Understanding the effect of Mn substitution for Fe in Co ferrite presents a challenge because there are three different transition-metal ions distributed among two distinct crystallographic and magnetic sublattices with complicated superexchange and anisotropic interactions. In this study, a series of six powder samples with compositions Co 1.0 Mn x Fe 2−x O 4 were investigated using transmission Mössbauer spectroscopy. Mössbauer spectroscopy provides an excellent tool for probing the local environment of the Fe atoms present in such materials. Results show two sets of six-line hyperfine patterns for all samples, indicating the presence of Fe in both A and B sites. Identification of sites is accomplished by evidence from hyperfine distribution width, integrated intensity, and isomer-shift data. Increasing Mn concentration was found to decrease the hyperfine field strength at both sites, but at unequal rates, and to increase the distribution width. This effect is due to the relative strengths of Fe-O -X superexchange ͑X = Fe, Co, or Mn͒ and the different numbers of the next-nearest neighbors of A and B sites. Results are consistent with a model of Mn substituting into B sites and displacing Co ions onto A sites.

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.

Ga-substituted cobalt ferrite oxides show promise as high magnetostriction, high sensitivity magnetoelastic materials for sensor and actuator applications, but their atomic-level behavior is not yet well understood. In this study, the magnetic environments of the Fe atoms in Ga-substituted cobalt ferrite have been investigated using Mossbauer spectroscopy. A series of five powder samples with CoGa x Fe 2−x O 4 compositions ͑x = 0.0-0.8͒ was investigated using transmission geometry. Results show two distinct six-line hyperfine patterns, which are identified as Fe in A ͑tetrahedral͒ and B ͑octahedral͒ spinel sites. Increasing Ga concentration is seen to decrease the hyperfine field strength for both A and B sites, as well as increasing the width of those distributions, consistent with the nonmagnetic nature of Ga 3+ ions. Effects are more pronounced for the B sites than the A sites. Results for Ga substitution show more pronounced effects than for previous studies with Cr 3+ or Mn 3+ substitution: the hyperfine fields decrease and distribution widths increase at greater rates, and the differences between A and B site behavior are more pronounced. Results indicate that at least for the lower Ga concentrations, the Ga 3+ ions substitute predominantly into the A sites, in contrast to Cr 3+ and Mn 3+ which substitute into the B sites. This interpretation is supported by measurements of magnetization at low temperatures. © 2008 American Institute of Physics. ͓DOI: 10.1063/1.2834721͔ INTRODUCTIONThe development of cobalt ferrite-based materials has received recent attention for the potential applicability of such materials as magnetoelastic sensors and actuators, and as the magnetoelastic component in composite "multiferroics." 1,2 Substitution of elements such as Mn or Cr for some of the Fe has shown promise of adjusting the magnetic and magnetoelastic properties of these materials through control of chemical composition. 3,4 In order to fully enable the practical applications of these compounds, a more complete family of materials is needed, such that the desired properties can be tailored to a specific application. In the present study, we report on the magnetic characterization of the series of Ga-substituted cobalt ferrites CoGa x Fe 2−x O 4 ͑x = 0.0-0.8͒ as a function of gallium concentration using transmission Mossbauer spectroscopy. The effects of this substitution were expected to be different for Mn and Cr substitution. [5][6][7] EXPERIMENTAL DETAILSThe samples used in this study were prepared at Ames Laboratory, USDOE. The samples were of the composition CoGa x Fe 2−x O 4 ͑where x = 0.0-0.8͒. Standard powder ceramic techniques were employed using Fe 2 O 3 , Ga 2 O 3 , and Co 3 O 4 powders as precursors. The powders were calcined twice, sintered at 1350°C for 24 h, and furnace cooled ͑see Ref. 7 for details͒. The final compositions of the samples were determined by energy-dispersive x-ray spectroscopy, and all samples were confirmed by x-ray powder diffractometry to be single phase and have the cubic spinel ...

An important component of many college and university science programs is that of community outreach. Some of the more typical kinds of outreach activities include teacher training workshops, public lectures, open house “science days,” and school demonstration visits. The latter activity usually consists of students and faculty transporting equipment from their institution to a local secondary school to provide “hands-on” demonstrations or activities to a few science classes. One problem with such visits is the short interaction time (usually an hour or so), which often comes and goes and is soon forgotten by the participating students. We discuss in this paper the specifics of an outreach program that has been successful in addressing this and other issues.

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