I. INTRODUCTION
Rare earth–transition metal (R-T) intermetallics are an important group of compounds with interesting magnetic and hydrogen absorption properties (Buschow, Reference Buschow1977). The magnetic performances of the intermetallics are caused by a combination of the complementary characteristics of 3d-itinerant and 4f-localized magnetisms. Their technological importance and interesting properties have resulted in a continuous experimental and theoretical research (Buschow, Reference Buschow1977; Richter, Reference Richter1998) over the past several decades. Among the R-T intermetallics, the hexagonal Haucke compounds RCo5 (CaCu5-structure type, space group P6/m m m) are one of the most interesting subclasses, which have been widely studied for a fundamental research as well as their possible applications as permanent magnets (Strnat, Reference Strnat1988). Studies of the structural and magnetic properties for DyCo4M (M=Al,Ga) (Klosek et al., Reference Klosek, Zlotea and Isnard2003, Reference Klosek, Zlotea and Isnard2004), DyCo5−xCux (Banerjee et al., Reference Banerjee, Bahadur, Suresh and Nigam2006), and Dy1−xYxCo5 (Banerjee et al., Reference Banerjee, Suresh and Nigam2008) have also been reported. The results show that the substitutions of nonmagnetic atoms, Al, Ga, and Cu for Co or Y for Dy, have remarkable influences on both crystal structure and magnetic properties, such as the Curie temperatures (T C), compensation temperatures T comp, spin-reorientation transitions, and magnetization of the compounds. To further study the magnetic properties of the 3d sublattice of RCo4Ga compounds, we reported in this paper the effects of the substitution of Fe for Co on both crystal and magnetic properties in DyCo4Ga.
II. EXPERIMENTAL
Polycrystalline samples DyCo4−xFexGa with x=0, 0.5, 1, and 1.5 were prepared by arc melting using a nonconsumable
Figure 1. (Color online) Rietveld refinement results for the DyCo3.5Fe0.5Ga sample (a) and the DyCo2.5Fe1.5Ga sample (b).
TABLE I. Structure and refined parameters for hexagonal DyCo4−xFexGa.
tungsten electrode and a water-cooled copper tray in argon. The alloy samples, each with 2 g in total weight, were prepared from Dy (99.99 wt %), Co (99.9 wt %), Fe (99.99 wt %), and Ga (99.999 wt %). The alloy samples were remelted at least three times to ensure homogeneity. No composition analysis was carried out since the weight lost of each sample was less than 1% during the preparation. Each sample was further sealed in an evacuated quartz tube, annealed at 950 °C for 1 week, and then cooled to room temperature. X-ray powder diffraction (XRD) data were collected using a Bruker D8 Advance SS/18 kW diffractometer with Cu K α radiation (40 kV and 250 mA) and a diffracted-beam graphite monochromator. A step-scan mode was employed with a step width of Δ2θ_=0.02° and a sampling time of 5 s. The scan range for all samples is from 20° to 100°2θ. JADE 5.0 and TOPAS 3.0 software programs were used for phase identification and structure determination. The temperature dependence and field dependence of the magnetization of the samples were measured using a vibrating sample magnetometer (VSM) (VSM-HH20, Nanjing University). The Curie temperature of each sample was determined from the minimum of the first derivatives of its magnetization curve.
III. RESULTS AND DISCUSSION
A. Crystal structure
Using the TOPAS 3.0 program, Rietveld refinements of the XRD data of the DyCo4−xFexGa samples were performed with the assumption that each DyCo4−xFexGa compound has a hexagonal CaCu5-type structure with space group P6/m m m, and the Dy atom occupies the 1a (0, 0, 0) site, two Co atoms occupy the 2c (1/3, 2/3, 0) and the 3g (1/2, 0, 1/2) sites, and the Ga and Fe atoms prefer to occupy the 3g sites, as those of DyCo4M and PrCo4−xFexM (M=Al and Ga) as reported by Klosek et al. (Reference Klosek, Zlotea and Isnard2003) and Zlotea and Isnard (Reference Zlotea and Isnard2003). Representative results obtained by the Rietveld refinements are depicted in Figure 1a for the DyCo3.5Fe0.5Ga sample and Figure 1b for DyCo2.5Fe1.5Ga. The refinement results for all four samples are listed in Tables I and II. As shown in Table I, the values of R factors are reasonably small from 2.72% to 5.24% and the values of goodness of fit (GOF) (or χ 2) are between 1.52 and 1.92. The refinement analysis reveals the presences of small amounts of dysprosium and/or iron oxides [for examples, see Figures 1a and 1b]. It should be noted that two forms of structures with different space groups of I a 3 and F m 3m were found to present the compound DyCo2.5Fe1.5Ga [see Figure 1b]. The larger the Fe content x, the larger the total amount of impurity oxides. The total amount of oxides presented in DyCo2.5Fe1.5Ga is almost 1.0% [Figure 1b]. Figures 1a and 1b reveal that Dy2Co7 only appears in the x=1.5 compound, not the x=0.5 compound. This suggests that the iron solubility limit in DyCo4Ga should be x<1.5, which is comparable to that of other CaCu5-type structure compounds. The iron solubility limits were previously reported to be x<2.0 in PrCo4−xFexAl by Zlotea and Isnard (Reference Zlotea and Isnard2003), x<1.5 in NdCo4−xFexAl by Konno et al. (Reference Konno, Ido and Maki1992), and x<1.8 in YCo4−xFexAl and x<1.5 in HoCo4−xFexAl phases by Thang et al. (Reference Thang, Tai, Liu, Thuy, Hien and Franse1995). The compositional dependences of lattice parameters a, c, c/a, and unit-cell volume V for DyCo4−xFexGa as a function of the Fe content x are depicted in Figures 2a – 2c. As shown in Figures 2a and 2b, both the values of the unit-cell parameters a and c increase with the content of Fe between x=0 and 1.0. However, a relatively smaller value of the unit-cell parameter a and a significantly larger value of c were obtained for the x=1.5 compound. As a result, the unit-cell volume V increases linearly with x in the
TABLE II. Atomic parameters for hexagonal DyCo4−xFexGa (P5/m m m).
Figure 2. (Color online) Compositional dependence of unit-cell parameters a and c (a), c/a (b), and unit-volume V (c) for DyCo4−xFexGa compounds with x=0, 0.5, 1.0, and 1.5.
range of 0≤x≤1.5 [see Figure 2c]. The expansion of the unit-cell parameters is more pronounced along the a axis at lower Fe substitutions with 0≤x≤1.0 but along the c axis at higher Fe substitutions with 1.0≤x≤1.5. This anisotropic expansion and contraction may be the cause for the nonstability of crystal structures reported by Taylor and Poldy (Reference Taylor and Poldy1975).
B. Magnetic properties
The temperature dependence and field dependence of the magnetization of the samples were measured using a VSM. Figure 3 shows the temperature dependence of the magnetization (M-T curves) for DyCo4−xFexGa with x=0, 0.5, 1.0, and 1.5 measured in the applied field of 0.1 T and the temperature range from 80 to 850 K. All DyCo4−xFexGa samples have magnetic ordering. The M-T curve of DyCo4Ga has the same magnetic behavior with that reported by Klosek et al. (Reference Klosek, Zlotea and Isnard2003). Compensation transition and a spin-reorientation transition were found in DyCo4Ga at T comp=286 K and T SRT=403 K, respectively. According to the results of a
Figure 3. (Color online) Temperature dependence of the magnetization (M-T curves) for DyCo4−xFexGa with x=0, 0.5, 1.0, and 1.5 measured in the applied field of 0.1 T and the temperature range from 80 to 850 K.
powder neutron diffraction analysis reported by Klosek et al. (Reference Klosek, Zlotea and Isnard2003), the Dy-sublattice magnetization is antiparallel to the Co-sublattice magnetization in DyCo4Ga. The composition dependences of the compensation temperature T comp, the spin-reorientation temperature T SRT, and the Curie temperature T C for the DyCo4−xFexGa with x=0, 0.5, 1.0, and 1.5 are shown in Figure 4. The magnetic parameters are summarized in Table III. The compensation temperatures T comp decrease with increasing Fe concentration from 286 K for x=0 to 238 K for x=1.5, which may due to the increase of the 3d-sublattice moment by substitution of Fe for Co in this compound. The Dy-sublattice magnetic moment was found to be much more temperature dependence than that of the Co sublattice. Almost cancellation of Dy- and Co-sublattice magnetizations was found to occur at its compensation temperature T comp. The spin-reorientation transitions appear in the DyCo4−xFexGa at T SRT=403 K for x=0 and at T SRT=530 K for x=0.5 but disappear for x=1.0 and 1.5, suggesting that a weakening of the 3d uniaxial anisotropy upon Fe substitution. Klosek et al. (Reference Klosek, Zlotea and Isnard2003) reported that the spin-reorientation transition occurs when the magnetic moments rotate continuously with temperatures near temperature T SRT, which is originated from the competition between axial Co (3d) and planar Dy (4f) magnetocrystalline anisotropic in DyCo4Ga. As shown in Figure 4, the Curie temperature (T C) of DyCo4−xFexGa increases from 498 K for x=0 to 673 K for x=1.5. The Curie temperature is mainly determined by the 3d-3d exchange interaction. The observed increase in T C is due to the magnitudes of both the Fe magnetic moment to be
Figure 4. (Color online) Composition dependences of the compensation temperature T comp, the spin-reorientation temperature T SRT, and the Curie temperature T C for DyCo4−xFexGa with x=0, 0.5, 1.0, and 1.5.
TABLE III. Magnetic properties for DyCo4−xFexGa with x=0, 0.5, 1.0, and 1.5.
slightly larger than Co as well as the Fe-Fe exchange interactions replacing Co-Co exchange interactions. The isothermal magnetization curves recorded at 173 and 300 K for the DyCo4−xFexGa samples are shown in Figures 5a and 5b, respectively. As shown in Figure 5, the saturation magnetization (M S) of DyCo4−xFexGa decreases from 20.49 to 11.09 A m2 kg−1 at 173 K but increases from 3.69 to 10.13 A m2 kg−1 at 300 K with increasing Fe content x. This was the result of an increase in the 3d-sublattice magnetic moment with increasing iron content, and the magnetic moment of the 3d sublattice is antiparallel to that of the dysprosium sublattice (Mayot et al., Reference Mayot, Isnard, Grandjean and Long2008). The magnetization of the dysprosium (4f) sublattice dominates that of the transition metal (3d) sublattice at a temperature lower than the compensation temperature T comp, whereas the opposite situation occurs at a temperature higher than T comp. An increase in the 3d-sublattice moment by substitution of Fe for Co in
Figure 5. (Color online) Isothermal magnetization curves recorded at 173 K (a) and 300 K (b) for DyCo4−xFexGa with x=0, 0.5, 1.0, and 1.5.
DyCo4−xFexGa leads to a decrease in the saturation magnetization recorded at 173 K but an increase in the saturation magnetization at 300 K.
IV. CONCLUSION
The crystal structural and magnetic properties of the DyCo4−xFexGa samples with x=0, 0.5, 1.0, and 1.5 were studied. The iron solubility limit in DyCo4Ga was found to be x<1.5. The values of the unit-cell parameters a, c, and V, and the 3d-sublattice moment of the hexagonal DyCo4−xFexGa increase with increasing Fe content x. The increase in the substitution of Fe for Co in DyCo4−xFexGa leads to a decrease in the compensation temperature but increases in the spin-reorientation and Curie temperatures. Our study also show that the value of saturation magnetization at 173 K increases but the value at 300 K decreases with increasing Fe content x.
ACKNOWLEDGMENTS
The work was supported by the National Natural Science Foundation of China (Grant Nos. 50871070 and 50861004) and Shenzhen Science and Technology Research Grant (Grant Nos. JC200903120081A and JC200903120109A).
