We have investigated the magnetic and magnetoelasticproperties of a series of Cr-substituted cobaltferriteCoCrxFe2−xO4 (x=0.0-0.79) samples. Substitution of Cr for some of the Fe in cobaltferrite reduced the Curie temperature, and the effect is more pronounced than that observed in Mn-substituted cobaltferrite samples. Cr substitution also caused the maximum magnetostriction to decrease at a greater rate than substitution of the same amount of Mn. The maximum of the strain derivative, dλ/dH, was reached for x=0.38. The behavior of the Curie temperature of Cr-substituted and Mn-substituted cobaltferrites has been analyzed using the Néel molecular field model and compared with recent Mössbauer spectroscopy results. We have investigated the magnetic and magnetoelastic properties of a series of Cr-substituted cobalt ferrite CoCr x Fe 2−x O 4 ͑x = 0.0-0.79͒ samples. Substitution of Cr for some of the Fe in cobalt ferrite reduced the Curie temperature, and the effect is more pronounced than that observed in Mn-substituted cobalt ferrite samples. Cr substitution also caused the maximum magnetostriction to decrease at a greater rate than substitution of the same amount of Mn. The maximum of the strain derivative, d / dH, was reached for x = 0.38. The behavior of the Curie temperature of Cr-substituted and Mn-substituted cobalt ferrites has been analyzed using the Néel molecular field model and compared with recent Mössbauer spectroscopy results.

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IntroductionCobalt ferrite and cobalt ferrite based composites have high magnetoelastic sensitivity, high magnetostriction and low cost, making the materials attractive for use in magnetoelastic sensors [1]. However, to enable practical applications a family of materials is needed, in which the magnetoelastic response, magnetic properties, and their temperature dependences can be tailored by adjusting the chemical composition, microstructure and site occupancies of cations. A series of Mn-substituted cobalt ferrite CoMn x Fe 2-x O 4 has recently been investigated for this purpose and showed promising results [2]. In the present study, a family of Cr-substituted cobalt ferrite CoCr x Fe 2-x O 4 is investigated. The results show that Cr substitution has more profound effects on the Curie temperature and magnetoelastic properties of cobalt ferrite than does Mn substitution. Experimental details and results A series of polycrystalline chromium-substituted cobalt ferrite samples with compositions of CoCr x Fe 2-x O 4 (where x ranges from 0 to 0.8) were prepared by standard powder ceramic techniques [1, 2]. The microstructure and final composition of the samples were characterized using a scanning electron microscope with energy-dispersive x-ray spectroscopy (EDX). The crystal structure of the samples was determined by x-ray diffractometry to be cubic spinel. Curie temperature, T C , was determined by measuring magnetization as a function of temperature in a vibrating sample magnetometer (VSM). Magnetostriction as a function of applied field was measured using the strain gauge method. These results are presented in Fig. 1. Discussion and ConclusionsAs Cr is substituted for Fe in CoCr x Fe 2-x O 4 , the Curie temperature T C decreases with increasing Cr content (Fig. 1b). This effect can be used to adjust magnetic properties through their temperature dependence. In fact, T C decreases at a somewhat greater rate with Cr substitution than with Mn substitution. The magnitude of magnetostriction also decreases with increasing Cr content for values of x up to 0.6 (Fig. 1a). For x = 0.6 magnetostriction becomes positive for fields above 2.5 kOe (200 kA/m), and for x = 0.8 magnetostriction is positive for all fields. Pure cobalt ferrite (x=0) is known to have <100> easy axes, λ 100 <0<λ 111 , and |λ 100 |> |λ 111 | [3]. Thus it appears that substitution of Cr progressively decreases λ 100 relative to λ 111 . Cr substitution also causes the maximum magnetostriction magnitude to decrease at a much greater rate than does Mn substitution (Fig. 1b). Both Cr 3+ and Mn 3+ are known to have strong preference for the octahedral sites in the ferrite structure [4], and Co 2+ has a stronger preference for the octahedral sites than Fe 3+ in pure cobalt ferrite. However, our Mossbauer spectroscopy investigations of these two series of samples appear to indicate that substitution of Cr or Mn for Fe in cobalt ferrite is not a simple substitution of one cation for the other on the octahedral sites [5,6]. Rather, it appears that Cr or Mn substi...