magnetization reversal process in sm dy gd co fe cu zr z magnets with different cellular structures

Magnetization reversal process in (Sm, Dy, Gd) (Co, Fe, Cu, Zr)z magnets with different cellular structures Lei Liu, Zhuang Liu, Xin Zhang, Yanping Feng, Chunxiao Wang, Yingli Sun, Don Lee, Aru Yan, and Qiong Wu Citation: AIP Advances 7, 056221 (2017); doi: 10.1063/1.4975701 View online: http://dx.doi.org/10.1063/1.4975701 View Table of Contents: http://aip.scitation.org/toc/adv/7/5 Published by the American Institute of Physics Articles you may be interested in The high squareness Sm-Co magnet having Hcb=10.6 kOe at 150°C AIP Advances 7, 056223056223 (2017); 10.1063/1.4976334 AIP ADVANCES 7, 056221 (2017) Magnetization reversal process in (Sm, Dy, Gd) (Co, Fe, Cu, Zr)z magnets with different cellular structures Lei Liu,1,2 Zhuang Liu,1,2 Xin Zhang,1,2 Yanping Feng,1,2 Chunxiao Wang,1,2 Yingli Sun,1,2 Don Lee,1,2 Aru Yan,1,2 and Qiong Wu3 Key Laboratory of Magnetic Materials and Devices, Ningbo Institute of Material Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, China Zhejiang Province Key Laboratory of Magnetic Materials and Application Technology, Ningbo Institute of Material Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, China China Jiliang University, Hangzhou 310018, China (Presented November 2016; received 20 September 2016; accepted 30 November 2016; published online February 2017) Magnetization reversal mechanism is found to vary with cellular structures by a comparative study of the magnetization processes of three (Sm, Dy, Gd) (Co, Fe, Cu, Zr)z magnets with different cellular structures Analysis of domain walls, initial magnetization curves and recoil loops indicates that the morphology of cellular structure has a significant effect on the magnetization process, besides the obvious connection to the difference of domain energy density between cell boundary phase (CBP) and main phase The magnetization of Sample (with a moderate cell size and uniformly continuous CBPs) behaves as a strong coherence domain-wall pinning effect to the domain wall and lead to a highest coercivity in the magnet The magnetization of Sample (with thin and discontinuous CBPs) shows an inconsistent pinning effect to the domain wall while that of Sample (with thick and aggregate CBPs) exhibits a two-phase separation magnetization Both the two cases lead to lower coercivities A simplified model is given as well to describe the relationships among cellular structure and magnetization behavior © 2017 Author(s) All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/) [http://dx.doi.org/10.1063/1.4975701] I INTRODUCTION 2:17 type SmCo permanent magnet has wide applications especially in high temperature for its large coercive force, excellent thermal stability, high Curie temperature and superior corrosion resistance.1–3 One of the hot topics in 2:17 type SmCo magnet is its coercivity mechanism because of its complex microstructure,3–5 which consists of 2:17 type cells surrounded by a Cu-rich 1:5 cell boundary phase (CPB) The major proposal is domain-wall pinning mechanism which suggests that H c should be proportional to the difference of the domain-wall energy density between the 2:17 phase and 1:5 phase.4,5 Another point is that the coercivity is controlled by the nucleation of reversed domain in some 2:17 type SmCo magnets with abnormal coercivity.3,6,7 Some studies8–11 also thought that the inter-grain exchange coupling is a significant factor to influence the coercivity of Sm(Co, Fe, Cu, Zr)z magnets Nevertheless, all points insist that the cellular structure is responsible for the coercivity of Sm(Co, Fe, Cu, Zr)z magnets.2 Magnetization behaviors, such as domain structure, initial magnetization curve and recoil loops, are closely related to the microstructure and reflect the coercivity of a magnet So it is necessary to illustrate the relationships among cellular structure, magnetization behavior and intrinsic coercivity in 2:17 type SmCo magnets In our previous study, we have found that Dy-Co liquid phase addition can regulate the microstructure of magnet and adjust its coercivity at the same time.12 In this paper, the magnetic behavior during magnetization and reversal magnetization process of (Sm, Gd, Dy) (Co, Fe, Cu, Zr)z magnets with adding wt%, wt% and wt% ratios of DyCo0.51 liquid phase, which possess different cellular structures, are deeply discussed 2158-3226/2017/7(5)/056221/6 7, 056221-1 © Author(s) 2017 056221-2 Liu et al AIP Advances 7, 056221 (2017) II EXPERIMENT Three (SmDyGd)(CoFeCuZr)z permanent magnets were prepared by liquid phase sintering DyCo0.51 (LP) and Sm0.67 Dy0.11 Gd0.22 (Co0.693 Fe0.201 Cu0.081 Zr0.0252 )7.94 (MP) alloys were prepared by induction melting under argon atmosphere The MP with adding different ratios of LP (0 wt%, wt%, wt%) were mixed and milled into powders Fine powders with particle size about 3-5µm were aligned and pressed in a magnetic field up to 3T and further compacted by using cold isostatic pressing The green compactions were sintered at 1200-1220 ◦ C for 0.5 h and later homogenized at 1160-1190 ◦ C for h The subsequent aging was at 830 ◦ C for 12 h, then slowly cooling to 400 ◦ C at a rate of 0.7 ◦ C/min, followed at 400 ◦ C for h and finally by water quenching The domain structure of magnets were measured by using a Lorentz Electron Microscopy The magnetic measurements were performed using a Physical Property Measurement System (PPMS) with a maximum field of 90 kOe The demagnetization factor of the specimens has been neglected because the magnetic measurements were performed along the longitudinal direction of the samples III RESULTS AND DISCUSSION The cellular microstructure of (Sm, Gd, Dy)(Co, Fe, Cu, Zr)z permanent magnets with adding wt%, wt% and wt% ratios of LP have been reported in Ref 12 The cell size, thickness of CBP, Cu content of CBP and intrinsic coercivity, H cj , in the samples are summarized in Table I With the adding ratios of LP increasing from to wt%, it can be seen clearly that the cell size decreases significantly from ∼200 nm to ∼100 nm, meanwhile, the thickness of the CBP is monotonously increasing from ∼5 nm to ∼15 nm and the Cu content of the CBP is gradually decreasing A big cell (∼200 nm) with thin and discontinuous CBP is obtained in the magnet without Dy-Co addition (Sample 1) A moderate cell (∼120 nm) with uniformly continuous CBP is obtained in the magnet with wt% Dy-Co addition (Sample 2) A small cell (∼100 nm) with thick and aggregate CBP is obtained in the magnet with wt% Dy-Co addition (Sample 3) Fig shows the domain walls (DWs) in the Sample analyzed by Lorentz TEM Fig 1(a) is the microstructure shown in over focus and Fig 1(b) is shown in sharp focus The white lines, which are regard as domain walls, along CBP are found in Fig 1(a) This result √ is consisted with that reported by Okabe et al.13 It indicates that the domain energy density (γ = |AK1 |, where K is the magnetocrystalline anisotropy constant and A is the exchange constant of the constituent phases) in the CBP is lower than that in the main phase The CBP exhibits an attractive pinning force to the DWs during applying magnetic field The pinning force is proportional to the difference of domain energy density between CBP and main phase, ∆γ = γ (2 : 17) − γ (1 : 5).4 Generally, the higher Cu content in the CBP, the lower K (1:5) and a bigger ∆γ are obtained According to pinning mechanism,14,15 the coercivity is proportional to ∆γ It’s worth noting that the biggest ∆γ is obtained in Sample while it possesses a lowest coercivity among the samples, which could be due to the effect of different cellular structure on domain-wall pinning process Initial magnetization curves and demagnetization curves of the samples are shown in Fig 2(a) All initial curves of the samples exhibit a low magnetic susceptibility, which reflects the domain-wall pinning process It can be seen that the initial curve in Sample is less steeper than that in Sample or 3, which means a higher domain-wall pinning force exists during the magnetization process, and obtains a biggest intrinsic coercivity, H cj The intrinsic coercivities of Sample and Sample are relatively close, but the initial curves are very different With external fields increasing, the moment of Sample TABLE I Cell size, thickness of CBP, Cu content of CBP and H cj in the samples Sample Dy-Co addition (wt %) Cell size (nm) Thickness of CBP (nm) Cu content of CBP (wt %) H cj (kOe) ∼200 110-130 ∼100 ∼5 10 ∼15 15.34 12.71 9.03 6.05 23.88 11.55 056221-3 Liu et al AIP Advances 7, 056221 (2017) FIG Lorentz TEM images for the magnet with 3wt% Dy-Co liquid phase addition is uniformly increasing before being magnetized to saturation, whereas that of Sample exhibits a magnetically two-phase behavior Recoil loops in the initial magnetization and demagnetization state for the samples are shown in Figs (b), (c) and (d) The recoil loops not close in Sample and Sample 3, while little opens are found in that of Sample According to the reports on nanocomposite permanent magnets in Refs 10 and 11, such a behavior is attributed to the effect of the stray field on the irreversible behavior of the soft phase There is no soft magnetic phase, but there is a big magnetic difference between the 2:17 main phase and 1:5 CBP, which could lead to the opens in recoil loops as well.10,11 Based on the analysis of initial magnetization curves, we think the irreversible movement of DWs is also a non-ignorable factor to the opens As a thin and discontinuous CBPs in Sample and a thick and aggregate CBPs in Sample 3, either the discontinuous CBPs or aggregate CBPs may result in an increase of stray field which could lead to the opens FIG Initial magnetization curves and demagnetization curves (a), recoil loops in the initial magnetization and demagnetization state (b), (c) and (d) for the series of magnets with different Dy-Co liquid phase addition 056221-4 Liu et al AIP Advances 7, 056221 (2017) FIG Irreversible magnetization (a), M irr /2M r , and irreversible magnetization susceptibility (b), χirr , for the series of magnets with different Dy-Co liquid phase addition Irreversible magnetization, M irr /2M r , and irreversible magnetization susceptibility, χirr = d(Mirr /2Mr )/d(H/i Hc ), for the three samples are analyzed modeled after Ref 10, as shown in Fig χirr curves show a narrow and high peak in Sample 2, a wide and low peak in Sample and two peaks in Sample The field at the χirr peak corresponds to the reversal field H n of irreversible magnetization, and the full width at half height suggests the H n distribution in magnets It can be deduced that a strong and narrow distribution domain-wall pinning force exist in Sample 2, a weak and a broad distribution domain-wall pinning force exist in Sample and two different reversal fields exist in Sample Fig shows the reversible magnetization, M rev /M r , for the Samples 1-3 A peak is found in each of Sample and 3, while no peak is discovered in the Sample According to previous reports,8,11 2:17 type SmCo magnet can be recognized as a nanocomposite magnet Generally the value of M rev /M r has a peak It is resulted from the competition between the external field and intergrain exchange coupling.10,16,17 However, they neglected the domain-wall displacement during the magnetization process In our opinion, the reversible movement of domain-wall cannot be ignored, as domainwall pinning by cell boundaries is widely recognized as the coercivity mechanism of 2:17 type SmCo magnet.18 The peak of M rev /M r is resulted from the competition between the external field and domain-wall pinning field Completed cellular structures are obtained in Sample and 3, the domain-wall depinning from 1:5 phase lead to a peak appears in each of the M rev /M r cuves The discontinuous CBPs in Sample cause a broad distribution domain-wall pinning force and lead to no peak appears in the M rev /M r cuve of Sample Combined with the analysis of microstructure of the magnets, the cellular structure should be responsible for the different magnetic behavior Sketch map for the series of magnets with different Dy-Co liquid phase addition are shown in Fig Fig (a) represents Sample As discontinuous FIG Reversible magnetization, M rev /M r , for the series of magnets with different Dy-Co liquid phase addition 056221-5 Liu et al AIP Advances 7, 056221 (2017) FIG Sketch map for the magnets with (a) 0, (b) wt% and (c) wt% Dy-Co liquid phase addition CBPs it possesses, the DWs at the gap of CBPs spread in the 2:17 main phase and there is no ∆γ barrier to block the movement of DWs It exhibits a steep initial magnetization curve, a broad peak in χirr curve, big opens in recoil loops and no peak in M rev /M r curves, which indicates that a discordant domain-wall pinning force is performing during the magnetization or demagnetization process Consequently, the Sample has a lower coercivity although a highest Cu content in CBP and a biggest ∆γ between CBP and 2:17 main phase are obtained Fig (b) represents Sample A cellular structure with a moderate cell size (∼120 nm) and uniformly continuous CBPs is obtained and all DWs are bound in the CBPs, so a relatively gentle initial magnetization curve, a narrow peak in χirr curve, little opens in recoil loops and a peak in M rev /M r curves, which indicates that a strong coherence domain-wall pinning force is existing during the magnetization process Moreover, a higher Cu content and a big ∆γ between CBP and 2:17 main phase are obtained, so a highest coercivity is acquired in Sample Fig (c) representing Sample 3, a cellular structure with small cell size and thick aggregate CBPs is obtained As the volume fraction of CBPs is too high, there is a certain space in CBP that DWs can displace Therefore, there is a separation magnetization of two phases during magnetization and demagnetization The first stage of magnetization belongs to the movement of DWs in CB phase and the second stage belongs to the DWs depinning from CBP At the same time, a lower Cu content and a smaller ∆γ between CBP and 2:17 main phase are achieved In consequence, a lower coercivity is got in Sample Based on these behaviors, it can be concluded that besides the obvious connection to the difference of domain energy density between CBP and main phase, ∆γ,4,5 the morphology of cellular structure also has a significant effect on the magnetization process The thin and discontinuous CBPs in Sample give an inconsistent pinning effect to the domain wall, so the magnet has a relatively low coercivity The uniformly continuous CBPs in Sample lead to a consistent pinning effect, so the magnet has a highest coercivity Two-phase separation magnetization behavior is found in Sample for its thick and aggregate CBPs, which also leads to a lower coercivity IV CONCLUSION In summary, the detailed investigations combining domain structure, initial magnetization curve and recoil loops analyses give clear insight into the magnetic behavior of the magnets with different microstructures Results show that, the cellular structure with a moderate cell size (∼120 µm) and uniformly continuous CBPs of (Sm, Dy, Gd)(Co, Fe, Cu, Zr)z magnet with 3wt% Dy-Co addition is what makes its magnetic behavior exhibits a low initial magnetic susceptibility, a narrow peak in χirr curve, little opens in recoil loops and a peak in M rev /M r curves, which indicates that a strong 056221-6 Liu et al AIP Advances 7, 056221 (2017) coherence domain-wall pinning force is existing during the magnetization process, which lead to a highest coercivity With a lower Dy-Co addition, a cellular structure with big cell size and thindiscontinuous CBPs is obtained, which will destroy the coherence domain-wall pinning effect With an excess Dy-Co addition, a cellular structure with small cell size and thick-aggregate CBPs is obtained, which give rise to a two-phase separation magnetization Both of these two cases can lead to a lower coercivity ACKNOWLEDGMENTS This project was supported by following foundations: China Postdoctoral Science Foundation funded project No 2015M581665, National Natural Science Funds of China No 51301190, National Natural Science Funds of China No 51601209, Zhejiang Province Technology Application Research No 2016C31027 O Gutfleisch, K H Muller, K Khlopkov, M Wolf, A Yan, R Schafer, T Gemming, and L Schultz, “Evolution of magnetic domain structures and coercivity in high-performance SmCo : 17-type permanent magnets,” Acta Materialia 54, 997–1008 (2006) X Y Xiong, T Ohkubo, T Koyama, K Ohashi, Y Tawara, and K Hono, “The microstructure of sintered Sm(Co0.72Fe0.20Cu0.055Zr0.025)7.5 permanent magnet studied by atom probe,” Acta Materialia 52, 737–748 (2004) L Liu, Z Liu, M Li, D Lee, R J Chen, J Liu, W Li, and A R Yan, “Positive temperature coefficient of coercivity in 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