Conversion of Hard to Soft Magnetic Ferrite Nanowires by Paramagnetic Shielding

Round 1

Reviewer 1 Report

In this manuscript entitled "Conversion of hard to soft magnetic ferrite nanowires by

paramagnetic shielding", the magnetization behavior of coaxial nanowires fabricated

through the sol-gel electrospinning method have been investigated.

The authors have found that a remarkable reduction in coercivity when a BaTiO3 shell was introduced to the CoFe2O4 core, leading to a shift from hard magnetic to soft magnetic behavior. This intriguing magnetization reversal effect was also observed when GdBa2Cu3O7 was used as the shell material. 

Generally, the present work shows some important results, but some issues need to be addressed before acceptance.

1. A very recently updated articles [Yan Wang et al., J. Appl. Phys. 132, 183907 (2022)] related to the magnetic properties of magnetic materials should be cited in the introduction or discussion part. The reference has detailed structural measurements (e.g. XRD, SEM and EDS, etc) and magnetic characterizations (e.g. MT curves and MH loops, MCE effects, etc).

2. The wavelength of X-ray diffraction is suggested to be provided in experiment part. 

3. In Figure 6B, BaTiO3 nanotubes do not show a PM state. What magnetic properties does this magnetization curve represent? This should be explained clearly. 

4. Figure 6B should be described in detail in results and discussion. 

 Minor editing of English language required

Author Response

1. A very recently updated articles [Yan Wang et al., J. Appl. Phys. 132, 183907 (2022)] related to the magnetic properties of magnetic materials should be cited in the introduction or discussion part. The reference has detailed structural measurements (e.g. XRD, SEM and EDS, etc) and magnetic characterizations (e.g. MT curves and MH loops, MCE effects, etc).

Response: 

Thanks for the comment. We have added this article to our introduction in the revision.

2. The wavelength of X-ray diffraction is suggested to be provided in experiment part. 

Response:

Thanks for the comment. The wavelength information has been added in the revision.

3. In Figure 6B, BaTiO3 nanotubes do not show a PM state. What magnetic properties does this magnetization curve represent? This should be explained clearly. 

4. Figure 6B should be described in detail in results and discussion. 

Response:

Thanks for the comment. We have added the discussion of the magnetic behavior of the BaTiO3 nanotubes in the revision.

Reviewer 2 Report

This is an interesting piece of work showing how a sheaths of paramagnetic BaTiO₃ and GdBa₂Cu₃O₇   reduce significantly the coercive field of ferromagnetic nanowires made of CoFe₂O₄, NiFe₂O₄ and Fe₂O₃. The effect is attributed to the paramagnetic susceptibility of the coating shield that enhances the switching of the total magnetization. Additionally a frequency-dependent  magnetic losses (impedance) in the range 10 MHz – 3000 MHz with the magnetic field almost perpendicular to the nanowires has been determined. An additional losses peak is attributed to imperfect alignment of the wires. The coated nanowires were obtained with the electrospinning method. The particular stages of fabrication and characterisation have been described and documented. The description is not always clear to a non-specialized reader; in particular the “electric field distribution” vs the orientation of the nanowires. May be a more detailed caption of Figure 3 indicating the position/orientation of the nanowires would help. In my opinion the paper deserves being published with amendments of the description of technical details. The Authors may also wish to attempt to establish, or at least to discuss, a quantitative relation of the coercive field with the size (thickness and radius) and susceptibility of the outer paramagnetic shield as compared with the ferromagnetic core. The language is mostly correct but the abbreviation GMI, presumably Giant Magnetoresistance and not Growth Modulation Index,  is not deciphered.

 The language is mostly correct but the abbreviation GMI, presumably Giant Magnetoresistance and not Growth Modulation Index,  is not deciphered.

Author Response

1. The description is not always clear to a non-specialized reader; in particular the “electric field distribution” vs the orientation of the nanowires. May be a more detailed caption of Figure 3 indicating the position/orientation of the nanowires would help.

Response:

Thanks for the comment. In the revision, we replotted the Figure 1 and Figure 2, the statements about the electrospinning setup and the mechanism of electric field distribution leading to the fiber alignment are added in text. In Figure 3, dash lines and inset image are added to assist reader recognizing the location and orientation of the nanowire.

2. In my opinion the paper deserves being published with amendments of the description of technical details. The Authors may also wish to attempt to establish, or at least to discuss, a quantitative relation of the coercive field with the size (thickness and radius) and susceptibility of the outer paramagnetic shield as compared with the ferromagnetic core.

Response:

Thanks for the comment. The further discussion about the magnetic properties of our paramagnetic shell (BaTiO3) is added in the text, including the size influence to its magnetic properties. And just as reviewer said, we did hope to establish a relationship between such coercivity reduction and the paramagnetic shell thickness. However the result from the TEM-EDAX mapping did not provide convincing enough result to support our statement, therefore, we used the other quantified parameter, the molar ratio of the compound in our discussion.

3. The language is mostly correct but the abbreviation GMI, presumably Giant Magnetoresistance and not Growth Modulation Index,  is not deciphered.

Response:

Thanks for the comment. The abbreviation GMI here is referred to Giant Magnetoimpedance. Correction and definition are added in the revision.

Reviewer 3 Report

This is a study on an interesting and potentially useful multiferroic system. My main concern is that the experiments and results are not well enough described, particularly for those not familiar with work on nanowires. 

• ‘CoFe2O4@BaTiO3’ – please explain your nomenclature.
• Figure 1 –  the text description, the figure caption and the lack of labels on the figure should all be improved. In the text introduce the figure in the order 1A, 1B, then 1C. Please label all major parts in the figures. Is the grounding collector shown anywhere?
• Figure 2A needs labels to understand what is being shown. What is the electric field distribution and how is this achieved? What are we looking for in the red circle area? What function are the screws performing. The text then goes from describing 2A to 2C, missing any description of 2B. How does the schematic of Fig 2C relate to 2A? There are no rectangular blocks in 2A.  Describe in more detail what is meant by ‘picked up’.
• Sentence “Due to the specific orientation of the nanowires…applying the electric pattern …allows for straightforward identification of the electric pattern…” This sentence makes no sense to me, can you please rephrase? The description is clumsy in this section, I presume the copper is only deposited over the electrode areas or is it deposited everywhere? Can you indicate the orientation of the nanowires in Fig 3? There is no label or explanation as to what the red areas are showing. Why not label all three electrodes?
• Section 3: How is the grain size determined? Grain size alone does not confirm the absence of superparamagnetism, but you could reference other work to say the grain size is likely too large to support superparamagnetism.
• Fig 4C – why does the EDAX have a large Cu signal but no Si? Wouldn’t this measurement be done on the material on the Si wafer?
• Section 3.3 – we need to know the orientation of the applied (H) field to the sample, and we also need to know the dimensions and density of the sample used for these measurements. Magnetic measurements are not independent of the sample geometry.
• Section 3.4 – can you add a diagram which illustrates the misalignment between current and field referred to? Please explain why this leads to multiple peaks.
• “…the contact pads may …catch more than one nanowire..’ This has not been explained properly. Earlier we were told about copper being deposited ‘on top of the nanowires’. How is a single nanowire isolated?
• P10 ‘In conclusion…’ This paragraph needs more detail to explain what is being proposed. The ‘helical anisotropy through joule heating’ requires a reference. Where exactly is the range of linear response?

p1 line 31, remove 'been'.

Section 3.4 heading 'coaxial'

Author Response

‘CoFe2O4@BaTiO3’ – please explain your nomenclature.

Response:

CoFe2O4@BaTiO3 means a compound with CoFe2O4 as the core material, which is put in front of the ‘@’ mark, while BaTiO3 is the shell material which is put behind the ‘@’ mark. We use the same nomenclature over the whole manuscript.

1. Figure 1 –  the text description, the figure caption and the lack of labels on the figure should all be improved. In the text introduce the figure in the order 1A, 1B, then 1C. Please label all major parts in the figures. Is the grounding collector shown anywhere?

Response:

Thanks for the comment. Figure 1 has been replotted with clear description of different parts of the setup.

2. Figure 2A needs labels to understand what is being shown. What is the electric field distribution and how is this achieved? What are we looking for in the red circle area? What function are the screws performing. The text then goes from describing 2A to 2C, missing any description of 2B. How does the schematic of Fig 2C relate to 2A? There are no rectangular blocks in 2A.  Describe in more detail what is meant by ‘picked up’.

Response:

Thanks for the comment. Figure 2 has been replotted. We provide additional statement about the distribution of the electric field, and the approaches of fiber collection.

3. Sentence “Due to the specific orientation of the nanowires…applying the electric pattern …allows for straightforward identification of the electric pattern…” This sentence makes no sense to me, can you please rephrase? The description is clumsy in this section, I presume the copper is only deposited over the electrode areas or is it deposited everywhere? Can you indicate the orientation of the nanowires in Fig 3? There is no label or explanation as to what the red areas are showing. Why not label all three electrodes?

Response:

Thanks for your comment. Actually we wrote in this sentence for the one who are not familiar with the technique of establishing electrical connection with nanowires. We rewrite the statement now and hope the revision is clear to everyone. The red areas show the location of the nanowire. A statement is given in text and the labels are added in Figure 3 in the revision.

4. Section 3: How is the grain size determined? Grain size alone does not confirm the absence of superparamagnetism, but you could reference other work to say the grain size is likely too large to support superparamagnetism.

Response:

The grain size is estimated according to the analysis of the XRD refinement data and the observation from TEM. We accept the idea that the grain size can not confirm the absence of superparamagnetism and we have rewritten the statement in the revision.

5. Fig 4C – why does the EDAX have a large Cu signal but no Si? Wouldn’t this measurement be done on the material on the Si wafer?

Response:

The EDAX anaylsis is conducted in the TEM, and our samples are mounted on the copper grid. Consequently, our data contains a background signal originating from the presence of copper (Cu).

6. Section 3.3 – we need to know the orientation of the applied (H) field to the sample, and we also need to know the dimensions and density of the sample used for these measurements. Magnetic measurements are not independent of the sample geometry.

Response:

Thanks for the comment. However, our case is different from measurements of bulk or thin film, the nanowires are stored in a capsule during the magnetic measurement just like the powder, influence from the sample geometry is negligible.

7. Section 3.4 – can you add a diagram which illustrates the misalignment between current and field referred to? Please explain why this leads to multiple peaks.

Response:

Thanks for the comment. In the revision we use dash lines to indicate the location of the nanowire, showing the non-perpendicular alignment of the nanowire to the electric pattern. The applied magnetic field is perpendicular to the electric pattern. That means the applied magnetic field is not parallel to the alignment of the nanowire, in other word, the current. This leads to the misalignment between the current and the field.

The explanation of the appearance of double peaks in impedance measurement and related reference are added in the revision.

8. “…the contact pads may …catch more than one nanowire..’ This has not been explained properly. Earlier we were told about copper being deposited ‘on top of the nanowires’. How is a single nanowire isolated?

Response:

Thanks for the comment. As discussed in the section 2.2 (page 3 line 77-79). When the nanofibers are parallelly aligned, in the area where the density of the nanofibers is low enough, it is possible the electric patterns are covering just a single nanofiber, as shown in Figure 3. The location of the fiber has been highlighted with dash lines in the revision. In order to avoid the misunderstanding, the sentence ‘“…the contact pads may …catch more than one nanowire..’’ is removed in the revision.

9. P10 ‘In conclusion…’ This paragraph needs more detail to explain what is being proposed. The ‘helical anisotropy through joule heating’ requires a reference. Where exactly is the range of linear response?

Response:

In our result, the range of linear response locates from maximum value of the peak at around 50 Oe to the bottom regime, where the sample meets the saturation field at around 100 Oe. If a field sensor is using this sample as the sensing ingredient, the field detection in this regime will be sensitive and easily calibrated. However, obvious this is not enough in practise. So what we are expecting is the sample presenting symmetric double peaks. Then the range of linear response starts from the 0 Oe, and ends at peak positions at both positive and negative sides, in this case, the peaks locate at the saturation field.

In the revision, we have added some detail about the idea of ‘inducing helical anisotropy through joule heating’ with a related reference.

comments on language:

1. p1 line 31, remove 'been'.

Response:

This is a passive tense as the subject is actually the researchers not ‘such nano-sized ferrites’.

2. Section 3.4 heading 'coaxial'

Response:

Thanks for the comment. The typo is corrected in the revision.

Round 2

Reviewer 1 Report

The paper can be accepted in current form.

Author Response

We appreciate your comments and your assistance in enhancing our manuscript. Thanks.

Reviewer 3 Report

Using the nomenclature 'CoFe2O4@BaTiO3' - please explain in the manuscript when it is first introduced. The reader should not have to work it out from the context after reading a few paragraphs.

The text discussion of Figure 1 remains confusing. The text introduces first figure 1b, then figure 1C (which now doesn't exist), then Figure 1A. My suggestion is to always construct figures and text so that first you discuss A. then B. then C. etc. 

Figure 3 and related discussion. The term 'electric pattern' is not standard. The common practice in scientific English is to talk about 'electrodes'. I would say something like (replacing text from line 98-101) 'A 300 nm copper layer is deposited by sputtering on the nanowires to form electrodes in the pattern shown in figure 3. The electrodes contact the ends of the nanowire.'

p1 line 31. The grammar is incorrect. My suggestions are

Such nano-sized ferrites have utility in magnetic...'

or

Such nano-sized ferrites have been found useful in magnetic...'

Author Response

1. Using the nomenclature 'CoFe2O4@BaTiO3' - please explain in the manuscript when it is first introduced. The reader should not have to work it out from the context after reading a few paragraphs.

Response: Thanks for the comment. The explanation about the ‘@’ mark has been given in the introduction of revision.

2. The text discussion of Figure 1 remains confusing. The text introduces first figure 1b, then figure 1C (which now doesn't exist), then Figure 1A. My suggestion is to always construct figures and text so that first you discuss A. then B. then C. etc. 

Response:

Thanks for the comment. The second passage of section 2.1 has been rewritten, as to re-order the description of Figure 1 A, B and C.

3. Figure 3 and related discussion. The term 'electric pattern' is not standard. The common practice in scientific English is to talk about 'electrodes'. I would say something like (replacing text from line 98-101) 'A 300 nm copper layer is deposited by sputtering on the nanowires to form electrodes in the pattern shown in figure 3. The electrodes contact the ends of the nanowire.'

Response: 

Thanks for the comment. We have replaced the term ‘electric pattern’ by ‘electrodes’ in the revision, and rewritten the mentioned statement as you described in the revision.

4. p1 line 31. The grammar is incorrect. My suggestions are ‘Such nano-sized ferrites have utility in magnetic...' or ‘Such nano-sized ferrites have been found useful in magnetic...'

Response:

Thanks for the comment. The sentence is corrected in the revision.