Magnetic and thermoelectric properties of Co2MnT (T = Ga, Si) Heusler compounds

doi:10.1016/j.physb.2020.412761

Physica B: Condensed Matter

Volume 603, 15 February 2021, 412761

https://doi.org/10.1016/j.physb.2020.412761 Get rights and content

Highlights

•
The Co2MnGa and Co2MnSi Heusler compounds were confirmed to be ferromagnetic.
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The Co2MnSi Heusler compound exhibited a high Curie temperature.
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The Co2MnGa and Co2MnSi Heusler compounds were n-type thermoelectric materials.
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The Co2MnSi Heusler compound showed a power factor of 1800 μW/mK2 at 600 K.

Abstract

Magnetic measurements confirmed that the Co₂MnSi Heusler compound is a superior ferromagnetic material to the Co₂MnGa Heusler compound. The Co₂MnGa Heusler compound showed a saturation magnetization of 81.3 emu/g and a Curie temperature of 700 K, while the Co₂MnSi Heusler compound exhibited both a high saturation magnetization of 130 emu/g and a high Curie temperature of 1015 K. Thermoelectric measurements revealed that the Co₂MnSi Heusler compound is also a superior thermoelectric material to the Co₂MnGa Heusler compound. The Co₂MnGa Heusler compound showed a large Seebeck coefficient of −35.2 μV/K and a low electrical resistivity value of 1.30 μΩm at room temperature, while the Co₂MnSi Heusler compound showed values of −14.3 μV/K and 0.198 μΩm, respectively. A power factor of 1800 μW/mK² at 600 K was obtained in the Co₂MnSi Heusler compound, which is comparable to or higher than that of the well-known Fe₂VAl Heusler compound.

Introduction

Co-based Heusler alloys have been studied as ferromagnetic shape memory alloys (FSMAs), and their development as smart materials is generating strong interest [[1], [2], [3], [4], [5], [6], [7]]. Unlike the case of conventional shape memory alloys (SMAs) such as Ti–Ni alloys, the martensite variant rearrangement is induced by an applied magnetic field [[8], [9], [10]]. The response time of the shape changes accompanying magnetically controlled martensitic transformation in the FSMAs is therefore much faster than that of the shape changes accompanying thermally controlled martensitic transformation in the SMAs. It has been reported that Co-based Heusler alloys show half-metallic properties, making them attractive candidates for application to spin-polarized current devices [[11], [12], [13]]. Thus, the magnetic properties of Co-based Heusler alloys have been intensively studied in recent years.

Since the Fe₂VAl Heusler alloy was reported to have exhibited attractive thermoelectric properties, the Heusler alloys have been both theoretically and experimentally studied as thermoelectric materials [[14], [15], [16], [17], [18]]. Co-based Heusler alloys are also possible thermoelectric materials, but they have mainly been studied as FSMAs and half-metallic materials [[19], [20], [21], [22]]. In this study, the thermoelectric properties of two typical Co-based Heusler alloys, Co₂MnGa and Co₂MnSi, were investigated [23,24]. As mentioned above, Co-based Heusler alloys are ferromagnetic. Therefore, both the magnetic and thermoelectric properties of these two Co-based Heusler alloys were investigated.

Section snippets

Experiment

Co₂MnGa and Co₂MnSi alloys were prepared by subjecting small ingots of Co (99.9 wt%), Mn (99.9 wt%), Ga (99.9 wt%), and Si (99.9 wt%) to induction melting. Small amounts of the alloy ingots were placed in a quartz crucible with an orifice of 0.6 mm at the bottom. The ingots were induction melted in an argon atmosphere and then ejected through the orifice with argon gas into a copper mold with a cavity of 3 mm in diameter and 50 mm in length. The cast specimens were annealed at 1073 K for 24 h

Results and discussion

In this study, we investigated the properties of Co₂MnGa and Co₂MnSi Heusler compounds produced by mold casting and subsequent annealing. Fig. 1 shows the XRD diffraction patterns of the Co₂MnGa and Co₂MnSi alloy ingots. It was found that the specimens were basically composed of the Heusler structure (L 2₁ structure).

The order of Heusler alloys is a significant determinant for their properties. Thus, the structures of the Co₂MnGa and Co₂MnSi specimens were further studied by TEM. Fig. 2 shows

Conclusion

It was found that the Co₂MnGa Heusler phase transformed into the B2-type phase at 1200 K, whereas the Co₂MnSi Heusler phase was stable up to the melting temperature of 1450 K. The magnetic measurements revealed that the Co₂MnSi Heusler compound is a superior ferromagnetic material to the Co₂MnGa Heusler compound. The Co₂MnSi Heusler compound exhibited a saturation magnetization of 130 emu/g and a Curie temperature of 1015 K, both of which were higher than those of the Co₂MnGa Heusler compound.

Data availability

The data required to reproduce these findings cannot be shared at this time as the data also form part of an ongoing study.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgment

This work was performed using the facilities of the Institute for Solid State Physics, The University of Tokyo.

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Citation Excerpt :
Heusler alloys have been widely concerned by researchers due to its potential applications in many important fields [1–24]. As the most representative Co-based Heusler, it has also become a research hotspot because of their 100% spin polarization and high Curie temperatures [25–34]. High spin polarization is a typical feature of half-metallic (HM) ferromagnets and spin gapless semiconductors [35–40].
The phase structures, magnetic properties, electronic structures, and anisotropy of the full Heusler alloy Co₂NbGa were studied using the first-principles method, and the results were compared with experiments. The L2₁-type cubic structure was the most stable, which was in good agreement with the experimental measurement. The Curie temperature obtained by the mean field theory was also very close to the experimental value. The electronic structures and magnetic properties showed that the Co₂NbGa alloy was a half-metallic ferromagnet with 100% spin polarization at equilibrium, and the robustness of the half-metallic feature was preserved in lattice ranges of 5.79 to 6.42 Å. The Co₂NbGa alloy was an anisotropic material with stable mechanical properties at L2₁-type cubic and XA-type tetragonal structures, and the influence of Hubbard U and vacancy defects on the physical nature was also studied and discussed. The 100% spin polarization, high Curie temperature, and stable equilibrium structure make Co₂NbGa alloy a promising candidate in spintronics and magnetoelectronic.

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Magnetic and thermoelectric properties of Co2MnT (T = Ga, Si) Heusler compounds

Highlights

Abstract

Introduction

Section snippets

Experiment

Results and discussion

Conclusion

Data availability

Declaration of competing interest

Acknowledgment

Scripta Mater.

J. Alloys Compd.

J. Alloys Compd.

J. Alloys Compd.

J. Alloys Compd.

J. Alloys Compd.

J. Phys. Chem. Solid.

J. Alloys Compd.

Appl. Phys. Lett.

Mater. Trans.

Scripta Mater.

J. Appl. Phys.

J. Appl. Phys.

Appl. Phys. Lett.

J. Appl. Phys.

Appl. Phys. Lett.

Magnetic and thermoelectric properties of Co₂MnT (T = Ga, Si) Heusler compounds