Microstructure and Magnetism of Heavily Helium-Ion Irradiated Epitaxial Iron Films
, Daiki Umeyama 1, Tomoki Oyake 1, Takeshi Murakami 1, Kazuyuki Shimizu 1, Satomi Fujisaki 1, Noriyuki Yoshimoto 1, Kazuhito Ohsawa 2 and Hideo Watanabe 2Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsThe effect of irradiation with 30 keV helium ions of 200 nm thick iron films epitaxially grown on MgO (001) were studied. Having carefully studied the structure and properties of the films (X-ray diffraction measurements and cross-sectional transmission electron microscope observations, magnetization curves) before and after irradiation, the authors observed microstructural changes, formation of many pores after irradiation. However, the shape of the magnetization curve remained unchanged. The authors discuss this by involving first-principles calculations. The paper is of interest because the issue of the effect of ionizing radiation on iron coatings, is important for practice. It is thoroughly done with careful application of advanced experimental approaches. The technology and observations are clearly stated. However, the interpretation of the results raises several issues that should be addressed before final acceptance of the article.
1. The authors assume that there is significant lattice expansion due to helium intercalation as a result of irradiation and claim that this is the case. This conclusion appears to be incorrect. The major reflexes in the XRD patterns are not shifted with respect to pure BCC Fe(Fig.4b), which means that there are no impurities in the lattice. The displacement of the minor reflex in the ~65 Deg angles is most likely due to long-wavelength strain. Helium in the body of the film is most likely placed in irradiation induced nanopores.
2. One would expect that the presence of a large number of pores should significantly affect the magnetization curves. However, this is not observed. This "contradiction" would be interesting to discuss in view of the work [E. Schlomann, Properties of Magnetic Materials with a Nonuniform Saturation Magnetization. I. General Theory and Calculation of the Static Magnetization, J. Appl. Phys. 38 (1967) 5027. https://doi.org/10.1063/1.1709271.] where it is shown that nanometer pores in a material have little or no effect on their magnetic homogeneity. This is worth discussing, since in its current form, the paper seems to lack of intrigue.
Author Response
Reviewer 1
Thank you for the thorough review and helpful advice. We have made the following revisions to the manuscript.
***********************
[Question & Comment 1]
The authors assume that there is significant lattice expansion due to helium intercalation as a result of irradiation and claim that this is the case. This conclusion appears to be incorrect. The major reflexes in the XRD patterns are not shifted with respect to pure BCC Fe (Fig.4b), which means that there are no impurities in the lattice. The displacement of the minor reflex in the ~65 Deg angles is most likely due to long-wavelength strain. Helium in the body of the film is most likely placed in irradiation induced nanopores.
[Answer 1]
As you pointed out, the 002 peak shifted to a lower angle after irradiation, but the 110 peak was almost unaffected (Fig. 4). This means that the iron undergoes lattice stretching in the vertical direction of the film, but not in the in-plane direction. The lattice expansion mentioned in this study is not a hydrostatic lattice expansion. It refers to the phenomenon in which the lattice vertically expands in the film specimen due to the effects of cavity formation and substrate constraint. This non-uniform and unidirectional expansion results in an increase in the average volume occupied by a single iron atom. To avoid confusion for reader, we clarified that the term "lattice expansion" refers to the lattice expansion perpendicular to the film plane. We have made the following revisions to the manuscript.
Page 1, Line 17.
“lattice expansion perpendicular to the film plane”
Furthermore, the Fe002 peak after irradiation exhibits a significantly broadened width. As you pointed out, this indicates that the lattice stretching is not uniform. This phenomenon is also important, so we have added a mention of the change in peak width to the main text.
Page 5, Lines 232-235.
“After irradiation, the intensity of the Fe 002 peak decreased, the peak width broadened, and the peak position shifted to a lower angle (Fig. 4(a)). These indicate that the lattice spacing perpendicular to the film plane is stretched with a broad distribution.”
We believe that helium does not dissolve atomically in iron, but rather accumulates in nanoscale cavities due to energy consideration. Additionally, we consider that the asymmetric arrangement of the cavities can induce strain over long distances. This strain, combined with the effect of substrate constraint, can lead to a broad distribution of lattice spacing that is stretched perpendicular to the film plane. Consequently, in the irradiated film, the iron lattice exhibits non-uniform and unidirectional volume expansion, which differs from the lattice model discussed in the section on first-principles calculations (Fig. 7). However, as mentioned in the section, there have been no reports on the adjacent effects of helium on the magnetic moment of iron. We believe that the results of this simple calculation model are useful for understanding the influence of helium.
***********************
[Question & Comment 2]
One would expect that the presence of a large number of pores should significantly affect the magnetization curves. However, this is not observed. This "contradiction" would be interesting to discuss in view of the work [E. Schlomann, Properties of Magnetic Materials with a Nonuniform Saturation Magnetization. I. General Theory and Calculation of the Static Magnetization, J. Appl. Phys. 38 (1967) 5027. https://doi.org/10.1063/1.1709271.] where it is shown that nanometer pores in a material have little or no effect on their magnetic homogeneity. This is worth discussing, since in its current form, the paper seems to lack of intrigue.
[Answer 2]
Thank you for the helpful advice and for introducing a reference paper. As you pointed out, we are also surprised that despite the formation of cavities with a very high density, it does not affect the magnetism of iron. To address this, we have added the following sentences to the manuscript and included the referenced paper [28] that you mentioned.
Page 8, Lines 361-368.
“However, despite the fact that the current specimen contains a large number of cavities, no significant changes were observed in the saturation magnetization and the shape of the magnetization curve. The cavities can be considered as very small, nonmagnetic inclusions. There has been a study on an analytical model that considers the deviation in the direction of magnetic moments around non-magnetic inclusions and their effects on magnetic properties [28]. The results of this study have led to interesting findings. In nanometer-scale non-magnetic inclusions, the deviation of surrounding magnetic moments is small and does not significantly affect the magnetism of iron.”
***********************
In addition, in order to make it easier for readers to understand, I have made some modifications to the English expressions in certain parts without changing the content. Thank you.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
Comments and Suggestions for AuthorsThis paper describes in high quality and accuracy the influence of He+ exposed epitaxial grown iron. However, despite the quality of the presenation the prime intention namely to explore the influence of high energy radiation on nuclear reactor steels and the findings presented are somehow failing as structered materials are completely distinct from what is presented here. So far, the study fails to explain the intended aspects. Structured material can't be compared with epitaxial grown iron. The plenty of dislocations, residual stresses, phases and inner boundaries are giving a complete different picture (literature attached). So far, the conclusions can't be valid for real constructions which weakens the paper significantly as the results might completely change. The authors are encouraged to point onto in the sense of being self critical within the introduction and the conclusion in order not to mislead the reader.
Comments for author File: Comments.pdf
No comments else than in the rating.
Author Response
Reviewer 2
Thank you for the thorough review and helpful advice. We have made the following revisions to the manuscript.
***********************
[Question & Comment 1]
This paper describes in high quality and accuracy the influence of He+ exposed epitaxial grown iron. However, despite the quality of the presentation the prime intention namely to explore the influence of high energy radiation on nuclear reactor steels and the findings presented are somehow failing as structured materials are completely distinct from what is presented here. So far, the study fails to explain the intended aspects. Structured material can't be compared with epitaxial grown iron. The plenty of dislocations, residual stresses, phases and inner boundaries are giving a complete different picture (literature attached). So far, the conclusions can't be valid for real constructions which weakens the paper significantly as the results might completely change. The authors are encouraged to point onto in the sense of being self critical within the introduction and the conclusion in order not to mislead the reader.
[Answer 1]
As you pointed out, the microstructure of thin film materials and practical bulk materials differs significantly. Taking this into consideration, we have added the following three sentences to the introduction and discussion sections to explain the significance of the magnetic study of ion-irradiated iron thin films in this study. We have also included a new reference [19].
Page 2, Lines 73-86.
“Investigating the saturation magnetization behavior of irradiated alloys is necessary for the safe operation of nuclear fusion reactors and is also of academic interest. When studying the magnetism of ion-irradiated films of iron and model alloys as a research method, it is important to consider the differences in microstructure between thin film materials and actual structural materials. The practical iron-chromium alloys of reduced activation ferritic/martensitic steel used for blankets have a tempered martensite structure that includes numerous grain boundaries, dislocations, and precipitates [1]. Considering the magnetic properties of iron-based alloys, there are structure-sensitive characteristics such as permeability, coercive force, and remanence, as well as structure-insensitive characteristics such as saturation magnetization [19]. Grain boundaries, dislocations, and precipitates have a significant effect on the former, but have little effect on the latter. This study primarily focuses on the latter characteristics. In this sense, the research approach that utilizes ion-irradiated film is considered beneficial for gaining insights into the magnetism of neutron-irradiated structural materials.”
Page 9, Lines 426-430.
“The present research focused on pure iron, which consists of only one type of 3d TM element. However, practical iron alloys are composed of multiple elements, so it is necessary to consider the effects of these elements. In fact, it has been confirmed that adding other 3d TM elements to iron significantly alters the irradiation effect on magnetism.”
Page 9, Lines 435-438.
“Further studies on the effects of the second and third elements, using ion-irradiated iron alloy films, are expected to provide a comprehensive understanding of the magnetic properties of neutron-irradiated iron-based structural materials.”
***********************
[Question & Comment 2]
Comments on the following text: “As far as the authors know, there are no systematic studies on the magnetic properties of iron alloys exposed to high doses of neutron irradiation,” and supplementary literature.
[Answer 2]
Thank you for providing multiple references. We believe those references are papers on the "microstructure" and "mechanical properties" of neutron-irradiated iron alloys. While research in that field has advanced, there are very few reports investigating the relationship between high doses of irradiation and magnetism. To enhance reader’ understanding of these aspects, we have made the following revision to the manuscript.
Page 1, Lines 37-43.
“Numerous studies have been carried out to examine the effects of high doses of neutron irradiation on the microstructure and mechanical properties of iron alloys. However, there has been a lack of systematic investigation regarding the magnetic aspects.”
***********************
[Question & Comment 3]
The referred SRIM method should be explained in detail, especially how it comes to the 18 dpa. This refers also to Figure 2 which is not to understand in its meaning without more and detailed explanation.
[Answer 3]
The value of the irradiation dose of 18 dpa was read from Figure 2 obtained from SRIM calculations. To make it easier to understand, two arrows were added to the figure, and the following sentence was added.
Page 3, Lines 123-124.
“As depicted in Figure 2, the level of irradiation-induced damage can be estimated to be around 18 dpa.”
***********************
[Question & Comment 4]
Figure 4: not to understand the meaning of variable q (nm-1).
[Answer 4]
The variable q represents the magnitude of the scattering vector of X-ray diffraction, and we added the following explanatory text in the caption of Figure 4.
Page 6, Lines 281-283.
“The diagrams on the right side illustrate the diffraction maps in reciprocal space and the scan directions. The scattering vector, denoted as q, is defined as q = (4π/λ) sinθ, where λ represents the wavelength of CuKa.”
***********************
[Question & Comment 5]
Comments on the following text we have written as “The magnified TEM image (e) revealed a high density of small white contrasts, which are helium cavities”. How proven the existence of cavities?
[Answer 5]
This time, we observed the under-focused image and over-focused image of TEM and determined the presence of a cavity by observing the change in contrast. We have revised the sentence as follows and added reference [24].
Page 6, Lines 301-303.
“The magnified and under-focused image (e) revealed numerous small, bright contrasts. When the observations were changing from under-focused to over-focused, the contrast was reversed. This indicates that the small contrasts are helium cavities [24].”
***********************
In addition, in order to make it easier for readers to understand, I have made some modifications to the English expressions in certain parts without changing the content. Thank you.
Author Response File: Author Response.pdf
Reviewer 3 Report
Comments and Suggestions for AuthorsIn this manuscript, the authors present a study on the microstructure and magnetism of pure iron that was exposed to high doses of helium ions. High-quality epitaxial iron films were prepared and subjected to helium ion irradiation. Interestingly, the magnetization curves remained unchanged, and the saturation magnetization was almost the same as that of the unirradiated film within experimental accuracy. This finding is consistent with the first-principles calculations of a simple model that considers the lattice expansion caused by helium implantation. The results demonstrate that the intrinsic magnetism of iron remains robust and unaffected under current irradiation conditions.
The manuscript is highly intriguing and presents significant findings within the field of interest.
The tables accurately present the data and the figures are precise with strong text support. The conclusion provides a clear, concise, and comprehensive summary of the findings.
Author Response
Reviewer 3
Thank you for the thorough review and your comment.
***********************
[Comment]
The manuscript is highly intriguing and presents significant findings within the field of interest. The tables accurately present the data and the figures are precise with strong text support. The conclusion provides a clear, concise, and comprehensive summary of the findings.
[Answer]
In addition to making revisions based on the suggestions of other reviewers, I have also changed the English expressions to make the text easier to read, while keeping the content unchanged. Thank you.
Author Response File: Author Response.pdf
Reviewer 4 Report
Comments and Suggestions for AuthorsThis study investigates the irradiation effect on the microstructure and magnetic properties of the pure Fe thin films. The paper is well organized with detailed discussion of the results and possible mechanisms. I would recommend the acceptance of this paper after addressing several minor questions.
1. The peak damage level is ~18 dpa with a He concentration of ~3000 appm. Such high damage level is rarely used for He ion irradiation. Is there a special reason why the authors chose to irradiate to such a high damage level? In most cases, the high dpa and the high He concentration would make it hard to accurately measure the density and size of bubbles due to the complicated defect structure.
2. The details regarding preparation of TEM samples using FIB such as the ion beam voltage should be given in the experimental section. Meanwhile, for the XRD scan, what is the scan range?
Author Response
Reviewer 4
Thank you for the thorough review and helpful advice. We have made the following revisions to the manuscript.
***********************
[Question & Comment 1]
The peak damage level is ~18 dpa with a He concentration of ~3000 appm. Such high damage level is rarely used for He ion irradiation. Is there a special reason why the authors chose to irradiate to such a high damage level? In most cases, the high dpa and the high He concentration would make it hard to accurately measure the density and size of bubbles due to the complicated defect structure.
[Answer 1]
The main objective of this study was to investigate the effects of high irradiation levels and cavity formation on magnetic properties at the level of a fusion reactor. In structural materials for fusion reactors, it is expected to encounter situations with damage levels of several tens of dpa and helium concentrations at the level of 1000 appm. We have added a new paper [7] that contains information about these aspects.
***********************
[Question & Comment 2]
The details regarding preparation of TEM samples using FIB such as the ion beam voltage should be given in the experimental section. Meanwhile, for the XRD scan, what is the scan range?
[Answer 2]
Regarding the experiment conditions you pointed out, we have made the following additions.
Page 4, Lines 176-179.
“Furthermore, thin sections were cut from both the unirradiated and irradiated specimens using the micro-sampling method of the focused ion beam of 30 keV Ga+ (FIB, Hitachi High-Tech MI-4050).”
Page 5, Lines 230-232.
“In the out-of-plane scan profile, which was measured by varying the diffraction angle 2θ from 40 degrees to 80 degrees, only the Fe 002 peak was observed, except for the MgO 002 peak.”
***********************
In addition, in order to make it easier for readers to understand, I have made some modifications to the English expressions in certain parts without changing the content. Thank you.
Author Response File: Author Response.pdf
Round 2
Reviewer 2 Report
Comments and Suggestions for AuthorsPaper is fine and recommended for being published
