Investigation of Cr substitution in Co ferrite (CoCrxFe2−xO4) using Mossbauer spectroscopy

Abstract

Substitution of other metals for Fe in cobalt ferrite has been proposed as a method to tailor the magnetic and magnetoelastic properties for sensor and actuator applications ͓H. Zheng et al., Science 303, 661 ͑2004͔͒. However, to understand the effect of Cr substitution, one needs atomic-level information on the local environments and interactions of the transition-metal ions. In this study, Mossbauer spectroscopy was used to investigate the local environments of the Fe atoms in these materials. A series of five powder samples with compositions CoCr x Fe 2−x O 4 ͑x = 0.0 to 0.8͒ was investigated using transmission geometry. Results show two distinct six-line hyperfine patterns, indicating Fe in A and B spinel sites. Increasing Cr concentration is seen to decrease the hyperfine field strength for both A and B sites, as well as increasing the width of those distributions. Results for Cr substitution show generally similar behavior to a prior study using Mn; however, Cr substitution has more pronounced effects: the hyperfine fields decrease and distribution widths increase at greater rates for Cr substitution, and the differences between A and B site behavior are more pronounced. Results are consistent with a model in which Cr has an even stronger B-site preference than Mn, and displaces more of the Co from the B to the A sites.

Investigation of Ga substitution in cobalt ferrite (CoGaxFe2−xO4) using Mossbauer spectroscopy

“…This indicates the presence of two different types of ferromagnetic Fe atoms in the material, which are identified as Fe in the A ͑tetrahedral͒ and B ͑octahedral͒ sites of the spinel structure. The spectra show a behavior that is observed to be qualitatively similar to that of the Cr-and Mn-substituted cobalt ferrites: 8,9 as the Ga concentration increases, the line spacing ͑indicative of the magnetic hyperfine field at the Fe nuclei and related to the exchange coupling͒ can be seen to decrease and the spacing of the two individual sextets that make up a͒ Electronic mail: krieblek@moravian.edu each spectrum varies relative to one another. Quantitative values for the mean hyperfine field strengths, hyperfine field distribution widths, and isomer shifts of Fe A and B sites in all the samples were calculated by fitting a composite of two hyperfine patterns to each spectrum using WMOSS analysis software.…”

“…4͒, the hyperfine field distribution width increases with increasing Ga content, with the B-site distribution width increasing faster than A. Both the A-site and B-site hyperfine fields decrease at a faster rate for Ga substitution than they do for Mn substitution 8 or even Cr substitution, 9 and the hyperfine field distribution widths increase at a faster rate. Furthermore, the difference in hyperfine field and hyperfine field distribution width between the A-sites and B-sites becomes greater for Ga 3+ substitution than for Mn 3+ or Cr 3+ substitution.…”

“…In the case of Mn 3+ and Cr 3+ substitutions, it was observed that Mn 3+ and Cr 3+ appear to substitute into the B sites and displace some of the Co 2+ onto the A sites. 8,9 In the case of Ga 3+ , a combination of the moredirect substitution onto the A sites and substitution of a nonmagnetic ion decreases the B site hyperfine field and increases the B-site distribution width even more rapidly. The A-site hyperfine field is also observed to decrease, and hyperfine field distribution width increases with increasing substitution of Ga 3+ .…”

Magnetic and magnetoelastic properties of Cr-substituted cobalt ferrite

“…10 A similar Mössbauer spectroscopy study of Crsubstituted cobalt ferrites showed that Cr 3+ ions have a stronger preference for the B sites than Mn 3+ . 11 Additionally, displacement of Co 2+ ions onto A sites was found to be more prevalent in Cr-substituted cobalt ferrites than in Mnsubstituted cobalt ferrites.…”

“…Recently cobalt ferrite based composites have received attention because of their high magnetostriction, high sensitivity of magnetic induction to applied stress, chemical stability and low cost. [9][10][11][12] These make the materials attractive for use in magnetoelastic sensors, 1 and as the magnetoelastic component in oxidebased "multiferroic" composites.…”