A novel strategy to synthesize anisotropic SmCo5 particles from Co/Sm(OH)3 composites with special morphology

Article abstract of DOI:10.1039/c8tc02048e

In this study, we report a new strategy for preparing magnetically anisotropic SmCo₅ single crystal particles by reductive annealing of precursors with specially designed nanostructures. The precursors containing urchin-shaped Co clusters and Sm(OH)₃ nanorods were synthesized by a hydrothermal process. Then we coated these precursors of Co/Sm(OH)₃ nanocomposites with Ca(OH)₂, and the following thermal reduction generated SmCo₅ particles, where the original size and morphology of the precursor were basically preserved after undergoing high temperature annealing. The single crystalline SmCo₅ particles exhibit strong magnetic anisotropy and excellent room temperature magnetic properties: remanence of 8.1 kG, coercivity of 25.7 kOe, and a maximum energy product of 15.8 MGOe, which is one of the highest maximum energy products for chemically synthesized nanostructured SmCo₅ particles presently reported.

Full text of DOI:10.1039/c8tc02048e

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Nguyên T. K. Thanh, Xiaodi Su et al.
Fine-tuning of gold nanorod dimensions and plasmonic properties using
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DOI: 10.1039/C8TC02048E
A novel strategy to synthesize anisotropic SmCo5 particles from Co/Sm(OH)3
composites with special morphology
Chenglin Li, Qiong Wu, Zhenhui Ma, Huanhuan Xu, Liying Cong, Ming Yue*
College of Materials Science and Engineering, Beijing University of Technology,
Beijing 100124, China
*Corresponding author: yueming@bjut.edu.cn
Keywords: SmCo5, nanocomposite precursor, magnetically anisotropic particle, high
magnetic energy product
Abstract
In this study, we report a new strategy for preparing magnetically anisotropic SmCo5
single crystal particles by reductive annealing of precursors with specialy designed
nanostructures. The precursors containing urchin-shaped Co clusters and Sm(OH)3
nanorods were synthesized by hydrothermal process. Then we coated these precursors
of Co/Sm(OH)3 nanocomposites with Ca(OH)2, and the following thermal reduction
generated SmCo5 particles, where the original size and morphology of precursor were
basically preserved after undergoing high temperature annealing. The single
crystalline SmCo5 particles exhibit strong magnetic anisotropy and excellent room
temperature magnetic properties of remanence of 8.1 kG, coercivity of 25.7 kOe, and
maximum energy product of 15.8 MGOe, which is one of the highest record of energy
product for chemical synthesized nanostructured SmCo5 particles presently reported.
1. Introduction
SmCo5 with exceptionally-large magnetocrystalline anisotropy (Ku = 2 ×108 erg/cm3)
and a high Curie temperature (Tc = 1020 K) is one of the widely used hard magnetic
materials 1-3. SmCo5 particles with high coercivity are frequently chosen as the hard

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4-13
DOI: 10.1039/C8TC02048E
phase to build the nanocomposite magnets with soft phase nanoparticles
, since
SmCo5 may maintain high coercivity of the magnets 14-16. Generally, the highest
coercivity of a single crystalline particle will be achieved at the critical size of the
single-domain particle, while the critical single domain size of SmCo5 particle is as
high as 0.9-1.8 μm 17-20. In addition, according to the Stoner-Wohlfarth model, the
ratio of the remanent magnetization Mr to the saturation magnetization Ms is 0.5 for
an assembly of non-interacting isotropic particles, while it is 1 for the assembly of
non-interacting and totally oriented particles 21, 22. Therefore, single crystalline
magnetic aligned SmCo5 particles with a uniform size just below 1 μm would be
promising candidates for the hard phase in the nanocomposite magnets. According to
16
the recent report by Li et al. , significant progress in bulk nanocomposite magnets
with the high (BH)max of 28 MGOe has been obtained in SmCo/FeCo nanocomposites
by multistep deformation, which realizes higher properties in the composite magnets
than single SmCo bulk for the first time. To further enhance the energy products,
SmCo hard-phase constituent with a large coercivity is highly required. The
motivation of our research is to provide another promising route to fabricate dense
SmCo bulk from a bottom-up strategy, which is consistent with the reports by Zeng et
al. 8 and Hu et al. 4.
To obtain SmCo5 particles with high magnetic performance, many efforts with
physical and chemical methods have been devoted 23-30. Compared to physical
technology, chemical strategy owns unique advantage in controlling morphology and
size, which would show substantial influence to magnetic property. Unfortunately,
electronegativity and instability of Sm element make the synthesis of pure phase
SmCo5 by direct liquid-phase chemical method extremely difficult. As a consequence,
the approach combining solution-phase chemical synthesis with reductive annealing
has been established 31-37. The difficulty to fabricate SmCo5 single crystal lies in the
bonding of each particle during high temperature. To address the issue, some
stabilizers such as CaO or graphene have been employed to prevent SmCo5 particles
from agglomeration 38-40. However, due to random distribution of Sm-containing and
Co-containing particles in precursor, these stabilizers could also restrain the

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DOI: 10.1039/C8TC02048E
interdiffusion between Sm and Co atoms. As a consequence, the emerging particles
present inhomogeneous Sm/Co ratio, even showing 2:17 phase, which causes the poor
magnetic property. In this protocol, we synthesize a special precursor of Co/Sm(OH)3
nanocomposite. Due to the high aspect ratio of urchin-like Co particles, the Sm(OH)3
rods are uniformly inlayed in the gap of Co urchin, forming a dense structure
composite. As a whole, Co/Sm(OH)3 nanocomposites were further coated with
Ca(OH)2. Therefore, during reductive annealing process, interdiffusion between Sm
and Co atoms can operate smoothly and the bonding phenomenon can be impeded
effectively by Ca(OH)2. Correspondingly, the homogeneous Sm/Co ratio and single
nature will contribute to the high performance.
In present study, we fabricate a specially designed precursor, uniform Co/Sm(OH)3
nanocomposites with two types of particles tightly contacted, which are further coated
with Ca(OH)2 to stabilize their size during annealing process and to prevent SmCo
particles form agglomeration or bonding. After calciothermic reduction, the precursor
converted into single crystalline SmCo5 particles with a size of 400-900 nm, where
the original size and morphology of the Co/Sm(OH)3 nanocomposites were basically
preserved with the help of the Ca(OH)2 coating layer. Good alignment and high
magnetic performance have been successfully achieved in these SmCo5 particles. This
approach may provide a novel route to chemically synthetic Co-based magnetic
particles.
2. Materials and methods
2.1. Synthesis of Sm(OH)3 nanorods
The Sm(OH)3 nanorods were first prepared via a hydrothermal process. 3.758 g (10.2
mmol) SmCl3·6H2O was dissolved in 40 ml deionized water. A 20 ml of 1.84 M
aqueous NaOH solution was added drop-wise into the SmCl3 solution with stirring at
room temperature for 30 min. The solution was then transferred into a teflon vessel
(100 ml) for hydrothermal reaction at 453 K for 12 h using an autoclave reactor. After
cooling to room temperature, the resultant was centrifuged and washed with deionized
water and then dried in an air oven at 333 K for 12 h.