Study of Multiferroic Tunnel Junctions

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Technical AbstractThis award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5). Multiferroic tunnel junction refers to a ferromagnetic tunnel junction using a ferroelectric insulator barrier. Theoretical calculations have shown that the ferroelectric polarization reversal will result in resistance switching behavior in a tunnel junction. Furthermore, the interplay between the ferroelectric and ferromagnetic layers can affect the electric polarization of the barrier, the electronic structures and the magnetic properties at the interface, and the spin polarization of the tunneling current. This type of junctions can thus display new functionalities, such as quaternary or octal logic state, which can be encoded by both magnetic and electric fields. It can also open up new ways to potentially control spin polarization electrically. However, to preserve ferroelectricity in a nanometer thick ferroelectric layer sandwiched between two ferromagnetic layers is extremely challenging and reliable multiferroic junctions have not been achieved previously. This individual investigator award supports a research plan to fabricate and systematically study the multiferroic tunnel junctions in manganite/Ba1-xSrxTiO3/manganite and manganite/Ba1-xSrxTiO3/Co structures. The objective of the project is to first establish the behaviors of such junctions under electric and magnetic fields, and then to systematically study the manipulation of the different states by both electric and magnetic fields. The project will provide both graduate and undergraduate students, including women and underrepresented minorities, with a stimulating environment for interdisciplinary experience in physics, materials sciences, and device fabrication and testing. Non- Technical AbstractThis award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5). Recent advances in magnetic tunnel junctions have made it possible to use these devices in computer read head, magnetic sensors, and non-volatile magnetic random access memories. A tunnel junction consists of two metal layers and a nanometer thick insulator layer or barrier in between. It was predicted theoretically that by using a very thin ferroelectric insulator--a material with a spontaneous charge polarization which can be switched by an electrical voltage--as the barrier, new functionalities can be achieved. For example, it can display quaternary logic states which can be encoded by both magnetic and electric fields, instead of the binary states in the current magnetic tunnel junctions. This could be very important for quantum information processing. The strong interaction between the ferromagnetic layer and the ferroelectric layer can also lead to control of the spin polarization electrically, which is a critical advance in achieving functional spintronic devices, a new device concept that relies on the spin polarized transport instead of the conventional electron transport. However, to achieve charge polarization in a nanometer thick material is extremely challenging and such junctions have not been made previously. This award supports a research plan to fabricate and systematically study several such junction structures using manganite as the electrode and Barium Strontium titanate as an insulator. The goal of the project is to first fabricate the junctions, and then to systematically study the junction properties and how to manipulate the different states by using electrical and magnetic fields. The project will provide both graduate and undergraduate students, including women and underrepresented minorities, with a stimulating environment for interdisciplinary experience in physics, materials sciences, and device fabrication and testing.