Magnetoelectric effect in BaTiO3/Ni particulate nanocomposites at microwave frequencies

We present a comprehensive study of the magnetic and microwave properties of piezoelectric BaTiO3/magnetostrictive Ni nanocomposites (NCs), fabricated under uniaxial compression, at room temperature. In the current work, we investigated samples in the compositional range between 0 <= f(Ni) <= 33.5 vol % and from 0.1 to 6 GHz using broadband microwave spectroscopy in combination with atomic and magnetic force microscopy (MFM), x-ray diffraction (XRD), electron transport, and broadband (6-28 GHz) ferromagnetic resonance (FMR) experiments in the microwave regime to correlate magnetization dynamics, electromagnetic materials parameters, and microstructural information. The static magnetic response is consistent with a model of a composite medium with an unmodified Ni phase in a nonmagnetic matrix. We provide the experimental evidence for a magnetoelectric (ME) effect, i.e., the effective permittivity at microwave frequencies can be controlled by an external magnetic field, which makes these nanostructures ready for microwave tunable devices, sensors, and transducers. We show in the analysis that this magnetic field dependence is inconsistent with expectations from magnetoresistance and magnetocapacitance effects, and propose as an alternative an explanation based on the striction across the interfaces between the magnetic and piezoelectric phases. By varying the Ni content and frequency, room temperature broadband FMR was performed in order to investigate the different contributions, e. g., inhomogeneous broadening, to the effective linewidth and microwave damping. The line broadening and asymmetry of the FMR features are not intrinsic properties of the metallic nanophase but reflects the local nonmagnetic environment in which they are embedded. The increase in the effective Gilbert damping coefficient as function of the Ni content is related to the strong increase in the damping experienced by the precessing magnetization in the Ni phase. One of the characteristic features of the present results is the significant correlation between the internal field probed by FMR and the ME coupling coefficient evaluated by microwave spectroscopy which was not observed in our previous study of ZnO/Ni NCs. The present results highlight the strong influence of interfaces of the composite constituent play a crucial role in the analysis of the ME coupling. In addition MFM has been successfully used to detect the strong magnetic contrast between the phases of these nanostructures which indicates local changes in composition and structure. (C) 2009 American Institute of Physics. [doi:10.1063/1.3225567]