High coercivity SmCo5 synthesized with assistance of colloidal SiO2

SmCo5 is one of the most promising candidates for achieving a hard magnet with a high coercivity. Usually, composition, morphology, and size determine the coercivity of a magnet, however, it is challenging to synthesize phase pure SmCo5 with optimal size and high coercivity. In this paper, we report on the successful synthesis of phase pure SmCo5 with spherical/prolate spheroids shape. Size control is obtained by utilizing colloidal SiO2 as a template preventing aggregation and growth of the precursor. The amount of SiO2 nanoparticles (NPs) in the precursor tunes the average particle size (APS) of the synthesized SmCo5 with particle dimension from 740 to 504 nm. As-prepared pure SmCo5 fine powder obtained from using 2 ml SiO2 suspension possesses an APS of 625 nm and exhibits an excellent coercivity of 2986 kA m−1 (37.5 kOe) without alignment of the particles prior to magnetisation measurements. Comparing with a reference sample prepared without adding any SiO2 NPs, an enhancement of 35% of the coercivity was achieved. The improvement is due to phase purity, stable single-domain (SSD) size, and shape anisotropy originating from the prolate spheroid particles.

. Sm-Co particles with the main phase of SmCo 5 and average particle size (APS) of approx. 816 nm exhibited a coercivity of 2176 kA m −1 (27.3 kOe). This simple method for synthesizing Sm-Co particles with stable single-domain (SSD) sizes has great potential for industrial applications. The reported high coercivity compound had small amounts of metastable Sm 2 Co 7 impurities and the size distribution was large and extending into the micrometer multi-domain region 19 .
In an earlier study, silica-protected annealing was applied to prepare Fe 2 O 3 nanoparticles (NPs) 20 . Annealing the sample in a stable matrix effectively prevents the pristine precursor particles from growing, maintaining a low average size. In this work, we have introduced amorphous SiO 2 nanoparticles as a confinement templates to prevent inter-growth between cobalt and samarium oxides during the precursor preparation. Consequently, the composition and size of the final product can be tuned by adding different volumes of colloidal SiO 2 suspensions, resulting in SmCo 5 particles with an APS ranging from 504 to 740 nm. The prepared compounds were investigated by powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) combined with Energy Dispersive X-ray Spectroscopy (EDS). A vibrating sample magnetometer (VSM) was used to measure magnetic properties. The SmCo 5 particles with an APS of 625 nm exhibit the highest coercivity in this study with a value of 2986 kA m −1 (37.5 kOe), which exceeds most coercivities reported for SmCo 5 21 .
The synthesis process is schematically illustrated in Fig. 1, starting from the metal salts to the final SmCo 5 product. The SiO 2 NPs with spherical shape have an average diameter of 25.1(3) nm (Fig. S1, supporting information). The PXRD pattern of SiO 2 NPs only has a broad peak at 2θ = 24.8° without any sharp features from crystalline phases, indicating that SiO 2 NPs are amorphous (see Fig. S2) 22 . The introduction of SiO 2 NPs during the preparation of precursor prevents the inter-growth of the precursor NPs, by keeping a reduced size of the precursors it is possible to reduce the size of the final SmCo 5 product. The PXRD patterns of the precursors displayed in Fig. 2a show that the main phase of the precursor is Co 3 O 4 . In addition, some small broad peaks allow identifying   www.nature.com/scientificreports/ CoO, SmCoO 3, and Sm 2 O 3 . It is easiest to identify the various phases for the synthesis without any colloidal SiO 2 (CSS_0) added. The colloidal silica suspension is added in steps of 0.5 ml between 0.0 and 3.0 ml, and the samples are named CSS_x, where x equals the amount of colloidal silica suspension added in ml. After reduction by H 2 , NPs with good crystallinity are formed of Co and Sm 2 O 3 , see Fig. 2b. Weak peaks identified as CoO can be detected, these are attributed to slight oxidation in air during the sample preparation and data collection. The CoO peaks becomes increasingly intense with increasing the volume of SiO 2 suspension. From the Co peak, observed at 2θ = 51.9°, it is observed how the peak width increases, when the volume of SiO 2 solution is increased, in other words how the Co size decreases. Therefore, it can be hypothesized that the addition of SiO 2 colloidal particles prevent the growth of Co 3 O 4 , which in turn results in smaller Co crystallites. The smaller Co finally leads to reduced size of the final SmCo 5 particles. In order to confirm this hypothesis, the PXRD data of CSS_0, CSS_1, CSS_2, and CSS_3 was refined and the crystalline size was extracted from the different phases, see supporting material, Fig. S3 and Table S1 for other important refinement parameters. The crystalline size of Co and Sm 2 O 3 NPs decreases when increasing the amount of SiO 2 NPs, meanwhile the crystalline size of CoO increases slightly. This can be attributed to smaller Co NPs being more reactive, thus being more prone to oxidize in air. The PXRD patterns of the final product are displayed in Fig. 2c and reveal the main phase in all samples to be SmCo 5 , while a small number of other phases like Sm 2 Co 7 -R (rhombohedral structure), Sm 2 Co 7 -H (hexagonal structure), and Sm 2 Co 17 can be identified for some samples. Often in the literature SmCo 5 is reported to be phase pure based on PXRD collected with Cu radiation (λ = 1.54 Å), this is problematic, because Cu radiation produces strong fluorescence when the sample contains Co and Sm, this strong background can easily hide impurities 8,9,17,22 . Ideally the samples should be measured using Co radiation (λ = 1.78 Å) or at a short wavelength synchrotron source. The sample CSS_2 was revealed to be phase pure SmCo 5 when investigated by Co radiation. The sample was taken to beamline P02.1 at Petra-III, Germany for synchrotron radiation (SR-PXRD) for high quality data collection. The refined synchrotron data is plotted in Fig. 2d and confirms the phase purity SmCo 5 .
In order to shed light on the crystallite morphology and the transformation of the precursors to, the final SmCo 5 TEM imaging were collected for the CSS_2 sample at the different synthesis stages, see Fig. 3. The initial combustion process produced many large platy shaped precursor aggregates, each plate is made of plenty of small NPs (Fig. 3a). Figure 3b indicates that amorphous SiO 2 NPs are stable during the burning process and prevented the inter-growth of the precursor NPs. The elemental mapping and the EDX spectrum in Fig. 3c indicate that SiO 2 NPs are homogeneously distributed in the precursor sample and no other elements are detected except C and Cu from the TEM grid. After being washed by NaOH aqueous solution, SiO 2 NPs are dissolved, and the morphology of the precursor NPs changed. Some randomly oriented nanosheets/nanoneedles are seen at the edges of the NPs (Fig. 3d,e). The PXRD pattern and elemental mapping of washed-precursor revealed the nanosheets/nanoneedles to be CoO(OH) or SmCoO 3 (Fig. S4). Figure 3f indicates the size of cobalt oxide NPs is around 20-40 nm, and samarium oxide NPs is about 15 nm. However, a weak Si signal was detected in the EDX spectrum (Fig. 3f). As precursor NPs will started to react with NaOH, the washing time or temperature were not increased to completely remove the SiO 2 NPs. The TEM image of H 2 -precursor, Fig. 3g reveal a large number of holes to be left behind after removal of SiO 2 NPs by NaOH. The Co NPs are clearly crystalline as observed from the HRTEM image and PXRD patterns (Figs. 2b, 3h). Figure 3i indicates that Co and Sm elements are distributed homogeneously and Co NPs are larger than Sm 2 O 3 NPs, which is consisted with the refined crystallite size extracted from Rietveld refinements shown in Fig. S3. The TEM image of SmCo 5 particles (Fig. 3j) suggests that the formation of SmCo 5 takes place after Sm 2 O 3 is reduced by Ca, and that Sm and Co metal subsequently diffused into each other. The intergrowth of some particles is inevitable, however the relative small starting size of Co results in a reduced size of the final SmCo 5 , in other words the SiO 2 colloidal susception prevents uncontrollable growth of Co and Sm 2 O 3 , which in turn leads to control over the final SmCo 5 particle size. Figure 3k displays several separate SmCo 5 particles with a prolate spheroid shape. The Co and Sm are evenly distributed as shown by the elemental maps in Fig. 3l, however oxygen signal has been detected on the surface of the SmCo 5 particles, which is attributed to the slight oxidation in air.
The final produced SmCo 5 particles were investigated by SEM, allowing extraction of morphology, size and size distribution, the results are shown in Fig. 4. The insert in Fig. 4a reaveal high-magnification SEM image giving a detailed impression of the SmCo 5 particles. The SmCo 5 particles in most cases resemble spheres or prolate spheroids, similar to those observations from TEM image (Fig. 3k). The size also agrees well between the SEM and TEM images. Without adding SiO 2 NPs, SmCo 5 particles are revealed to be large imperfect spheres with an APS of 740(9) nm (Fig. 4b). Adding a small amount of SiO 2 NPs (CSS_0.5), does not cause significant changes to the APS with respect to the pristine sample. As the volume of SiO 2 NPs increases, it is observed that the APS decreases, see Fig. 4c. The CSS_1 sample has an APS of 721(4) nm, followed by CSS_1.5 (677(38) nm) and CSS_2 (625(17) nm). CSS_3 sample has the lowest APS of 504(25) nm. The sizes extracted from the SEM images corroborate the hypothesis that adding SiO 2 colloidal suspension reduces the size of the final synthesis SmCo 5 .
The initial magnetization curves and hysteresis loops are shown in Fig. 5, and the important magnetic properties are extracted and listed in Table 1. The initial magnetization curves reveal three stages during the magnetizing process: (I) fast magnetization changes caused reversible domain wall displacements under low applied magnetic field; (II) when continually increasing the magnetic field, the magnetization changes slow down, this is interpreted as pinning sites causing irreversible domain wall displacements; (III) the magnetization increases gradually with increased applied magnetic field, here rotation of the magnetic moment in SSD particles takes place, this requires high magnetic fields to overcome the energy barrier from preferred orientation and shape anisotropy [23][24][25][26] . SmCo 5 particles cannot be saturated completely at the maximum applied magnetic field (9 T) at the PPMS system at Aarhus University. Most samples show a single-phase magnetic behavior except CSS_2.5 and CSS_3 samples, which both have a kink in the second quarter (Fig. 5b). PXRD results reveal that CSS_2.5 contains a small amount of Sm 2 Co 7 , while CSS_3 has a relative large Sm 2 Co 17 impurity, these have lower coercivity than the SmCo 5 phase 27,28 . The weak exchange-coupling between the main phase and the impurity phase lead ). An impressive improvement of 35% was achieved comparing with CSS_0 reference sample. The M r /M s ratio has a similar trend as coercivity. The M r /M s ratios in most samples exceed 70%, except CSS_3 sample, which has a ratio of 59%, this is due to the weak exchange-coupling between the two phases as demonstrated by Henkel plots and δM plots shown in Fig. S5. www.nature.com/scientificreports/

Conclusions
Herein, we have developed an inorganic chemical synthesis method for preparation of size controlled SmCo 5 particles. The particle size control is achieved through adding colloidal SiO 2 nanoparticles. The introduction of SiO 2 NPs as a matrix template plays a significant role in preventing the inter-growth of the precursors during the combustion process. The size control of the precursor in turn gives control over the size of the SmCo 5 particles. The APS of SmCo 5 particles can be tuned from 740 to 504 nm by controlling the volume of the added colloidal SiO 2 suspension. As-prepared SmCo 5 particles with an APS of 625 nm reveal the largest coercivity of 2986 kA m −1 (37.5 kOe), a 35% improvement compared with the reference sample without adding any SiO 2 NPs. The coercivity is attributed to reversible and irreversible domain-wall displacement and the rotation of the single-domains. Phase purity, single-domain particles, and shape anisotropy from the prolate spheroid particles The particle size distributions from the seven samples measured by the ImageJ software 29 . More than 300 particles were measured in each sample; the data is fitted by a lognormal distribution function to extract the APS. For the spheroid particles, the short diameter is given as the particle size. An approximate polydispersity index (PI) is shown, it is given by PI = (σ/APS) 2  In the next step the precursor is reduced using 5% H 2 /Ar gas, producing the H 2 -precursor. The H 2 -precursor is mixed with Ca granular and KCl powder in an Ar filled glove box. Finally, the mixture is reacted at 900 °C for half an hour under Ar atmosphere to form SmCo 5 particles. The above product was washed by water and weak acetic acid several times to remove Ca, CaO, and KCl. Different volume of colloidal silica suspension, 0 ml, 0.5 ml, 1 ml, 1.5 ml, 2 ml, 2.5 ml, and 3 ml, were added for tuning the final particle size, and the samples are named CSS_0, CSS_0.5, CSS_1, CSS_1.5, CSS_2, CSS_2.5 and CSS_3, respectively.
Characterization. The phase identification was analyzed from conventional laboratory powder X-ray diffraction (PXRD) patterns collected with Rigaku SmartLab diffractometer equipped with a Co Kα 1,2 radiation source, using parallel beam optics (Rigaku, Japan) and synchrotron radiation powder X-ray diffraction (SR-PXRD) data was collected at P02.1 beamline, Petra III, DESY using a PerkinElmer XRD1621 (2048 × 2048 pixels, with pixel dimensions 200 × 200 µm 2 ) and a wavelength of λ = 0.20714 Å 30 . The morphology and microstructure  www.nature.com/scientificreports/ characterization was conducted by transmission electron microscopy (TEM, FEI TALOS F200A) and scanning electron microscopy (SEM, FEI Nova Nano SEM 600). The hysteresis loops were measured by a vibrating sample magnetometer (VSM) attached to a Physical Property Measurement System (PPMS, Quantum Design, US). The powder samples were cold-pressed into a thin pellet with a thickness of ~ 1 mm and diameter of 3 mm without applying an external magnetic field or fixing the crystallites using glue or vax. The applied magnetic field is parallel to the pressing direction.