λ reduced with increase in quenching temperature

1. μm ENHANCEMENT OF HIGHLY MAGNEOSTRICTIVE COBALT FERRITE FOR ADVANCED SENSOR AND ACTUATOR APPLICATIONSI. C Nlebedim Wolfson Centre for Magnetics, Cardiff University, Cardiff, CF24 3AA. UK. Email†- nlebedimci@cardiff.ac.uk; Phone† - +44 (0) 2920875936 INTRODUCTION MAGNETOSTRICTION (λ) AND SENSITIVITY (dλ/dH) The ability to tune the magnetostrictive properties of Co-ferrite offers the potential to develop high performance magnetomechanical stress sensor and actuator devices. Magnetostrictive properties such as strain sensitivity and magnetostriction can be tuned by cation doping and heat treatment. In this study, the capability of altering the magnetoelastic properties of cobalt ferrite by varying the cation distribution through quenching heat treatment has been demonstrated. Results from this study are compared with those obtained from cation substituted studies. SAMPLE PREPARATION AND CHARACTERIZATION CoFe2O4 was prepared by mixing the constituent metallic oxides at appropriate ratios, calcining twice at 1000 oC and sintering at 1350 oC all in air for 24 hours. Selected samples were reheated to and quenched from 600, 800, 1000, 1200 and 1400 oC . • λ reduced with increase in quenching temperature • Compared to the unquenched sample, the slope of λ changed shape at high field region • dλ/dH also decreased with increase in quenching temperature • Decrease in both λ and dλ/dH can be explained in terms of changes in anisotropy and increase in stress due to quenching • (dλ/dH)max of samples are higher than previous report in literature (1.37x10-9 A-1m) • Compared to literature values, these high values of sensitivity (especially for the unquenched sample) may be due to differences in oxygen content of the samples. EDX, SEM and XRD RESULTS • Cation ratio from EDX analysis is Co : Fe = 1.02 : 1.98 for all samples • EDX analysis was not sensitive to oxygen content • XRD results showed single phase spinel structure for all samples. • SEM micrographs showed uniform microstructure which confirms single phase samples. Results were similar for all samples. • These results indicate that changes after quenching are not due to crystal structure or composition changes. RESULT COMPARISON MAGNETIC PROPERTIES • Room temperature magnetization at H = 4 MA/m shows an increase in magnetization with increase in quenching temperature. • This might be due to cation redistribution among the spinel sites. • First cubic anisotropy coefficient decreased with increase in quenching temperature. • The coercive field also decreased with increase in the quenching temperature • Change in coercive field may be due to change in anisotropy following cation redistribution • Higher strain sensitivity was obtained for samples 2 and 3 • Cation substituted samples (samples 5 to 9) also gave improved sensitivity • High strain sensitivity correspond to high signal to background noise ratio • Except for Ge4+/Co2+ substitution (sample 6), amplitudes of magnetostriction were reduced for all samples compared to the previous study • The obtained amplitudes of magnetostriction are still sufficient for many device developments CONCLUSION • Quenching Heat treat resulted in changes in magnetic and magnetostrictive properties of samples • XRD, SEM and XRD results indicate that these changes are not due to changes in crystal structure or composition • High strain sensitivity with sufficient magnetostriction obtained for samples are indicative of suitability of material for high sensitivity sensor and efficient actuator development. Acknowledgement This research was supported by the UK Engineering and Physical Sciences Research Council under grant number EP/D057094 and by the US National Science Foundation under grant number DMR-0402716.