Contactless Inductive Sensors Using Glass-Coated Microwires

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The proposed review paper represents a valuable summary of the state of the art in contactless sensors based on magnetic microwires. It gives a complete introduction of the main magnetic processes involved and about the excitation and signal processing and read out methodologies.

The following recommendations should be considered in order to improve the quality of the work.

1.- On page 3, a reference should be given regarding the spectral analysis for nonlinear systems

2.- On figure 1, the meaning of area and area/2 is not evident. Also in this figure the size of the …..should be increased

3.- On page 4, line 129, the following paragraph should be reconsidered “ For soft magnetic materials, two types of magnetization loops can be expected. The first type is close to rectangular but has a small hysteresis field (𝐻𝐶 ), while the second type is nearly non-hysteretic and has a small characteristic anisotropy field (𝐻𝐾) that determines the saturation regime. These are shown in Figures 1a-b, respectively.”

On one side, it should be quantitatively defined soft magnetic by means of coercivity values. It is not acceptable to say “small hysteresis field”. It should something like “ switching field below 1 Oe or even less” It is not clear the meaning of hysteresis field, it should be coercive field or switching field in the case of bistability. The same with small anisotropy field. This should be, again, quantified.

Also, instead categorically affirming that “ For soft magnetic materials, two types of magnetization loops can be expected” it should be said “ The control of the composition and geometry of soft magnetic materials allows the tailoring of their hysteresis loops giving a broad spectrum of behaviors that, in a simplified way, can be divided in two types i.e.: close to rectangular due to high axial anisotropy with switching field below…, and nearly non-hysteretic due to certain transversal anisotropy with anisotropy field below….¨

4.- On page 4, line 139 where it says “ The shape of the hysteresis loop is influenced by numerous physical parameters, which can be quantitatively assessed by analyzing the voltage spectrum”. It should be given more detail about the parameters to control the hysteresis loops and a reference regarding this aspect.

5.- On page 5, line 202 the use of the word “inclinated” to describe an hysteresis loop should be reconsidered

6.-  On page 6, a reference related to Vitroperm should be given

7.- On page 6, figure 2 is not clear. The axis should include the magnitudes they are representing and the meaning of the concept “ changing the slope of the excitation current” should be better explained.

8.- On page 7, line 268, after this paragraph “ However, in Fe-Ni based amorphous alloys, increasing Ni content results in considerable decrease in saturation magnetization and Curie temperature and therefore vanishing 𝜆𝑠 values correspond to transition to a paramagnetic state.” a reference should be given

9.- On equation 6 each term of the sum should be better explained

10.- On page 8, a reference related with nanocrystalline magnetic microwires of composition FeSiBCuNb should be included

11.- On page 9, the information given in the last paragraph could be clarified by means of a figure mainly regarding this sentence “ In the context of amorphous materials, the short-range ordering can be modified through various annealing treatments. This is significant for fine-tuning the magnetic structure and controlling the magnetic parameters such as uniaxial anisotropy, magnetostriction,  and Curie temperature [53–55].”

12.- On page 10, it would be interesting to add a reference and a figure regarding this aspect “ The coercivity mechanisms can be more complex, leading to deviations from Equation 9, particularly when the wire has almost a rectangular loop but lacks bistable behavior. In the case of temperature sensing, harmonics detection is therefore more suitable.

The area under the pulse 𝑆 = ∫ 𝑉(𝑡)𝑑𝑡 is proportional to 𝑀𝑠. Consequently, the harmonics amplitudes show pronounced alterations near to 𝑇 𝐶 , as illustrated in Figure 1e, which presents the harmonics amplitudes for two values of S.”

13.- On page 15, line 571 it says “ In the case of negative magnetostriction microwires” this should be correlated with the shape of the hysteresis loops and a distinction should be made between the microwires considered here and those with axial hysteresis on line 595 and with circular domain structure on line 611b

Comments on the Quality of English Language

The quality of the English is average, but a text review is recommended to improve its clarity and flow

Author Response

We are very thankful to the reviewer for his/her valuable suggestions which really helped us to improve the manuscript. 

1.- On page 3, a reference should be given regarding the spectral analysis for nonlinear systems

Response 

Two references are given- current numbers are 13 and 38.  

2.- On figure 1, the meaning of area and area/2 is not evident. Also in this figure the size of the …..should be increased

Response 

Figure 1 was improved increasing the font size.  Meaning of areas is explained. This example simulates the approach to the Curie temperature when the area S under the voltage pulse decreases following the decrease in the saturation magnetization. The comparison was made for the harmonic amplitudes calculated for two values of this area, S and S/2.

3.- On page 4, line 129, the following paragraph should be reconsidered “ For soft magnetic materials, two types of magnetization loops can be expected. The first type is close to rectangular but has a small hysteresis field (?? ), while the second type is nearly non-hysteretic and has a small characteristic anisotropy field (??) that determines the saturation regime. These are shown in Figures 1a-b, respectively.”

On one side, it should be quantitatively defined soft magnetic by means of coercivity values. It is not acceptable to say “small hysteresis field”. It should something like “ switching field below 1 Oe or even less” It is not clear the meaning of hysteresis field, it should be coercive field or switching field in the case of bistability. The same with small anisotropy field. This should be, again, quantified.

Also, instead categorically affirming that “ For soft magnetic materials, two types of magnetization loops can be expected” it should be said “ The control of the composition and geometry of soft magnetic materials allows the tailoring of their hysteresis loops giving a broad spectrum of behaviors that, in a simplified way, can be divided in two types i.e.: close to rectangular due to high axial anisotropy with switching field below…, and nearly non-hysteretic due to certain transversal anisotropy with anisotropy field below….¨

Response

Many thanks for your suggestions. We fully agree and modified the statement according to your suggestion. The new text reads as

Soft magnetic materials are distinguished by low values of the anisotropy field () and coercivity field (),  typically ranging from 100 to 200 A/m, and even lower. The composition and geometry of soft magnetic materials can be tailored to control their magnetic structure, thereby resulting in a broad spectrum of magnetization behaviors and observed hysteresis loops. In essence, considering ferromagnetic wires subjected to a magnetic field applied along their axis, the resulting hysteresis loops can be categorized into two distinct types. The first type is characterized by a close-to-rectangular loop, which is attributed to an axial easy magnetization direction. The second type is nearly non-hysteretic, resulting from specific easy magnetization along circumference. The corresponding circumferential anisotropy field in turn determines the saturation regime. These are shown in Figures 1a-b, respectively.

4.- On page 4, line 139 where it says “ The shape of the hysteresis loop is influenced by numerous physical parameters, which can be quantitatively assessed by analyzing the voltage spectrum”. It should be given more detail about the parameters to control the hysteresis loops and a reference regarding this aspect.

Response

This was clarified as following.

The shape of the hysteresis loop and the critical fields are influenced by physical parameters, such as mechanical stress, strain, temperature, etc. [22,39] which can be quantitatively assessed by analyzing the voltage spectrum.

Two references were given, current numbers are 22 and 39

5.- On page 5, line 202 the use of the word “inclinated” to describe an hysteresis loop should be reconsidered

Response

We now use “non-hysteretic magnetization behavior”

6.-  On page 6, a reference related to Vitroperm should be given

Reference was given, current number  47.   

7.- On page 6, figure 2 is not clear. The axis should include the magnitudes they are representing and the meaning of the concept “ changing the slope of the excitation current” should be better explained.

Response

Figure 2- (current Figure 3) was modified and figure caption was changed.  Actually, we only provided a schematic representation of the detecting method rather than conducting the experiments. The explanation for the slope change was that it occurs when the field suddenly shifts from increasing to decreasing after the apex of a triangle waveform.

8.- On page 7, line 268, after this paragraph “ However, in Fe-Ni based amorphous alloys, increasing Ni content results in considerable decrease in saturation magnetization and Curie temperature and therefore vanishing ?? values correspond to transition to a paramagnetic state.” a reference should be given

References were given. Current references 23 and 24.  

9.- On equation 6 each term of the sum should be better explained

Response

Explanation of equation 6  is given

It is useful to consider the total magnetic energy Em , which is contributed by averaged magnetocrystalline anisotropy Ecr, induced anisotropy Eu , and magnetoelastic anisotropy Eme  (inverse magnetostriction effect):

In amorphous cobalt- based alloys the contribution of  Ecr could be larger than that of a magnetostrictive origine owing to clustering of Co-nanocrystals  with high anisotropy.

10.- On page 8, a reference related with nanocrystalline magnetic microwires of composition FeSiBCuNb should be included

Reference related with nanocrystalline magnetic microwires of composition FeSiBCuNb is given. Current reference 57.

 

11.- On page 9, the information given in the last paragraph could be clarified by means of a figure mainly regarding this sentence “ In the context of amorphous materials, the short-range ordering can be modified through various annealing treatments. This is significant for fine-tuning the magnetic structure and controlling the magnetic parameters such as uniaxial anisotropy, magnetostriction,  and Curie temperature [53–55].”

Response

We gave an illustration of magnetostriction change and corresponding change in hysteresis loop (Figure 4). We also provided more references (current references 50, 64-65), and gave an  example of Curie temperature change due to annealing

For example, the Curie temperature of CoFeCr- allows can be reduced by up to 7 C  by annealing at 250-300 C due to antiferromagnetic coupling of Co-Cr and Fe-Cr atoms.

12.- On page 10, it would be interesting to add a reference and a figure regarding this aspect “ The coercivity mechanisms can be more complex, leading to deviations from Equation 9, particularly when the wire has almost a rectangular loop but lacks bistable behavior. In the case of temperature sensing, harmonics detection is therefore more suitable.

Response

We gave a reference on coercivity mechanisms (current reference 74). And also example on irregular temperature dependence of the switching field in  microwires with addition of tungsten used to decrease its Curie temperature (current reference 75).

13.- On page 15, line 571 it says “ In the case of negative magnetostriction microwires” this should be correlated with the shape of the hysteresis loops and a distinction should be made between the microwires considered here and those with axial hysteresis on line 595 and with circular domain structure on line 611b

Response

We have made clarifications on all three cases.

In the case of negative magnetostriction microwires typically having circumferential easy magnetization and exhibiting hysteresis loops of the type shown in Figures 1b and 5b, it is possible to use spectral analysis to measure mechanical stress, since the application of stress alters the effective anisotropy and, consequently, the susceptibility, as seen from the hysteresis loops in Figure 7.

It is interesting to consider the stress influence on the harmonic spectrum in microwires having a negative magnetostriction but a nearly rectangular hysteresis loop in as-prepared state. Example of such magnetic hysteresis is shown in Figure 7a [37,87]) for Co₇₁Fe₅B₁₁Si₁₀Cr₃ microwire.

In Co-based microwires with an easy circumferential magnetization and a well-defined circular domain structure characterized by a hysteresis loop shown in Figure 5b

Reviewer 2 Report

Comments and Suggestions for Authors

This work presents interesting results on inductive sensors based on ferromagnetic glass-coated microwires. In general, the work is very well structured and well written. Some aspects to be clarified before publication are stated below.

1.For the “Introduction” section, please include a brief description of the specific compositions and properties of amorphous ferromagnetic microwires relevant for the applications you are describing. Also make a amention about the typical physical dimensions characterizing such microwires.

2.In line 128 you mentioned the “hysteresis field (Hc)” characterizing rectangular hysteresis curves. However, such Hc field corresponds to the “coercivity field”, which is a more common term for designing such field. Please, make a comment on this.

3.In line 132 you use again “Hc” to refer a “critical field”. Thus “Hc” has three different meanings in a couple of paragraphs. Please, use different labels for each type of field.

4.In line 158 you mentioned the term “easy anisotropy”. Did you mean “easy axis of magnetization”?

5.In line 164 you mentioned “Lanzheven function”, Did you mean “Langevin function”?.

6.In line 314 include a brief defintion of Curie temperature, please.

7.In lines 330-332, please give an idea about the dimension of axial and closure domains.

8.For the wires described in section 3.2 with negative magnetostriction, Is there an inner region with axial magnetization?.

9.For Figures 4, 5, 7, 9, 10 you say “Adopted from [ ] ”. Did you really mean “Adapted from”?.

10. If possible, please provide specific examples of commercially available sensors based on the described ferromagnetic microwires.

 

Author Response

We are very thankful to the reviewer for his/her valuable suggestions which really helped us to improve the manuscript. 

This work presents interesting results on inductive sensors based on ferromagnetic glass-coated microwires. In general, the work is very well structured and well written. Some aspects to be clarified before publication are stated below.

1.For the “Introduction” section, please include a brief description of the specific compositions and properties of amorphous ferromagnetic microwires relevant for the applications you are describing. Also make a amention about the typical physical dimensions characterizing such microwires.

Response 

The following wire description was added in Introduction (lines 90-99)

Amorphous materials prepared by rapid quenching from the melt in the form of wires and ribbons have not only excellent soft magnetic properties, but also good mechanical properties and small dimensions. Glass-coated microwires have typical metal core diameter of 1-50 microns and glass thickness of 3-15 microns.  The primary parameter that allows magnetic property tuning is the magnetostriction. By selecting a specific composition of ferromagnetic elements such as CoxFe1−x and  NixFe1−x with a positive or negative magnetostriction.

2.In line 128 you mentioned the “hysteresis field (Hc)” characterizing rectangular hysteresis curves. However, such Hc field corresponds to the “coercivity field”, which is a more common term for designing such field. Please, make a comment on this.

Response

Of course, we agree that the term 'coercivity field' should be used.

3.I n line 132 you use again “Hc” to refer a “critical field”. Thus “Hc” has three different meanings in a couple of paragraphs. Please, use different labels for each type of field.

Response

We made different designations for different fields.

4.In line 158 you mentioned the term “easy anisotropy”. Did you mean “easy axis of magnetization”?

Response

Yes, we meant easy axis of magnetization. This was corrected.

5.In line 164 you mentioned “Lanzheven function”, Did you mean “Langevin function”?.

Response. You are right. This should be Langevin function and it was corrected

6.In line 314 include a brief defintion of Curie temperature, please.

Response.  Definition of a Curie temperature was included:

When microwires are used as sensing elements, the ferromagnetic alloy should have a high temperature of the transition to a paramagnetic state known as the Curie temperature, Tc  (lines 386,387)

7.In lines 330-332, please give an idea about the dimension of axial and closure domains.

Response.

We consider that in as-prepared glass coated microwires with positive magnetostriction the axial domain may occupy almost  the entire volume of the wire, as evident from almost rectangular hysteresis loops. (lines 405-407)

8.For the wires described in section 3.2 with negative magnetostriction, Is there an inner region with axial magnetization?.

We included the following discussion (lines 476-480)

Magnetic microwires with low negative magnetostriction typically have a circular anisotropy and a so-called bamboo domain structure at least in the outer region.  This is related to the coupling of a negative magnetostriction  with internal tensile stresses. If this stress is predominant, the inner region with axial magnetization is small as evident from hysteresis loops with small remanence magnetization and coercivity field, shown in Figure 5b.   

9.For Figures 4, 5, 7, 9, 10 you say “Adopted from [ ] ”. Did you really mean “Adapted from”?.

Response. Yes, you are right. In fact, we corrected this and asked the technical editor to replace the manuscript but probably it was not done.

10. If possible, please provide specific examples of commercially available sensors based on the described ferromagnetic microwires.

Response. There are a number of companies (as RVmagnetics, Aichi Steel) dealing with magnetic microwires and various products from them. But we are hesitating to directly referencing this.

Reviewer 3 Report

Comments and Suggestions for Authors

Comments to Author:

1. The manuscript currently reads mostly as a review, and several sections overlap substantially with existing review papers in this field. The authors should clearly state what experimental results or methodological developments are new in abstract section.
2. The paper exclusively discusses microwire-based sensing, without providing a meaningful comparison to alternative contactless sensing approaches. A comparative discussion would significantly strengthen motivation.
3. The harmonic-based measurement section should provide more methodological details, without details, it is difficult to evaluate reproducibility and practical applicability.
4. The biomedical discussion is not sufficiently developed. The manuscript directly concludes that microwires are “suitable for biomedical use” and “accepted by tissues”, but there is no discussion regarding biocompatibility, safety, chronic implantation, or alternatives. In recent biomedical sensing literature, high-performance ultrasound sensing and wearable ultrasonic devices have demonstrated promising results, e.g. Coded Excitation Imaging Platform for 100 MHz Ultrasound Applications, and wearable ultrasound array belt for small animal echocardiography. A short comparison or contextual discussion would considerably improve this section.

Author Response

We are very thankful to the reviewer for his/her valuable suggestions which really helped us to improve the manuscript. 

1. The manuscript currently reads mostly as a review, and several sections overlap substantially with existing review papers in this field. The authors should clearly state what experimental results or methodological developments are new in abstract section.

Response 1. Thank you for your comment. We would like to clarify that our manuscript is indeed a review; however, it differs from existing review articles on magnetic microwires. Specifically, we focus on the nonlinear magnetization dynamics of microwires in remote sensing applications, as well as the various methods for controlling and tailoring their magnetization response. To emphasize this e aspect, we have revised the abstract accordingly.

2. The paper exclusively discusses microwire-based sensing, without providing a meaningful comparison to alternative contactless sensing approaches. A comparative discussion would significantly strengthen motivation.

Response 2. 

We have included short discussion on wireless measuring techniques at the beginning of introduction and emphasized that in this review we will focus on short range interrogation using inductive methods based on nonlinear magnetization. (lines 47-55)

In addition to covering detection techniques based on microwire magnetization, we have included general aspects of spectral analysis and discussed its application to magnetic particle detection, which was generalized for measuring parameters of magnetic elements.  

3. The harmonic-based measurement section should provide more methodological details, without details, it is difficult to evaluate reproducibility and practical applicability.

Response 3. 

We have included additional section 2.1 devoted to harmonic-based measurements and additional Figure 2.

Lines 220-236. 

4. The biomedical discussion is not sufficiently developed. The manuscript directly concludes that microwires are “suitable for biomedical use” and “accepted by tissues”, but there is no discussion regarding biocompatibility, safety, chronic implantation, or alternatives.

Response 4. 

More discussions are provided on biocompatibility of glass coated microwires along with additional references.This part of the manuscript is (lines 569-581).

Non-contact methods of measurements of stress/strain and temperature with the use of bistable microwires were proposed for medical applications [30–32,69,82]. Due to their Pyrex glass coating, microwires are biocompatible, as confirmed by several studies. In [83], cytotoxicity tests were conducted using the human hepatocellular carcinoma cell line (Huh7). Fe- and Co-based microwires with a metal core diameter of 20–30 µm and a glass coating thickness of 3–5 µm were placed in a nutrient medium alongside the cell culture. After a 24-hour incubation, propidium iodide staining was used to identify dead cells, and the labeled cells were analyzed using epifluorescence microscopy. In [84], the glass-coated microwires were evaluated with a human embryonic fibroblast cell culture. Cell viability was assessed using fluorescent staining with ethidium bromide and the MTT assay. No significant differences were observed between cultures with and without microwires, and these tests were conducted in the presence of a magnetic field. These results confirm that the microwires are non-toxic to cell cultures.

5. In recent biomedical sensing literature, high-performance ultrasound sensing and wearable ultrasonic devices have demonstrated promising results, e.g. Coded Excitation Imaging Platform for 100 MHz Ultrasound Applications, and wearable ultrasound array belt for small animal echocardiography. A short comparison or contextual discussion would considerably improve this section.

Response 5. We added the following discussion and provided reference (current number 85). Lines 594-601.

The use of bistable microwires for stress and pressure measurements in organs can be viewed in the context of recent progress in ultrasonic sensing. Wearable ultrasonic devices can be employed not only for imaging, but also for monitoring physiological parameters such as blood pressure and shear-wave velocities during muscle motion, thereby providing dynamic information on muscle mechanical properties [85]. However, the development of such devices still faces several challenges, including the need to improve piezoelectric materials, whose performance degrades as the size of the vibrating element decreases.

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The authors have given a detailed response and have made a good effort  to improve the  revising manuscript. All the concerns and suggestions raised during the previous round of review have been addressed satisfactorily.

The manuscript is now much clearer and more robust, and I believe it makes a valuable contribution to the field. I am happy to support the acceptance of the work for publication.