1 Key Laboratory of Solidification Control and Digital Preparation Technology (Liaoning Province), School of Materials Science and Engineering, Dalian University of Technology, Dalian 116024, China 2 Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan
Cite this article:
Dianguo MA,Yingmin WANG,Kunio YUBUTA,Yanhui LI,Wei ZHANG. Effect of Co Content on the Structure and Magnetic Properties of Melt-Spun Fe55-xCoxPt15B30 Alloys. Acta Metall Sin, 2017, 53(5): 609-614.
Fe-Pt-B nanocomposite magnets have attracted much attention because of their excellent hard magnetic properties, in which the face-centered-tetragonal FePt (L10) phase ensures high coercivity (iHc) and the Fe2B phase provides high magnetic saturation. A high iHc, however, is hard to reach at low Pt concentrations in these nanocomposite magnets. It is known that a high concentration of B favors the formation of L10 phase in Fe-Pt-B alloys with low Pt concentration, but the annealed microstructure is usually coarse-grained due to their low amorphous-forming abilities, and the magnetic properties get deteriorated. Replacement of Fe with Co is expected to enhance the amorphous-forming ability of Fe-Pt-B alloys with low Pt and high B concentrations, and to improve their magnetic properties. In this work, the structure and magnetic properties of as-quenched and annealed Fe55-xCoxPt15B30 (x=0~45, atomic fraction, %) alloys have been investigated. Melt-spun ribbons were prepared by melt spinning, followed by vacuum annealing at different temperatures. The structure and magnetic properties of the samples were examined by XRD, TEM and a vibrating sample magnetometer (VSM). The results indicate that single amorphous phase is formed in the alloys at x=15~45. After appropriate annealing, a nanocomposite structure consisting of L10 and (Fe, Co)2B phases is obtained at x=0 and 15, and an additional (Fe, Co)B phase gets formed at x=30 and 45. A fine microstructure with mean grain size of ~18 nm has been obtained in the annealed alloys with x=15~45. In these nanocomposite alloys, the best hard magnetic property with an energy product of 94.4 kJ/m3 is reached at x=15. With increasing Co content, the iHc gradually increases to a maximum value of 413.7 kA/m at x=30, and then decreases at higher Co contents, which are attributed to the change of the magnetocrystalline anisotropy in L10 phases with different c/a ratios.
Fig.1 XRD spectra of melt-spun Fe55-xCoxPt15B30 alloy ribbons
Fig.2 XRD spectra of the Fe40Co15Pt15B30 amorphous alloy annealed at different temperatures for 900 s
Fig.3 Demagnetization curves of the Fe40Co15Pt15B30 alloy annealed at different temperatures for 900 s (H—applied magnetic field, J—magne-tic polarization)
x
a / nm
c / nm
c/a
0
0.3859
0.3699
0.9585
15
0.3851
0.3693
0.9589
30
0.3843
0.3690
0.9604
45
0.3839
0.3681
0.9588
Table 1 Lattice constants of L10 phase of the Fe55-xCoxPt15B30 alloys annealed at 823 K for 900 s
x
iHc
Br
Mr / Ms
(BH)max
kAm-1
T
kJm-3
0
225.1
1.02
0.81
64.7
15
275.9
0.97
0.84
94.4
30
413.7
0.50
0.75
33.5
45
184.8
0.47
0.75
27.7
Table 2 Magnetic properties of the Fe55-xCoxPt15B30 alloys annealed at 823 K for 900 s
Fig.4 XRD spectra of the Fe55-xCoxPt15B30 alloys annealed at 823 K for 900 s
Fig.5 Hysteresis loops of the Fe55-xCoxPt15B30 alloys annealed at 823 K for 900 s
Fig.6 Bright-field TEM images and corresponding SAED patterns (insets) of the Fe55- xCoxPt15B30 alloys
annealed at 823 K for 900 s (a) x=0 (b) x=15 (c) x=30 (d) x=45
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