Structural , Magnetic , and Mössbauer Studies of Transition Metal-Doped Gd 2 Fe 16 Ga 0 . 5 TM 0 . 5 Intermetallic Compounds ( TM = Cr , Mn , Co , Ni , Cu , and Zn )

The effect of transition metal substitution for Fe and the structural and magnetic properties of Gd2Fe16Ga0.5TM0.5 (TM = Cr, Mn, Co, Ni, Cu, and Zn) compounds were investigated in this study. Rietveld analysis of X-ray data indicates that all the samples crystallize in the hexagonal Th2Ni17 structure. The lattice parameters a, c, and the unit cell volume show TM ionic radii dependence. Both Ga and TM atoms show preferred site occupancy for 12j and 12k sites. The saturation magnetization at room temperature was observed for Co, Ni, and Cu of 69, 73, and 77 emu/g, respectively, while a minimum value was observed for Zn (62 emu/g) doping in Gd2Fe16Ga0.5TM0.5. The highest Curie temperature of 590 K was observed for Cu doping which is 15 and 5% higher than Gd2Fe17 and Gd2Fe16Ga compounds, respectively. The hyperfine parameters viz. hyperfine field and isomer shift show systematic dependence on the TM atomic number. The observed magnetic and Curie temperature behavior in Gd2Fe16Ga0.5TM0.5 is explained on the basis of Fe(3d)-TM(3d) hybridization. The superior Curie temperature and magnetization value of Co-, Ni-, and Cu-doped Gd2Fe16Ga0.5TM0.5 compounds as compared to pure Gd2Fe17 or Gd2Fe16Ga makes Gd2Fe16Ga0.5TM0.5 a potential candidate for high-temperature industrial magnet applications.


Introduction
The rare-earth intermetallic compounds R2Fe17 have energy product (BH)max and Hc to be about 26 MGOe and 15 kOe, respectively [1]. In spite of these properties, they exhibit low Curie temperature (Tc). For example, 473 K for Gd2Fe17 and 300 K for Dy2Fe17 along with low magnetic anisotropies [2]. Various strategies have been employed addressing issues related to improving magnetic anisotropy, magnetization and Curie temperature of R2Fe17 compounds. Metalloids such as C, N, and H atoms are added to improve the magnetic anisotropy and Curie temperature [3,4,5,6]. However, high-temperature processing of these interstitial modified compounds is difficult. Subsequently, the addition of non-magnetic atoms such as Al, Ga, and Si for iron in the R2Fe17-xMx compound was investigated which in fact showed Curie temperature enhancement at high non-magnetic atom content. Among Al, Si, and Ga, Ga substituted compounds show high Tc, e.g., Sm2Fe16Ga, Tc ~485 K [7] and Dy2Fe16Ga, Tc~462 K [8]. However, this improvement in Tc is overshadowed by a concomitant deterioration in saturation magnetization as iron atoms are being replaced by non-magnetic atoms.
The Curie temperatures TC in the R2Fe17 compounds was explained on the base of strength of exchange interaction between Fe-Fe pairs [9]. It is based on the assumption that the exchange interactions favor ferromagnetic (r>rc) or antiferromagnetic (r<rc), where rC ~2.5 Å. Hence, Tc is assumed dependent on the competition between ferromagnetic and antiferromagnetic exchange interactions between neighboring pairs of Fe-Fe ions located at various crystallographic positions. This means that Tc enhancement can be achieved via unit cell lattice expansion, except in Si-substituted RE2Fe17-xSix, favoring ferromagnetic exchange interaction between Fe-Fe pairs. Usually, such lattice expansion is possible either via substituting for Fe ions by ions with the larger ionic radii [10,11] or via insertion of interstitial atoms in the unit cell [12,13]. It was observed that there are two ingredients influencing TC value: local magnetic moment values and exchange interaction values [14].
Among R2Fe17 intermetallic, Gd2Fe17 is of special interest as it has the highest Curie temperature, TC. Among the doped R2Fe17-xMx (M=Al. Si. Ga), Ga doped compounds display higher Tc value [15]. Given the above, the present work investigates effect of doping transition metal (TM) atoms in Ga doped Gd2Fe16Ga0.5TM0.5 compounds and compare the results with Gd2Fe17. It is expected that the doping of TM atoms with ionic radii greater than Fe will bring unit cell volume expansion and hence improve Fe-Fe exchange interaction enough to couple Fe-Fe moments ferromagnetically; thus, improving the Curie temperature of the compound. Furthermore, there also lies the possibility of improving magnetic moment of Fe via Fe-TM 3d band hybridization which can either bring band narrowing or increase exchange splitting by moving the 3d states below the Fermi level or allow charge transfer out of the 3d band, provided the spin-down density of states exceed the spin-up density [16].
This study discusses the changes in the structural and magnetic properties in R2Fe17 compounds when Fe is substituted in R2Fe16Ga0.5TM0.5 compounds with transition metal TM = Cr, Mn, Co, Ni, Cu, and Zn. The main aim of the study is to bring structural and band related changes to R2Fe17 compounds such as to improve Tc without impacting the saturation magnetization.

Experimental
The raw materials of Gd, Fe, Ga and TM (TM=Cr, Mn, Co, Ni, Cu, and Zn) with 99.9% purity were purchased from Sigma Aldrich. The parent alloys Gd2Fe16Ga0.5TM0.5 were prepared by arc melting the stoichiometric amount of aforementioned elements under a high purity argon atmosphere. The ingots were melted several times to ensure the high degree of homogeneity.
XRD was carried out with CuK (~1.5406 Å) radiation on a Bruker (D8 Advance) diffractometer. The powder X-ray data sets were collected in the 2θ range from 20° to 75° with a step size of 0.042°. The XRD analysis was performed by the well-known structural refinement Rietveld [17] method using the JANA2006 [18] software package to fit the experimental and calculated diffraction patterns. The initial crystal structure parameters were used as given by Liao et al. [19]. In the hexagonal setting, Gd was fixed at the 2b and 2d site (0, 0, 0.25) and (0.333, 0.667, 0.75), Fe is fixed at 4f, 6g, 12j, and 12k (0.333 0.667 0.105), (0.5 0 0), (0.333 0.969 0.25), and (0.167 0.333 0.985). The profile was constructed using a pseudo-Voigt function. Profile asymmetry was introduced by employing the multiterm Simpson rule integration devised by Howard [20]. A surface roughness correction was also applied using the Pitschke, Hermann, and Matter [21] model. In this technique, structural parameters, lattice parameters, peak shift, background profile shape and preferred orientation parameters are used to minimize the difference between a calculated profile and the observed data.
Magnetic properties of the powder sample were investigated at room temperature using vibrating sample magnetometer (VSM) in the maximum field of 1.2T. In order to minimize the effect of demagnetizing field, the samples were compacted at 3000psi and cut into rectangular parallelepiped with the ratio of length to a width larger than three times and embedded in epoxy. Modified thermogravimetric analyzer (DuPont 910) equipped with a permanent magnet was used to determined Curie temperature of composite samples. In this procedure, a magnetic material is placed inside an empty, tared TGA pan located near a strong magnet. The material is then heated. At the Curie temperature (Tc), the magnetic properties disappear (i.e., the material goes from diamagnetic to paramagnetic) and the reduced attraction for the magnet results in a sharp apparent weight loss or gain (depending on the TGA design).
The Mossbauer spectra of the samples were obtained at room temperature (RT) using a 25 mCi 57 Co source in an Rh foil mounted on a constant acceleration drive system (SEE Co. Minneapolis, USA) in transmission geometry. The velocity scale of the Mossbauer spectrometer was calibrated by measuring the hyperfine field of -Fe foil, at room temperature. The Mossbauer spectra were analyzed using WMoss software from SEE Co.
The spectra were least-square fitted with the hyperfine field (f&f), isomer shift (IS) and quadrupole splitting (QS) as variables.

Results and Discussion
The raw powder profile for Gd2Fe16Ga0.5TM0.5 systems is presented in Fig. 1 (a). The inset with the increasing size of the substitution atom whose metallic radii increase as going from TM=Cr to Zn, Table I. The refined Rietveld profiles are presented in Fig. 1 Gd2Fe16Ga0.5TM0.5 systems. The refined structural parameters, lattice parameters a, c, c/a ratio, unit cell volume, and the reliability indices are given in Table I. From the Rietveld analysis, the refined profile indicates that Gd2Fe16Ga0.5TM0.5 compounds crystallize in hexagonal Th2Ni17 structure with P63/mmc symmetry group. Fig. 2 show the lattice parameters a function of TM atomic number in Gd2Fe16Ga0.5TM0.5. It is observed from compounds. This is also evident from the variation in c/a ratio (Table I)    The atomic site occupancy for Gd, Fe, Ga and TM atoms derived from Rietveld refinement is listed in Table II. The site notations are given for rhombohedral structure with Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 5 October 2018 doi:10.20944/preprints201810.0075.v2 corresponding hexagonal notation viz. 6c(4f), 9d(6g), 18f(12j) and18h(12k). The crystallographic site preference exhibited by TM in Gd2Fe16Ga0.5TM0.5 is listed in Table II.  The Fe-Fe site-to-site bond distances are listed in Table III and are plotted in Fig. 3. It is observed from the   Room temperature magnetization vs. field plot for Gd2Fe16Ga0.5TM0.5 is shown in Fig. 4.

It is evident from
The room temperature magnetic parameters derived from VSM are plotted in Fig. 5 and are listed in Table IV    The measured Curie temperature, Tc, of Gd2Fe16Ga0.5TM0.5 compounds is plotted in Fig. 5 as a function of TM atomic number. It is evident from Fig.5 Fig. 3, which is less than 2.45 Å needed for ferromagnetic ordering [37]. It is to be noted that increase in Tc has been reported earlier with higher Al, Ga, and Si content (at x>2) in R2Fe17-xMx (M=Al, Ga, Si) [15] however with a concomitant reduction in Ms due to large Fe replacement with nonmagnetic atoms. A Tc value of 581 K has been reported earlier in YGdFe16CoGa [38] compound but reported Tc~586 K of Gd2Fe16Ga0.5Co0.5 exceeds that of the former compounds. Thus, the observed increase in Tc in TM doped Gd2Fe16Ga0.5TM0.5 compounds is highest with a minimum replacement of Fe atoms.  with Gd2Fe17.
Friedel model [39] can also be used to explain the observed variation in Tc. According to this model the strength of interaction between two magnetic moments would be strong and ferromagnetic, if /d>1, where distance "d" between these magnetic atoms is smaller than the distance "" covered by the main peak of the Friedel oscillations. In compounds containing 3d transition metals, it has been established that the magnetic coupling is governed mainly by the nearest-neighbor interactions and is proportional to the lattice parameters. Furthermore,  is found to be inversely proportional to the Fermi wave vector, kf. For the 3d band in the R2Fe17 compounds, kf is large. Substitution of TM decreases the holes in the 3d band and hence decreases kf. The substitution of Ga leads to lattice expansion and hence increases "d", which will have an effect of reducing /d ratio. Since the substitution of Co, Ni and Cu brings in lattice volume reduction as compared to R2Fe16Ga; there is hence an increase in the, /d>1 and Tc [37,38]. The reported theoretical studies attribute changes in the Curie temperature in substituted R2Fe17-xTx (T=Al, Si, Ga, and Ti) intermetallics to be electronic in origin other than due to the simple volume expansion effect and hence bond-distances [40,41,42]. The effect of the substitution is to fill out the Fe-3d spin-up sub-bands which alter the magnetic moment of the compound and hence the strength of exchange interaction [39,43]. In fact, theoretical calculations performed using LSDA+U method showed that enhancement between Fe-Fe atoms in the presence of Ga in Gd2Fe17-xGax compounds, which intern was shown to enhance Curie temperature for low Ga (x<3) content [44]. Thus, observed higher Tc values of Gd2Fe16Ga0.5TM0.5 as compared to that of pure Gd2Fe17 could be attributed to this effect as well. In comparison to various doped intermetallics such as Gd2Fe16Ga (~410 K) [45], Gd2Fe16Ga0.5Ti0.5 (556 K) [46], Dy2Fe16Ga (~462K) [8], Ce2Fe16Ga (~320K) [47], Sm2Fe16Ga (~505K) [48], or Sm2Fe16.2Ti0.8 (~435K) [49], the reported compound Gd2Fe16Ga0.5TM0.5 with Co, Ni, and Cu substitution certainly exhibits higher Tc and Ms, thus ensuring their potential use as high temperature permanent magnet applications.
The R2Fe17 intermetallic with Th2Ni17 structure, have the easy direction of magnetization and hyperfine field lying in the basal plane along a or b axes of the unit cell [50,51]. This basal plane easy direction of magnetization complicates the Mossbauer spectral analysis of R2Fe17 compounds because it involves four crystallographically inequivalent iron sites. The reason for the inequivalent iron site is the vector character of the hyperfine field and tensor character of the electric field gradient [52]. Thus, this inequivalency demands further magnetic splitting of g, j, and k iron sites. Mössbauer studies of Gd2Fe16Ga0.5TM0.5 have been conducted accordingly, either with 8 or 10 magnetic sextets, with absence or presence of impurity phase, respectively [46,53,54,55]. The Mössbauer spectral analysis was carried out with magnetic sextets assigned to the 4f, 6g, 12j, and 12k sites in Gd2Fe17. The 6g, 12j, and 12k sites were further split into 2, 3, and 2 corresponding to the site occupancies of Fe atoms in the crystal structure of R2Fe17 with the planar anisotropy. The intensities of the six absorption lines of each sextet were assumed to follow the 3:2:1 intensity ratio expected for randomly oriented powder samples in zero magnetic field and a single common line-width was assumed for all the eight sextets. The isomer shifts (IS,) for the magnetically inequivalent sites were constrained to be the same, whereas the hyperfine field (HF, Bhf) were expected to vary at pairs of magnetically inequivalent sites due to variations in the dipolar and orbital contributions to the magnetic hyperfine fields [56].  The hyperfine parameters derived from the fitting are listed in Table V, and weighted average (Wt.Avg.) hyperfine field (HF) and isomer shifts (IS, ) are plotted in Fig. 7. There exists a direct correlation between hyperfine field values of a site to its near neighbor (NN) iron sites. In case of Th2Ni17 structure, 12k site has 9 NN Fe sites (1 (4f), 2 (6g), 4 (12j), 2(12k)), 12j has 10 NN Fe sites (2 (4f), 2 (6g), 2 (12j), 4 (12k)), 6g has 10 NN Fe sites (2 (4f), 0 (6g), 4 (12j), 4(12k)), and 4f site has 11 NN Fe sites (1 (4f), 3 (6g), 6 (12j), 3 (12k)).  [57,58]. It is obvious that 4f (6c) site has the maximum hyperfine field, since it has the maximum number of Fe nearest neighbors, whereas, the 18h (12k) site has the minimum number of Fe neighbors and consequently has the least HF value.
Although 6g(9d) and 12j(18f) sites have the same number of Fe neighbors, the former has comparatively smaller Fe-Fe distances and hence a larger hyperfine field, Table III and   is proportional to the total s-electron charge density at the iron nucleus, which is the sum of 12 the spin-up and spin-down s-electron density and lattice site volume; an increasing s-13 electron density at iron nucleus is indicated by a decreasing isomer shift. The observed 14 behavior of IS value could be attributed to the competition between lattice site volume and 15 complex nature of hybridization between Fe-Ga-TM [62,63], which all affect the s-electron 16 charge density at the iron nucleus. A volume contraction is observed until TM=Ni followed 17 with unit cell expansion till TM=Zn doping in Gd2Fe16Ga0.5TM0.5. However, the Wt. Avg.

18
IS value becomes less negative with TM=Co and onward. Thus, this behavior of IS 19 indicates electronic effects at play in dictating IS behavior of Gd2Fe16Ga0.5TM0.5 compound. 20 The increased IS value for with Co, Ni, Cu, Zn, and Ga in Gd2Fe16Ga0.5TM0.5 could be 21 associated with the increased number of the 3d electrons which increases the shielding of 22 the s-electrons from the nucleus. In earlier TM atoms viz. Cr and Mn, the 3d band is 23 broader and heavily hybridized with the conduction band [38]. These make electrons freer 24 and thus have a greater presence at the Fe nucleus, which makes IS more negative. The  of 587 K for cobalt substitution, which is 15% higher than Tc value of Gd2Fe17. 38 Furthermore, 15% and 14% enhancement in Ms was observed for Cu substituted Gd2Fe16Ga0.5TM0.5 compound as compared to Dy2Fe17 and Dy2Fe16Ga1 compounds, 40 respectively. Furthermore, unlike other doped compounds of RE2Fe17-xMx (M=Al, Si, Ga) 41 intermetallic, where improvements in Tc is comprised with the reduction in Ms, in the 42 present studied compound Gd2Fe16Ga0.5TM0.5, even small TM doping (TM=Co, Ni, and 43 Cu) brings in a simultaneous enhancement in Ms and Tc. The combined magnetic and 44 Mossbauer study points to the fact that the observed improvement in Tc and Ms could be 45 attributed to electronic effects resulting from Fe-3d hybridization with substituted TM atom 46 electronic shell. A concomitant improvement in Ms and Tc is desirable for the magnetic industry. The study elucidates that the judicious selection of dopants and its content can 48 improve Ms and Tc of the R2Fe17 intermetallic compounds. 49