Exchange-coupled SmCo₅/Co nanocomposites synthesized by a novel strategy

Abstract

SmCo₅/Co nanocomposites with exchange-coupling are synthesized by a reverse design where Co is decomposed from SmCo₅ nanoparticles by hydrogen disproportionation process to fabricate two-phase nanocomposites. The novel design makes SmCo₅ nanoparticles uniformly surrounded by Co nanoparticles, which avoids the alloying in traditional methods and achieves exchange-coupling.

---

Nanocomposite magnets are desirable for promising applications like modern aerospace, electric motors, high-density magnetic storage, mediators for therapeutics and etc.^1–9 Particularly, SmCo-based nanocomposites exhibit unique advantages since SmCo₅ permanent magnet has exceptionally-large magnetocrystalline anisotropy (K_u = 2 × 10⁸ erg cm⁻³) and a high Curie temperature (T_c = 1020 K),^10–12 being expected to develop into a new generation of permanent magnetic materials.^5–9

For SmCo-based nanocomposites, effective exchange-coupling is an expected target to achieve high magnetic performance.^1–3 Theoretically, to realize effective exchange-coupling, the size of soft phase should be reduced to about twice the domain wall width of the hard phase, which requires the size of soft phase less than 10 nm.^13–16 Meanwhile, soft phase should uniformly distribute around the hard phase. Many efforts have been focused on SmCo₅/Fe(Co) nanocomposites by mechanical ball-milling method.^17–19 Nevertheless, there are three problems to be settled: (1) a large size for hard and soft phases; (2) an uneven distribution for two phases; (3) serious alloying between SmCo₅ and Fe/Co (Fe or Co is prone to enter into the lattice of SmCo₅ during ball-milling or high temperature process).

Chemical route exhibits distinct advantage in fabrication of nanocomposites since the chemically synthetic particles could be accurately controlled at nanoscale.^20–23 To date, SmCo₅/Fe nanoparticles have been prepared by reductive annealing of samarium cobalt oxide and ferric oxide.^24,25 However, alloying phenomenon still exist for high temperature annealing (>800 °C), which could not fully exploit the potential of nanocomposites.²⁴ Recently, SmCo₅/Co core–shell nanocomposites have been synthesized via calciothermic reduction of Sm[Co(CN)₆]·4H₂O coated with graphene oxide (GO) by Yang et al.²¹ Although the GO could restrain the phenomenon of alloying between soft and phases to some extent, there is still slight interface diffusion (alloying) in the nanocomposites. Therefore, it is still an arduous task to fabricate independently two-phase SmCo-based nanocomposites and demonstrate the exchange-coupling.

Herein, we report a facile approach to chemically synthesize exchange-coupled SmCo₅/Co nanoparticles. Different from all previous methods, a reverse design is proposed to decompose Co nanoparticles from SmCo₅ nanoparticles by hydrogen disproportionation process at 400 °C. The facile strategy exhibits three obvious advantages. Firstly, for low temperature, this novel method yields independently two-phase SmCo₅/Co other than the alloy of SmCo–Co. Secondly, in this process, the sizes of SmCo₅ and Co can be accurately controlled at nanoscale. Finally, this approach makes SmCo₅ nanoparticles uniformly surrounded by Co nanoparticles due to in situ synthesis. The three advantages contribute to effectively exchange-coupling.

The synthetic route is outlined in Fig. 1. SmCo₅/SmCo₅H_x nanoparticles were first obtained by hydrogenation process under high hydrogen pressure. Then hydrogenated nanoparticles were further decomposed at 400 °C under hydrogen to produce the core–shell SmCo₅@SmH_2±x&Co, according to the reaction: SmCo₅ + (2 ± x)H₂ ↔ SmH_2±x + 5Co.^26,27

Fig. 1 Schematic illustration of the synthesis of SmCo₅/Co nanocomposites by the hydrogenation of SmCo₅ nanoparticles: (a) hydrogenation process; (b) heating to 400 °C; (c) washing by deionized water and acetic acid.

And the proportion of soft phase can be manipulated by tuning the hydrogenation time. After washing the extra SmH_2±x, exchange-coupled SmCo₅/Co nanocomposites were achieved. The in situ synthesis offers a facile approach for the preparation of exchange-coupled SmCo-based nanocomposites. And the method avoids alloying of soft and hard phase, achieving uniform distribution of soft-hard phases.

SmCo₅ nanoparticles were first synthesized by reductive annealing of SmCo–OH at 870 °C, which has been described elsewhere.²⁸ The mixture of Sm(OH)₃ and Co(OH)₂ were prepared by co-precipitation method; then the precursor were further reduced by Ca at 870 °C for 1.5 h. After wash and separation treatment, 30–50 nm SmCo₅ nanoparticles were obtained. Then 200 mg synthesized SmCo₅ nanoparticles were heated up to 400 °C in vacuum. Then the sample would be hydrogenated under 4 MPa H₂. The hydrogen pressure was kept for different time to control the proportion of Co. After the reaction, the H₂ atmosphere was quickly replaced by Ar atmosphere. The powders were allowed to cool to room temperature quickly and further washed by deionized water and 5% acetic acid for 10 min to dissolve the extra SmH_2±x according to the reaction: 2SmH_2±x + 6H₂O = 2Sm(OH)₃ + (5 ± x)H₂. The hydrogen absorption data is illustrated in Fig. 2a. It can be clearly seen that, the hydrogen absorption content increases obviously with reaction time going on. According to the reaction equation: 2SmCo₅ + (2 ± x)H₂ = 2SmH_2±x + 10Co, when x = 0, SmCo₅ : Co = (1 − n) : 5n can be adopted to roughly estimate Co content, where n is H₂ absorption content. As shown in Fig. 2a (inset: labeled line), the n value is 1.3%, 2.25%, 3.3% and 5.75% with reaction time of 10 min, 20 min, 30 min and 40 min, respectively. On the basis of this calculation, SmCo₅/Co_0.07, SmCo₅/Co_0.12, SmCo₅/Co_0.17 and SmCo₅/Co_0.29 are obtained with reaction time of 10 min, 20 min, 30 min and 40 min, respectively.

Fig. 2 (a) Dependence of hydrogen absorption content of SmCo₅ nanoparticles on time; (b) XRD patterns of SmCo₅/Co with different hydrogenation time.

The structure of as-prepared powders has been characterized by X-ray diffraction (XRD). To clarify the existence of precipitated Co, the large current of 200 mA (generally, 40 mA) was employed to obtain strong diffraction peak. Fig. 2b is the XRD patterns of SmCo₅/Co nanoparticles with different hydrogenation time. For these hydrogenated samples, there are weak diffraction peaks at 44.2°, corresponding to the (111) planes of cubic structure Co (JCPDS No. 15-0806), which confirms that SmCo₅ nanoparticles are decomposed by hydrogenation process. No obvious SmH_2±x diffraction peak was found, indicating SmH_2±x in the materials is amorphous. Furthermore, with the reaction time increasing, the diffraction peaks of Co become stronger, indicating that the Co content in nanocomposites also increases with the hydrogenation time going on, in good agreement with the hydrogen absorption curve. The magnification of diffraction patterns of Co is given in Fig. S1 (ESI†).

The quantitative analysis of hard and soft phases is carried out by Reference Intensity Ratio (RIR) method using the JADE software. According to this method, the phase contents can be estimated by the formula as followed:

Here W_A is weight percent of A phase, K^B_A = K_B/K_A, I_A and I_B are maximum integrated intensity (I_area) of all peaks in A phase and B phase respectively. According to the data in JADE, K_SmCo5 = 7.29, K_Co = 6.35. The related data is shown in Table 1. In SmCo₅/Co nanocomposites, SmCo₅/Co_0.06, SmCo₅/Co_0.11, SmCo₅/Co_0.19 and SmCo₅/Co_0.26 are obtained with reaction time of 10 min, 20 min, 30 min and 40 min, respectively, extremely close to the calculation results by hydrogen absorption data. The detailed description of integration method is given in the ESI.†

Table 1 Integrated intensity and calculative content of SmCo₅ and Co with different hydrogenation time

Time	ISmCo5	ICo	WSmCo5	SmCo5 : Co (at)
10 min	100	0.9	0.992	1 : 0.06
20 min	100	1.6	0.986	1 : 0.11
30 min	100	3.0	0.975	1 : 0.19
40 min	100	3.9	0.967	1 : 0.26

---

The ratio of elements in SmCo₅ with 30 min hydrogenation at different steps is further measured by Inductively Coupled Plasma-Atomic Emission Spectrometry (ICP-AES). The data displayed in Table 2 demonstrates that hydrogenation process makes no loss of elements. And the SmCo₅ : Co is 1 : 0.182 in washed SmCo₅/Co hydrogenated for 30 min, which is in accordance with results calculated by RIR method (0.19). The difference of Sm : Co, between the SmCo₅ before hydrogenation and washed SmCo₅/Co, is mainly attributed to the loss of washed SmH_2±x.

Table 2 The ratio of elements in SmCo₅ by ICP-AES with 30 min hydrogenation at different steps

Steps	Phases	Sm : Co (at)	SmCo5 : Co
Before hydrogenation	SmCo5	1 : 4.9099	1 : 0
After hydrogenation	SmCo5/Co/SmH2±x	1 : 4.9106	—
After washing	SmCo5/Co	1 : 5.0919	1 : 0.182

---

The nanoparticles feature of both SmCo₅ and Co in composite is characterized with transmission electron microscopy (TEM). Fig. 3a–c are TEM images of SmCo₅/Co_0.19 nanocomposites without washing. The unwashed composites exhibit core–shell structure with core size of 30–50 nm and shell thickness of 3–5 nm (Fig. 3a and b). The high resolution TEM (HRTEM) in Fig. 3c further demonstrates the core has lattice at 2.17 Å, corresponding to the (200) planes in SmCo₅, and the shell comprises of an amorphous phase (may be SmH_2±x) with Co nanoparticles embedded in. This core–shell structure may be developed from the SmCo₅@SmCo₅H_x core–shell structure. The TEM images of washed nanocomposites (Fig. 3d–f) suggests the core–shell structure has disappeared after washing with acid solution and the nanocomposites show apparent two phases: large sized nanoparticles (30–50 nm) and small nanoparticles (3–5 nm). Fig. 3e exhibits the HRTEM of zone A in Fig. 3d, where the masked lattice fringe image and periodic Bragg dots by Fourier transform are inset. The masked lattice fringes imply that large sized nanoparticle has lattices at 2.11 and 2.17 Å, matching well with the (111) and (200) planes in hexagonal SmCo₅, respectively. The periodic Bragg dots present a hexagonal array, further confirming existence of hexagonal SmCo₅. The HRTEM of zone B is displayed in Fig. 3f, where mono-dispersedly small sized nanoparticles have lattice spacing value of 1.77 Å, corresponding to the (200) planes in cubic Co (JCPDS No. 15-0806). Meanwhile, the two-dimensional Bragg dot matrix fits well with cubic Co, further confirming that the existence of Co phase.

Fig. 3 (a) TEM image of unwashed SmCo₅/Co_0.19 nanocomposites prepared with 30 min hydrogenation; (b) an amplified image of (a); (c) HRTEM image of red circle in (b); (d) TEM image of washed SmCo₅/Co_0.19 nanocomposites; (e) HRTEM image of zone A (large sized particle) in (d) (inset: masked lattice fringe image and periodic Bragg dots by Fourier transform); (f) HRTEM image of zone B (small sized particle) in (d) (inset: periodic Bragg dots by Fourier transform).

The phenomenon (the disappearance of core–shell structure and formation of two phases) can be attributed to the cleanout of amorphous SmH_2±x, which weakens the adhesive ability between SmCo₅ and Co nanoparticles. And the rough surface of SmCo₅ nanoparticles may be caused by decomposing SmCo₅H_x. The independent two-phase distribution other than the alloy of SmCo–Co demonstrate the superiority of in situ synthesis. Moreover, the Co nanoparticles with 3–5 nm are smaller than twice of domain wall width in SmCo₅ (domain wall width of the SmCo₅ is about 3.7 nm),¹³ which contributes to the effective exchange-coupling.

Magnetic properties of synthetic SmCo₅/Co nanoparticles are also measured by the Physical Property Measurement System (PPMS). Fig. 4a shows room temperature magnetic hysteresis loop of SmCo₅/Co composite nanoparticles with different hydrogenation time. It proves that SmCo₅/Co nanocomposites are ferromagnetic at room temperature. Decomposed Co increases the saturation magnetization (M_s) and remanent magnetization (M_r) of nanocomposite but decreases the coercivity. The SmCo₅/Co nanoparticles with smooth hysteresis curves are confirmed to be exchange-coupled nanocomposites. Fig. 4b exhibits the change in coercivity and M_r with reaction time in the SmCo₅/Co nanocomposites. As reaction time increases, the coercivity decreases obviously and M_r increases with near linear relationship, further confirming the Co content in SmCo₅/Co nanocomposites increases with hydrogen absorption time. The coercivity value of 17.7 kOe, 14.7 kOe, 12.3 kOe, 11.9 kOe, 10.8 kOe, and M_r of 34.9 emu g⁻¹, 38.4 emu g⁻¹, 44.2 emu g⁻¹, 49.5 emu g⁻¹, 50.9 emu g⁻¹ are achieved with SmCo₅, SmCo₅/Co_0.06, SmCo₅/Co_0.11, SmCo₅/Co_0.19 and SmCo₅/Co_0.26, respectively. Fig. 4c is the dependence of (BH)_max on reaction time. The (BH)_max increases first with increasing time, to a maximum of 5.35 MG Oe with reaction time of 30 min, and then falls. The (BH)_max of SmCo₅/Co_0.19 nanocomposites is about twice as high as pure SmCo₅ nanoparticles.

Fig. 4 (a) Room temperature magnetic hysteresis loop of SmCo₅/Co nanocomposites with different hydrogenation time; (b) the change in coercivity and M_r with reaction time and (c) dependence of (BH)_max on reaction time in the SmCo₅/Co nanocomposites.

The exchange-coupled two-phase structure is further characterized by the δM method (Henkel plot).^20,24 The δM–H plot of SmCo₅/Co_0.19 is shown in Fig. 5. In this nanocomposite, a strong positive peak in the δM plot suggested the strong exchange coupling interaction between the hard and soft phases. And δM drops from positive to negative values, indicating magnetostatic interactions in the composite due to the presence of soft magnetic Co phase. This observation further confirms the exchange coupling interaction in SmCo₅/Co nanocomposites, consistent with previous reports.^24,25

Fig. 5 δM–H plot of the nanocomposite SmCo₅/Co_0.19.

Conclusions

In summary, exchange-coupled SmCo₅/Co nanocomposites have been first synthesized by hydrogen disproportionation of SmCo₅ nanoparticles at 400 °C. In this process, the core–shell SmCo₅@SmH_2±x&Co is first prepared by hydrogenation. Then homogeneously two-phase SmCo₅/Co nanocomposites are obtained by washing off the amorphous Sm hydride, where SmCo₅ and Co nanoparticles have size of 30–50 nm and 3–5 nm respectively. And Co content could be tuned by controlling the hydrogenation time to optimize the magnetic properties of nanocomposites. The exchange-coupled nanocomposites of SmCo₅/Co_0.19 exhibit the largest (BH)_max value of 5.35 MG Oe, twice single SmCo₅ nanoparticles. The facile strategy can effectively avoid the alloying of hard-soft phases, and yield exchange-coupled nanocomposites with soft-hard phases uniform distribution.

Acknowledgements

This work was supported by National Natural Science Foundations of China (NSFC) under Grant No. 51471016, and the Key Natural Science Foundation of Beijing No. 2151002.

Notes and references

1. R. Hao, R. Xing, Z. Xu, Y. Hou, S. Gao and S. Sun, Adv. Mater., 2010, 22, 2729.
2. O. Gutfleisch, M. A. Willard, E. Brück, C. H. Chen, S. G. Sankar and J. Ping Liu, Adv. Mater., 2011, 23, 821.
3. Y. L. Hou, Z. C. Xu, S. Peng, C. B. Rong, J. P. Liu and S. H. Sun, Adv. Mater., 2007, 19, 3349.
4. W. F. Li, H. Sepehri-Amin, L. Y. Zheng, B. Z. Cui, A. M. Gabay, K. Hono, W. J. Huang, C. Ni and G. C. Hadjipanayis, Acta Mater., 2012, 60, 6685.
5. E. F. Kneller and R. Hawig, IEEE Trans. Magn., 1991, 27, 3588.
6. R. Skomski and J. M. D. Coey, Phys. Rev. B: Condens. Matter Mater. Phys., 1993, 48, 15812.
7. H. Zeng, J. Li, J. P. Liu, Z. L. Wang and S. Sun, Nature, 2002, 420, 395.
8. P. Sharma, J. Waki, N. Kaushik, D. V. Louzguine-Luzgin, H. Kimura and A. Inoue, Acta Mater., 2007, 55, 4203.
9. W. B. Cui, Y. K. Takahashi and K. Hono, Adv. Mater., 2012, 24, 6530.
10. K. J. Strnat, in Ferromagnetic Materials, ed. E. P. Wohlfarth and K. H. J. Buschow, North-Holland, Amsterdam, 1988, vol. 4, p. 131.
11. P. Larson, I. I. Mazin and D. A. Papaconstantopoulos, Phys. Rev. B: Condens. Matter, 2003, 67, 214405.
12. W. F. Li, A. M. Gabay, X. C. Hu, C. Ni and G. C. Hadjipanayis, J. Phys. Chem. C, 2013, 117, 10291.
13. J. M. D. Coey, J. Magn. Magn. Mater., 1995, 140–144, 1041.
14. A. Szlaferek, J. Alloys Compd., 1998, 278, 260.
15. H. Fukunaga, M. Ikeda and A. Inuzuka, J. Magn. Magn. Mater., 2007, 310, 2581.
16. T. Schrefl, R. Fischer, J. Fidler and H. Kronmüller, J. Appl. Phys., 1994, 76, 7053.
17. J. Zhang, S.-Y. Zhang, H.-W. Zhang and B.-G. Shen, J. Appl. Phys., 2001, 89, 5601.
18. C. Rong, Y. Zhang, N. Poudyal, I. Szlufarska, R. J. Hebert, M. J. Kramer and J. Ping Liu, J. Mater. Sci., 2011, 46, 6065.
19. M. Ito, K. Majima, T. Umemoto, S. Katsuyama and H. Nagai, J. Alloys Compd., 2001, 329, 272.
20. F. Liu, Y. Hou and S. Gao, Chem. Soc. Rev., 2014, 43, 8098.
21. C. Yang, L. Jia, S. Wang, C. Gao, D. Shi, Y. Hou and S. Gao, Sci. Rep., 2013, 3, 1.
22. H. W. Zhang, S. Peng, C. B. Rong, J. P. Liu, Y. Zhang, M. J. Kramer and S. H. Sun, J. Mater. Chem., 2011, 21, 16873.
23. N. Poudyal and J. Ping Liu, J. Phys. D: Appl. Phys., 2013, 46, 043001.
24. Y. Hou, S. Sun, C. Rong and J. Ping Liu, Appl. Phys. Lett., 2007, 91, 153117.
25. G. S. Chaubey, N. Poudyal, Y. Liu, C. Rong and J. Ping Liu, J. Alloys Compd., 2011, 509, 2132.
26. S. Rivoirard, J. Lyubina, E. Beaugnon and O. Gutfeisch, J. Appl. Phys., 2008, 93, 172509.
27. K. H. J. Buschow and R. C. Sherwood, J. Appl. Phys., 1978, 49, 1480.
28. Z. Ma, T. Zhang and C. Jiang, Chem. Eng. J., 2015, 264, 610.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra15079e

---