Strategy to control magnetic coercivity by elucidating crystallization pathway-dependent microstructural evolution of magnetite mesocrystals

Mesocrystals are assemblies of smaller crystallites and have attracted attention because of their nonclassical crystallization pathway and emerging collective functionalities. Understanding the mesocrystal crystallization mechanism in chemical routes is essential for precise control of size and microstructure, which influence the function of mesocrystals. However, microstructure evolution from the nucleus stage through various crystallization pathways remains unclear. We propose a unified model on the basis of the observation of two crystallization pathways, with different ferric (oxyhydr)oxide polymorphs appearing as intermediates, producing microstructures of magnetite mesocrystal via different mechanisms. An understanding of the crystallization mechanism enables independent chemical control of the mesocrystal diameter and crystallite size, as manifested by a series of magnetic coercivity measurements. We successfully implement an experimental model system that exhibits a universal crystallite size effect on the magnetic coercivity of mesocrystals. These findings provide a general approach to controlling the microstructure through crystallization pathway selection, thus providing a strategy for controlling magnetic coercivity in magnetite systems.

As shown in the TEM images in Supplementary Figs. 1a and b, the Fe3O4 mesocrystals in S1 are composed of smaller primary crystallites than those in S2. The XRD patterns represent the magnetite phase of the inverse spinel structure with crystallite sizes of 23 and 43 nm for S1 and S2, respectively. The S2 sample, with relatively large crystallites, exhibits higher Ms and Hc values than those of the S1 sample. Supplementary Figs. 1e and f show photographs of samples refluxed for 0 to 8 h, with the color gradually changing from orange to black. The color of iron oxides is a clue for distinguishing iron oxide phases; here, it indicates that the black magnetite phase is gradually formed. The reacting solution in Supplementary Fig. 1f (S2) turns black more slowly than that in From the fitting, the strain is extracted from the slope, and the crystallite size is extracted from the y-intercept of the linear fit. As shown in the TEM images in Supplementary Fig. 4, the reaction rate of S2 is much slower than that of S1. The Fe3O4 phase appears mainly after 3 h. Fe3O4 primary crystallites rarely appear before 3 h of refluxing. For the S1 sample, the primary Fe3O4 Rectangles on the HRTEM image indicate regions from which FFT patterns were obtained. c-h, Primary crystallite coarsening and crystallite formation (S1). TEM and FFT images of S1 after reflux times of (c,f) 3.5 h, (d,g) 4.  Supplementary Fig. 9. As the NaOAC concentration increases, the diameter tends to gradually decrease. The diameter is inversely proportional to the NaOAC/FeCl3•6H2O ratio, even if the FeCl3•6H2O concentration is fixed. The diameter becomes larger when the concentration of FeCl3•6H2O is increased from 1 to 3 mmol at the same NaOAC/Fe ratio; the graph shifts upward on the y-axis. The crystallite size tends to decrease as the NaOAC concentration increases after reaching a maximum value at a NaOAC/FeCl3•6H2O ratio of 5-6.
We confirmed that the NaOAC/FeCl3•6H2O ratio can affect the diameter and crystallite size of the Fe3O4 mesocrystals even when the Fe precursor content is controlled while the NaOAC and H2O contents are constant (for TEM images, see Supplementary Fig. 10). To analyze the effects of the Fe precursor, we increased the FeCl3•6H2O content from 1 to 3 mmol in 0.5 mmol increments while keeping the NaOAC and H2O contents (150 mmol) constant. As the Fe concentration increases, the diameter of the Fe3O4 mesocrystals increases in inverse proportion to the NaOAC/FeCl3•6H2O ratio. When the NaOAC content increases from 6 to 15 mmol, the diameter decreases more gradually with variation in the NaOAC/FeCl3•6H2O ratio. Similar to the changes it shows when the NaOAC concentration is varied, the crystallite size exhibits a maximum value at NaOAC/FeCl3•6H2O = 5-6. When the concentration of the Fe precursor is increased such that the NaOAC/FeCl3•6H2O ratio becomes smaller than 5-6, the crystallite size tends to decrease, and the diameter tends to increase steeply. These phenomena are presented more clearly in Supplementary Fig. 11, where the x-axis represents the Fe content.
To analyze the effect of H2O, its amount is varied from 100 to 300 mmol while the FeCl3•6H2O and NaOAC contents are kept constant (for TEM images, see Supplementary   Fig. 12). The diameter decreases as the amount of H2O increases. As we add more Fe precursors (from 1 to 3 mmol), the diameter increases. As the amount of H2O increases, the crystallite size exhibits a maximum value at a certain H2O content and then tends to decrease. The effect of the H2O content on the crystallite size is similar to that of the NaOAC content. The reason is that the concentration of OH − ions is determined by the equilibrium reaction between NaOAC and H2O. There is a limited range of H2O content in which Fe3O4 mesocrystals can be synthesized. As the FeCl3•6H2O content increases from 1 to 3 mmol, the range of H2O content in which Fe3O4 can form becomes narrower.
For example, if more than 200 mmol of H2O is added and the FeCl3•6H2O content is kept constant at 3 mmol, goethite (α-FeOOH) and hematite (α-Fe2O3) would be synthesized rather than magnetite.
We have shown that two pathways are competitive in the formation of Fe3O4 mesocrystals and that the crystallite size differs depending on which pathway is dominant.
It can be inferred that the increase in crystallite size when the NaOAC/FeCl3•6H2O ratio increases is related to the formation pathway.
The crystallite size decreases when the NaOAC content decreases or the FeCl3•6H2O content increases for NaOAC/FeCl3•6H2O < 5, indicating that Pathway 1 gradually accounts for the entire reaction. The reason could be the effect of excess Fe 3+ cations, which could be reduced to Fe 2+ by the subsequent reaction with ethylene glycol. The addition of Fe 2+ to preexisting ferrihydrite can determine the subsequent phase 2-4 .
Goethite and lepidocrocite appear mainly at low concentrations of Fe 2+ , and magnetite can be synthesized directly at high concentrations of Fe 2+ . As the Fe 2+ concentration increases, it is adsorbed onto the surface of the iron (oxyhydr)oxide phases, and it transfers electrons to the bulk iron (oxyhydr)oxide, thus promoting the growth of magnetite. Here, increasing the supply of NaOAC causes a gradual decrease in the ratio of excess Fe 3+ , and consequently, the proportion of Pathway 2 on which magnetite is formed by lepidocrocite and goethite gradually increases. Conversely, the supply of the Fe precursor increases the excess Fe 3+ , and thus Pathway 1 dominates the synthesis of Fe3O4 mesocrystals and reduces the crystallite size. The crystallite size tends to decrease for NaOAC/Fe > 5, owing to the effect of the reduced diameter.
It was confirmed that both the crystallite size and diameter tend to behave differently 18 above and below NaOAC/FeCl3•6H2O = 5-6. The diameter increases slowly at NaOAC/FeCl3•6H2O > 5-6 and abruptly at NaOAC/FeCl3•6H2O < 5-6. Thus, the diameter can also be affected by the formation pathway. In the classical nucleation and growth model, rapid nucleus formation produces larger numbers of small mesocrystals 5 .
For Fe3O4 mesocrystal synthesis, the concentration of OH − increases, and hydrolysis and condensation of FeCl3•6H2O are accelerated, resulting in small Fe3O4 mesocrystals.
However, our study suggests that the results depend on the formation pathway. As the NaOAC/FeCl3•6H2O ratio becomes smaller than 5-6, Pathway 1 dominates, and Furthermore, we found that the water content has an effect similar to that of the NaOAC content, but it has a greater impact on the predominance of Pathway 2.
Dehydration occurs during magnetite formation from ferrihydrite to ferric (oxyhydr)oxides. Increases in the H2O content can interfere with dehydration, resulting in slower reactions (Pathway 2) and larger crystallites.