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Article

Magnetic and Thermoelectric Properties of Fe2CoGa Heusler Compounds

by
Tetsuji Saito
* and
Hayai Watanabe
Graduate School of Engineering, Chiba Institute of Technology, Narashino 275-8588, Chiba, Japan
*
Author to whom correspondence should be addressed.
Inorganics 2025, 13(2), 33; https://doi.org/10.3390/inorganics13020033
Submission received: 25 December 2024 / Revised: 20 January 2025 / Accepted: 21 January 2025 / Published: 23 January 2025
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Abstract

:
The investigation of the properties of Heusler compounds is an important task that will pave the way for new applications in various fields related to magnetics and thermoelectrics. This study examines the magnetic and thermoelectric properties of Fe2CoGa Heusler compounds prepared by casting and subsequent annealing. The Fe2CoGa Heusler compound was found to be ferromagnetic, with a large saturation magnetization of 110 emu/g and a high Curie temperature of 1011 K. The Fe2CoGa Heusler compound was a good thermoelectric material, with a negative Seebeck coefficient of −44 μV/K, a low electrical resistivity of 0.60 μΩm, and a high-power factor of 3000 μW/mK2 at room temperature. The maximum power factor of 3230 μW/mK2 for the Fe2CoGa Heusler compound was obtained at 400 K. In order to improve the magnetic and thermoelectric properties of the Fe2CoGa Heusler compound, Fe2-xCo1+xGa (x = 0–1) Heusler compounds were also prepared by casting and subsequent annealing. In the Fe2-xCo1+xGa (x = 0–1) Heusler compounds, the saturation magnetization slightly decreased, but the Curie temperature increased with increasing Co content (x). As regards the thermoelectric properties, the electrical resistivity of the Fe2-xCo1+xGa (x = 0.25–1) Heusler compounds was smaller than that of the Fe2CoGa Heusler compound. The Seebeck coefficient and power factor of the Fe1.75Co1.25Ga Heusler compound were more significant than those of the Fe2CoGa Heusler compound. An increase in the Co content of the Fe2CoGa Heusler compound did not improve the saturation magnetization but improved the Curie temperature and thermoelectric properties of the Fe2CoGa Heusler compound. The Fe1.75Co1.25Ga Heusler compound exhibited a high-power factor value of over 4000 μW/mK2, which was comparable to that of the Bi2Te3 compound.

1. Introduction

Burning fossil fuels is still necessary to secure energy supplies, but this has a significant environmental impact; in particular, it can induce global warming. The growing concern over global warming makes it essential to reduce greenhouse gas emissions. Currently, we lose more than 60% of the energy generated from fuels worldwide, mainly in the form of waste heat [1]. To prevent the increase in global warming, it is essential to recover as much heat energy as possible and, thus, avoid wasted. However, many industries still fail to engage in waste heat utilization, since effective waste heat recovery systems have yet to be developed. As a result, various technologies for waste heat recovery are now being investigated, such as heat pipe systems, heat-to-power conversion systems, and thermoelectric systems [2,3,4,5,6]. Of these approaches, thermoelectric systems using thermoelectric materials are attracting increasing attention because they can directly convert heat to electrical energy [1]. These systems are environmentally friendly, as they emit no pollutants, are highly reliable, and require minimal maintenance thanks to their lack of moving parts. The systems can be used over a wide temperature range, from the low-temperature waste heat of automobiles to the high-temperature waste heat of power plants and factories.
Thermoelectric materials exhibit the Seebeck effect, where heat is converted into electricity. It is well known that thermoelectric materials supply the electric power used in NASA’s spacecraft, such as Voyager 1 and 2 [7]. Thermoelectric materials also exhibit the Peltier effect, by which electricity is converted into heat, namely the reverse of the Seebeck effect. Various cooling devices used with thermoelectric materials, such as thermoelectric coolers and devices used for the electric cooling of computers, are commercially available [8,9]. As regards thermoelectric materials, the energy conversion efficiency is usually evaluated by the dimensionless figure of merit (ZT = (PF/κ)T, where PF is the power factor (PF = S2σ, where S is the Seebeck coefficient, σ is the electrical conductivity), κ is the thermal conductivity, and T is absolute temperature). A bismuth telluride (Bi2Te3) compound, which exhibits high PF and ZT values [10], has been employed as the thermoelectric material in thermoelectric devices [11,12,13,14]. However, the Bi2Te3 compound cannot be used for waste heat recovery because it contains the scarce and toxic element tellurium. New thermoelectric materials with nontoxic and abundant elements should be realized while increasing energy and environmental conservation awareness.
Since thermoelectric materials can directly convert waste heat into electricity, several materials, such as Pb-Te, Fe-Si, Mg-Si, skutterudite, and Heusler alloys, have been developed [15,16,17,18,19]. Fe-based alloys are the most promising candidates for waste heat recovery applications with respect to resource and environmental issues. The iron silicide (β-FeSi2) compound is a typical Fe-based thermoelectric material [16]. Although the β-FeSi2 compound exhibits a high S value, it has a small PF value due to its low electrical conductivity. As a result, there have been several efforts to improve the thermoelectric properties of the β-FeSi2 compound for thermoelectric applications [20,21,22,23,24,25,26].
Another Fe-based thermoelectric material is the Fe-based Heusler compound. Heusler compounds have an ordered L21-type structure with the formula X2YZ, in which X and Y are transition metals and Z is an element of the IIIA, IVA, or VA group on the periodic table [27,28,29,30]. The Heusler compounds were initially known as magnetic materials but have attracted much attention because of their potential for new applications, such as magnetic-field-induced shape memory materials [31,32]. Since the Fe2VAl Heusler compound exhibits good thermoelectric properties, the Heusler compounds have attracted considerable attention as thermoelectric materials [33]. The thermoelectric properties of various Fe-based Heusler compounds, such as Fe2CoAl, Fe2TiAl, Fe2TiSi, and Fe2TiSn, have been studied [34,35,36,37]. Since the discovery that the Fe2VAl Heusler compound exhibited semiconductor-like behavior as regards electrical resistivity, this compound has been intensively investigated as a thermoelectric material [33]. Although the ZT value of the Fe2VAl Heusler compound is still smaller than that of the Bi2Te3 compound, the former exhibits a higher PF value, larger than the latter [38,39].
Another Fe-based Heusler compound, the Fe2CoGa Heusler compound, is stable and has an L21 structure [40]. There have been several studies on the Fe2CoGa Heusler compound [41,42,43,44], but they have mainly focused on the magnetic properties: the Fe2CoGa Heusler compound displays a large saturation magnetization and a high Curie temperature. Recent work has shown that the Fe2CoGa Heusler compound exhibits thermoelectric properties [44]. In this study, we investigate the thermoelectric properties of the Fe2CoGa Heusler compound. Since the Fe2CoGa Heusler compound is ferromagnetic, the substitution of Co for Fe in this compound will affect not only the magnetic properties, but also the thermoelectric properties. Therefore, we discuss the magnetic and thermoelectric properties of the Fe2+xCo1-xGa (x = 0–1) Heusler compounds.

2. Results and Discussion

2.1. Fe2CoGa Heusler Compound

First, we examined the structures of the Fe2CoGa specimen. The XRD patterns of an as-cast Fe2CoGa ingot and an annealed specimen are shown in Figure 1. The diffraction peaks in the XRD pattern of the as-cast Fe2CoGa ingot were indexed to those of the Fe2CoGa Heusler compound (L21 phase), but small unidentified peaks were also found in the XRD pattern. On the other hand, the diffraction peaks in the XRD pattern of the annealed specimen were well indexed to those of the Fe2CoGa Heusler compound. This confirms that, if we are to obtain the Fe2CoGa Heusler compound, we must anneal the as-case ingot for a long period at a high temperature. We investigated the magnetic and thermoelectric properties of the Fe2CoGa Heusler compound.
Figure 2 shows the magnetization curve of the Fe2CoGa Heusler compound. The specimen exhibited a high saturation magnetization (Ms) of 110 emu/g, which corresponds to 1.15 T in SI units. Thus, the saturation magnetization of the Fe2CoGa Heusler compound (0.240 kg/mol) was calculated to be 4.76 μB. This obtained value of 4.76 μB was slightly smaller than the reported saturation magnetization values of 5.09–6.27 μB for a bulk specimen but comparable to those of 3.89–5.3 μB for nanoparticles [42,43,44].
Since the Fe2CoGa Heusler compound was found to be ferromagnetic, we examined its thermomagnetic curve. The result is shown in Figure 3. The thermomagnetic curve of the specimen exhibited a large magnetic transition at 1011 K. This magnetic transition is considered to be the Curie temperature (Tc) of the Fe2CoGa Heusler compound. The obtained Curie temperature was smaller than the previously reported value of 1210 K for Fe2CoGa nanoparticles [43] but agrees with the value shown in the Fe-Co-Ga phase diagram [45]. This confirms that the ferromagnetic Fe2CoGa Heusler compound has a high Curie temperature.
According to the Fe-Co-Ga phase diagram, no Heusler compound (L21 phase) exists at a temperature of 1123 K. Therefore, we examined the stability of the Fe2CoGa Heusler compound (L21 phase) using DTA. Figure 4 shows the DTA curve of the Fe2CoGa Heusler compound. The specimen exhibited a small endothermic peak at around 1000 K (arrow #1) and a large endothermic peak at 1059 K (arrow #2). The first small endothermic peak was the magnetic transition. We were able to detect the magnetic transition, although we detected the Curie temperature more clearly in the thermomagnetic measurements (see Figure 3). The second large peak was the order–disorder phase transition from the ordered L21 phase to the disordered B2 phase. The observed order–disorder phase transition temperature was slightly higher than the reported value of 1043 K [41].
Next, we examined the thermoelectric properties of the Fe2CoGa Heusler compound. Figure 5 shows the Seebeck coefficient (S) of the Fe2CoGa Heusler compound as a function of the ambient temperature. The Fe2CoGa specimen was found to be an n-type thermoelectric material characterized by a negative Seebeck coefficient. The Seebeck coefficient was −44 μV/K at room temperature. The Seebeck coefficient of the specimen increased with increasing temperature. The maximum Seebeck coefficient of −48 μV/K was achieved in the Fe2CoGa Heusler compound when measured at an ambient temperature of 600 K. Thus, the Fe2CoGa Heusler compound can be considered a candidate as a thermoelectric material. A further increase in the ambient temperature did not increase the Seebeck coefficient. Therefore, in this experiment, we examined the thermoelectric properties of the Fe2CoGa Heusler compound up to an ambient temperature of 600 K.
Another essential property of a thermoelectric material is its electrical conductivity. In this experiment, we examined electrical resistivity (resistivity is the inverse of conductivity). The result is shown in Figure 6. The Fe2CoGa Heusler compound exhibited a low electrical resistivity in the temperature range investigated in this study, with a value of 0.60 μΩm at room temperature. Therefore, the electrical resistivity of the Fe2CoGa Heusler compound is sufficiently small for these compounds to be thermoelectric materials.
To evaluate the thermoelectric properties of the Fe2CoGa Heusler compound, we calculated the power factor (PF = S2/ρ). Figure 7 shows the power factor (PF) of the Fe2CoGa Heusler compound. The power factor of the specimen increased from 3000 μW/mK2 at room temperature to 3200 μW/mK2 at 400 K and then decreased with increasing ambient temperature. We found that the Fe2CoGa Heusler compound exhibited a high-power factor. The achieved power factor of the Fe2CoGa Heusler compound was almost the same as that of the Fe2VAl Heusler compound, but the power factor was lower than that of the Bi2Te3 compound [39].

2.2. Fe2-xCo1+xGa (x = 0–1) Heusler Compounds

It is known that the Seebeck coefficient of the Fe2VAl Heusler compound can be improved by substituting Co for Fe [45,46]. In the Fe2VAl Heusler compound, the shift of the Fermi level by doping or off-stoichiometry causes an enhancement in the Seebeck coefficient. As with the Fe2VAl Heusler compound, the thermoelectric properties of the Fe2CoGa Heusler compound might be improved by substituting Co for Fe. Therefore, we prepared Fe2-xCo1+xGa (x = 0–1) Heusler compounds and investigated their magnetic and thermoelectric properties.
The structures of the Fe2-xCo1+xGa (x = 0–1) annealed specimen were found to be the Heusler compound (L21 phase). The isothermal section at 973 K for the Fe-Co-Ga phase diagram is shown in Figure 8. According to the Fe-Co-Ga phase diagram shown in Figure 8, the equilibrium phase in the Fe2-xCo1+xGa (x = 0–1) annealed specimens is the Heusler compound (L21 phase) at 973 K. Thus, Fe2-xCo1+xGa (x = 0–1) Heusler compounds were obtained by annealing the as-cast ingots (see Figure 9). We investigated the magnetic and thermoelectric properties of these compounds.
We examined the magnetization curves of the Fe2-xCo1+xGa (x = 0–1) Heusler compounds, and we plot the saturation magnetization values that we obtained in Figure 10. The saturation magnetization of the Fe2-xCo1+xGa (x = 0–1) specimens decreased slightly from 110 emu/g to 106 emu/g as the Co content increased. Although cobalt’s saturation magnetization is lower than iron’s, Fe-Co alloys have a higher saturation magnetization than Fe [48,49]. Thus, substituting Co for Fe in Fe-based alloys such as Nd-Fe-B magnets results in an increase in the saturation magnetization [50]. However, the substitution of Co for Fe in the FeCo2Ga Heusler compound resulted in a decrease in the saturation magnetization.
We examined the thermomagnetic curves of the Fe2-xCo1+xGa (x = 0–1) Heusler compounds, and the Curie temperature values that we obtained are summarized in Figure 11. The Curie temperature of the Fe2-xCo1+xGa (x = 0–1) Heusler compounds increased with the Co content. Unlike the case of the saturation magnetization, the substitution of Co for Fe in the Fe2CoGa Heusler compound increased its Curie temperature from 1011 K for the Fe2CoGa Heusler compound to 1109 K for the FeCo2Ga Heusler compound. Cobalt is known as the only alloying element that substantially increases the Curie temperature of iron [49]. For example, the substitution of Co for Fe in Nd-Fe-B magnets increases their Curie temperature [50]. It was found that the substitution of Co for Fe in the FeCo2Ga Heusler compound also resulted in an increase in the Curie temperature.
The Seebeck coefficient of the Fe2-xCo1+xGa (x = 0–1) Heusler compounds is shown in Figure 12. The substitution of Co for Fe in the Fe2CoGa Heusler compound did not change the nature of the thermoelectric material; that is to say, Fe2-xCo1+xGa (x = 0–1) Heusler compounds are n-type thermoelectric materials. However, the Co substitution significantly affects the Seebeck coefficient value. Regardless of the ambient temperature, a small Co substitution resulted in an increase in the Seebeck coefficient, but a large Co substitution resulted in a decrease in the Seebeck coefficient. The maximum Seebeck coefficient of −52 μV/K was achieved in the Fe1.75Co1.25Ga Heusler compound when measured at an ambient temperature of 600 K. This indicates that the Co substitution can improve the Seebeck coefficient of the Fe2CoGa Heusler compound as with the Fe2VAl Heusler compound.
The electrical resistivity of the Fe2-xCo1+xGa (x = 0–1) Heusler compounds is shown in Figure 13. Regardless of the ambient temperature, the electrical resistivity of the Fe2-xCo1+xGa (x = 0–1) Heusler compounds decreased with increasing Co content. The Fe2CoGa Heusler compound had a low electrical resistivity of 0.60 μΩm at room temperature, but the FeCo2Ga Heusler compound exhibited a lower electrical resistivity of 0.20 μΩm at room temperature. This indicates that the Co substitution can also improve the electrical resistivity of the Fe2CoGa Heusler compound.
Finally, the power factors (PF) of the Fe2-xCo1+xGa (x = 0–1) Heusler compounds are shown in Figure 14. In the power factor (PF = S2/ρ), the effect of the Seebeck coefficient (S) is stronger than that of the electrical resistivity (ρ). In this experiment, the maximum Seebeck coefficient was achieved in the Fe1.75Co1.25Ga Heusler compound, so the maximum power factor was also achieved in the Fe1.75Co1.25Ga Heusler compound. The power factor of the Fe1.75Co1.25Ga Heusler compound increased from 4000 μW/mK2 at room temperature to 4270 μW/mK2 at 400 K and then decreased as the ambient temperature increased. This confirms that the Co substitution can improve the thermoelectric properties of the Fe2CoGa Heusler compound. The achieved power factor of the Fe2CoGa Heusler compound was almost the same as that of the Bi2Te3 compound (PF = 3800 μW/mK2 at 320 K) [39]. However, this value is not yet comparable to that of the Ta-doped Fe2VAl compound (PF = 6500 μW/mK2 at 340 K) [40]. The power factor of the Fe2CoGa Heusler compound would be further improved by doping other elements.

3. Materials and Methods

3.1. Sample Preparation

Fe-Co-Ga alloys with a nominal composition of Fe2CoGa were prepared from Fe (99.9%), Co (99.9%), and Ga (99.9%) metals. The metal mixtures were induction-melted and then cast into a Cr-coated copper mold in an argon atmosphere. The as-cast ingots were cylindrical, 3 mm in diameter, and 30–50 mm long. The ingots were wrapped in tantalum foil and then annealed in an argon atmosphere at 1073 K for 20 h to stabilize them and obtain a Heusler structure. Fe-Co-Ga alloys with a nominal composition of Fe2+xCo1-xGa (x = 0–1) were also prepared under the same conditions.

3.2. Characterization

For the property measurements, disc-shaped specimens (0.5 mm thickness and 3 mm in diameter) and cylindrical specimens (8 mm thickness and 3 mm in diameter) were cut from the cast ingots. The phases of the specimens were characterized by X-ray diffraction (XRD) using Cu Kα radiation (MiniFlex600, Rigaku, Tokyo, Japan) and differential thermal analysis (DTA) in an argon atmosphere at a heating rate of 0.16 K/s (STA7300, Hitachi Hightech, Tokyo, Japan). The magnetic properties of the specimens were characterized using a vibrating sample magnetometer (VSM) with an applied magnetic field of 25 kOe (BHV-525RSCM, Riken Denshi, Tokyo, Japan). Disc-shaped specimens were measured parallel to the disc surface in the hysteresis measurements to avoid the need for demagnetization correction. Thermomagnetic curves for the specimens were obtained using DTA equipped with a permanent magnet. The thermoelectric properties of the specimens were characterized using a Seebeck coefficient/electric resistance measurement system (ZEM-3M8, Advance Riko, Yokohama, Japan). The Seebeck coefficient (S) of the cylindrical specimens was determined by measuring the upper and lower temperatures with thermocouples, and the electric resistance (ρ) was measured with the standard DC four-terminal method in a low-pressure helium atmosphere. The thermoelectric measurements are detailed in Ref. [34]. The power factors (PF) of the specimens were determined using the equation PF = S2/ρ, where S is the Seebeck coefficient and ρ is the electrical resistance (ρ) value.

4. Conclusions

The Fe2CoGa Heusler compound was found to be ferromagnetic, with a large saturation magnetization (Ms) of 110 emu/g and a high Curie temperature (Tc) of 1011 K. It was found that the Fe2CoGa Heusler compound is a good thermoelectric material, with a Seebeck coefficient (S) of −44 μV/K and a high power factor (PF) of 3000 μW/mK2 at room temperature. The magnetic and thermoelectric properties of the Fe2CoGa Heusler compound were improved by substituting Co for Fe in the Fe2CoGa Heusler compound. The Co substitution did not increase the saturation magnetization but led to an increase in the Curie temperature. Regarding the thermoelectric properties, a small Co substitution greatly improved the thermoelectric properties, but a large Co substitution decreased the thermoelectric properties. The Fe1.75Co1.25Ga Heusler compound exhibited a high-power factor (PF) value of over 4000 μW/mK2 at room temperature, comparable to that of the Bi2Te3 compound.

Author Contributions

T.S.: methodology, writing, formal analysis, original draft preparation; H.W.: methodology, investigation, formal analysis. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

Data are contained within the article.

Acknowledgments

The technical support of K. Ito is acknowledged.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. XRD patterns of (a) the as-cast Fe2CoGa ingot and (b) the annealed specimen.
Figure 1. XRD patterns of (a) the as-cast Fe2CoGa ingot and (b) the annealed specimen.
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Figure 2. Magnetization curve of the Fe2CoGa Heusler compound.
Figure 2. Magnetization curve of the Fe2CoGa Heusler compound.
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Figure 3. Thermomagnetic curves of the Fe2CoGa Heusler compound.
Figure 3. Thermomagnetic curves of the Fe2CoGa Heusler compound.
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Figure 4. DTA curve of the Fe2CoGa Heusler compound.
Figure 4. DTA curve of the Fe2CoGa Heusler compound.
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Figure 5. Seebeck coefficient as a function of the ambient temperature for the Fe2CoGa Heusler compound.
Figure 5. Seebeck coefficient as a function of the ambient temperature for the Fe2CoGa Heusler compound.
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Figure 6. Electrical resistivity as a function of the ambient temperature for the Fe2CoGa Heusler compound.
Figure 6. Electrical resistivity as a function of the ambient temperature for the Fe2CoGa Heusler compound.
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Figure 7. Power factor as a function of ambient temperature for the Fe2CoGa Heusler compound.
Figure 7. Power factor as a function of ambient temperature for the Fe2CoGa Heusler compound.
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Figure 8. Fe-Co-Ga phase diagram with the composition of Fe2-xCo1+xGa (x = 0–1) [47].
Figure 8. Fe-Co-Ga phase diagram with the composition of Fe2-xCo1+xGa (x = 0–1) [47].
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Figure 9. XRD patterns of the Fe2-xCo1+xGa (x = 0–1) Heusler compounds.
Figure 9. XRD patterns of the Fe2-xCo1+xGa (x = 0–1) Heusler compounds.
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Figure 10. Saturation magnetization of the Fe2-xCo1+xGa (x = 0–1) Heusler compounds.
Figure 10. Saturation magnetization of the Fe2-xCo1+xGa (x = 0–1) Heusler compounds.
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Figure 11. Curie temperatures of the Fe2-xCo1+xGa (x = 0–1) Heusler compounds.
Figure 11. Curie temperatures of the Fe2-xCo1+xGa (x = 0–1) Heusler compounds.
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Figure 12. Seebeck coefficient as a function of the ambient temperature for the Fe2-xCo1+xGa (x = 0–1) Heusler compound.
Figure 12. Seebeck coefficient as a function of the ambient temperature for the Fe2-xCo1+xGa (x = 0–1) Heusler compound.
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Inorganics 13 00033 g013
Figure 13. Electrical resistivity as a function of the ambient temperature for the Fe2-xCo1+xGa (x = 0–1) Heusler compound.
Figure 13. Electrical resistivity as a function of the ambient temperature for the Fe2-xCo1+xGa (x = 0–1) Heusler compound.
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Figure 14. Power factors (PF) as a function of the ambient temperature for the Fe2-xCo1+xGa (x = 0–1) Heusler compound.
Figure 14. Power factors (PF) as a function of the ambient temperature for the Fe2-xCo1+xGa (x = 0–1) Heusler compound.
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Saito, T.; Watanabe, H. Magnetic and Thermoelectric Properties of Fe2CoGa Heusler Compounds. Inorganics 2025, 13, 33. https://doi.org/10.3390/inorganics13020033

AMA Style

Saito T, Watanabe H. Magnetic and Thermoelectric Properties of Fe2CoGa Heusler Compounds. Inorganics. 2025; 13(2):33. https://doi.org/10.3390/inorganics13020033

Chicago/Turabian Style

Saito, Tetsuji, and Hayai Watanabe. 2025. "Magnetic and Thermoelectric Properties of Fe2CoGa Heusler Compounds" Inorganics 13, no. 2: 33. https://doi.org/10.3390/inorganics13020033

APA Style

Saito, T., & Watanabe, H. (2025). Magnetic and Thermoelectric Properties of Fe2CoGa Heusler Compounds. Inorganics, 13(2), 33. https://doi.org/10.3390/inorganics13020033

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