Room Temperature Multiferroicity In CoFe₂O₄-based Spinel Films

Multiferroic materials are functional materials with more than two kinds of ferroics at the same time,which have a great application prospects in new memory devices,magnetoelectric sensors,spintronic devices and other fields.This work aims to seek for single-phase multiferroic materials with ferroelectric and ferromagnetic coexisting at room temperature.Based on ferromagnetic CoFe₂O₄ spinel,doping of non-magnetic ions(Al³⁺,Mg²⁺)at B or A sites in the spinel structure is carried out,and CoFe_1.6Al_0.4O₄ and Mg_0.3Co_0.7Fe₂O₄ thin films exhibit room temperature multiferroic properties.The ferroelectric order of CoFe_1.6Al_0.4O₄ and Mg_0.3Co_0.7Fe₂O₄ thin films might origin from the atomic order in cationic sub-lattices and the local deformation induced by Al³⁺or Mg²⁺substitution in CoFe₂O₄.This work provides theoretical and experimental basis for the development of single-phase multi-iron materials.The specific research contents are as follows:Al doped CoFe₂O₄ thin films were prepared by RF magnetron sputtering.The effects of different power and argon-oxygen ratio on the morphology and ferroelectric properties of the thin films were analyzed,and the preparation parameters of the thin films were optimized.According to the optimized process,CoFe_1.6Al_0.4O₄ nanofilm was prepared.The ionic valence and elemental composition of the film were analyzed by XPS and EDS,and the crystal structure of the film was characterized by XRD and Raman spectroscopy.At room temperature,the saturation polarization intensity of the hysteresis loop(M-H)curve is 75 emu/cm³,the saturation polarization intensity of the electrohysteresis loop(P-E)curve is 2.3?C/cm².The maximum displacement of the piezoelectric curve is 19(?)at a voltage of 5 V,which proves the ferroelectric and magnetic orders in the films.Mg doped CoFe₂O₄thin films were prepared by RF magnetron sputtering.The argon-oxygen ratio and thickness of the thin films were optimized according to the morphologies and the ferroelectric properties of the thin films.The Mg_0.3Co_0.7Fe₂O₄nanofilm was prepared according to the optimized parameters,and the crystal structure,ion valence and elemental composition of the film were characterized by Raman spectroscopy and XPS.At room temperature,the in-plane and out-of-plane saturation magnetization of M-H curve is 149 emu/cm³ and 136 emu/cm³,respectively.The saturation polarization of P-E curve of thin film is 1.7?C/cm²,and the maximum displacement of piezoelectric curve is 30(?)at 5 V,which proves the existence of ferroelectric property and magnetic order of thin film.The activation energy of the thin film is estimated to be 0.489 eV,which is related to the electric potential barrier over which the electrons jump.