Study Of Anisotropy And Magnetostriction In Co-ferrites

Cobalt ferrite-based materials have advantages of high magnetostriction amplitude,high resistivity,low eddy current loss.The magnetostriction of cobalt ferrites in[100]easy directions can reach-590×10-6.Because of the large magnetocrystalline anisotropy,the magnetostriction of isotropic polycrystalline sample is only about-200×10-6.The objective of this paper is to prepare polycrystalline cobalt ferrites with high magnetostriction,high piezomagnetic coefficient and low driving field.In this work,the preparation technology of oriented polycrystalline cobalt ferrites have been studied.Injection molding methods and magnetic aligning methods were combined to prepare oriented samples with a strong<001>fiber texture.?sof-500×10-6 at 800 Oe and(d?/dH)max of-0.97×10-6/Oe for oriented samples had been obtained.Magnetostrictive properties of oriented samples have been significantly enhanced compared with isotropous samples.Orientation distribution of grains had been connected to the magnetostriction.Effects of the magnetic treatment and HIP treatment on domain structure and microstructure had been studied.The anisotropy induced by magnetic treatment had been illustrated.Effects of the element doping on the magnetocrystalline anisotropy and magnetostriction of cobalt ferrites had been studied.Magnetoelastic properties of cobalt ferrites under the coupling load of stress and magnetic field had been studied.Changes of the magnetostriction and elastic modulus with stress have been identified.The preparation technology of oriented polycrystalline cobalt ferrites had been studied.Ferrite powders in single crystal forms were prepared by high energy ball mill method.Injection molding methods and magnetic aligning methods were combined to prepare oriented ferrite slurries.While slurries were heat treated to exclude the binder and sintered to increase the density,we got the oriented cobalt ferrites with a strong<001>fiber texture.Magnetostriction was calculated based on the texture information and results showed well consistency between the magnetostriction and the intensity of<001>fiber texture.Hot isostatic pressing treatment had eliminated the internal defects and brought the density up to 99%.An uniaxial anisotropy were induced by magnetic heat treatment and the proportion of 90° magnetic domain was increased.As a result,?s of-564×10-6 at 500 Oe and(d?/dH)max of-1.54×10-6/Oe had been obtained after magnetic heat treatment for oriented sample.Effects of the element addition on the magnetocrystalline anisotropy and magnetostriction of cobalt ferrites had been studied.The co-doping of Mn and Zn had made more Co2+ into B site.The magnetocrystalline anisotropy K1 and magnetostriction ?s had been reduced simultaneously,but the value of KI/?s and(d?/dH)max increased.Magnetoelastic properties of cobalt ferrites under the coupling load of stress and magnetic field had been studied.Changes of magnetostrictive curves in constant stress and stress-strain curves in constant magnetic field had been studied by using multi-field coupling measuring device.Results showed that?s decreased linearly with the increase of stress.Elasticity modulus loss of up to 26%had been observed when no field was applied.The in-situ observation of magnetic domains had been performed by the magnetic force microscopy and it revealed that domains became homogenous in the interior of grains because of the uniaxial anisotropy induced by the stress.