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Acta Metall Sin  2016, Vol. 52 Issue (5): 561-566    DOI: 10.11900/0412.1961.2015.00517
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Weidan LI,Xiaohua TAN,Kezhi REN,Jie LIU,Hui XU
Institute of Materials, School of Materials Science and Engineering, Shanghai University, Shanghai 200072, China
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A well known feature of ferromagnetic materials is the time dependent behavior of the magnetic polarization, i.e. magnetic viscosity, which arises from thermal activation over energy barriers. It is found that magnetic parameters, such as the fluctuation field (Hf) and the exchange interaction length (lex), have a close relationship with the microstructure of the materials. Therefore, investigation on magnetic viscosity is helpful to understand the coercivity mechanism of ferromagnetic materials. In this work, ingots with nominal composition Nd8.5Fe76Co5Zr3B6.5Dy1, Nd9.5Fe75Co5Zr3B6.5Nb1 and Nd9.5Fe75.4Co5Zr3B6.5Ga0.6 were prepared by arc-melting pure metals Nd, Fe, Co, Zr, Dy, Nb, Ga and Fe-B alloy in an argon atmosphere. A small portion of an ingot weighing about 5 g was re-melted in a quartz nozzle and ejected onto a rotating copper wheel in a range of 10~30 m/s. The annealing treatment was carried out at 690~710 ℃ for 4~5 min. Vibrating sample magnetometer (VSM), XRD and TEM were used to study magnetic viscosity behavior and exchange interaction for Nd2Fe14B/α-Fe nanocomposite permanent alloys. Furthermore, the relationship among exchange interaction, microstructure and magnetic property was discussed. For the nanocomposite Nd8.5Fe76Co5Zr3B6.5Dy1, Nd9.5Fe75Co5Zr3B6.5Nb1 and Nd9.5Fe75.4Co5Zr3B6.5Ga0.6 alloys, Hf and lex were obtaind by sweep rate measurement. The Hf were 4.80, 4.87 and 5.09 kA/m, and lex were 4.53, 4.41 and 4.20 nm for permanent Nd8.5Fe76Co5Zr3B6.5Dy1, Nd9.5Fe75Co5Zr3B6.5Nb1 and Nd9.5Fe75.4Co5Zr3B6.5Ga0.6 alloys, respectively. It suggested that the lex had a minor change. The Nd9.5Fe75Co5Zr3B6.5Nb1 alloy had the strongest exchange interaction among three alloys in this work. It is due to a refined microstructure and uniform distribution of grains. Furthermore, the behavior of the irreversible susceptibility (χirr) as a function of applied magnetic field (H) was investigated. A single sharp peak could be seen near coercive field in the χirr-H curve in three alloys, suggesting that the magnetization reversal was a uniform reversal process. The Nd9.5Fe75.4Co5Zr3B6.5Ga0.6 alloy exhibited a sharper and narrower peak, indicating a more rapid change in magnetization and a strong interaction between adjacent magnetic phases. Since exchange interaction of neighboring grains favors the nucleation of reversed domains, remanence enhancement is generally achieved at the expense of coercivity. Among three alloys, Nd9.5Fe75.4Co5Zr3B6.5Ga0.6 alloy showed the optimum magnetic properties, that is, the coercivity Hc=687.56 kA/m, the remanence Br=0.92 T, the maximum magnetic energy product (BH)max=120.88 kJ/m3. It was mainly due to consisting of well-coupled grains with near perfect alignment of the easy magnetization direction, which improved the remanence and maximum energy product.

Key words:  nanocomposite permanent alloy      magnetic viscosity      exchange interaction      microstructure     
Received:  08 October 2015     
Fund: Supported by National Natural Science Foundation of China (Nos.51171101 and 51471101)

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Alloy Abbreviation Wheel speed / (ms-1) Annealing temperature / ℃ Time / min
Nd8.5Fe76Co5Zr3B6.5Dy1 Dy1 15 690 5
Nd9.5Fe75Co5Zr3B6.5Nb1 Nb1 22 710 4
Nd9.5Fe75.4Co5Zr3B6.5Ga0.6 Ga0.6 18 710 5
Table 1  The abbreviation, wheel speed and heat treatment conditions of nanocomposite permanent alloys
Fig.1  XRD spectra of nanocomposite permanent alloys Dy1, Nb1 and Ga0.6
Fig.2  Demagnetization curves of nanocomposite permanent alloys Dy1, Nb1 and Ga0.6 (H─applied magnetic field, J─magnetic polarization)
Alloy Hc / (kAm-1) Br / T (BH)max / (kJm-3) Hf / (kAm-1) lex / nm Average grain size / nm
Dy1 872.45 0.73 88.18 4.80 4.53 18
Nb1 759.86 0.82 110.92 4.87 4.41 15
Ga0.6 687.56 0.92 120.88 5.09 4.20 40
Table 2  The magnetic properties, fluctuation field Hf, exchange length lex and average grain size for the nanocomposite permanent alloys Dy1, Nb1 and Ga0.6
Fig.3  Deviation of normalized direct current demagnetization and isothemal remanent magnetic intensity (δM) as a function of H for nanocomposite permanent alloys Dy1, Nb1 and Ga0.6
Fig.4  Hc as a function of sweep rate (r) for nanocomposite permanent alloys Dy1, Nb1 and Ga0.6
Fig.5  Bright-field TEM images for nanocomposite permanent alloys Dy1 (a), Nb1 (b) and Ga0.6 (c)
Fig.6  Irreversible susceptibility (χirr) as a function of applied magnetic field (H) for nanocomposite permanent alloys Dy1, Nb1 and Ga0.6
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