Magnetic clusters in ilmenite-hematite solid solutions

We report the use of high-field ${}^{57}\text{F}\text{e}$ Mössbauer spectroscopy to resolve the magnetic ordering of ilmenite-hematite $\left[x{\text{FeTiO}}_3-\left(1-x\right){\text{Fe}}_2{\text{O}}_3\right]$ solid solutions with $x>0.5$. We find that nanometer-sized hematite clusters exist within an ilmenite-like matrix. Although both phases are antiferromagnetically ordered, the hematite clusters show ferrimagnetic behavior due to superexchange coupling with ${\text{Fe}}^{2+}$ in ilmenite. For ilmenite-rich samples $\left(x=0.95\right)$, the clusters are isolated and superparamagnetic. For more hematite-rich samples with $x=0.80$ and $x=0.70$, the clusters interact to form a cluster glass.

Figure 1

(Color online) Mössbauer spectra of Ilm95, Ilm80, and Ilm70 at different temperatures and in zero (0 T) or 6 T applied field as indicated. Colored lines represent fits to data.

Figure 2

(Color online) Schematic representation of cluster formation in Ilm95 and Ilm80. (a) ${\text{Fe}}^{3+}$ ions in Ilm95 form isolated antiferromagnetic clusters (red - dark gray in print - arrows represent magnetic spins of ${\text{Fe}}^{3+}$ ions) and surrounding ${\text{Fe}}^{2+}$ (blue - light gray in print - arrows) couple to the ${\text{Fe}}^{3+}$ cluster leading to a net “ferromagnetic” moment. Mainly ${\text{Fe}}^{2+}$ ions at the rim of a ${\text{Fe}}^{3+}$ cluster contribute to the net moment. The large blue arrow to the right of the panel shows resulting magnetization. (b) ${\text{Fe}}^{3+}$ ions in Ilm80 form antiferromagnetic clusters. Most surrounding ${\text{Fe}}^{2+}$ ions couple magnetically to ${\text{Fe}}^{3+}$ clusters. As spin frustration may exist, especially further away from the clusters, the ferromagnetic alignments of ${\text{Fe}}^{2+}$ moments may not be complete in zero field. The large blue arrow below the panel indicates the net magnetization.