Magnetic and magnetoelastic properties of M-substituted cobalt ferrites (M= manganese, chromium, gallium, germanium)

Magnetic and magnetoelastic properties of a series of M-substituted cobalt ferrites, CoMxFe2-xO4 (M=Mn, Cr, Ga; x=0.0 to 0.8) and Ge-substituted cobalt ferrites Co1+xGexFe 2-2xO4 (x=0.0 to 0.6) have been investigated. The Curie temperature TC and hysteresis properties were found to vary with substitution content x, which indicates that exchange and anisotropy energies changed as a result of substitution of those cations for Fe. The maximum magnitude of magnetostriction decreased monotonically with increase in substitution contents x over the range x=0.0 to 0.8. However, the rate in change of magnetostriction with applied magnetic field (dlambda/dH) showed a maximum value of 5.7 x 10-9 A-1m at x=0.1 Ge sample, which is the highest value among recently reported cobalt ferrite based materials. The slope of magnetostriction with applied field dlambda/dH is one of the most important properties for stress sensor applications because it determines the sensitivity of magnetic induction to stress (dB/dsigma). The results of Ga- and Ge-substituted cobalt ferrite were compared with those of Mn- and Cr-substituted cobalt ferrites, and it was found that the effect of the substituted contents x on magnetic and magnetoelastic properties was dependent on the ionic distribution between two possible interstice sites within the spinel structure: Mn3+ and Cr3+ prefer the octahedral sites, whereas Ga3+ and Ge4+ prefer the tetrahedral sites. Temperature dependence of the absolute magnitude of the magnetic anisotropy constant |K1| of Ga-substituted cobalt ferrites CoGaxFe 2-xO4 (x=0.0 to 0.8) was investigated based on the law of approach to saturation and the results were compared with those of magnetostriction measured at the same temperatures. Based on the results, it was considered that there was a change in sign of K1 around 200 K for Ga-substituted cobalt ferrites. Comparison of the results between Ga- and Ge-substituted cobalt ferrites showed that substitution of Ge4+ ions for Fe made more pronounced effects on magnetic and magnetoelastic properties at room temperature than that of Ga3+ ions. Especially the enhanced value in dlambda/dH by Ge-substitution suggests that adjusting Ge content substituted into cobalt ferrite can be a promising route for controlling critical magnetic properties of the material for practical sensor applications.