Studied effect of oxygen defects on spin tunneling (TMR) in magnetic tunnel junctions

1. Spin transport studies in band and interface tailored materials: towards total spin polarization for spin electronics Jagadeesh Moodera, Massachusetts Institute of Technology, DMR 0504158 i) Observed a large spin decay length in an amorphous organic semiconductor (OS) leading to its potential as a spin transporter and logic media towards spintronics [Phys. Rev. Lett. 100 (June 2008)] ii) Disturbance of Tunnel Coherence by Oxygen Vacancy[Phys. Rev. Lett. 101 (July 2008)]:The large TMR in magnetic tunnel junctions (MTJ) with MgO barrier is critically dependent on the coherent spin tunneling, which is extremely sensitive to the defects inside the barrier. This is investigated and the causes for decoherence is elucidated in this work. This is important from fundamental as well as application viewpoint. The direct observation of slow decay of spin polarization in OS Rubrene is shown above Experimental result on the defect influence in spin tunneling is shown on the left for cleaner and defective MgO, whereas the model in the middle explains the result. Sample is Fe/MgO/Fe • Studied effect of oxygen defects on spin tunneling (TMR) in magnetic tunnel junctions • - Discovered that O vacancy leads to symmetry scatterings, decreased TMR • - Modelled that oxygen defects (more at RT than 180°C) also create a defect level in the barrier, thus opens tunneling path that mediate non-coherent tunneling (lost symmetry, thus lower TMR) • Studied spin transport through organic semiconductors and directly showed for the first time that spins can travel large distances without loosing spin memory even in amorphous materials: important both from fundamental science and for molecular electronics development.

2. iii) Modulating Superconductivity with Spin Polarized Current :Jagadeesh Moodera, MIT, DMR 0504158 Spin polarized (SP) currents have profound effect on a superconductor. A BCS type superconductor (SC) forms Cooper pairs with opposite momentum and spin. Injecting SP current into a SC results in the suppression of superconductivity and also a huge magnetoresistance (MR). This hypothesis was tested inaSC spin valve effect in bcc-Fe/V/Fe epitaxial trilayer planar structures. This is seen in the data below showing infinite MR, going from spin parallel to spin antiparallel state. The effect of band symmetry at the bcc-Fe/V interface is shown for the first time.(submitted to Phys. Rev. Lett.). Far left: Resistance vs applied field showing the effect of spin current on a superconductor and infinite resistance change. Left: Tunnel junction resistance (R) vs temp. showing the huge drop in R below the magnetic ordering temperature of EuO, showing large exchange splitting in ultrathin spin filter EuO films useful for spin injection into a semiconductor. • (iiv)Spin filter effect: Magnetic semiconductors such as EuS and EuO in ultra thin film form as tunnel barriers show excellent spin filter properties. Right figure shows a novel method to determine exchange splitting in EuO spin filter barrier by tunneling. In these ultra thin layers near perfect spin polarization of tunnel current has been achieved - an important step towards efficient spin injection into semiconductors (submitted to Phys. Rev. Lett.) • Education, Outreach and Publications:One undergrad and two HS students participated in summer research. Three undergrad students from Europe visited to carry out collaborative/diploma thesis experiment. (Three among them are female students). Two visiting students are expected from Europe in Sept 2008. The female PhD student joined Argonne National Lab as a distinguished postdoctoral and another female postdoc joined LG Co. • - 12 international conf. presentations, university colloquia and invited talks. Published three articles (2 in PRL and one in JAP. Three more submitted to PRL, Nature Mater. Our 1995 PRL article was the fourth most cited paper. Three patent applications under preparation. • - Collaboration: J. Borchers (NIST), Y. Idezerda (Brookhaven Nat. Lab/ Montana State U), M. Baldo and C. Ross (MIT), M. Muenzenberg and G. Reiss (Germany)