Physics Colloquium, Febraury 21, 2012
Multifunctional Electronics: Probing the fundamental coupling between charge, structure, spin and optical degrees of freedom in electronic materials Ezekiel Johnston-Halperin

The Ohio State University

The success of solid state electronics over the past sixty years has both driven and been driven by an increasingly sophisticated understanding of the electronic properties of condensed matter systems. Modern electronic devices exploit a deep knowledge of charge transport in semiconducting, metallic and organic materials and their various heterostructures. As the size of these devices approaches atomic and molecular length scales it has become necessary to extend these models from the semiclassical to the fully quantum regime; however even this level of sophistication is approaching fundamental limits when considered within traditional device design and circuit architectures. These constraints have generated renewed interest in the exploration of additional state variables for computing, and consequent focus on the fundamental coupling between charge, structure, spin and optical degrees of freedom in electronic materials. A principle aim of our research is to expand this understanding through the investigation of multifunctional behavior in magnetic materials and heterostructures. In this talk I will present two specific examples of this approach: strain-induced multiferroicity in EuTiO3 and organic(ferromagnetic)/inorganic(nonmagnetic) semiconducting heterostructures. In the first example the theoretical prediction [1] of a novel spin-phonon coupling approach to achieving multiferroic (ferromagnetic/ferroelectric) behavior is realized in strained EuTiO3/DyScO3 heterostructures, yielding a multiferroic material with both strong ferromagnetism (~ 4 ?B/Eu) and strong ferroelectric moment (~ 10 ?C/cm2) [2]. In the second example we employ the organic-based ferromagnet V[TCNE]x (TCNE: tetracyanoethelyne; x~2; TC > 400 K) to inject spin currents into an n-p AlGaAs light emitting diode (LED) [3]. Our measurements demonstrate electrically driven spin injection across the organic/inorganic interface by monitoring the polarization state of the photons emitted from the LED structure (the optical selection rules in this structure couple optical polarization and electron spin polarization). In contrast to previous hybrid structures, these measurements employ the organic layer as the source of spin polarization, setting the stage for the first room temperature all-semiconductor spintronic devices as well as added chemical functionality (photosensitivity, chemical sensing, etc.) via the organic layer. [1] C.J. Fennie and K. M. Rabe, “Magnetic and electric phase control in epitaxial EuTiO3 from first principles”, Phys. Rev. Lett. 97, 267602 (2006). [2] J. H. Lee, L. Fang, E. Vlahos, X. L. Ke, Y. W. Jung, L. F. Kourkoutis, J. W. Kim, P. J. Ryan, T. Heeg, M. Roeckerath, V. Goian, M. Bernhagen, R. Uecker, P. C. Hammel, K. M. Rabe, S. Kamba, J. Schubert, J. W. Freeland, D. A. Muller, C. J. Fennie, P. Schiffer, V. Gopalan, E. Johnston-Halperin and D. G. Schlom, “A strong ferroelectric ferromagnet created by means of spin-lattice coupling,” Nature (2010), DOI: 0.1038/nature09331. [3] L. Fang, K. D. Bozdag, C.-Y. Chen, P.A. Truitt, A. J. Epstein and E. Johnston-Halperin, “Electrical spin injection from an organic-based magnet in a hybrid organic/inorganic heterostructure”, Phys. Rev. Lett. 106, 156602 (2011).

Dr. Johnston-Halperin's Web Site

4:00 p.m., Physics Research Building (PRB), Room 1080

Reception at 3:45 p.m., Atrium, PRB