Magnetic clusters in ilmenite-hematite solid solutions

Abstract

We report the use of high-field 57 Fe Mössbauer spectroscopy to resolve the magnetic ordering of ilmenitehematite ͓xFeTiO 3 − ͑1−x͒Fe 2 O 3 ͔ solid solutions with x Ͼ 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 Fe 2+ in ilmenite. For ilmenite-rich samples ͑x = 0.95͒, 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.

Influence of cation disorder on the magnetic properties of ball-milled ilmenite (FeTiO3)

“…The spectrum obtained before ball-milling consists of a quadrupole doublet with isomer shift of 1.18 mm s À1 and quadrupole splitting of 1.00 mm s À1 (green fit lines). These parameters are typical for paramagnetic ilmenite [8,17,35]. After ball-milling for 1 h, a second Fe 2þ -doublet (blue fit lines) with isomer shift of 1.14 mm s À1 and quadrupole splitting of 2.06 mm s À1 has appeared.…”

“…However, especially the orbital contribution is very sensitive to the local environment of the Fe 2þ ions, and therefore the cation disorder induced by ball-milling is expected to result in larger magnetic hyperfine fields. Similarly, in quenched solid-solutions of ilmenite-hematite (x ¼ 0.80 and x ¼ 0.70), where the local environment of Fe 2þ also differ from that of pure ilmenite, hyperfine fields up to 25e40 T have been observed [17]. In order to estimate the Néel temperature of the ball-milled material we have obtained Mössbauer spectra at different temperatures of the sample, ball-milled for 64 h. The spectra are shown in Fig.…”

“…In a recent Mössbauer study of synthetic, rapidly cooled solutions of ilmenite and hematite (a-Fe 2 O 3 ), with x ¼ 0.95, it was found that nm-size hematite clusters formed in an ilmenite matrix [17]. Mössbauer spectra obtained at 60 K showed that part of the Fe 3þ doublet in the zero-field spectrum transformed to a magnetically split component when a large field was applied.…”

“…The two other sextets (blue fit lines) have very broad lines, indicating a broad distribution of magnetic hyperfine fields. Relaxation effects may also contribute to the line broadening of such spectra [17]. A fit with two sextets is therefore an oversimplification.…”

“…The ferrimagnetic properties and the increased magnetic ordering temperature may be explained by the increased amount of magnetic Fe 3þ ions in the B layers and nanometer-sized hematite clusters within ilmenite that interact to form a cluster-glass [12,17]. Recently, the solid-solution ilmeniteehematite system, with its ability to be a magnetic semiconductor at room temperature, has also been found promising for spintronics applications [18e21].…”