The Impact of the Composition Effect on Ferromagnetic Properties of Tb2Co2Ga

Tanaka, Seiya; Terada, Hirotaka; Shirakawa, Naoki; Tsubota, Masami; Kitagawa, Jiro

doi:10.3390/met9121242

Open AccessArticle

The Impact of the Composition Effect on Ferromagnetic Properties of Tb₂Co₂Ga

by

Seiya Tanaka

¹,

Hirotaka Terada

¹,

Naoki Shirakawa

²

,

Masami Tsubota

³

and

Jiro Kitagawa

^1,*

¹

Department of Electrical Engineering, Faculty of Engineering, Fukuoka Institute of Technology, 3-30-1 Wajiro-higashi, Higashi-ku, Fukuoka 811-0295, Japan

²

Flexible Electronics Research Center, National Institute of Advanced Industrial Science and Technology, Tsukuba, Ibaraki 305-8565, Japan

³

Physonit Inc., 6–10 Minami-Horikawa, Kaita Aki, Hiroshima 736-0044, Japan

^*

Author to whom correspondence should be addressed.

Metals 2019, 9(12), 1242; https://doi.org/10.3390/met9121242

Submission received: 18 October 2019 / Revised: 11 November 2019 / Accepted: 19 November 2019 / Published: 20 November 2019

(This article belongs to the Special Issue Advanced Magnetic Materials)

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Abstract

:

The ferromagnetic properties of Tb₂Co₂Ga, crystallizing into an orthorhombic W₂CoB₂-type structure, were investigated by preparing 11 polycrystalline samples with different starting atomic compositions. We found that Tb₂Co₂Ga possesses a homogeneity range in the ternary phase diagram. The Curie temperature T_C is sensitive to the atomic composition and ranges rather widely, i.e., from 75 to 145 K. For the samples with a T_C above 90 K, the nearest Tb–Tb and the Tb–Co distances would be important factors deciding T_C, considering the RKKY interaction through the hybridization between Tb and Co atoms. An anisotropic change of two kinds of Co–Tb–Co angles in the octahedron formed by two Tb and four Co atoms occurs in the samples with a T_C lower than 90 K. Such a change of octahedral parameters seems to be related to a difference of shapes in the ac magnetization anomaly at T_C between the samples in the lowest T_C (~ 75 K) group and those in the other groups.

Keywords:

ferromagnetism; W₂CoB₂-type structure; composition effect; magnetic measurements; lattice parameters

1. Introduction

Several ferromagnetic compounds of R₂Co₂Al and R₂Co₂Ga (R = heavy rare earth) with an orthorhombic W₂CoB₂ (Mo₂NiB₂) -type structure [1] are attractive as magnetic refrigeration materials [2,3,4,5]. Recently, we studied the orthorhombic W₃CoB₃-type Tb₃Co₃Ga that is located near Tb₂Co₂Ga in the ternary phase diagram [6]. A polycrystalline Tb₃Co₃Ga sample also showed ferromagnetism, but the Curie temperature T_C ranged from 90 K to 117 K, depending on the starting atomic composition. The actual atomic composition that correlates with the starting composition has revealed the existence of a homogeneity range, especially along the Ga concentration. The crystal structure parameters, which slightly change with the atomic composition, severely affect T_C.

It has been recently reported [7] that Tb₂Co₂Ga is a ferromagnet with T_C = 86 K. Figure 1a,b shows the crystal structure of Tb₂Co₂Ga. In the orthorhombic structure, with the space group of Immm (No.71), Tb, Co, and Ga atoms occupy the crystallographic 4h, 4f, and 2a sites, respectively. Two Tb and the nearest neighboring four Co atoms build an octahedron, the arrangement of which can be regarded as a rectangular lattice in the a-b plane (see Figure 1b). The rectangular lattice stacks along the c-axis. The isostructural Tb₂Co₂Al has been reported to be a ferromagnet [4] below 111 K. A neutron diffraction study [4] has clarified that Tb and Co sublattices form a c-axis ferrimagnetic ordering. The magnetic moment of Tb at 2 K is 8.86 μ_B, which dominates over that of Co with 0.26 μ_B.

As in the case of Tb₃Co₃Ga, we can expect a composition effect for Tb₂Co₂Ga. If a homogeneity range exists in a metallurgical phase diagram, the magnetic ordering temperature often varies depending on the atomic composition. Such a composition effect is actually observed [8] in Nd₃Pd₂₀Ge₆, which shows a slight decrease of the Néel temperature as the Pd atomic composition is increased from 19.2 to 21. Another example is Mn_1+xGa, which is a ferromagnet showing a reduced T_C with decreasing Mn concentration [9]. The study that found this effect presented a detailed composition effect of ferromagnetic properties that depend on the crystal structure parameters.

If Tb₂Co₂Ga shows a composition effect based on ferromagnetic properties, the effect would depend on the crystal structure parameters, which may play a crucial role in the design of magnetic refrigeration materials with a W₂CoB₂ -type structure. Therefore, it is important to know the relationships between the crystal structure parameters and ferromagnetic properties in W₂CoB₂ -type compounds; however, this has not been well explored. In this paper, we report the composition effect of the magnetic properties of polycrystalline Tb₂Co₂Ga by a metallographic examination and measurement of the magnetic susceptibility and the magnetization curve.

2. Materials and Methods

Eleven polycrystalline samples, A to K, with different starting atomic compositions, as listed in Table 1, were synthesized by a home-made arc furnace. The constituent elements of Tb (Nippon Yttrium, Omuta, Japan, bulk, 99.9%), Co (Kojundo Chemical Laboratory, Sakado, Japan, bulk, 99.9%) and Ga (Kojundo Chemical Laboratory, Sakado, Japan, bulk, 99.99%), placed on a water-cooled Cu hearth, were arc-melted in an Ar atmosphere. Each as-cast sample was sealed in an evacuated quartz tube, which was heated to 650 °C and held at that temperature for 4 days. In Table 1, all the samples are labeled into four groups (G1, G2, G3, and G4) based on the value of T_C, and the samples in the G4 group are further divided into three subgroups (SG4-1, SG4-2 and SG4-3), as explained below. The starting atomic composition covers Tb: 39–45%, Co: 36–42% and Ga: 17–24%. The location of each sample, with its starting atomic composition, is depicted in the Tb–Co–Ga ternary phase diagram (see Figure 2a), in which the impurity phases detected in this study are also marked.

X-ray diffraction patterns were recorded by using a powder X-ray diffractometer (XRD-7000L Shimadzu, Kyoto, Japan,) with Cu-Kα radiation. The 2θ range was between 10° and 90°. The microstructure of the prepared sample was checked by a field emission scanning electron microscope (JSM-7100F FE-SEM; JEOL, Akishima, Japan,), and the atomic composition in each area of the sample was determined by an energy dispersive X-ray (EDX) spectrometer attached to the FE-SEM.

The temperature dependence of ac magnetic susceptibility χ_ac (T) between 3 K and 300 K was measured by a home-made system in a closed-cycle He gas cryostat. The amplitude and frequency of the ac field were 5 Oe and 800 Hz, respectively. The temperature dependence of dc magnetic susceptibility χ_dc (T) between 5 K and 300 K, and the magnetization curves were measured by a MPMS magnetometer (Quantum Design, San Diego, CA, USA).

3. Results and Discussion

3.1. XRD Pattern and Microstructure

The X-ray diffraction (XRD) patterns of all samples are shown in Figure 3, which also includes the simulated pattern of Tb₂Co₂Ga. Each pattern of the main phase matches with that of Tb₂Co₂Ga, while obvious impurity peaks appear in the samples H to K. The impurity phases are discussed in the results of EDX measurement. From samples A to K, each peak position shifts, indicating a change of lattice parameters and the existence of a homogeneity range. The composition dependences of the crystal structure parameters are explained after showing the results of metallographic examinations.

Back-scattered electron images obtained by FE-SEM with electron beams of 15 keV are shown in Figure 4a–f for the samples A to F, and in Figure 5a–e for samples G to K, respectively. The atomic compositions obtained by EDX measurement of all the samples are listed in Table 1 and Table 2. The number of data collection points per sample is 10. The composition of the main phase for each sample is close to Tb₂Co₂Ga. In the samples A to C, the compositions of the Tb₂Co₂Ga phases are almost stoichiometric. Going on to samples C to K, a gradual replacement of Tb and Co atoms by Ga atom is observed (see Table 1). Thus, Tb₂Co₂Ga possesses a homogeneity range as indicated by black broken lines in Figure 2b. In this study, the homogeneity range is defined as the composition range detected by EDX for a target compound. The samples A and B contain a small amount of Tb₃Co₃Ga phase, located near these samples in the ternary phase diagram (see Figure 2a). When the Ga concentration of the Tb₂Co₂Ga phase is below 21.7%, it is accompanied by the N phase (see Table 1 and Table 2), the amount of which is rather large in samples B and D (see Figure 4b,d). This is consistent with the phase relation displayed in Figure 2b. The pseudo binary TbCo_2−xGa_x appears in samples A, C, G, H, I, and K, which are located near TbCo_2−xGa_x (see Figure 2a). Because the locations of samples G to K are rather far away from that of the N phase, these samples do not contain the N phase; however, another impurity phase is detected, shown as small bright areas in Figure 5a–e. The impurity phase is presumably Tb_1−xCo_xGa for the samples G, H, I, and K, or TbCo_xGa_1−x for sample J. In sample J, the Tb₂Co₂Ga phase seems to be inhomogeneous, which is deduced by the observation of two phases with slightly different atomic compositions, and the χ_ac (T) results, as mentioned below.

The lattice parameters of all samples were obtained by the least square method with the help of RIETAN-FP program [10,11] using the XRD patterns listed in Table 3. The actual fitting range of 2θ was between 10° and 90°, in which impurity phases with known crystal structures are also considered, but diffraction peaks originating from impurity phases with unknown structures are eliminated. Focusing on the Ga-concentration dependence of the obtained lattice parameters, it can be said that a increases with the increase of Ga concentration (see also Figure 9a). On the other hand, b (c) monotonously increases (decreases) from sample A to F (in the G1, G2 and G3 groups); the sample dependence is not so straightforward in the G4 group. Samples G to I show the slight contraction of b compared to sample F, but the opposite behavior is observed in samples J and K. Compared to c of sample F, c’s of samples G and H are decreased, although samples I, J, and K show an increase of c.

3.2. Magnetic Properties

Figure 6a–d show the χ_ac (T) of all samples, categorized by the T_C of the Tb₂Co₂Ga phase, indicated by arrows. The ferromagnetic states of several samples were checked by magnetization curves, as mentioned below. As in other ferromagnetic compounds [12,13], the peak position of χ_ac (T) is employed as T_C, which would be systematically governed by the atomic composition (see also Table 3). In the inset of Figure 6a, the χ_ac (T) of the sample with the starting composition Tb_42.9Co₅₀Ga_7.1 is exhibited. This sample contains the pseudo binary TbCo_2-xGa_x alloy and the N phase [6]. Two pronounced peaks are observed at approximately 250 and 60 K, for which the pseudo binary TbCo_2−xGa_x alloy and the N phase are responsible, respectively [6]. The x value of TbCo_2−xGa_x is estimated to be 0.22, corresponding to that of TbCo_2−xGa_x exhibiting ferromagnetic transitions at approximately 260 K [12]. The χ_ac peaks of sample A at approximately 260 K and 55 K can also be ascribed to the parasitic phases of TbCo_2−xGa_x and the N phase, respectively. Another small hump at approximately 90 K is due to the impurity phase of Tb₃Co₃Ga. These assignments are supported by the metallographic examination combined with the EDX measurement of sample A (see Table 1 and Table 2). In sample B, the χ_ac peak at 55 K evolves in accordance with the increased area of the N phase observed by the FE-SEM image. The T_C intrinsic to the Tb₂Co₂Ga phase is approximately 145 K for each sample. As shown in Figure 6b, sample C shows almost a single peak at 101 K. The small amount of N phase detected by EDX is responsible for a small hump at 60 K. The enlarged N phase in sample D leads to an obvious χ_ac peak at 60 K, in addition to the ferromagnetic transition due to the Tb₂Co₂Ga phase at 102 K. In samples E and F (see Figure 6c), the T_C of the Tb₂Co₂Ga phase is further reduced to approximately 90 K, and a small anomaly due to the N phase is observed in sample E. In sample F, an χ_ac anomaly due to the N phase might be masked by a large χ_ac peak at 90 K.

Figure 6d shows the χ_ac (T) of the G4 group. Except for sample J, two small anomalies due to impurity phases are observed, indicated by filled triangles in each sample. Tb_1−xCo_xGa and TbCo_2−xGa_x are responsible for the low and high temperature anomalies, respectively. The value of x in TbCo_2−xGa_x is around 0.47 for each sample of G, H, and I, and 0.7 for sample K, respectively. The reduction trend of the magnetic transition temperature in TbCo_2−xGa_x with increasing x is in agreement with the results in the literature [12]. In sample J, the peak at approximately 140 K would be induced by TbCo_1−xGa_x, and the χ_ac peak due to the Tb₂Co₂Ga phase is rather broad, reflecting the coexistence of two Tb₂Co₂Ga phases with slightly different atomic compositions. Tb_1−xCo_xGa and TbCo_1−xGa_x may be related to TbGa with T_C = 154 K [14], which probably supports the assignment of magnetic impurity phases in Figure 6d. We remark that the shapes of the χ_ac peaks at T_C of the Tb₂Co₂Ga phases in the G4 group somewhat differ from those of the G1 to G3 groups. For the G4 group (Figure 6d), χ_ac shows a λ-type anomaly, and the χ_ac peaks of the G1 to G3 groups (Figure 6a–c) are more symmetric with respect to the temperature-axis.

Figure 7a shows χ_dc (T) under the external field of 10 Oe of samples A, C, F, and G, respectively. In each sample, χ_dc steeply increases below the approximate T_C of the Tb₂Co₂Ga phase. Figure 7b shows the temperature dependences of the inverse χ_dc of the four samples, which follow the Curie-Weiss law above 285 K, 275 K, 180 K, and 255 K, respectively (see the lines in Figure 7b). The effective magnetic moment μ_eff and the Weiss temperature Θ are summarized in Table 3. The value of μ_eff is near 9.72 μ_B/Tb, as expected for a free trivalent Tb ion, except in sample A, in which the narrow temperature range for the Curie–Weiss fitting may disturb an accurate extraction of μ_eff. From the χ_dc data, it is not clear whether the Co atom carries a magnetic moment or not.

Figure 8a–d exhibit the M-H (M: magnetization and H: external field) curves measured at several temperatures denoted in the figures for samples A, C, F and G, respectively. In samples A, C, and F, below the T_C of the Tb₂Co₂Ga phases obvious hystereses grow, which are characteristic of ferromagnetism. For sample G in the G4 group, soft ferromagnetic behavior is observed down to 40 K, which might suggest that the ferromagnetic nature differs from those of samples A (G1), C (G2), and F (G3).

3.3. Relationship between the Ferromagnetic Properties and Crystal Structure Paremeters

Hereafter, we discuss the relationship between the crystal structure parameters and T_C. Figure 9a–c are the 3D bar graphs of T_C as a function of a and b, a and c, and b and c, respectively. In all figures, the G1 to G3 groups show systematic lattice parameter dependences of T_C; the increase of a(b) and the decrease of c reduce T_C. On the other hand, from the G3 to G4 group, the systematic dependence is not observed. All figures indicate that the samples in the G4 group can be divided into three subgroups of SG4-1, SG4-2, and SG4-3. Moreover, in the ternary phase diagram shown in Figure 2b, the starting atomic compositions of samples in the SG4-1 (SG4-3) subgroup are located in the region with a Tb concentration higher (lower) than that of sample I (SG4-2). As shown in Table 1, the order of the actual Tb compositions in the G4 group is consistent with that of the starting compositions, although the difference between the samples is smaller for the actual composition. Therefore, there might be some correlation between the lattice parameters and the Tb concentration for the G4 group. We note that the T_C of the sample in the G4 group seems to be slightly reduced with decreasing Tb concentration (see Table 3).

Shown in Figure 10a,b are the 3D bar graphs of T_C as a function of Tb–Tb and Tb–Co, and θ₁ and θ₂, respectively, especially focusing on the octahedron formed by the Tb and Co atoms (see also Figure 1). The Tb–Tb (Tb–Co) means the nearest Tb–Tb (Tb–Co) distance, and θ₁ and θ₂ are Co–Tb–Co angles in the octahedron. From the G1 to G3 groups, as the nearest Tb–Tb (Tb–Co) distance monotonously decreases (increases), T_C is reduced from ~145 K to ~90 K. Then, both θ₁ and θ₂ become wider, and Figure 10a,b for the G1 to G3 groups suggest that the octahedron being compressed along the c-axis leads to the T_C reduction. The expansion of Tb–Co distance in particular means a weakened RKKY interaction through the weakened hybridization between the Tb and Co atoms. This might be responsible for the reduction of T_C down to 90 K. At the present stage, the origin of further reductions of T_C down to ~75 K is unclear; however, a possible change of ferromagnetic properties in the G4 group can be conjectured as below. From the G3 to the G4 group, θ₁ becomes narrower, while on the other hand, θ₂ apparently continues to increase. The anisotropic behavior of the octahedral parameters is related to the difference of shapes of the χ_ac peaks at T_C between the samples in the G1 to G3 groups and those in the G4 group, as mentioned above.

4. Conclusions

The composition effect of the ferromagnetic properties in Tb₂Co₂Ga, crystallizing into an orthorhombic W₂CoB₂-type structure, was investigated. The atomic composition obtained by the EDX measurement revealed the existence of a homogeneity range. The compound shows a remarkable composition effect of T_C that is sensitive to the atomic composition, and ranges rather widely, i.e., from 75 to 145 K. The composition dependence of T_C is ascribed to the differences in the crystal structure parameters. For the samples in the G1 to G3 groups, the Tb–Tb and Tb–Co distances would be important factors in determining T_C. The decrease of the Tb–Tb distance leads to an increase of the Tb–Co distance, which weakens the RKKY interaction through weakened hybridization between the Tb and Co atoms. The samples in the G4 group can be divided into three subgroups by the lattice parameter dependences of T_C, but the θ₁ and θ₂ angles in each sample show similar values. The anisotropic change of two angles below a T_C of ~90 K makes the crystal feature of the octahedron different from that of the samples in the G1 to G3 groups. In reality, the anisotropic modification of the octahedron in the sample of the G4 group would be reflected in the λ-type χ_ac anomaly at T_C, which differs from those of the samples in the G1 to G3 groups.

Author Contributions

Conceptualization, J.K., N.S. and M.T.; methodology, J.K., N.S and M.T.; formal analysis, J.K. and M.T.; investigation, S.T., H.T. and N.S.; writing—original draft preparation, J.K.; writing—review and editing, J.K., N.S. and M.T.

Funding

This research received no external funding.

Acknowledgments

J.K. is grateful for the support provided by the Comprehensive Research Organization of Fukuoka Institute of Technology.

Conflicts of Interest

The authors declare no conflict of interest.

References

Rieger, W.; Nowotny, H.; Benesovsky, F. Die kristallstruktur von W₂CoB₂ und isotypen phasen. Mon. Chem. 1966, 97, 378–382. [Google Scholar] [CrossRef]
Sengupta, K.; Iyer, K.K.; Sampathkumaran, E.V. Large magnetoresistance and magnetocaloric effect above 70 K in Gd₂Co₂Al, Gd₂Co₂Ga, and Gd₇Rh₃. Phys. Rev. B 2005, 72, 054422. [Google Scholar] [CrossRef]
Fu, H.; Zou, M.; Guo, M.S.; Zheng, Q.; Zu, X.T. Structural, magnetic, and magnetothermal properties of R₂Co₂Al (R = Tb, and Dy) compounds. Mater. Charact. 2011, 62, 451–455. [Google Scholar] [CrossRef]
Morozkin, A.V.; Genchel, V.K.; Garchev, A.V.; Yapaskurt, V.O.; Isnard, O.; Yao, J.; Nirmala, R.; Quezado, S.; Malik, S.K. Magnetic ordering of Mo₂NiB₂-type (Gd, Tb, Dy)₂Co₂Al compounds by magnetization and neutron diffraction study. J. Magn. Magn. Mater. 2017, 442, 36–44. [Google Scholar] [CrossRef]
Morozkin, A.V.; Garchev, A.V.; Yapaskurt, V.O.; Yao, J.; Nirmala, R.; Quezado, S.; Malik, S.K. Mo₂NiB₂-type Sm₂Co₂Al and Sm₂Co₂Ga compounds: Magnetic properties and giant low-temperature coercivity. J. Solid State Chem. 2018, 260, 95–100. [Google Scholar] [CrossRef]
Kitagawa, J.; Terada, H.; Shirakawa, N.; Tsubota, M.; Nose, A.; Tanaka, S. Composition effect in ferromagnetic properties of Tb₃Co₃Ga. Res. Phys. 2019, 15, 102591. [Google Scholar] [CrossRef]
Morozkin, A.V.; Yao, J.; Knotko, A.V.; Yapaskurt, V.O. Low-temperature coercivity of Mo₂NiB₂-type Tb₂Co₂Ga and Tb₂Co₂Al-based solid solutions. J. Solid State Chem. 2019, 277, 406–414. [Google Scholar] [CrossRef]
Kitagawa, J.; Takeda, N.; Sakai, F.; Ishikawa, M. Effect of composition in (RE) ₃Pd ₂₀Ge ₆ (RE=La, Ce and Nd). J. Phys. Soc. Jpn. 1999, 68, 3413–3416. [Google Scholar] [CrossRef]
Lu, Q.M.; Yue, M.; Zhang, H.G.; Wang, M.L.; Yu, F.; Huang, Q.Z.; Ryan, D.H.; Altounian, Z. Intrinsic magnetic properties of single-phase Mn1+xGa (0 < x < 1) alloys. Sci. Rep. 2015, 5, 17086. [Google Scholar] [PubMed]
Izumi, F.; Momma, K. Three-dimensional visualization in powder diffraction. Solid State Phenom. 2007, 130, 15–20. [Google Scholar] [CrossRef]
Tsubota, M.; Kitagawa, J. A necessary criterion for obtaining accurate lattice parameters by Rietveld method. Sci. Rep. 2017, 7, 15381. [Google Scholar] [CrossRef] [PubMed]
Dębiec, I.; Chełkowska, G. Magnetic, electric and XPS study of Tb(Co_1−xGa_x)₂ compounds. J. Magn. Magn. Mater. 2003, 261, 73–77. [Google Scholar] [CrossRef]
Miyahara, J.; Shirakawa, N.; Setoguchi, Y.; Tsubota, M.; Kuroiwa, K.; Kitagawa, J. Hill plot focusing on Ce compounds with high magnetic ordering temperatures and consequent study of Ce₂AuP₃. J. Supercond. Nov. Magn. 2018, 31, 3559–3564. [Google Scholar] [CrossRef]
Zheng, X.Q.; Chen, J.; Shen, J.; Zhang, H.; Xu, Z.Y.; Gao, W.W.; Wu, J.F.; Hu, F.X.; Sun, J.R.; Shen, B.G. Large refrigerant capacity of RGa (R = Tb and Dy) compounds. J. Appl. Phys. 2012, 111, 07A917. [Google Scholar] [CrossRef] [Green Version]

Figure 1. (a) Unit cell of Tb₂Co₂Ga. (b) Crystal structure of Tb₂Co₂Ga, emphasizing the polyhedral arrangement. The solid line represents the unit cell. θ₁ and θ₂ are Co–Tb–Co angles in the octahedron formed by two Tb atoms and the nearest neighboring four Co atoms.

Figure 2. (a) Ternary phase diagram of the Tb–Co–Ga system for prepared samples plotted based on their starting compositions (filled colored circles). The impurity phases (Tb₃Co₃Ga, the N phase, TbCo_2−xGa_x, Tb_1−xCo_xGa and TbCo_xGa_1−x) are also added. The area surrounded by broken lines indicates a range of the solid solution detected by EDX measurement. The homogeneity range of Tb₃Co₃Ga is depicted by the black triangles. (b) Expanded diagram around the Tb₂Co₂Ga phase. The area surrounded by black broken-lines indicates a range of the solid solution of Tb₂Co₂Ga detected by EDX measurement.

Figure 3. XRD patterns of prepared samples. The simulated pattern of Tb₂Co₂Ga with a = 4.103 Å, b = 5.421 Å and c = 8.433 Å is also shown. The origin of each pattern is shifted by an integer value.

Figure 4. Back-scattered electron (15 keV) images of samples (a) A, (b) B, (c) C, (d) D, (e) E, and (f) F, respectively.

Figure 5. Back-scattered electron (15 keV) images of samples (a) G, (b) H, (c) I, (d) J, and (e) K, respectively.

Figure 6. Temperature dependences of χ_ac for samples (a) A and B, (b) C and D, (c) E and F, and (d) G to K, respectively. The origin of each χ_ac is shifted by a value for clarity. The inset of (a) is the χ_ac (T) of the sample, with the starting composition of Tb_42.9Co₅₀Ga_7.1.

Figure 7. (a) Temperature dependences of χ_dc for samples A, C, F, and G. The external field is 10 Oe. (b) Temperature dependences of 1/χ_dc for samples A, C, F, and G.

Figure 8. M-H curves measured at several temperatures as denoted in the figures for samples (a) A, (b) C, (c) F, and (d) G, respectively.

Figure 9. 3D bar graphs of T_C as a function of (a) a and b, (b) a and c, and (c) b and c, respectively.

Figure 10. 3D bar graphs of T_C as a function of (a) Tb–Tb and Tb–Co, and (b) θ₁ and θ₂, respectively. The errors of Tb–Tb and Tb–Co distances are 0.0005 Å, 0.0003 Å, respectively. That of θ₁ and θ₂ is 0.007°.

Table 1. Starting atomic compositions and determined compositions of the main phase by EDX measurement for prepared samples.

Group	Subgroup	Sample	Starting Composition	Tb₂Co₂Ga Main Phase
G1	-	A	Tb_41.7Co_41.7Ga_16.6	Tb_40.1(6)Co_39.8(8)Ga_20.1(4)
G1	-	B	Tb_45.0Co_38.0Ga_17.0	Tb_39.9(8)Co_40.7(4)Ga_19.4(9)
G2	-	C	Tb_41.4Co_39.4Ga_19.2	Tb_39.9(3)Co_39.4(2)Ga_20.6(5)
G2	-	D	Tb_43.9Co_36.8Ga_19.3	Tb_40.3(8)Co_38.8(9)Ga_21.0(2)
G3	-	E	Tb_42.0Co_38.0Ga_20.0	Tb_39.7(8)Co_38.6(7)Ga_21.7(5)
G3	-	F	Tb_42.9Co_35.7Ga_21.4	Tb_39.8(6)Co_39.7(5)Ga_21.5(4)
G4	SG4-1	G	Tb_41.0Co_37.0Ga_22.0	Tb_39.6(7)Co_37.7(9)Ga_22.7(6)
G4	SG4-1	H	Tb_40.6Co_38.7Ga_20.7	Tb_39.5(4)Co_38.5(9)Ga_22.1(8)
G4	SG4-2	I	Tb_40.0Co_40.0Ga_20.0	Tb_39.4(5)Co_37.7(6)Ga_22.9(9)
G4	SG4-3	J	Tb_38.9Co_37.0Ga_24.1	Tb_39.1(1)Co_38.4(6)Ga_22.5(8), Tb_40.9(6)Co_38.5(6)Ga_20.6(9)
G4	SG4-3	K	Tb_39.0Co_39.0Ga_22.0	Tb_39.1(3)Co_38.0(4)Ga_22.9(2)

Table 2. Atomic compositions of impurity phases determined by EDX measurement for prepared samples.

Group	Subgroup	Sample	Tb₃Co₃Ga	N phase	TbCo_2−xGa_x	Tb₁_−xCo_xGa (G,H,I,K) or TbCo_xGa_1−x (J)
G1	-	A	Tb_42.6(4)Co_42.5(1)Ga_14.9(3)	Tb_59.2(6)Co_22.5(7)Ga_18.3(3)	Tb_37.3(8)Co_52.4(9)Ga_10.3(9)	-
G1	-	B	Tb_43.1(2)Co_40.9(5)Ga_16.0(7)	Tb_59.9(6)Co_21.4(8)Ga_18.8(6)	-	-
G2	-	C	-	Tb₅₆₍₁₎Co₁₃₍₁₎Ga₃₁₍₁₎	Tb₃₈₍₁₎Co₅₀₍₁₎Ga₁₂₍₁₎	-
G2	-	D	-	Tb_58.4(8)Co_15.8(8)Ga_25.8(3)	-	-
G3	-	E	-	Tb₅₇₍₁₎Co₆₍₂₎Ga_36.8(8)	-	-
G3	-	F	-	Tb_55.7(8)Co₅₍₁₎Ga_39.3(9)	-	-
G4	SG4-1	G	-	-	Tb_33.8(8)Co_50.8(9)Ga_15.4(3)	Tb_45.6(3)Co_4.2(8)Ga_50.3(3)
G4	SG4-1	H	-	-	Tb₃₆₍₁₎Co₄₈₍₁₎Ga₁₆₍₁₎	Tb₄₅₍₁₎Co₅₍₁₎Ga₄₉₍₁₎
G4	SG4-2	I	-	-	Tb_34.0(5)Co_51.4(8)Ga_14.5(5)	Tb_44.9(3)Co₇₍₂₎Ga₄₈₍₂₎
G4	SG4-3	J	-	-	-	Tb₄₉₍₁₎Co₇₍₂₎Ga₄₃₍₁₎
G4	SG4-3	K	-	-	Tb_34.0(5)Co_42.8(9)Ga_23.2(5)	Tb₄₇₍₁₎Co_2.0(9)Ga₅₀₍₁₎

Table 3. Atomic composition, lattice parameters, cell volume V, T_C determined by χ_ac, μ_eff and Θ of each Tb₂Co₂Ga sample.

Group	Subgroup	Sample	Composition of Tb₂Co₂Ga	a (Å)	b (Å)	c (Å)	V (Å³)	T_C (K)	μ_eff (μ_B/Tb)	Θ (K)
G1	-	A	Tb_40.1(6)Co_39.8(8)Ga_20.1(4)	4.0889 (7)	5.407 (1)	8.448 (2)	186.8 (1)	146	7.70	190
G1	-	B	Tb_39.9(8)Co_40.7(4)Ga_19.4(9)	4.0882 (7)	5.405 (1)	8.451 (2)	186.7 (1)	145
G2	-	C	Tb_39.9(3)Co_39.4(2)Ga_20.6(5)	4.0952 (5)	5.4182 (9)	8.436 (1)	187.18 (8)	101	9.83	98
G2	-	D	Tb_40.3(8)Co_38.8(9)Ga_21.0(2)	4.0955 (6)	5.4196 (9)	8.435 (1)	187.22 (9)	102
G3	-	E	Tb_39.7(8)Co_38.6(7)Ga_21.7(5)	4.0969 (5)	5.4204 (8)	8.430 (1)	187.21 (8)	92
G3	-	F	Tb_39.8(6)Co_39.7(5)Ga_21.5(4)	4.0983 (5)	5.4230 (7)	8.427 (1)	187.28 (7)	90	10.6	45
G4	SG4-1	G	Tb_39.6(7)Co_37.7(9)Ga_22.7(6)	4.0976 (7)	5.418 (1)	8.423 (2)	187.0 (1)	78	10.1	90
G4	SG4-1	H	Tb_39.5(4)Co_38.5(9)Ga_22.1(8)	4.0983 (6)	5.4189 (9)	8.424 (1)	187.08 (9)	75
G4	SG4-2	I	Tb_39.4(5)Co_37.7(6)Ga_22.9(9)	4.1026 (5)	5.4211 (7)	8.433 (1)	187.56 (7)	76
G4	SG4-3	J	Tb_39.1(1)Co_38.4(6)Ga_22.5(8)	4.1046 (7)	5.425 (1)	8.437 (2)	187.9 (1)	76
G4	SG4-3	K	Tb_39.1(3)Co_38.0(4)Ga_22.9(2)	4.1037 (7)	5.425 (1)	8.443 (2)	188.0 (1)	71

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Tanaka, S.; Terada, H.; Shirakawa, N.; Tsubota, M.; Kitagawa, J. The Impact of the Composition Effect on Ferromagnetic Properties of Tb₂Co₂Ga. Metals 2019, 9, 1242. https://doi.org/10.3390/met9121242

AMA Style

Tanaka S, Terada H, Shirakawa N, Tsubota M, Kitagawa J. The Impact of the Composition Effect on Ferromagnetic Properties of Tb₂Co₂Ga. Metals. 2019; 9(12):1242. https://doi.org/10.3390/met9121242

Chicago/Turabian Style

Tanaka, Seiya, Hirotaka Terada, Naoki Shirakawa, Masami Tsubota, and Jiro Kitagawa. 2019. "The Impact of the Composition Effect on Ferromagnetic Properties of Tb₂Co₂Ga" Metals 9, no. 12: 1242. https://doi.org/10.3390/met9121242

APA Style

Tanaka, S., Terada, H., Shirakawa, N., Tsubota, M., & Kitagawa, J. (2019). The Impact of the Composition Effect on Ferromagnetic Properties of Tb₂Co₂Ga. Metals, 9(12), 1242. https://doi.org/10.3390/met9121242

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