Magnetoelectric PVDF-Cobalt Ferrite Films: Magnetostrictive and Magnetorotational Effects, Synergy, and Counteraction

Numerical modeling of the direct magnetoelectric (ME) effect in a PVDF-cobalt ferrite (CFO) composite film has been performed. The problem is solved within the framework of the mesoscopic RVE approach, where each elementary cell contains three particles with varying mutual positions. Both modes of mechanical stress generation are taken into account: magnetostrictive and magnetorotational, i.e., changes in the shape and rotation of the particle as a whole. Depending on the sign of the magnetostriction constants, these sources of piezopolarization can either enhance or reduce the overall ME effect. A significant dependence of the ME effect on the mutual arrangement of CFO particles in the cell has been discovered; for instance, the effect is minimal when the particles are closest to each other. In other words, clustering is a negative factor. In a system where the magnetic moments of the magnetically hard CFO particles are ordered, the maximum ME effect is attained when the poling direction is at an angle of about 40∘ to the film plane. As it turns out, a fairly good estimate of this angle can be obtained from the solution of a single-particle problem; the main contribution here comes from the 'diagonal' components of the piezotensor: d31 and d33. The 'tangential' component d15 plays a special role: changing its sign can reverse the polarity of the charge generated on the film.