so on.

Journal abbreviation are due to CAS Source Index with the exception of Phys. Rev. B which was adapted to MDPI style.
Punctuation was now checked again. Book pages are given using MDPI reference style in the text.