Anisotropic characteristics and improved magnetic performance of Ca–La–Co-substituted strontium hexaferrite nanomagnets

Recent studies on next-generation permanent magnets have focused on filling in the gap between rare-earth magnets and rare-earth-free magnets, taking into account both the cost-effectiveness and magnetic performance of the magnetic materials. As an improved rare-earth-free magnet candidate, here, Ca-substituted M-type Sr-lean hexaferrite particles within a nano- to micro-scale regime, produced using an ultrasonic spray pyrolysis method, are investigated. Theoretically, the maximum coercivity (Hc) can be achieved in submicron Sr-ferrite crystals (i.e., 0.89 μm). The plate-like resultants showed a significant enhancement in Hc, up to a record high of 7880.4 Oe, with no deterioration in magnetization (M: 71–72 emu/g). This resulted in more favorable magnetic properties than those of the traditional Sr–La–Co ferrites. On the basis of microstructural analysis and fitting results based on the law of approach to saturation method, the Ca-substitution effects on the change in size and anisotropic characteristics of the ferrite particles, including pronounced lateral crystal growth and a strong increase in magnetocrystalline anisotropy, are clearly demonstrated. The cost-effective, submicron, and Ca-substituted Sr-ferrite is an excellent potential magnet and moreover may overcome the limitations of traditional hard magnetic materials.

As the world pursues higher energy efficiency for miniaturized devices such as small motors in hybrid electric vehicles, the demand for ultra-high-performance permanent magnets is rapidly increasing. There are two types of permanent magnets: rare-earth (RE) magnets (e.g., SmCo and NdFeB) with broadly outstanding magnetic characteristics but they have high concentrations scarce RE elements; and the diametrically opposed RE-free magnets (e.g., Sr-ferrite and Ba-ferrite), which possess relatively poor magnetic properties, but are less expensive and have excellent oxidation resistance 1 . So-called "gap magnets" have been introduced as a compromise to fill the gap between RE magnets and RE-lean magnets, given both their lower material costs and good magnetic performance 2,3 .
To date, many reports have suggested M-type Sr-hexaferrite (SrFe 12 O 19 ) with elemental substitutions as a viable candidate for a high-performance magnet, by the substitution of RE ions and other cations such as Sm 3+4 , Sm 3+ -Co 2+5 , La 3+ -Sm 3+6 , La 3+ -Co 2+7, 8 , Nd 3+9 , Nd 3+ -Y 3+10 , and Nd 3+ -Co 2+11 . Among these gap magnets, partial La 3+ -Co 2+ -substituted Sr-ferrite has received great attention due to the successful enhancement in its intrinsic coercivity (H c ) 8,12 . Interestingly, upon non-RE substitution, Al 3+ for Fe 3+ in SrFe 12 O 19 , the H c increased up to tens of kOe, although this came at the expense of the saturation magnetization (M s ), which dropped from ~ 60 emu/g to ~ 10 emu/g. This led to a deterioration in the maximum energy product ((BH) max ), which is the most important magnetic parameter 13,14 .
In this regard, elemental substitution should enhance H c without sacrificing the M value. We have found that Sr-ferrites with earth-abundant Ca-substitution need further research in terms of their microstructure and magnetic properties. There are a few previous reports on M-type hexaferrites with Ca-substitution using Scientific RepoRtS | (2020) 10:15929 | https://doi.org/10.1038/s41598-020-72608-0 www.nature.com/scientificreports/ conventional solid-state reaction routes [15][16][17][18][19][20] ; however, the solid-state process does not allow adequate control of the particle size, morphology, or homogeneity, resulting in ambiguities in the relationship between the effect of elemental substitution and the microstructural properties 21 . Some reports developed an enhancement in H c upon Ca-substitution; however, the reason has not been fully corroborated. From microstructural, magnetic, and anisotropic points-of-view, we have found that the Ca-substitution effect is still in need of further research. The goal of this study is to synthesize Ca-substituted Sr-ferrites possessing enhanced H c without M s deterioration and to elucidate the Ca-substitution effects on the morphological, crystallographic, and magnetic performance of the ferrite particles. Motivated by the past success of La-Co-substitution in Sr-ferrite, we concurrently searched for the optimal amount of La-Co-substitution. We used salt-assisted ultrasonic spray pyrolysis (SA-USP) to prepare Ca-substituted, M-type Sr-hexaferrites, Sr 0.75-x La 0. 25  For the USP process, the precursor solution, which was homogeneously stirred for 3 h, was fed into a cylindrical quartz tube with side arms (see the schematic of a laboratory-scale USP setup in Fig. S1a). The precursor solution was first atomized by an ultrasonic mist generator (1.7 MHz of frequency), and the atomized droplets were introduced into a tube furnace by an O 2 carrier gas (flow rate of 2 L/min), and then thermally pyrolyzed through the heating stage in O 2 currents at 650 °C. A subsequent calcination process of the trapped intermediate particles was performed at 1050 °C (in air, for 1 h) to complete the SrFe 12 O 19 phase formation 23 . The calcined samples were rinsed with distilled water to remove most of the residual NaCl, and were dried overnight in a vacuum oven. (See the phase and morphology of the synthesized particles in Fig. S1b and c, respectively.) The overall procedure was slightly modified from a previous method that we describe in detail elsewhere 7,22 . characterization. Morphological characterization and particle size measurement of the Ca-La-Co-substituted SrFe 12 O 19 nanoparticles were performed by using field emission scanning electron microscopy (FE-SEM; MIRA-3, Tescan, Czech Republic). X-ray diffractometry (XRD; D/MAX-2500/PC, Rigaku Co., Japan) was employed for a crystal-structural characterization of the magnetic powder. The Rietveld refinements on the XRD patterns were performed by using the JAVA-based refinement program (Materials Analysis Using Diffraction; MAUD). The magnetic performance of the Sr-ferrite powder at room temperature were examined by vibrating sample magnetometry (VSM; VSM7410, Lake Shore Cryotronics, Inc., USA). There was no additional magnetic alignment and sintering processes.

Morphology of the ferrites with different Ca contents. FE-SEM micrographs of the series of Ca-La-
Co-substituted ferrite samples and their size distributions are depicted in Fig. 1a as a function of Ca content (x). Table 1 contains the numerical data, including the mean particle sizes with corresponding standard deviations. As the amount of Ca increased up to 0.40, there was a noticeable change in not only the thickness, but also in the diameter of the Sr-ferrite particles (ranging from the submicron to micro scale), while they became more plate-like in shape with a high aspect ratio, up to 9.31. Thus, it can be inferred that the Ca-substitution affected the anisotropic parameters, inducing the predominantly lateral crystal growth in the hexagonal ferrites 24 . Even though there was salt (i.e., NaCl) introduced to prepare the resultant particles with precisely controlled homogeneity of the dimension, additional Ca-substitution (x > 0.40) led to abnormal grain growth, resulting in a broad particle diameter distribution that mainly reflected Ostwald ripening 25 . Figure 1b presents a schematic of the evolving hexagonal crystal structures of the Sr-ferrites, with their preferred orientation of particle-stacking. Even though there was no applied external magnetic field, the pronounced crystal growth perpendicular to the c-axis with a high aspect ratio (i.e., the <00l> direction) coming from the Ca-substitution led to the easy stacking of the plate-like ferrite particles along the c-axis, in good agreement with the experimental data (Fig. 1c).
crystallographic characteristics upon ca-substitution. Figure Fig. S2). For the samples with Ca content (x) ranging from 0.00 ≤ x ≤ 0.20, the diffraction pattern indicated a pure hexagonal SrFe 12    with diameters ranging from 10 to 50 μm, which exceeds the diameter of the ferrites with x = 0.60 (2.5 μm). That is, the Sr-free hexagonal microplates clearly back up the Ca substitution effect on the growth of particles to be more plate-like in shape, with improved crystallinity (see the data in Fig. S4). The shift in the SrFe 12 O 19 peak position toward a higher angle, which results from a considerable change in the lattice parameters ( Figure S5), was observed as the Ca content in Sr 0.75−x La 0.25 Ca x Fe 11.8 Co 0.2 O 19 continuously increased. This could be mainly attributed to the fact that the ionic radius of the Ca 2+ ion (0.099 nm) was smaller than that of the Sr 2+ ion (0.110 nm), leading to lattice shrinkage during Sr-ferrite phase formation 29 . Interestingly, the relative intensity ratio of reflections (008) to (110) (= I (008) /I (110) ) clearly increased with increasing x from 0.00 to 0.40 (Fig. 2a). It can be inferred that the plate-like Ca-substituted hexaferrites, originating from dominant crystal growth perpendicular to their c-axis, are able to partially and spontaneously orient themselves uniaxially (<00l>), thereby leading to the change in relative intensity ratio of the reflections without an external magnetic field 30 . When the Ca content increased up to 0.60, the plate-like microparticles were aligned in a haphazard manner, similar to the non-substituted ferrite nanoparticles, due to their broad size distribution and the incorporated byproduct (i.e., Fe 2 O 3 and CaFe 2 O 4 ). Likewise, the behavior of I (107) /I (114) can be understood in the same way (Fig. 2b).

Magnetic performance as a function of ca content. Magnetic measurements of the Sr-hexaferrite
particles with different Ca contents were conducted at room temperature (Fig. 3a). Regardless of the quantity of Ca-substitution in the Sr-ferrite, all hysteresis loops showed single-phased ferromagnetic behavior without kinks, even though a small amount of the antiferromagnetic Fe 2 O 3 phase was incorporated in the samples with x ≥ 0.30 31 . Fig. 3b illustrates the dependence of the maximum magnetization at 25 kOe (M 25kOe ) and the intrinsic coercivity (H c ) of each hexaferrite sample on the amount of Ca. Table 2 provides the numerical data, including M 25kOe , remanence (M r ), H c , and squareness. Clearly, from x = 0.00 to 0.30, the M 25kOe values did not degrade, whereas the maximum value of H c peaked at (~ 7880.4 Oe) at x = 0.20 and then decreased, but remained above the value of the non-substituted ferrites. Generally speaking, the inherent magnetic parameter M can fall from the theoretical value (e.g., ~ 72 emu/g for pristine Sr-ferrites 32 ) mainly due to a decrease in either phase purity or www.nature.com/scientificreports/ in the crystallinity of the magnetic particles. The saturation magnetization (M s ) of nano-scaled Fe 2 O 3 as reported in a previous study is only ~ 10 emu/g 33 . Furthermore, the extrinsic factor, H c , can vary according to a complex set of variables such as the grain size, particle shape, degree of particle orientation, and the particle density 19,34 ; due to the increase of the demagnetization factor, H c can decrease a fair amount when the magnetic particles become more plate-like 18 . Accordingly, with cationic substitution, the value of M continued to deteriorate from the theoretical M, even when H c was maintained or slightly increased to a value more than that of the pristine powder, as has been well documented in many previous studies 11, [13][14][15][16] . From this viewpoint, the plate-like, Ca-substituted ferrite shows intriguing results. As a measure of crystallinity, the apparent full width at half maximum intensity (FWHM) of the (107) peak and the calculated crystallite size were determined from the XRD data, as shown in Fig. S6. The decrease in FWHM with increasing Ca 2+ content resulted in an increasing crystallinity of the Ca-substituted Sr-ferrite particles, maintaining the level of M in spite of the foreign Ca introduction. The onset of a decline in M from x = 0.30 is attributed to the presence of byproducts, which in good agreement with the XRD data.  www.nature.com/scientificreports/ The Ca-substitution also induced a change in the microstructural characteristics, specifically from a spherical particle shape to a flat hexagonal plate, and this could have led to a strong decrease of H c . Nevertheless, up to x = 0.60, H c remained at a high level, increasing up to 12% for x = 0.20 without a significant deterioration of M. Thus, the H c tendency was not greatly influenced by the change in particle diameter and aspect ratio, implying that there must be another predominant factor having the greatest effect on H c .
To confirm the effect of Ca-substitution on the magnetocrystalline anisotropy, which can determine the highest achievable H c , we determined the first anisotropy constant through the law of approach to saturation (LAS) method. causality between ca-substitution and coercivity enhancement. The LAS theory is a popular method for determining the local crystalline anisotropy of magnetic materials, describing the empirical H dependency on M, in the form Eq. 3: where A/H is the inhomogeneity of the materials, χ p H is the field-induced forced magnetization term, and B/H 2 is a term associated with the magnetocrystalline anisotropy parameter 13 . Through Eq. 3, a typical curve fitting of experimental data with the output statistical parameter R 2 (representing the goodness of the curve fit) is shown in Fig. 4. The results fit the curve with high reliability, with an R 2 coefficient of determination values above 0.999. This indicates that all of the hexaferrite particles possess a good relationship between M and H and the M does not depend on any one specific term (see the data fitting to the LAS in different equation forms in Fig. S7 and Table S1).
Along with the R 2 , the fitted parameters also provide important information associated with the magnetic properties (Table 3): the drastic increase in the inhomogeneity parameter A for x > 0.30 can be understood, as the increase in structural defects and nonmagnetic ion inclusions resulted in a secondary phase formation 35 . For the hexagonal crystal structure, the anisotropy factor B can be expressed as: where H A is the anisotropy field, and K 1 is the magnetocrystalline anisotropy constant. On the basis of the fitting results, K 1 and H A were calculated by using Eqs. 4 and 5:   Through microstructural and magnetic studies, we found that the additional Ca-substitution led to dramatic changes in the anisotropic characteristics of the Ca-substituted Sr-ferrite, specifically, more plate-like-shaped particles ascribed to pronounced lateral growth, and a strong increase in its magnetocrystalline anisotropy, K 1 , far beyond the optimized La-Co-substituted Sr-ferrite. As a result, the optimal composition contained earthabundant Ca, to the benefit of both enhanced magnetic performance and lower composition costs as compared to the current commercial ferrite magnets. We expect that this new Sr-lean composition, possessing enhanced magnetic properties, will find applications far beyond the limitations of traditional Sr-ferrite-based magnetic materials, and we envision that it can be widely used in new types of affordable magnets.