Papers

AVS2008 Session MI-WeA: New Directions in Spintronics

Wednesday, October 22, 2008 1:40 PM in Room 206

Wednesday Afternoon

Start	Invited?	Item
1:40 PM		MI-WeA-1 Spin Tunneling and Transport through Organic Semiconductors - Towards Large Spin Relaxation Length J. Moodera (Massachusetts Institute of Technology) The emerging field of organic spintronics is merging the two hot fields - organic electronics and spintronics. Chemical tunability of electrical properties in organic semiconductors (OS) with a bottom-up approach, along with the mechanical flexibility and low-cost fabrication processes has given rise to organic-electronic devices, such as light-emitting diodes (OLED) and field effect transistors (OFET). From the spintronics viewpoint, of growing interest is the potential to transport and manipulate spin information in OSs. Spin-orbit and hyperfine interactions, the main cause of spin-decoherence, being weak in OSs, suggest a large λ_s in these materials. Electron spin polarized tunneling is explored with ultrathin layers of the molecular organic semiconductor tris(8-hydroxyquinolinato)aluminum (Alq3) and Rubrene (C₄₂H₂₈). Significant tunnel magnetoresistance (TMR) was measured in magnetic tunnel junctions at room temperature, which increased when cooled to low temperatures. Spin polarization of the tunnel current through these OS layers directly measured using superconducting Al as the spin detector, shows that minimizing formation of an interfacial dipole layer between the metal electrode and organic barrier significantly enhanced elastic tunneling characteristics and greatly improves the spin transport. For example, directly measured spin diffusion length (λ_s) in amorphous rubrene by spin polarized tunneling is large in comparison to amorphous Si or Ge, where no spin-conserved transport has been reported. These results will be discussed. Based on our findings, λ_s in single crystalline OS can be expected to reach even millimeters, showing the potential for organic spintronics development. Work done in collaboration with Tiffany Santos, Jenny Shim, Karthik V. Raman and supported by KIST-MIT project fund and ONR grant.
2:20 PM		MI-WeA-3 Magnetism and Magnetoresistance in Multilayer Thin Film Rings C.A. Ross (Massachusetts Institute of Technology) Magnetic data storage devices, including magnetic random access memories and patterned media, are based on thin film magnetic nanostructures. Magnetic multilayer thin film rings present a particularly interesting geometry, and their rich behavior offers opportunities for development of multibit magnetic memories and programmable, non volatile logic devices. A single layer magnetic ring can adopt a variety of stable and metastable magnetic states characterized by different numbers of domain walls, and the behavior of a multilayer ring is further complicated by magnetostatic and exchange interactions between the individual magnetic layers. In this study, rings with nanoscale to micron scale dimensions are made using electron beam lithography and self-assembled block copolymer lithography. We will describe the behavior of single layer, multilayer and exchange-biased magnetic rings, including control of the chirality of the magnetization direction, and magnetotransport measurements made on electrically contacted rings that show large relative changes in resistance, and we will discuss how these structures may be used in multibit memory cells and logic devices.
4:00 PM		MI-WeA-8 Electrical Spin Injection into Silicon: A Comparison between Fe/Schottky and Fe/Al₂O₃ Tunnel Contacts G. Kioseoglou, A.T. Hanbicki, C.H. Li, P.E. Thompson, O.M.J. van 't Erve, M. Holub, C. Awo-Affouda, R. Goswami, G. Spanos, B.T. Jonker (Naval Research Laboratory) Electrical spin injection is a prerequisite for a semiconductor spintronics technology. While significant progress has been realized in GaAs, little has been made in Si, despite its overwhelming dominance of the semiconductor industry. Si is an ideal host for spin-based functionality due to its smaller spin orbit than GaAs (responsible for spin relaxation) and to its long spin lifetimes (microseconds). Recently¹ we have reported successful injection of spin-polarized electrons from an Fe film through an Al₂ O₃ tunnel barrier into Si (001). The circular polarization of the electroluminescence (EL) resulting from radiative recombination in Si and in GaAs (in Si/AlGaAs/GaAs structures) tracks the Fe magnetization, confirming that these spin polarized electrons originate from the Fe contact. The polarization reflects Fe majority spin. We determined a lower bound for the Si electron spin polarization of ~30% at 5K, with significant polarization extending to at least 125K. Here we compare electrical spin injection from Fe into MBE grown Si n-i-p heterostructures using different tunnel barriers- a reversed biased Fe/Si Schottky contact and an Fe/Al₂ O₃ barrier. For both types of structures the EL spectra are dominated by transverse acoustic and optical phonon emissions in the Si and the circular polarization of the EL due to radiative recombination in the Si tracks the Fe out of plane magnetization. However, the polarization is almost 50% lower for the Fe/Si contact than that of the Fe/Al₂ O₃/Si system. This could be due to different interface structure or it may result from changes in the transport mechanism involved. Systematic TEM analysis has been performed to correlate the interface structure with the observed optical polarization, and reveals some Fe/Si intermixing that is absent in the Fe/ Al₂ O₃/Si structure. While the zero bias resistance for the Fe/ Al₂ O₃/Si system shows very weak temperature dependence, the resistance for the Fe/Si system increases orders of magnitude with decreasing temperature. This implies that two different transport mechanisms may be responsible for the spin injection. ¹B.T. Jonker, G. Kioseoglou, A.T. Hanbicki, C.H. Li, and P.E. Thompson, Nature Physics 3, 542 (2007). This work was supported by ONR and core programs at NRL.
4:20 PM		MI-WeA-9 Current Perpendicular to Plane Giant Magnetoresistance in Magnetic Multilayers* W.P. Pratt, Jr. (Michigan State University) Giant magnetoresistance (GMR) in magnetic multilayers, consisting of alternating ferromagnetic and non-magnetic (F/N) layers, is now a major field of study in metallic magnetic materials both for fundamental physics and important sensor applications, especially read heads in computer hard drives. Until recently, applications of GMR mostly used Current-In-Plane (CIP) geometry. However, the Current-Perpendicular-to-Plane (CPP) GMR can be larger, and the CPP geometry has certain fabrication advantages. Indeed, CPP tunneling-MR read heads are now in computers, and CPP-GMR in metallic multilayers is competing for next-generation read heads. There is also great theoretical and experimental interest in the inverse phenomenon to CPP-GMR, where a high-density (~10⁷ A/cm²) spin-polarized CPP current exerts a large enough torque on a given nano-size F-layer to cause its magnetization to precess and then switch. Such current-induced magnetization switching (CIMS) has potential applications in magnetic random access memories. Progress in this field is tied to understanding the spin-polarized transport parameters of existing and new materials. The CPP-GMR usually gives more direct access to these fundamental parameters: F/N interface resistances, asymmetries of conduction electron scattering in the bulk of F-layers and at F/N interfaces, and the length scales for electron spin-memory loss due to spin-flip scattering. After a brief review of the CPP-GMR and CIMS phenomena, I will present examples of important CPP-transport parameters that we have quantified for a wide variety of F and N metals. I will then illustrate applications of this knowledge of the CPP parameters to CIMS in F/N/F trilayer structures. *Work supported by US National Science Foundation, the MSU Keck Microfabrication Facility and Seagate Technology.