Magnetostructural Transition and Magnetocaloric Effect with Negligible Magnetic Hysteresis in MnCoGe_1.02−xGa_x Alloys

Round 1

Reviewer 1 Report

Dear editor and authors,

thank you for the opportunity to review the manuscript “Magnetostructural transition and magnetocaloric effect with negligible magnetic hysteresis in MnCoGe1.02-xGax alloys”.

The manuscript shows structural, magnetic and caloric measurements of MnCoGe1.02-xGax alloys with four different Ga concentrations 0, 0.02, 0.04 and 0.06 at%. The authors mention that the first-order magneto-structural transition from high temperature hexagonal to orthorhombic structure decrease with increasing Ga content. In addition, the authors mention that the negligible magnetic hysteresis in these alloys makes the alloy interesting for magnetic refrigeration.

The conclusions and interpretation of the data is partially incorrect and the authors give also contradicting conclusions in the manuscript. Furthermore, a microstructural and compositional analysis on the alloys is missing, this analysis however is essential to evaluate the homogeneity of the samples and to exclude multiple phases with can cause a misinterpretation of the magnetic and magnetocaloric data. I recommend to rejected the manuscript. In the following I listed the most important point for my decision. I hope that the remarks are helpful for future or current studies on this topic.

• In the introduction the authors mention that for materials with first-order magnetic transition the magnetic hysteresis can reduce the refrigeration efficiency. Most magnetocaloric materials are soft magnetic and the magnetic hysteresis is negligible same as in MnCoGeGa. The important property which limits the refrigeration efficiency of first-order materials is the thermal hysteresis. In the manuscript is not clear if the authors discuss the magnetocaloric effect of the first-order transition or the effect at the Curie-temperature.
• In the experimental part the authors mention that the samples are homogenized at 1123K for 5 days and are furnace cooled to room temperature. However, the homogeneity of the samples is not confirmed by electron microscopy (backscattered electron detection) and for example energy dispersive x-ray (EDX) analysis. Especially for small cooling rates the material can decompose during cooling and the formation of multiple secondary phases can lead to a misinterpretation of the magnetic measurements.
• The XRD pattern in Fig.1 cannot exclude the presence of secondary phases since these have often a comparable structure than the targeted phase and the reflexes are overlapping. This can be also the explanation of the inversion of the peak intensity relation I(102)h/I(110)h between the reference sample x=0 and x=0.02,0.04 and 0.06 in Fig. 1. This effect is not discussed in the manuscript, furthermore the authors does not show a Rietveld refinement of the XRD data to confirm the structure. It would be also beneficial to compare the x=0 sample with literature values.
• A further hint for the presence of secondary phases and an inhomogeneous sample except for x=0 are the “step-like” transition shown in Fig. 4.
• The missing magnetic hysteresis in the M-H curves in Fig. 6 indicates that the first-order magneto-structural transition cannot be induced by the external magnetic field. It is not explained in detail why this absence of a field-induced transition is interesting for magnetic refrigeration since the field induced transition is the key aspect for the giant MCE in magnetocaloric materials with first-order transformation.
• Additional aspects which require more detailed analysis and discussion
  • Thermal hysteresis in Fig. 5 and the small shift of the transition temperature with field limit the reversibility of the MCE.
  • DSC measurements Fig. 2c) and 2d) show a negative heat flow for heating and cooling which shouldn’t be the case.
  • In general, the MCE should be measured directly by adiabatic temperature change measurements or heat conductivity measurements but the entropy can be also determined by the DSC measurements which can be used to validate the values calculated by the Maxwell relation and the M-H data.

Best regards,

Author Response

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Reviewer 2 Report

In this manuscript entitled "Magnetostructural transition and magnetocaloric effect with negligible magnetic hysteresis in MnCoGe1.02-xGax alloys", the authors have investigated the structure, magnetic properties and magnetocaloric effect of the  MnCoGe1.02-xGax (x=0, 0.02, 0.04, 0.06) alloys using XRD, DSC and Magnetic Measurements.

Near room temperature, the magnetic entropy change of MnCoGeGa0.02 alloy reaches 23.47 J/kg∙K under 5T magnetic field, and its refrigeration capacity reaches 387 J/kg.

The low magnetic hysteresis and good refrigeration effect can be obtained in the MnCoGeGa0.02 alloy. It is found that the MnCoGeGa0.02 alloy has a better magnetocaloric effect, making it a promising new type of refrigeration material.

The research of this work is impressive, but some revisions are needed to make the work acceptable.

1. In the Experimental part, the authors should list the wavelength of XRD.
2. Besides the isothermal magnetization and field curves of MnCoGeGa0.02, the Arrott curves of MnCoGeGa0.02 alloy should also be provided in this manuscript. Magnetization measurements, isothermal magnetization, Arrott curves and magnetocaloric effect have been studied in some related papers (Journal of Applied Physics 131, 043901 (2022); Journal of Physics: Condensed Matter 33 (2021) 285802). The authors should refer to these references and cite them in the Results and Discussion part. 
3. Magnetocaloric effect of MnCoGeGa0.02 has been investigated in detail. Why didn't the authors study the isothermal magnetization and magnetocaloric effect of the MnCoGe1.02-xGax alloys with x= 0.04 and 0.06 ?
4. Some minor errors need to be corrected, such as “For the MnCoGeGa0.02 alloy, Ms is 300K...”. The authors are advised to recheck the manuscript carefully.

Author Response

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Reviewer 3 Report

The paper “Magnetostructural transition and magnetocaloric effect with negligible magnetic hysteresis in MnCoGe_1.02-xGa_x alloys” by L. Gao, Y. Feng, S.H. Hu and X.Y. Xin is devoted to the optimization of composition MnCoGe alloys by partial germanium substitution by gallium to obtain a material high value of magnetocaloric effect and low magnetic hysteresis. From my point of view the authors have obtained several quite interesting results in this work to be considered for publication in Metals. However some point should be cleared up before.

1. I would like to see some speculations about applicability of the Maxwell relation for magnetic entropy change for these alloys. The magnetoelastic contribution should be also discussed for the compound where the structural transformation coincides with magnetic one.
2. From the first glance the magnetic data for the structural transformation area do not fit to DSC data. For all samples temperature hysteresis of the magnetization is observed near the Curie point, but this is not so for DSC curves. I would like to see also some explanation about nonmonotonic character of the temperature behavior of magnetization for alloys with 0.04 and 0.06 Ga content.
3. The partial magnetization curves in Fig. 6 are not clear for me. What is the reason of the divergence with the field increase?
4. One more small comment. Too many words “obvious” are in the text.

Author Response

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Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

A detailed report is attached.

Comments for author File: Comments.pdf

Author Response

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Author Response File: Author Response.pdf

Reviewer 2 Report

The manuscript has been improved and can be accepted in present form.

Author Response

Thanks for your affirmation and recommendation!

Reviewer 3 Report

I think that the authors have cleared up all the questions arisen and the paper now can be published in Metals.

Author Response

Thanks for your affirmation and recommendation!