840 Vol.28 No.4 CHENG Gang et al: Effect of Dy on Structure and Magnetic Properties of...

Effect of Dy on Structure and Magnetic Properties of CoPt Alloys

CHENG Gang1, PANG Kuang1, MA Lei1, GU Zhengfei1, HE Wei2, RAO Guanghui1

(1.School of Materials Science and Engineering, Guilin University of Electronic Technology, Guilin 541004, China; 2.Key Laboratory of Nonferrous Metals and New Processing Technology of Materials Ministry of Education, Institute of Materials Science, Guangxi University,

Nanning 530004, China)

Abstract: The effects of Dy on the microstructure and magnetic properties of DyxCo50xPt50 alloys were investigated. The XRD results indicate that all the alloys homogenized at 1000 ℃ contain only a single A1 (fcc) phase, while the alloys annealed at 675 ℃ consist of a hard-magnetic face-centered-tetragonal (fct) phase and a magnetically soft face-centered-cube (fcc) phase. Maximum values for the coercivity Hc and remanence ratio mr were achieved in Dy0.4Co49.6Pt50 alloys annealed at 675 ℃ for 80 min. For the series of DyxCo50xPt50 alloys annealed at 675 ℃ for 60 min, Hc decreases monotonically with increasing Dy concentration, but mr is fi rst enhanced and then weakened.

Key words: magnetic properties; degree of ordering; Co-Pt alloys; crystal structure

©Wuhan University of Technology and SpringerVerlag Berlin Heidelberg 2013(Received: Sep. 20, 2012; Accepted: Jan. 8, 2013)

CHENG Gang (成钢): E-mail: cheng59@guet.edu.cn Funded by the National Natural Science Foundation of China

(No.51261004&50661002) and the National Science foundation of Guangxi Province (2012GXNSFGA060002)

DOI 10.1007/s11595-013-0779-1

1 Introduction

Co-Pt alloys with L10-ordered structure have been recently studied as candidates for the next gen-eration of magnetic materials because of their ex-tremely high magnetic anisotropy constant Kk (4.9×107

ergs/cc)[1]. Generally, the structure of stoichiometric CoPt alloys at high temperatures (above 830 ℃) is a face-centered-cubic (fcc) disordered A1 phase, and shows soft magnetic behavior. The disordered fcc phase can be transformed into a hard magnetic face-centered-tetragonal (fct) L10 phase via annealing at an appropriate temperature. Several efforts have been made to enhance the permanent magnetic properties via phase transformation and control of the microstruc-ture[2-5]. Recently, Qiu et al[6] reported that a maximum intrinsic coercivity of about 342.3 kA/m was obtained in a CoPt alloy annealed at 700 ℃ for 30 min, in which both ordered and disordered phase coexist. Xiao QF et al[7] studied the effect of the atomic disorder-order transformation on the remanence enhancement and

coercivity in CoPt alloys, using isothermal annealing at various temperatures (that were well below the transformation point). Many researchers have proposed that reducing the ordering temperature of CoPt alloy films should be possible via doping with Sn, Pb, Sb, and Bi, which do not form solid solutions with CoPt[8]. However, there have few reports on the effects of Dy on the phase transformations and magnetic properties of CoPt alloys. In the present study, the effects of Dy additives on the microstructure and magnetic properties of CoPt alloys were investigated.

2 Experimental

Ingots of composition DyxCo50xPt50 (x =0, 0.2, 0.4, 0.6, 0.8, 1.0) were prepared by arc-melting of pure metals (the purity of the ingredients is better than 99.9wt%) under purifi ed argon. They were remelted no less than four times to ensure good homogeneity. The as-arc-melted alloys were cast in the form of a thin cyl-inder and then were pressed into plates of about 0.4 mm in thickness, 3 mm in width and 10 mm in length for XRD. The samples were sealed in quartz tubes pre-evacuated and refilled with some purified argon, homogenized at 1100 ℃ for 3 h and then quenched into ice water. Subsequently, the as-quenched samples were annealed, respectively, at 675 ℃ and 750 ℃ for different times. The analysis of crystal structure and

Journal of Wuhan University of Technology-Mater. Sci. Ed. Aug.2013 841

the phase composition was carried out using XRD with Cu Kα radiation. The magnetic properties were measured at room temperature using a vibrating sample magnetometer (VSM). The average grain sizes of fct phase were calculated from the broadening of the X-ray diffraction profi les.

3 Results and discussion

Fig.1 (a) shows X-ray diffraction patterns for DyxCo50xPt50 alloys (x= 0, 0.2, 0.4, 0.6, 0.8, 1.0) annealed at 1100 ℃ for 3 h. For all homogenized samples, the XRD patterns show a single A1 (fcc) phase. The lattice parameters of the face-centered-cubic (fcc) phase are derived, and the results are shown in Fig.1(b). Fig.1(b) shows that the lattice parameter a increases in a near-linear fashion with increasing Dy concentration. It indicates that the Dy atoms dissolve into fcc phase, forming a solid solution, which leads to a cell-volume expansion resulting from the fact that the Dy atom radius is larger than the radii of both the Co and Pt atoms.

Fig.2 displays X-ray diffraction patterns for the DyxCo50xPt50 alloys annealed at 750 ℃ of 60 min. Compared with the XRD patterns shown in Fig.1 (a), it is clear that the (200), (220) and (311) refl ections split into both (200) and (002), (220) and (202) and (311) and (113), respectively. The superstructure reflections (001), (110), (201) and (112) appear in Fig.2. From the XRD data obtained, it follows that all DyxCo50xPt50 alloys annealed at 750 ℃ for 60 min contained only a single L10 (fct) phase. The grain size of fct phase and the degree of ordering S, were calculated from the

XRD data, and the results are displayed in Fig.3. Here, S is defi ned as[9]:

(1)

where, {I(001)/I(111)}measured and {I(001)/I(111)}S=1 are the integrated intensity ratio of (001)/(111) peaks for the measured sample and the perfect ordering phase, re-spectively. It can be seen that the grain size of fct phase did not signifi cantly change with Dy content up to 0.4, but subsequently increased rapidly with Dy content. On the contrary, S first decreases slowly and then shows a strong reduction. A possible reason for this could be that the existence of Dy may introduce local strain, leading to restrain the ordering. The results suggest that Dy element can promote grain growth of fct phase but postpones the process of ordering.

The XRD results obtained for Dy0.4Co49.6Pt50 al-loys annealed at 675 ℃ for various times are shown in Fig.4. Fig.4(b) shows that the (200) refl ection shifts to a lower angle, and the (002) reflection appears in the XRD patterns when the annealing time is longer than

842 Vol.28 No.4 CHENG Gang et al: Effect of Dy on Structure and Magnetic Properties of...

60 min. The phase transformation from fcc to fct phase is not completed even with an annealing time of 100 min, because the (200) refl ection has not fully split into two refl ections ((200) and (002)). Therefore, there ex-ists a nanocomposite alloy phase, namely, an fct or-dered hard-magnetic precipitated phase coexisting with an fcc disordered soft-magnetic matrix phase. From the line width of the refl ections belonging to the fct phase, we can derive the values of the average grain size of the fct phase by means of Scherrer’s formula. These values are included on the left side in Fig.4(a). Fig.5 shows the effect of annealing time on the magnetic properties of the Dy0.4Co49.6Pt50 alloys. Peak value for the coercivity Hc (6450 Oe) reaches at an annealing time 80 min. The rise in Hc with annealing time is mainly deu to the improvement in the chemical ordering of the fct phase, leading to an increase in the anisotropy constant Kk. The relationship between the coercivity and the anisotropy constant can be expressed as follows:

Hc=Kk/0Msm (2)

where, Msm is the saturation magnetization of the soft magnetic phase and Kk is the anisotropy constant of the hard magnetic phase. The reduction in Hc with annealing time is ascribed to excessive grain growth in the fct phase, an effect that has preiously been reported by Ka-neko et al[10]. The higher remanence ratio suggests that the exchange-coupling occurs between the soft magnetic phase and the hard magnetic phase.

Fig.6 shows X-ray diffraction patterns for DyxCo50xPt50 alloys annealed at 675 ℃ for 60 min. The average grain sizes for fct phase were calculated from

Journal of Wuhan University of Technology-Mater. Sci. Ed. Aug.2013 843

the XRD data obtained and the results are shown in Fig.6. Fig.7 illustrates the effect of Dy concentration on the magnetic properties of the DyxCo50xPt50 alloys. The inset in Fig.7 shows that the coercivity Hc of the alloys first decreases rapidly as the Dy content increases to 0.4, and then shows a more gradual change for x values from 0.4 to 0.6. Hc again decreases rapidly with further increasing Dy content. The marked reduction in the coercivity can be mainly attributed to the decrease in the anisotropy constant produced by the addition of Dy, as discussed above. In addition, the exchange-coupled interaction decayed because of the increase in grain size in the fct phase, which leads to further decreases in Hc. This is why Hc decreases with increasing Dy content for CoPt-based alloys.

The derived values for mr are shown in Fig.7. For an isotropic system of tetragonal symmetry, one expects mr=0.5. The higher remanence ratio shown here suggests that the magnetic particles in the alloys are exchange-coupled. A maximum value of mr=0.81 is obtained for x=0 to 0.2. The relationship between mr and Dy content can be explained as follows: the exchange-coupling between the soft magnetic and hard magnetic phases relates to the grain size, and an appropriate grain size in the fct phase can enhance exchange-coupling[11]. On the other hand, the hard magnetic phase is not true hard magnetic phase because of the decrease in S resulting from the addition of Dy, which leads to a decrease in the exchange coupling between the soft and hard magnetic phase. These two factors result in a peak remanence ratio value of 0.81 at x=0.2.

4 Conclusions

When small amounts of Dy were substituted for Co, the Dy atoms dissolved into the fcc phase, forming a solid solution. The addition of Dy led to a decrease in the degree of ordering S and an increase in the average grain size D in the fct phase. These results suggested that Dy played an important role in promoting the grain growth and retarding the ordering process. For

Dy0.4Co49.6Pt50 alloys, both the maximum values of the coercivity Hc (about 6450 Oe) and remanence ratio mr (about 0.82) were obtained in alloys annealed at 675 ℃ for 80 min. With the appropriate substitution of Dy for Co, Dy was able to enhance exchange-coupling between hard magnetic phase (fct) and soft magnetic phase (fcc), with a peak value of the remanence ratio mr (about 0.82) at x=0.4.

References[1] Brissonneau P, Blanchard A, and Bartholin H. Magnetic Properties of

Ordered Pt-Co[J]. IEEE Trans. Magn., 1966, 21: 479-482

[2] Elena V, Gomonaj, Eugeniusz, et al. Atomic and Mmagnetic Ordering

in CoPt Alloy [J]. J. Appl. Phys., 1995, 77(5): 2 160-2 165

[3] Liou SH, Huang S, Klimek E, et al. Enhancement of Coer-civity in

Nanometer-size CoPt Crystallites[J]. J. Appl. Phys., 1999, 85: 4 334-

4 336

[4] Yi Ding and Sara A Majetich. Size Dependence, Nucleation, and Phase

Transformation of FePt Nanoparticles[J]. Appl. Phys. Lett., 2005, 87:

022508-1-3

[5] Yokota T, Gao L, Liou SH, et al. Effect of Au Spacer Layer on L10

Phase Ordering Temperature of CoPt Thin Films[J]. J. Appl. Phys.,

2004, 95: 7 270-7 272

[6] Qiu N, Teubert JA, Overfelt RA, et al. Microstructural and Magnetic

Characterization of Rapidly Solidifi ed and Annealed Pt-Co-B Alloys[J].

J. Appl. Phys., 1991, 70: 6 137-6 139

[7] Xiao QF, Bruck E, Zhang ZD, et al. Remanence Enhancement and

Coercivity in Two-phase CoPt Bulk Magnets[J]. J. Appl. Phys., 2002,

91: 304-307

[8] Kitakami O, Shimada Y, Oikawa K, et al. Low-temperature Ordering

of L10–CoPt Thin Films Promoted by Sn, Pb, Sb, and Bi Additives[J].

Appl. Phys. Lett., 2001, 78: 1 104-1 106

[9] Takahashi YK, Koyama T, Ohnuma M, et al. Size De-pendence of

Ordering in FePt Nanoparticles[J]. J. Appl. Phys. 2004, 95: 2 690-2

696

[10] Kaneko H, Homma M, Suzuki K. A New Heat-treatment of Pt-Co

Alloys of High-grade Magnetic Properties[J]. Trans. JIM., 1968, 9:

124-127

[11] Kneller EF, Hawig R. The Exchange-spring Magnet: A New Material

Principle for Permanent Magnet[J]. IEEE Trans. Magn., 1991, 27:

3 588-3 600