  In this study we have compared magnetic, magnetostrictive and piezomagnetic
properties of isotropic and anisotropic cobalt ferrite pellets. The isotropic
sample was prepared by the ceramic method while the sample exhibiting uniaxial
anisotropy was made by reactive sintering using Spark Plasma Sintering (SPS).
This technique permits to induce a magnetic anisotropy in cobalt ferrite in the
direction of the applied pressure during SPS process. Sample with uniaxial
anisotropy revealed a higher longitudinal magnetostriction and piezomagnetism
compared to the isotropic sample, but the transversal magnetostriction and
piezomagnetism were dramatically reduced. In the case of magnetoelectric
layered composite, the magnetoelectric coefficient is directly related to the
sum of the longitudinal and transversal piezomagnetic coefficients. These two
coefficients being opposite in sign, the use of material exhibiting high
longitudinal and low transversal piezomagnetic coefficient (or vice versa) in
ME devices is expected to improve the ME effect. Hence, ME bilayer devices were
made using isotropic and anisotropic cobalt ferrite stuck with a PZT layer. ME
measurements at low frequencies revealed that bilayer with anisotropic cobalt
ferrite exhibits a ME coefficient three times higher than a bilayer with
isotropic cobalt ferrite. We also investigated the behavior of such composites
when excited at resonant frequency.
