Influence of vacuum sintering on microstructure and magnetic properties of magnetostrictive cobalt ferrite I.C. Nlebedim, N Ranvah, P.I. Williams, Y Melikhov, FAnayi, J.E. Snyder, A.J. Moses, D.C. Jiles à Wolfson Centre for Magnetics, School of Engineering, Cardiff University, Cardiff CF24 3AA, United Kingdom article info Article history: Received 8 December 2008 Available online 20 March 2009 Keywords: Anisotropy coefficient Cobalt ferrite Magnetoelastic property Magnetostriction Vacuum sintering abstract Differences in the microstructure and magnetic properties of highly magnetostrictive cobalt ferrite resulting from the effects of different vacuum sintering temperatures and times have been investigated. A vacuum environment was chosen to allow direct comparison of results with air-sintered samples which are more often reported in the literature. It was found that vacuum sintering resulted in the development of a solid solution second phase with composition Co 1x Fe x O 4 (x0.33). There was a decrease in magnetostriction as a result of the formation of the second phase. Furthermore, differences in sintering temperatures were found to have a greater effect on the magnetostriction than differences in sintering times. It was found that the first order cubic anisotropy coefficient initially increased with both sintering temperature and time, before peaking and decreasing to its lowest measured value. The lowest anisotropy was therefore achieved with samples sintered at higher temperatures and longer times. & 2009 Elsevier B.V. All rights reserved. 1. Introduction The use of magnetostrictive materials for development of non- contact stress sensors and actuators in a wide range of engineer- ing applications has attracted considerable research interest. This includes the need to develop low-cost materials with improved mechanical, thermal and chemical properties of which ferrites are good candidates. Among the ferrites, cobalt ferrites (including the parent material CoFe 2 O 4 and derivatives CoM x Fe 2x O 4 ) have been investigated as practical alternatives to the rare earth based magnetostrictive materials such as Terfenol (Tb x Dy 1x Fe 2 ) and other magnetostrictive metals such as Galfenol (Fe 80 Ga 20 ) for sensor and actuator development [1,2]. CoFe 2 O 4 has a spinel crystal structure. For normal spinels, all the divalent metallic ions are on A-sites (tetrahedral sites) and all the trivalent ions are on B-sites (octahedral sites). On the other hand, inverse spinels have half of the trivalent ions on the A-site and the other half plus all the divalent metallic ions on the B-site. It is widely accepted [3] that CoFe 2 O 4 is neither completely normal spinel nor completely inverse spinel. This is because the cation distribution between the tetrahedral and octahedral sites is intermediate between that of normal and inverse spinel materials. Since the magnetostrictive properties of CoFe 2 O 4 depend largely on the position and concentration of the Co 2+ ions, it follows that changes in the site occupancy of these ions will affect the magnetic and magnetostrictive properties of CoFe 2 O 4 . Studies have shown that magnetic and magnetostrictive properties can be altered by both chemical substitutions of the cations [4–6] and by heat treatment [7,8], the latter leading to a different distribution of cations as they migrate towards their most stable (that is the lowest energy) state. The time constants associated with this process are prohibitively long at room temperature, but are reduced at elevated temperature because of the higher thermal energy per ion which enables them to migrate more easily. Most of the studies on the improvement of magnetostrictive properties of CoFe 2 O 4 have concentrated on samples prepared at a particular temperature in air. Since site occupancy of ions is crucial to the magnetostrictive properties, and this can be altered depending on sintering conditions, there is a need for a systematic approach to vary the heat treatment and sintering conditions to produce desired properties. This can also lead to a better under- standing of magnetostrictive properties. Such understanding is particularly important due to the varying levels of magnetostric- tion reported for CoFe 2 O 4 in different studies. Values ranging from 100 to 225ppm have been reported previously [8–10]. In this work, we present the results of a study on the effect of vacuum sintering, with different sintering temperatures and sintering times, on the microstructure, magnetic and magnetos- trictive properties of bulk samples of CoFe 2 O 4 . Since oxygen partial pressure influences the compositional variation of CoFe 2 O 4 [7], vacuum condition was chosen in order to compare and contrast results with those of air-sintered samples reported [8,10]. 2. Experimental details CoFe 2 O 4 powder was ball milled, pressed into buttons and sintered in vacuum at a pressure of 10 5 Torr. Nine samples were ARTICLE IN PRESS Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/jmmm Journal of Magnetism and Magnetic Materials 0304-8853/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2009.03.021 à Corresponding author. E-mail address: jilesd@cf.ac.uk (D.C. Jiles). Journal of Magnetism and Magnetic Materials 321 (2009) 2528–2532