Co-relation between organic spin-valves and magnetoresistance in organic diodes (OMAR)

Himadri S. Majumdar 1Sayani Majumdar 1,2R Osterbacka 1

1. Abo Akademi University, Department of Physics, Porthansgatan 3, Turku 20500, Finland
2. Wihuri Physical Laboratory, University of Turku (UTU), Vesilinnantie 5, Turku 20014, Finland

Abstract
Study of spin-injection and -transport in pi-conjugated organics is a relatively new and less explored territory. It provides a whole new scope of physics along with immense potential for application. Major challenges for spintronics, namely optimization of spin lifetimes across relevant length scales and spin-transport through hetero-interfaces are of importance for organic spintronics as well. In this respect, organic semiconductors (OS) are envisioned to be better than their inorganic counterparts. OS comprise of mainly light molecules like carbon, hydrogen etc and possess low spin-orbit interaction and hyperfine interaction leading to longer spin-correlation length. Polymers are especially the better choice as their conjugation lead to better transport properties than non-crystalline small molecules and oligomers in films made with spin-coating and/or various printing methods. The ability to manipulate electron spin in organic molecules offers an alternative route to conventional electronics, both from fundamental and technological points of view.

 Magnetic field effects on OS were demonstrated already in the early ‘90s. However, demonstration of successful spin injection and transport in lateral [1] and vertical [2] organic spin-valves, with inorganic metals and half-metals as the ferromagnetic (FM) electrodes and OS or oligomers as spacers, caused resurgence of interest. Successful demonstration of efficient spin injection and transport in organic spin–valve structures at room temperature has been done [3] even though the magnitude of response is small (~ 1-2%). Room temperature magnetoresistance in organic diodes (OMAR) have also been demonstrated [4]. The magnitude of OMAR at room temperature is considerably higher (~20%) than the spin-valves and have potential for immediate application.

 In our presentation we will co-relate the OMAR and the spin-valve effect in light of the new experimental results on the magnetic behaviour of the ITO electrode and ferromagnetism in OS. Contrary to popular beliefs the ITO electrode and OS used in the diodes were found to have ferromagnetic signatures. We will discuss about these results especially in context of “d0 ferromagnetism” observed in oxide semiconductors. We will further discuss whether the OMAR effect is the namesake of spin-valve operation or if it is a completely different physical phenomenon.

 *Corresponding author: himadri.majumdar@abo.fi

 References:

[1]      V. Dediu et. al., Solid State Communication 122, 181 (2002).

[2]      Z. H. Xiong et. al., Nature (London) 427, 821 (2004).

[3]      S. Majumdar et. al., Appl. Phys. Lett 89, 122114 (2006).

[4]      T. L. Francis et. al., New J. Phys. 6, 185 (2004).

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Presentation: Oral at E-MRS Fall Meeting 2009, Symposium E, by Himadri S. Majumdar
See On-line Journal of E-MRS Fall Meeting 2009

Submitted: 2009-05-11 12:53
Revised:   2009-06-07 00:48
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