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Oxide Heterostructures for Memory Applications, Dr. Stuart Wolf

Abstract
In this talk I will describe the efforts at the University of Virginia to develop a new toolkit of oxide materials for memory applications including both phase change and spin based approaches. We have been exploring novel compounds such as pure and doped vanadium oxide which has a hysteretic metal to insulator phase transition above room temperature that can be driven both thermally and electrically and in a novel nearly amorphous form exhibits a highly non-linear current voltage characteristic that we believe can also be made hysteretic. We are also exploring tunneling structures that include ruthenium and vanadium doped chrome oxide. Chrome oxide is a very good metal that exhibits a ferromagnetic phase transition above room temperature and is a strong candidate for a novel spin based memory called spin transfer torque random access memory (STTRAM). Finally we have prepared epitaxial films of bismuth iron oxide, barium strontium titanate, lanthanum strontium manganate, lanthanum barium manganate, and Fe3O4 to explore both naturally occurring multiferroics (BiFeO3) or metamaterial based multiferroics (the rest). These materials offer the potential of electrically controlled magnetism that can be used in various information storage paradigms.

Bio
Prof. Stu Wolf is currently the Director of the University of Virginia Institute for Nanoscale and Quantum Scientific and Technological Advanced Research (nanoSTAR) and also serves as a professor in the Materials Science and Engineering Department as well as the Physics Department. Stu Wolf was previously a Program Manager at DARPA and a Senior Scientist at the Naval Research Laboratory. At DARPA he conceived and initiated several projects on functional materials that pushed the frontiers of materials science for electronics and he both started the Spintronics program (and invented the word "spintronics") to develop Magnetic Random Access Memory (MRAM). At the University of Virginia he is continuing to push the frontiers in spintronics and quantum information science. His group utilizes the spin degree of freedom in novel oxide heterostructures using spin torque to manipulate the magnetism in nanomagnetic structures. He is also developing a new spintronic logic based on controlling the exchange in magnetic quantum dots. He has an AB from Columbia College (1964) and an MS (1966) and PhD (1969) from Rutgers University. He was a Research Associate at Case Western Reserve University (1970-73) and a Visiting Scholar at UCLA (1981-82). He is a Fellow of the APS (1984), and was a Divisional Councilor for the Condensed Matter Division (1990-91) and for the Forum on Industrial and Applied Physics. He has authored or co-authored two books over 300 articles and has edited numerous conference proceedings.

Multiferroics
Multiferroics are materials that have coupled electric, magnetic and structural order parameters that result in simultaneous ferroic properties (i.e. ferroelectric, ferromagnetic, or ferroelastic). Some materials having not ferromagnetic but antiferromagnetic, ferrimagnetic, ferrotoroidic, or helimagnetic ordering are accepted as multiferroics as well.

Only a few single phase multiferroic materials exist. This is, amongst other reasons, because the classical ferroelectric perovskites (BaTiO3, PZT, etc) contain d ions with empty shells (e.g. Ti4+ is 3d0) and thus bear no magnetic moment. Exceptions include some orthorhombic manganites, like TbMnO3, and Bi-based perosvkites like BiFeO3 or BiMnO3. In addition, most multiferroics are antiferromagnetic or weak-ferromagnets, BiMnO3 being one of the very few ferromagnetic and ferroelectric multiferroics. Some multiferroics, like TbMnO3, exhibit the magnetoelectric effect. The scientific community has a large interest in this subgroup of multiferroic materials, because promising phenomena exist in these materials for technical applications.

Bismuth ferrite
Bismuth ferrite (BiFeO3) is an inorganic chemical compound. It is one of the most promising lead-free piezoelectricity by exhibiting multiferroic properties at room temperature and has a perovskite structure. Multiferroic materials exhibit ferroelectric or anti-ferroelectric properties in combination with ferromagnetic (or anti-ferromagnetic) properties in the same phase. As a result, an electric field can induce change in magnetization and an external magnetic field can induce electric polarization. This phenomenon is known as the magnetoelectric effect (ME) effect and materials exhibiting this effect are called magnetoelectrics or seignetto magnets. Further proof of it being ferromagnetic is that it produces a hysteresis loop during ferroelectric characterization. The ability to couple to either the electric or the magnetic polarisation allows an additional degree of freedom in device designs.

However one of the major drawbacks of the material is its high current leakage. Therefore it allows current to pass through when a high voltage is applied. Attempts to improve the electrical properties have been made by doping it with rare earth elements such as lanthanum (La), samarium (Sm), gadolinium (Gd), terbium (Tb) and dysprosium (Dy) etc. The dopant can be at the A site or the B site. A site being the edges of the perovskite cell and the B site being the centre of the perovskite cell.