ABSTRACT

Based on X-ray diffraction, microscopic and magnetic analysis, the structure and magnetic properties of Co₇₇Zr₁₈W₅ melt-spun ribbons were studied in this paper. A new element to stabilize the metastable Co₅Zr phase was found and the coercivity observed in Co-Zr alloys can be obviously enhanced by proper tungsten substitution. The Curie temperature of Co₇₇Zr₁₈W₅ ribbons is 475˚C which suggests that W doped Co-Zr alloys may become an attractive candidate permanent magnets for practical applications in high temperature. Annealing of the Co₇₇Zr₁₈W₅ ribbons results in a decrease of the coercivity which confirmed that the hard magnetic phase is Co₅Zr phase in Co₇₇Zr₁₈W₅ melt-spun ribbons.

1. Introduction

To enhance the thermal stability of permanent magnetic materials for high temperature applications, recent developments are largely concentrated on the non-rare-earth containing permanent magnets in the magnet society. One of the candidates for the new permanent magnetic materials is Co-Zr system alloys [1]. In recently years, a great deal of effort has been devoted to improve the magnetic properties of Co-Zr system alloys by doping or optimizing the annealing conditions of rapidly quenched ribbons [2-5]. The reported maximum values of energy products in Co-Zr-B(-Si) melt-spun ribbons are comparable to those of the hard ferrites [6,7]. According to previous reports, either Co₅Zr phase or Co₁₁Zr₂ phase is the hard magnetic phase in Co-Zr system alloys [7-11]. To further improve the hard magnetic properties of Co-Zr system alloys, it is essential to determine the hard magnetic phase in Co-Zr system alloys.

Previous studies show that the rapidly quenched CoZr-based alloys with near 18 at% Zr concentration have a good hard magnetic property. To our knowledge, works on Co-Zr system alloys were mostly performed by small atomic radius elements additions, such as B, Si or C [5]. In this work, we will broaden the range of admixture by doping the relatively large atomic radius tungsten element to Co-Zr alloys and focus on the structure and magnetic properties of Co₇₇Zr₁₈W₅ melt-spun ribbons. Moreover, we will try to identify the hard magnetic phase in Co-Zr system alloys.

2. Experiment

The ingot of Co₇₇Zr₁₈W₅ was prepared by arc melting the constituent elements, Co and Zr of purity 99.99% and W of purity 99.95%, in an arc furnace under argon atmosphere. To ensure homogeneity, the melting was carried out six times and the weight loss after the final melting was less than 0.3%. Then, small amounts of the alloy ingot were placed in a quartz crucible with an orifice of 0.8 mm at the bottom. The alloy ingot was induction melted in an argon atmosphere and then ejected through the orifice with argon on to a copper wheel rotating at a surface velocity (V_s) of 40 m/s. The resultant melt-spun ribbons were typically 2 - 3 mm wide and 20 - 40 µm thick.

The phases of the ribbons were identified by X-ray diffraction (XRD) using CuKα radiation in a Rigaku D/ MAX2200PC diffractometer. The lattice parameters were evaluated by least-squares refinement from powder X-ray diffractograms. Thermomagnetic analysis (TMA) was performed in a magnetic balance in a field of 1 kOe. Magnetic properties of the ribbons were measured by a vibrating sample magnetometer (VSM) with a maximum applied field of 12 kOe. VSM was calibrated with a pure nickel flake. The microstructures of the specimens were examined by a scanning electron microscope (SEM).

3. Result and Discussion

Figure 1 shows the initial magnetization and roomtemperature hysteresis curves of Co₇₇Zr₁₈W₅ melt-spun ribbons produced at a wheel speed of 40 m/s. The hysteresis curve is very smooth and the remanence ratio (Mr/Ms) exceeds 0.6, reflecting that an exchangecoupling effect might have occurred in these ribbons. The inset shows the hysteresis curves of Co₇₇Zr₁₈W₅ ingot as a function of applied magnetic field. We can see that the as-cast alloy nearly has no coercivity, suggesting that the process of quenching change the phase in the ribbons. For the Co₇₇Zr₁₈W₅ melt-spun ribbons, it has the coercive value of 4.3 kOe, which is higher than that of Co-Zr-B melt-spun ribbons (3.6 kOe) [7]. On the other hand, the large slope of magnetization at 12 kOe indicates that the specimens are not saturated. Much lager field is required to completely saturate the coercivity.

The magnetic properties of ingot and melt-spun ribbons are listed in Table 1. In the calculation of the maximum energy product (BH)_max, the density of the melt-spun ribbons is assumed to be 8.82 g/cm³. The maximum value of coercive field H_C were obtained under 12 kOe magnetic field. As can be seen, a much larger H_C and (BH)_max improvement are obtained for the Co₇₇Zr₁₈W₅ ribbons than the ingot. This value of coercivity is also much larger than the value of the undoped Co₈₂Zr₁₈ meltspun ribbons and annealed samples. It can be considered that the magnetic performance in polycrystalline Co-Zr alloys can be improved by the substitutions of W for Co. This improvement can be understood based on the dependence of the crystal structure and microstructure.

To understand the phases that exist in the studied ribbons, X-ray diffraction analysis was carried out. Figure 2 shows the X-ray diffraction pattern of Co₇₇Zr₁₈W₅

melt-spun ribbons. From the X-ray diffraction analysis, only a single Co₅Zr phase is found in the ribbons. The structure of the binary Co₅Zr phase is of fcc type having a F43m or F23 space group with a = 0.67 nm. It has been reported that the XRD patterns of Co₈₂Zr₁₈ melt-spun ribbons (the surface velocity of 40 m/s) and of that annealed consist of Co₁₁Zr₂, Co₂Zr, fcc-Co, Co₂₃Zr₆ and Co₅Zr phase [5]. The XRD pattern suggests that W doping can decrease the amount of other phases. By leastsquares refinement from powder X-ray diffractograms we get that the substitution of Co by the relatively larger atomic radius W leads to an increase of the Co₅Zr phase lattice constants of a = 0.6821 nm. We also detected the XRD of Co₇₇Zr₁₈W₅ ingot and almost no Co₅Zr phase was found. However, in the investigated Co₇₇Zr₁₈W₅ melt-spun ribbons only the Co₅Zr phase is detected. This indicates that the W additive plays an important role in stabilizing Co₅Zr phase and W is an effective element to stabilize the metastable Co₅Zr phase.

Figure 3 shows the thermomagnetic curve of Co₇₇ Zr₁₈W₅ melt-spun ribbons. We can see that the thermomagnetic curve exhibits a single magnetic transition near 475˚C. It is clear that the Co₇₇Zr₁₈W₅ melt-spun ribbons consist a single Co₅Zr phase. The Curie temperature 475˚C is lower than that of the binary Co₅Zr phase (T_C = 500˚C) in Co-Zr-B alloys [5]. Similar results have been observed in Co-Zr-B-Si alloys [8]. As far as we know, the decreasing of Curie temperature is possibly due to the increasing of lattice constants by the size effect, because of the change of the 3d band structure resulting from the increasing of lattice constants [12]. Corresponding to the XRD studies above, the investigated specimens, consisting more Co₅Zr phase, exhibits a higher coercivity. These results lead to the conclusion that Co₅Zr phase is the hard magnetic phase in Co-Zr system alloys.

It is well known that annealing of rapidly solidified

alloys results in the change in the grain size and the crystalline phase, which should give rise to the change in the magnetic properties. So the ribbons were annealed at temperatures between 550˚C and 750˚C for 20 min. Figure 4 shows the dependence of coercivity of Co₇₇Zr₁₈W₅ melt-spun ribbons on the annealing temperature. We can find that the coercivity of the annealed melt-spun ribbons decreases initially with increasing the annealing temperature from 4.0 kOe at 550˚C to the minimum value 2.4 kOe at 700˚C, then gradually increasing from 2.4 kOe at 700˚C to 2.6 kOe at 750˚C. Virtually, all the annealed specimens have a lower coercivity value than the as melt-spun ribbons. Heat treatment of the Co₇₇Zr₁₈W₅ melt-spun ribbons results in a decrease of the coercivity, as was also found in Co-Zr-M (M = Mo, C, Si, Al) melt-spun ribbons [3,13].

The corresponding XRD pattern of the Co₇₇Zr₁₈W₅ ribbons annealed at 700˚C is shown in Figure 5. Three phases, namely, Co₅Zr, Co₂₃Zr₆ and fcc Co phase, were found in the annealed Co₇₇Zr₁₈W₅ ribbons. It is clear that the amount of Co₅Zr phase in the annealed Co₇₇Zr₁₈W₅ ribbons is less than that of the as-prepared ribbons. Obviously no detection of Co₁₁Zr₂ phase was found in both the as-prepared and annealed ribbons. This may be due to that the equilibrium Co₁₁Zr₂ should be a soft magnetic phase in Co-Zr system as reported [5].

The microstructures of the Co₇₇Zr₁₈W₅ ribbons of before heat treatment and the Co₇₇Zr₁₈W₅ ribbons annealed at 700˚C for 20 min were analyzed by SEM, as we can see in Figure 6. The grain size of Co₇₇Zr₁₈W₅ melt-spun ribbons is too small to determine by SEM (less than 0.1 μm) and he ribbons annealed at 700˚C for 20 min have fine grains (about 2 μm). This suggests that heat treatment of the Co₇₇Zr₁₈W₅ melt-spun ribbons results in the dramatic change in the grain size. According to the micro-structural studies, the change of the coercivity for the

annealed specimens is considered to the change of the grain size of Co₅Zr phase, that is, the annealing temperature. The same results have been found in Co-Zr alloys [11]. On the other hand, compared with the as-prepared ribbons, we can see that the decrease of the coercivity for the annealed specimens is suggested to originate from the decreasing of the amount of Co₅Zr phase. These results further confirm that Co₅Zr phase is the hard magnetic phase in Co-Zr system alloys.

We also measured the coercivity of the minor hysteresis loops of Co₇₇Zr₁₈W₅ ribbons in dependence with the applied magnetic field. It was found that the coercivity of the ribbons increases drastically at the low applied field until the external field exceeds the coercivity value, whereupon it reaches the maximum value. Moreover, the lowfield susceptibility, determined by reversible displacement of the walls, is very large. Magnetic saturation is

obtained in comparatively low applied field. These behaviors indicate that the coercivity of the studied Co₇₇Zr₁₈W₅ ribbons is mainly governed by the nucleation of the reversed domain [6,14]. The physical mechanism should be complex and detailed work for that is required in the future.

4. Conclusion

In conclusion a single metastable Co₅Zr phase is obtained in the Co₇₇Zr₁₈W₅ melt-spun ribbons. The high coercivity of 4.3 kOe is proved coming form the hard magnetic phase of Co₅Zr. Substitution of Co by the relatively larger atomic radius W can lead to an increase of the Co₅Zr phase lattice constants and exhibit a Curie temperature of about 475˚C. The coercivity of the annealed specimens decreases than the as-prepared ribbons because of the decrease of hard magnetic phase and the change of grain size. The excellent hard magnetic properties may make the Co-Zr alloys become very attractive candidate materials for permanent magnetic applications. The related physics is complex and require a further fundamental investigation.

5. Acknowledgements

This work was supported by the National Natural Science Foundation of China grant No. 11074092 and 51101105.

REFERENCES

NOTES

^*Corresponding author.