Induced ferromagnetism and magnetoelectric coupling in ion-beam synthesized BiFeO₃–CoFe₂O₄ nanocomposite thin films

Ferrimagnetic CoFe₂O₄ (cobalt ferrite) is formed within an epitaxial BiFeO₃ (bismuth ferrite) thin film matrix by Co channeled ion implantation and subsequent annealing. The presence of nanoscale CoFe₂O₄ crystals in the matrix is confirmed by x-ray diffraction using synchrotron radiation. The significantly increased magnetic moment and the low-temperature coercive field of the composite system evidence the formation of ferrimagnetic cobalt ferrite and its nanoscale character, respectively. The results demonstrate that ion beam synthesis is an appropriate method to controllably transform a planar system into a granular one, increasing the interface area between cobalt ferrite and bismuth ferrite. The ferroelectric nature of the BiFeO₃–CoFe₂O₄ composite is confirmed by several scanning probe microscopy techniques. At room temperature, the composite exhibits a magnetoelectric voltage coefficient of α _ME  =  17.5 V (cm · Oe)⁻¹, while a single-phase BiFeO₃ thin film shows a α _ME value of 4.2 V (cm · Oe)⁻¹. The high magnetoelectric voltage coefficient is interpreted to be the result of the interfacial interaction between the ferrimagnetic CoFe₂O₄ nanocrystallites and the multiferroic BiFeO₃ matrix. (paper)