Abstract
The functional properties of nanocrystals can be finely tuned through controlled morphology and size. However, this can be challenging for metastable nanostructures that require harsh synthesis conditions, such as high temperatures. Here, we present a method for preparing large ε-Fe(2)O(3) nanorods that are not affected by magnetic relaxation. This study presents a novel growth mechanism in which high-aspect-ratio rods evolve from spherical ε-Fe(2)O(3) particles in a silica matrix containing Y(3+). With the presence of Y(3+), the glassy matrix undergoes a metastable binodal decomposition yielding the formation of nanodroplets of a Y-rich silicate of composition ∼Y(2)Si(2)O(7). This Y silicate selectively coats the ε-Fe(2)O(3) planes perpendicular to the rod axis along the [100] direction but is not observed in the rod apexes. Structural optimizations and energy calculations of different crystal faces of ε-Fe(2)O(3) in contact with Y(2)Si(2)O(7) obtained using machine-learning force fields provide an atomistic interpretation of these observations: the affinity of Y with the oxygen atoms exposed at ε-Fe(2)O(3) surfaces explains the preferential capping of ε-Fe(2)O(3) surfaces that present a large density of oxygen atoms and its absence in surfaces such as (100), where this density is significantly lower. The presence or absence of the silicate capping layer results in different surface energies and/or mass transfer coefficients across the interface, originating two independent Ostwald ripening processes, which drive the high aspect ratio growth. By using La(3+) instead of Y(3+), ε-Fe(2)O(3) rods with even larger aspect ratios are obtained. Notably, this synthetic approach counteracts the progressive diminution of the average nanoparticle size observed in ε-(Fe(1-x)Cr(x))(2)O(3) upon Cr(3+) addition, enabling to elucidate the effect of this substitution on the intrinsic magnetic anisotropy and the anisotropy fields that determine the high-frequency ferromagnetic resonances of this phase.