1. Investigation of cobalt ferrite based materials for stress sensor and actuator design INTRODUCTION Stress sensors can be used to monitor the health of buildings and structures –The change is dimensions of a material due to a change in its magnetisation is called magnetostriction ( ). –The inverse magnetostriction effect (Villari effect): magnetisation changes upon changing the dimensions. –Magnetic stress sensors can be made using magnetostrictive materials. Surface field changes on change in dimensions, which can be used to measure stress. WHY COBALT FERRITE?  Exhibits magnetostriction sensitivity (d /dH)  Is cheap, and mechanically and chemically robust SENSITIVITY AND RANGE N. Ranvah, I.C. Nlebedim, Y. Melikhov, J. E. Snyder, P. I. Williams, A. J. Moses and D. C. Jiles Wolfson Centre for Magnetics, School of Engineering, Cardiff University, The Parade, Cardiff – CF24 4QU Magnetoelastic properties of cobalt ferrite can be improved by chemical substitution CURRENT STUDY RESULTS Effect of non-magnetic Ge 4+ ion substitution  Lower anisotropy Lower anisotropy leads to  Higher sensitivity Ge 4+ goes into A-sites and more Co 2+ into B-sites  Higher range CONCLUSIONS SELECTED REFERENCES  Chikazumi, S. (1997), Physics of Ferromagnetism, Oxford University Press, Great Clarendon Street, Oxford, OX2 6DP.  Lee, S. J.; Lo, C. C. H.; Matlage, P. N.; Song, S. H.; Melikhov, Y.; Snyder, J. E. & Jiles, D. C. (2007), 'Magnetic and magnetoelastic properties of Cr-substituted cobalt ferrite', Journal of Applied Physics 102(7), 073910.  Melikhov, Y.; Snyder, J. E.; Jiles, D. C.; Ring, A. P.; Paulsen, J. A.; Lo, C. C. H. & Dennis, K. W. (2006), 'Temperature dependence of magnetic anisotropy in Mn- substituted cobalt ferrite', JOURNAL OF APPLIED PHYSICS 99, 08R102.  Song, S..H. (2007), 'Magnetic and magnetoelastic properties of M-substituted cobalt ferrites (M=Mn, Cr, Ga, Ge)', PhD thesis, Iowa State University, Ames, Iowa. RESULTS ACKNOWLEDGEMENTS 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. Power consumption  A Co 1.1 Ge 0.1 Fe 1.8 O 4 based device will need 93.75% less power if operated at the same range as pure cobalt ferrite. Email: nranvah@gmail.com Material Magnetostrictive material Sensor Electronics Material Magnetostrictive material Sensor Electronics 2 1 is the magnetostriction 1 2 H1H1 H 1,max H 2,max 1 2 CoFe 2 O 4 Ge 4+ /Co 2+ Substitution Co 1+x Ge x Fe 2-2x O 4 CUBIC SPINEL LATTICE Tetrahedral Octahedral B A The two sites on cobalt ferrite lattice: Fe 3+ Co 2+  Properties depend on site preference of substituted ions  Complete inverse spinel: [Fe] A [CoFe] B O 4  Cobalt ferrite is reported to have some Co in A-sites as well SITE OCCUPANCY OF IONS Co 2+ in B-sites  Higher magnetostriction Higher Anisotropy  Lower sensitivity Non-magnetic ion substitution  Lower Anisotropy Mn 3+ Cr 3+ B-sites (Octahedral) Ga 3+ Ge 4+ A-sites (Tetrahedral) SITE OCCUPANCY OF IONS Higher Moment  Higher Magnetostriction (higher range of operation) Internal coupling reduces  Anisotropy reduces (higher sensitivity) Co 2+ Fe 3+ Ge 4+ ++ B-site A-site B-site Before substitution = Net Moment Substitution IN OUT B-siteA-siteB-site ++ After substitution = Net Moment Energy Saved 93.75% More Cobalt in B -sites Co 2+ /Ge 4+ substitution Lower Exchange coupling Lower Anisotropy Higher Sensitivity ( d /dH ) Higher Magnetostriction ( ) Higher range of operation, lower power consumption, less hysteresis  Candidate for sensor or actuator applications