Electric Control of Magnetization in Biferroic Heterostructures with Patterned Interfaces: A Phase-Field Micromagnetic Study.

ALVANDI-TABRIZI, YOUNESS. Electric Control of Magnetization in Biferroic Heterostructures with Patterned Interfaces: A Phase-Field Micromagnetic Study (Under the direction of Dr. Justin Schwartz and Dr. Thomas Dow). Magnetoelectric coupling in biferroic composite films with ferroelectric and ferromagnetic phases enables control of dielectric polarization via magnetic field or manipulating the magnetization via electric field. The coupling of the ferroic orders is usually strain-mediated and orders-of-magnitude stronger than that in rarely existing single-phase biferroic systems. Potential for practical applications are thereby much higher. Yet, there are fundamental challenges in fully exploiting the emergent properties. In this project, a new design framework based on using patterned interfaces between the constituents is studied with the goal of enhancing electrically induced reconfigurability of the magnetic hysteresis in biferroic heterostructures. A three-dimensional continuum based micromagnetic model is developed to simulate the magnetization process and strain-mediated magnetoelectric coupling. The model employs the Landau–Lifshitz–Gilbert equation along with mechanical equilibrium and Gauss’ Law for magnetism to calculate the temporal and spatial distributions of the magnetic moments. Thus, this approach falls within the category of phase-field methods used for non-conserved systems. Finite element method is used to solve the partial differential equations in fully coupled fashion while using a different discretization method for each equation. To perform a quantitate verification of the model, the magnetization process in polycrystalline thin films with different crystallographic texturing and substrate-induced strain is computed. The obtained results are compared with a set of experimental measurements of sol-gel deposited NiFe2O4 thin films. The model demonstrate how the magnetization process is altered by adopting different microstructural orientations and the presence of substrate-induced strain. A comprehensive parametric study is then conducted to analyze the influence of material and geometric variations on the hysteretic behavior of thin films with periodically-ordered nanostructures. Material parameters are magnetostriction coefficient, magnetocrystalline anisotropy constant, and saturation magnetization. Geometric parameters are feature shape, size, spacing, and inclusion of a base layer. The results indicate that patterning could drastically change the magnetization process. Depending on the selection of material and geometric parameters, and the application of mechanical strain, remanence and coercivity may either increase or decrease. Such behavior is explained by considering magnetic anisotropy of different physical origins. Finally, the magnetization process in biferroic heterostructures of NiFe2O4 and PbTiO3 is simulated. Composite thin films with flat, stripe-patterned, and square-patterned interfaces are considered. The results show that the interfacial patterning dramatically improves the magnetoelectric coupling and hence the electrically induced reconfigurability of the magnetic properties. This is attributed to the elimination of the so-called substrate clamping effect and increase in interfacial surface area. © Copyright 2018 by Youness Alvandi-Tabrizi All Rights Reserved Electric Control of Magnetization in Biferroic Heterostructures with Patterned Interfaces: A Phase-Field Micromagnetic Study by Youness Alvandi-Tabrizi A dissertation submitted to the Graduate Faculty of North Carolina State University in partial fulfillment of the requirements for the degree of Doctor of Philosophy Mechanical Engineering Raleigh, North Carolina 2018