Reconfigurable Electronics Based on Multiferroics and Nanomagnetism

Schematic of the BFOCFO/SRO/PMN-PT heterostructure. Out-of-plane is perpendicular to the sample surface, and in-plane is parallel to the sample surface.

February 1, 2021

Air Force Research Laboratory, Arlington, Virginia

Multifunctional magnetoelectric materials with high exchange represent a missing “holy grail” of materials physics. To combine polarization and magnetization in the same solid is nothing short of actually controlling the fundamental nature of electromagnetism in matter. Although magneto-electricity (ME) is an intrinsic phenomenon in some natural materials at low temperature, such single-phase materials suffer from an extremely weak ME exchange.

In contrast, composites consisting of magnetostrictive and piezoelectric phases can feature dramatically larger ME coefficients. This proposed program focuses on achieving the disruptive potential of emerging multifunctional magnetoelectrics, and in so doing lay the foundations for their use as a materials platform that would benefit future AFOSR applications.

The purpose of this research is to develop new ME thin-layers. Efforts focus on heterostructures grown on piezoelectric single crystal substrates. The program objectives are to (1) optimize epitaxial ME thin layers with regards to the influence of film composition, epitaxy, deposition parameters, defects and substituents; and influence of substrate composition with proximity to the morphotropic phase boundary (MPB) and crystallographic orientation; (2) study the reconfigurable properties of these ME heterostructures and nanostructures under multiple fields for application in logic, memory, and tunable microwave applications; and (3) understand EM interactions on the nanometer scale in these two phase systems.

The anticipated outcome and impact of this effort will be realized in the development of new materials with large magnetoelectric (ME) coupling for next-generation multifunctional devices, including, multi-state (neuromorphic-like) circuits and memories, and E-field tunable microwave resonators for secure communications.

In general, self-assembled BiFeO₃-CoFe₂O₄ (BFO-CFO) thin films were deposited on (100) Pb(Mg_1/3Nb_2/3)_0.62 Ti₀.₃₈O₃ (PMN-38PT) single crystal substrates. These heterostructures were used for the study of real-time changes in the magnetization with applied DC electric field (E_DC). With increasing E_DC, a giant magnetization change was observed along the out-of-plane (easy) axis. The induced magnetization changes of the CFO nanopillars in the BFO/CFO layer were about delta M=80%. A giant converse magnetoelectric (CME) coefficient of 1.3 × 10^-7s/m was estimated from the data. By changing E_DC, multiple (N ≥ 4) unique possible values of a stable magnetization with memory were found on removal of the field.

To be specific, a SRO buffered BFO-CFO nanopillar structure was deposited on (100) PMN- 38PT substrates, e.g. BFO-CFO/SrRuO₃/PMN-38PT, as schematically shown in the accompanying figure. A 65%BFO-35%CFO composition ratio was chosen. All thin films were deposited by pulsed laser deposition.

The single crystal PMN-38PT substrates were grown by the Shanghai Institute of Ceramics Chinese Academy Sciences. The crystal structure was determined by X-ray diffraction (XRD, Philips X’Pert system) line and mesh scans. Magnetic hysteresis loops were recorded using a vibrating sample magnetometer (VSM, Lakeshore 7300 series) along both out-of-plane (perpendicular to the sample surface) and in-plane (parallel to the sample surface) directions. Atomic force (AFM) and magnetic force microscopy (MFM) images were obtained (Dimension 3100, Vecco), which were used to study the surface and magnetic domain structures.

This work was done by Dwight Viehland and Jie-Fang Li of Virginia Polytechnic Institute and State University for the Air Force Office of Scientific Research. For more information, download the Technical Support Package below. AFRL-0301