Induced magnetoelectric coupling at a ferroelectric-ferromagnetic interface
Abstract
Preparation and characterization of multiferroic materials in which ferroelectricity and ferromagnetism coexist would be a milestone for functionalized materials and devices. First, electric properties of polyvinylidene (PVDF) films fabricated using the Langmuir-Schaefer method have been studied. Films of different thickness were deposited on silicon substrates and analyzed using several techniques. X-ray diffraction (XRD) data showed that PVDF films crystallize at an annealing temperature above 130 °C. Polarization versus electric field (PE) ferroelectric measurements were done for samples prepared with electrodes. PE measurements show that the coercivity of the films increases as the maximum applied electric field increases. The coercivity dependence on the frequency of the applied electric field can be fitted as f 0.6. The results also show that the coercivity decreases with increasing the thickness of PVDF film due to the pinning effect. Next, we have demonstrated that those PVDF properties can be controlled by applying an external magnetic field. Samples were created in a layered heterostructure, starting with a Fe thin film, PVDF above that, and followed by another thin film of Fe. Extended X-ray absorption fine structure (EXAFS) spectroscopy was used to study the interface between PVDF polymer films and ferromagnetic iron thin films. Conventional EXAFS was applied to identify the structure of a Fe film sandwiched between two PVDF layers. An electric signal was then applied to the polymer to study the effects polarizing the polymer has on the Fe atoms at the interface. This shows that the Fe atoms diffuse into the PVDF layer at the interface between the two layers. Polarizing the film causes further diffusion of Fe atoms into the polymer. We also found that as the applied magnetic field is changed, the switching of electric polarization for the PVDF displayed a dependence on the external magnetic field. We also noticed that both the coercivity and polarization for the PVDF polymer display hysteretic features as the applied magnetic field is changed. We also found that the thickness of both the iron layers and the PVDF layer has an effect on the magnetoelectric coupling in our samples. The same strain applied to a thicker PVDF layer becomes tougher to flip the polarization compared to a thinner PVDF layer. As the iron film thickness increases, the strain also increases, and the polarization of the PVDF polymer is more easily flipped. We also found that the magnetoelectric sensitivity increases as both the PVDF and iron layers increase in thickness. We have shown that it is possible to control the ferroelectric properties of a PVDF film by tuning the magnetic field in a heterostructure. Our experiments show a coupling between the electric polarization and applied magnetic field in multiferroic heterostructures much larger than any previously reported values. Previous reports have used inorganic materials for the ferroelectric layer. Organic polymers have an electric dipole originating at the molecular level due to atoms with different electronegativity that are free to rotate. To flip the polarization, the chains must rotate and the position of the atoms must change. This increases the force felt locally by those chains. Using this polymer, we are able to increase the magnetoelectric coupling.
Degree
Ph.D.
Advisors
Cheng, Purdue University.
Subject Area
Electromagnetics|Condensed matter physics
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