Development and analysis of highly-efficient magnetoelectric cobalt ferrite/barium titanate composites using a scalable synthesis technique

Abstract

Direct magnetoelectric (ME) coupling in single-phase multiferroic materials is limited to high magnetic fields and/or cryogenic temperatures. More robust ME behavior has been achieved in multi-phase composites that combine magnetostrictive and piezoelectric materials in a matrix 1–3. The ME effect in such composites is due to the induced stresses within the magnetostrictive or piezoelectric phases controlled by the application of external magnetic or electric fields, which transfer through the interface to the piezoelectric or magnetostrictive phases, respectively 4. Therefore, two factors play a crucial role in the effectiveness of the ferroic phases conjugation: resistivity of the ceramic, which directly governs the electrical poling effectiveness of the ferroelectric phase, and the interface of the two materials, which influences the strain transfer 5. Wide choice of materials, volume fraction, and microstructure in multi-phase composites allow addressing these challenges 1,3. There are multiple applications for magnetoelectric (ME) materials in sensors, energy harvesting, magnetoelectric random access memory, antennas, drug delivery, etc 2. The implementation of ME materials in some of these applications demands the use of nano-multiferroic samples. Although, the characterization methods to study the ME coupling in bulk materials are well-developed; however, the measurement of ME coupling at the nanoscale is still challenging 6.In this work, high magnetoelectric coupling coefficient CoFe2O4-BaTiO3 composites were developed using commercial CoFe2O4 and BaTiO3 powders as precursors. The bulk magnetoelectric coefficient in the optimized CoFe2O4-BaTiO3 composite where individual CoFe2O4 grains are fully enclosed by the BaTiO3 matrix was found to be comparable with the numbers reported for the composites based on core-shell structures 7, 8. This presentation will detail the physical (magnetic, electric, magnetoelectrical, and magneto-mechanical) and microstructural properties of the developed composites, offer the analysis of the observed behaviors, and outline the routes for further improvements. The composites were produced by dry ball-milling of precursor powders followed by annealing at 1200 °C. Properties of the composite as a function of synthesis precursor choice and precursor mixture composition were explored. The microstructural characterization (focused ion-beam depth profiling of pellet composites etched in hydrochloric acid to selectively remove CoFe2O4 and piezo-force microscopy of polished pellets) of the synthesized CoFe2O4-BaTiO3 composited clearly revealed a dramatic dependence on the size and phase (cubic vs. tetragonal) of BaTiO3 precursor. This dependence has translated into a dramatic difference between magnetoelectric coupling coefficients observed in the composites synthesized with cubic and tetragonal BaTiO3 precursors. Utilizing 50 nm cubic BaTiO3 precursor powder results in a material, in which relatively large individual CoFe2O4 grains are fully embedded into and well-insulated by a BaTiO3 matrix. This microstructure enhances the strain-mediated coupling between CoFe2O4 and BaTiO3 grain. Furthermore, it leads to higher electrical resistivity, which, in turn, enables effective electrical poling. The highest magnetoelectric coupling coefficient is exhibited in CoFe2O4-BaTiO3: 20-80% composites that utilize a 50 nm cubic BaTiO3 powder as precursor with a value as high as 4.3 mV/Oe.cm. The dependence of magnetoelectric coupling coefficient on the value of the external DC magnetic field for different compositions of CoFe2O4-BaTiO3 composites is shown in Fig. 1 and, as will be shown in this presentation, correlates well with magnetic and magnetostrictive properties and is in agreement with theoretical analysis. In addition to the detailed characterization of bulk properties, the dependence of the local surface potential on the magnitude of external DC magnetic fields was studied using Kelvin Probe Force Microscopy (KPFM). The line profile of the surface potential for the (CoFe2O4-BaTiO3 (50 nm cubic):30-70%) sample for different values of external magnetic field is shown in Fig. 2. **