The Impact of the Composition Effect on Ferromagnetic Properties of Tb 2 Co 2 Ga

: The ferromagnetic properties of Tb 2 Co 2 Ga, crystallizing into an orthorhombic W 2 CoB 2 type structure, were investigated by preparing 11 polycrystalline samples with different starting atomic compositions. We found that Tb 2 Co 2 Ga 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.


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 TC 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 TC.
It has been recently reported [7] that Tb2Co2Ga is a ferromagnet with TC = 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 μB, which dominates over that of Co with 0.26 μB. 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 Mn1+xGa, which is a ferromagnet showing a reduced TC 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.

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 TC, 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 χac (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 χdc (T) between 5 K and 300 K, and the magnetization curves were measured by a MPMS magnetometer (Quantum Design, San Diego, CA, USA).

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.  Tables 1 and 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 Tables 1 and 2), the amount of which is rather large in samples B and D (see Figures 4b and 4d). This is consistent with the phase relation displayed in Figure 2b. The pseudo binary TbCo2−xGax appears in samples A, C, G, H, I, and K, which are located near TbCo2−xGax (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 Figures 5a-e. The impurity phase is presumably Tb1−xCoxGa for the samples G, H, I, and K, or TbCoxGa1−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 χac (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.

Magnetic Properties
Figures 6a-d show the χac (T) of all samples, categorized by the TC 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 χac (T) is employed as TC, which would be systematically governed by the atomic composition (see also Table 3). In the inset of Figure 6a, the χac (T) of the sample with the starting composition Tb42.9Co50Ga7.1 is exhibited. This sample contains the pseudo binary TbCo2-xGax alloy and the N phase [6]. Two pronounced peaks are observed at approximately 250 and 60 K, for which the pseudo binary TbCo2−xGax alloy and the N phase are responsible, respectively [6]. The x value of TbCo2−xGax is estimated to be 0.22, corresponding to that of TbCo2−xGax exhibiting ferromagnetic transitions at approximately 260 K [12]. The χac peaks of sample A at approximately 260 K and 55 K can also be ascribed to the parasitic phases of TbCo2−xGax 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 Tables 1 and  2). In sample B, the χac peak at 55 K evolves in accordance with the increased area of the N phase observed by the FE-SEM image. The TC 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 χac 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 TC 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 χac anomaly due to the N phase might be masked by a large χac peak at 90 K.  Figure 6d shows the χac (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. Tb1−xCoxGa and TbCo2−xGax are responsible for the low and high temperature anomalies, respectively. The value of x in TbCo2−xGax 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 TbCo2−xGax 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 TbCo1−xGax, and the χac peak due to the Tb2Co2Ga phase is rather broad, reflecting the coexistence of two Tb2Co2Ga phases with slightly different atomic compositions. Tb1−xCoxGa and TbCo1−xGax may be related to TbGa with TC = 154 K [14], which probably supports the assignment of magnetic impurity phases in Figure 6(d). We remark that the shapes of the χac peaks at TC of the Tb2Co2Ga phases in the G4 group somewhat differ from those of the G1 to G3 groups. For the G4 group (Figure 6d), χac shows a λ-type anomaly, and the χac peaks of the G1 to G3 groups (Figures 6ac) are more symmetric with respect to the temperature-axis. Figure 7a shows χdc (T) under the external field of 10 Oe of samples A, C, F, and G, respectively. In each sample, χdc steeply increases below the approximate TC of the Tb2Co2Ga phase. Figure 7b shows the temperature dependences of the inverse χdc 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 μeff and the Weiss temperature Θ are summarized in Table 3. The value of μeff is near 9.72 μB/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 μeff. From the χdc data, it is not clear whether the Co atom carries a magnetic moment or not.

Relationship between the Ferromagnetic Properties and Crystal Structure Paremeters
Hereafter, we discuss the relationship between the crystal structure parameters and TC. Figures 9a-c are the 3D bar graphs of TC 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 TC; the increase of a(b) and the decrease of c reduce TC. 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 TC of the sample in the G4 group seems to be slightly reduced with decreasing Tb concentration (see Table 3).  Figures 10a,b are the 3D bar graphs of TC 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), TC is reduced from ~145 K to ~90 K. Then, both θ1 and θ2 become wider, and Figures 10a,b for the G1 to G3 groups suggest that the octahedron being compressed along the c-axis leads to the TC 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 TC down to 90 K. At the present stage, the origin of further reductions of TC 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 χac peaks at TC between the samples in the G1 to G3 groups and those in the G4 group, as mentioned above.

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 TC that is sensitive to the atomic composition, and ranges rather widely, i.e., from 75 to 145 K. The composition dependence of TC 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 TC. 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 TC, but the θ1 and θ2 angles in each sample show similar values. The anisotropic change of two angles below a TC 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 χac anomaly at TC, which differs from those of the samples in the G1 to G3 groups.