Bulk SmCo3 nanocrystalline magnets with magnetic anisotropy

Journal Pre-proofs Bulk SmCo3 nanocrystalline magnets with magnetic anisotropy Tiancong Li, Bo Jiang, Li Lou, Yingxin Hua, Jieqiong Gao, Jinyi wang, Xiaohong Li PII: DOI: Reference:

S0304-8853(19)33842-9 https://doi.org/10.1016/j.jmmm.2020.166552 MAGMA 166552

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Journal of Magnetism and Magnetic Materials

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7 November 2019 16 January 2020 1 February 2020

Please cite this article as: T. Li, B. Jiang, L. Lou, Y. Hua, J. Gao, J. wang, X. Li, Bulk SmCo3 nanocrystalline magnets with magnetic anisotropy, Journal of Magnetism and Magnetic Materials (2020), doi: https://doi.org/10.1016/ j.jmmm.2020.166552

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Bulk SmCo3 nanocrystalline magnets with magnetic anisotropy Tiancong Li a,b, Bo Jiang a, Li Lou a, Yingxin Hua a, Jieqiong Gao a, Jinyi wang a, and Xiaohong Li, a,b,* aState

Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao 066004, China

bKey

Laboratory for Microstructural Material Physics of Hebei Province, School of Science, Yanshan University, Qinhuangdao 066004, P. R. China

Abstract In bulk SmCo3 nanocrystalline materials with small grain sizes of 10-20 nm, it is a challenge to realize a strong crystal texture that is crucial for them as high-performance permanent magnets. Here we fabricated an anisotropic bulk SmCo3 nanocrystalline magnet with a strong (00l) texture combined with a small grain size of 10-20 nm using a high-pressure thermal compression (HPTC) approach from amorphous precursors. This magnet had an obvious magnetic anisotropy with Mr///Mr⊥ = 1.53 and a maximum energy product of 4.2 MGOe (33.8 kJ/m3) which are obviously higher than those Mr///Mr⊥ = 1.01 and 1.9 MGOe (15.1 kJ/m3) of the isotropic magnets prepared using the same precursors. These anisotropic magnets shows a coercivity temperature coefficient of β = -0.31 %/°C and a remanence temperature coefficient of α = -0.14 %/°C. The deformation and annealing temperatures played important roles on the microstructures and magnetic properties of these anisotropic SmCo3 nanocrystalline magnets. These findings make an important step toward the fabrication of oriented bulk SmCo3 magnets for practical applications. Keywords: Magnetic anisotropy, Nanocrystalline, Bulk, Microstructure, Magnetic properties *Corresponding

author, e-mail address: [email protected] (X. Li) 1

1. Introduction SmCo-based

magnets

have

attracted

strong

attentions

due

to

their

good

high-temperature magnetic properties and intense potential applications in aerospace, military, environment-friendly technologies and so forth [1-3]. Besides the well-developed commercial SmCo5 and Sm2Co17 permanent magnets [4], nanocrystalline Sm-Co magnets have attracted much scientific and technical interest due to some magical magnetic properties originating from small grain sizes and numerous interfaces [5-8] and their potential, as constituent components, for creating high-energy-product nanocomposite magnets [6]. For example, high energy products of 28-31 MGOe have been successfully achieved in SmCo-based hybrid nanostructures [6,9,10]. In the last decades, many Sm-Co nanocrystalline magnets such as Sm2Co7, SmCo5, SmCo7, Sm2Co17, Sm(FeCo)12 etc have been studied [11-16]. And big progresses have been made for the fabrication of isotropic and anisotropic SmCo5, SmCo7 or Sm2Co17 bulk nanocrystalline magnets [12,17-20]. But studies on bulk nanocrystalline SmCo3 magnets were rarely paid attention to, though they were reported to possess a high coercivity of 33 kOe [21]. Furthermore, at present, most of studies for bulk SmCo3 nanocrystalline magnets focuse on isotropic magnets, as it is challenging for fabricating nanocrystalline materials with small grain sizes of 10-20 nm and a strong crystallographic texture [6,22,23]. The anisotropic bulk SmCo3 nanocrystalline magnets with a strong texture have not been reported till now. Here we fabricated an anisotropic bulk SmCo3 nanocrystalline magnet with a strong (00l) texture combined with a small grain size of 10-20 nm using a high-pressure thermal compression (HPTC) approach from amorphous precursors. This magnet had an obvious magnetic anisotropy with Mr///Mr⊥ = 1.53 and a

2

maximum energy product of 4.2 MGOe (33.8 kJ/m3) which was obviously higher than that 1.9 MGOe (15.1 kJ/m3) of the isotropic magnets prepared using the same precursors. 2. Material and methods Sm-Co alloys with the atomic ratio Sm:Co = 1:3 were prepared by argon arc melting from pure Sm and Co metals with appropriate excess of Sm about 2 wt% to compensate the evaporation loss. The alloys were then crushed into powders and the crushed powders were subjected to high-energy ball milling using a SPEX 8000M mill with a milling time of 4.5 h and a ball-to-power weight ratio of 20:1 in argon atmosphere. The milled powders were consolidated into bulk cylinders with a relative density of 80 % in argon atmosphere at room temperature. The consolidated bulks were sealed in a steel tube and subjected to high-pressure thermal compression (HPTC) deformation under the stress of approximately 1 GPa with the deformation temperature T = 640-720 °C, deformation time t = 30 s, and height reduction  = 80 %, respectively. As a comparison, a high-pressure annealing (HPA) experiment was carried out on the milled precursors at 660 °C under a pressure of 3 GPa. The microstructures of the synthesized magnets were characterized using a transmission electron microscopy (TEM) and an X-ray diffractometer (XRD) with Co Kα radiation. The grain size of crystalline phase was calculated by the Rietveld refinement procedure from the measured XRD patterns. The magnetic properties at room and high temperatures were measured using a physical property measurement system (PPMS) and a vibrating sample magnetometer (VSM) with a maximum field of 8.5 T and 3 T, respectively. 3. Results and Discussion The as-milled Sm-Co powders exhibited an amorphous-nanocrystalline mixture

3

structure [the inset of Fig. 1(a)], in which a few SmCo3 nanocrystals distributed in an amorphous matrix. From the XRD pattern, the volume fraction of the amorphous matrix was determined to be about 67.5 %. XRD patterns (Figs. 1(a) and (b)) show that both the HPTC and HPA magnets exhibit a SmCo3 nanocrystalline structure. The average grain size of the HPTC and HPA magnets was determined to be ~12.3 and 10.6 nm, respectively, by analyzing the their XRD patterns using the Rietveld refinement procedure. The HPTC magnet possessed a strong (0012) texture along the pressure direction which was indicated by the enhanced intensity of the (0012) peak on the face perpendicular to the pressure direction [Fig. 1(c)], where the intensity ratio of I(0012) / I(116) = 1.58 is much larger than the value (0.125) for isotropic SmCo3 crystals [see the Inorganic Crystal Structure Database (ICSD): 625225]. (a)

(c)

(b)

(d)

Fig. 1 Characterization of bulk SmCo3 nanostructures made by HPTC deformation and HPA. (a) The XRD pattern of the bulk SmCo3 magnet made by HPTC deformation at the 4

temperature of T = 680 °C measured perpendicular to the pressure direction, inset is the XRD pattern of as-milled powders. (b) The XRD pattern of the magnet made by HPA at the temperature of T = 660 °C. (c, d) Zoomed views of the XRD patterns marked with the dotted line in panel (a) and (b), and the separated individual diffractions.

The determination of the texture could be further verified by the strengthened (0012) diffraction on the selected area electron diffraction [see the inset of Figs. 2(a) and (b)]. While the HPA magnets had no texture with the value of I(0012) / I(116) = 0.124 similar to the value (0.125) on the ICSD card [Fig. 1(d)]. The HPTC magnet exhibits an elongated grain shape with the short axis of ~12 nm along [Fig. 2(c)] and the long axis of ~15 nm [Fig. 2(e)], which was consistent with the XRD studies [Fig 1(a)]. These results demonstrated that the oriented bulk SmCo3 nanostructure with a strong (0012) texture and a small grain size can be fabricated by the HPTC technique. Previous studies demonstrate that strain-energy anisotropy has an important role in aligning nanograins in an amorphous alloy [24-26]. A small strain energy (along a specific crystallographic direction) lowers the energy barrier for crystal nucleation [6,24], facilitating grain alignment along the direction. We suggest that the strain-energy anisotropy yielded in the HPTC process might also be the reason for the (00l) texture formation in the HPTC magnet.

5

Fig. 2 (a, b) Bright-field and dark-field TEM images of the HPTC SmCo3 magnet, inset is the corresponding SAED pattern. (c) The statistical distribution of the boundary spacing along the short-axis direction, (e) the statistical distribution of the boundary spacing along the long-axis direction. (d) High-resolution TEM image and Fourier transformation (see the inset) of the HPTC SmCo3 nanograins indicating that the c-axis alignment was approximately parallel to the pressure direction.

The HPTC magnets exhibited an obvious magnetic anisotropy along and perpendicular to the applied pressure which further verified the analysis about the crystalline textures. As a result, this magnet had an enhanced maximum energy product of 4.2 MGOe (33.8 kJ/m3) and a remanence ratio Mr/Ms = 0.86 compared with those (BH)max = 1.9 MGOe (15.1 kJ/m3) and Mr/Ms = 0.73 for the HPA magnet [Fig. 3]. The enhanced Mr/Ms value mainly originated from the strong (0012) texture of the SmCo3 nanocrystals [27]. The enhancement of the energy

6

product was attributed to the enhanced remanence due to the formation of the (0012) texture. Though a high remanence ratio can be obtained due to the strong texture, the energy product is not so high, which was attributed to the low Ms of the SmCo3 compound [28]. (a)

(b)

Fig. 3 (a) The demagnetization curves of the bulk SmCo3 magnets made by HPTC deformation measured parallel (blue) and perpendicular (red) to the pressure direction (b) The demagnetization curves of HPA magnets along two perpendicular directions.

The magnetic properties of the HPTC magnets at high temperatures were investigated [Fig. 4]. The HPTC magnet showed a coercivity temperature coefficient of β = -0.31 %/°C and a remanence temperature coefficient of α = -0.14 %/°C [Fig. 4(b)]. These results suggest that our SmCo3 nanostructure has large potential for high-temperature applications. (a)

(b)

7

Fig. 4 Characterization of high temperature magnetic properties of HPTC SmCo3 magnet. (a) The demagnetization curves at 25-300 °C. (b) Dependence of coercivity Hci andremanence Br, on the measurement temperatures.

The deformation temperatures for the HPTC played important roles on the microstructure and magnetic properties of the fabricated magnets [Fig. 5]. With increasing the deformation temperatures from 640 to 720 °C, the (0012) texture becomes stronger, which is indicated by the increase of the I(0012) / I(116) ration from 0.76 to 2.02 [Figs. 5(a)]; moreover, the average grain size increases from ~11.8 to 18.9 nm, demonstrating the magnets still have small grain size. The remanence increases and coercivity decreases with increasing annealing temperatures [Figs. 5 (b), (c) and (d)]. We suggest that the increase of the remanence originated from the strengthened (0012) texture and the decrease of the coercivity may be attributed the grain growth of the SmCo3 nanocrystals at higher temperatures [7,29].

8

(a)

(c)

(b)

Fig. 5 (a) The XRD patterns of SmCo3 magnets prepared by HPTC at 640-720 °C, (b) dependence of the relative intensity ratios I(0012) / I(116) on different temperatures for HPTC deformation, (c) The demagnetization curves of bulk SmCo3 magnets prepared by HPTC at 640-720 °C, (d) dependence of the coercivity Hci and remanence Br of SmCo3 magnets on different temperatures for HPTC deformation.

Annealing the HPTC magnets (with a deformation temperature of 680 °C) under the temperatures from 700-850 °C also played important effects on microstructures and magnetic properties [Fig. 6]. With the increase of the annealing temperatures, the (0012) textures became stronger demonstrated by the increasement of the I(0012) / I(116) values from 1.58 to 2.27 for the annealing from 700 to 850 °C; at the same time, the average grain size increases 9

from ~12.3 to 52.4 nm [Fig. 6(a) ]. As a result, the remanence increased and the coercivity decreased and the energy product increased [Figs. 6(b) and (c)]. We suggest that the remanence enhancement results from the strong (00l) texture, and the coercivity decrease might be caused from grain growth at high annealing temperatures [7,29]. (a)

(c)

(b)

Fig. 6 Characterization of microstructures and properties of HPTC SmCo3 magnets after annealing at different temperatures. The HPTC magnet was made at a temperature of 680 °C. (a) The XRD patterns of HPTC SmCo3 magnets annealed at 700-850 °C, (b) demagnetization

10

curves of HPTC SmCo3 magnets annealed at 700-850 °C, (c) dependence of the relative intensity ratios I(0012) / I(116), coercivity Hci, remanence Br and the energy products (BH)max on annealing temperatures.

4. Conclusions In conclusion, an anisotropic bulk SmCo3 nanocrystalline magnet with a strong (0012) texture combined with a small grain size of 12-15 nm was prepared by using the high-pressure thermal compression. This magnet had an obvious magnetic anisotropy with Mr///Mr⊥ = 1.53 and a maximum energy product of 4.2 MGOe (33.8 kJ/m3) which was obviously higher than that 1.9 MGOe (15.1 kJ/m3) of the isotropic magnet prepared using the same precursors. These anisotropic magnets show a coercivity temperature coefficient of β = -0.31 %/°C and a remanence temperature coefficient of α = -0.14 %/°C. With increasing the deformation and annealing temperatures, the textures of SmCo3 phase and the energy product of these magnets increased and the coercivity decreased. These findings make an important step toward the fabrication of oriented bulk SmCo3 magnets for practical applications.

Acknowledgments We acknowledge financial support from National Natural Science Foundation of China (No. 51771163, 51931007, 51931196), Natural Science Foundation of Hebei province (No. E2018203142), project of Department of Education of Hebei Province (No. SLRC2019038, CXZZBS2019054).

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Bulk SmCo3 magnet with (00l) texture and nano grain size were prepared. Bulk SmCo3 magnet has an obvious magnetic anisotropy and 4.2 MGOe. The energy product of anisotropic magnet is 120 % larger then that of isotropic. Bulk SmCo3 magnet has potential for high-temperature applications.

Declaration of interests

☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Author statement

Tiancong Li: Conceptualization, Methodology, Supervision, Investigation. Bo Jiang: Methodology, Software, Investigation. Li Lou: Software, Validation, Formal analysis. Yingxin Hua: Funding acquisition, Validation. Jieqiong Gao: Data Curation. Jinyi wang: Investigation. 16

Xiaohong Li: Writing-Reviewing and Editing, Writing-Original Draft, Resources, Funding acquisition.

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