Session R2: Invited Session: New Developments in Organic Spintronics

Electrically detected magnetic resonance in organic light emitting diodes

Due to the built-in weak spin-orbit coupling of carbon based materials, electronic transitions in organic semiconductors are subjected to strong spin-selection rules that are responsible for a number of interesting electron spin- and even nuclear spin-dependent electrical and optical properties of these materials, including device efficiencies of organic light emitting diodes and solar cells or magnetoresistive and magneto-optic effects. In recent years, we have studied how these effects work and how they can be utilized for organic semiconductor device improvement and new device applications. Our focus has been in particular on the effects of spin on $\pi$-conjugated polymer based bipolar injection devices (more commonly known as organic light emitting diodes, OLEDs). In OLEDs, spin-interactions between recombining charge carriers do not only control electroluminescence rates but also the magnetoresistance. We have shown that spin-coherence can be observed through current measurements [1] and that these effects can be utilized for a coherent, pulsed electrically detected magnetic resonance spectroscopy (pEDMR) which enables us to encode the qualitative nature of spin-dependent mechanisms (the polaron pair mechanism [2,3] and the triplet polaron recombination [4]) and the their dynamical nature (spin-relaxation, electronic relaxation, hopping times [5]). The insights gained from these studies have led to the invention of a robust absolute magnetic field sensor based on organic thin film materials with absolute sensitivities of $<$50nT/Hz$^{1/2}$ [6].\\[4pt] [1] D. R. McCamey, et al., Nature Materials, 7, 723 (2008).\\[0pt] [2] D. R. McCamey, et al., Phys. Rev. Lett. 104, 017601 (2010).\\[0pt] [3] S.-Y. Lee, et al., J. Am. Chem. Soc. 133, 072019 (2011).\\[0pt] [4] Baker et al., Phys. Rev. B 84, 165205 (2011).\\[0pt] [5] Baker et al., Phys. Rev. Lett. 108, 267601 (2012). [6] Baker et al., Nature Communications 3, 898 (2012).   

Direct measurements of spin propagation in organic spin valves by low-energy muon spin rotation

Organic semiconductors fall into a class of materials that shows significant potential for future applications, but many of the fundamental mechanisms of spin relaxation and transport are not understood. As a result, the field is becoming extremely topical, but there is a need for suitable techniques that can yield information on intrinsic spin dynamics and transport in organic materials. I will present Low Energy Muon Spin Rotation measurements and demonstrate that this technique can directly measure the depth resolved spin polarisation of charge carriers in organic spin injection devices [1]. I will then go on to show that it is possible to separate out the various contributions to spin decoherence, differentiating between interface and bulk effects. By correlating macroscopic measurements with these separated interfacial and bulk effects, I will present evidence that it is possible to engineer interfaces in organic spintronic devices [2]. Finally, I will present some of the latest results on how spin injection and transport depend on bias voltage [3].\\[4pt] [1] A. J. Drew et al., Nature Materials 8, 109 (2009)\\[0pt] [2] L. Schulz et al., Nature Materials 10, 39 (2011)\\[0pt] [3] L. Nuccio et al., in preparation.   

Percolative Theory of Organic Magnetoresistance and Fringe-Field Magnetoresistance

A recently-introduced percolation theory [1,2] for spin transport and magnetoresistance in organic semiconductors describes the effects of spin dynamics on hopping transport by considering changes in the effective density of hopping sites, a key quantity determining the properties of percolative transport. Increases in the spin-flip rate open up ``spin-blocked'' pathways to become viable conduction channels and hence, as the spin-flip rate changes with magnetic field, produce magnetoresistance. Features of this percolative magnetoresistance can be found analytically in several regimes, and agree with measurements of the shape and saturation of measured magnetoresistance curves [3-5]. We find that the threshold hopping distance is analogous to the branching parameter of a phenomenological two-site model [6], and that the distinction between slow and fast hopping is contingent on the threshold hopping distance. Regimes of slow and fast hopping magnetoresistance are uniquely characterized by their line shapes. Studies of magnetoresistance in known systems with controllable positional disorder would provide an additional stringent test of this theory. Extensions to this theory also describe fringe-field magnetoresistance, which is the influence of fringe magnetic fields from a nearby unsaturated magnetic electrode on the conductance of an organic film [7]. This theory agrees with several key features of the experimental fringe-field magnetoresistance, including the applied fields where the magnetoresistance reaches extrema, the applied field range of large magnetoresistance effects from the fringe fields, and the sign of the effect. \\[4pt] All work done in collaboration with N. J. Harmon, and fringe-field magnetoresistance work in collaboration also with F. Maci\`a, F. Wang, M. Wohlgenannt and A. D. Kent. This work was supported by an ARO MURI.\\[4pt] [1] N. J. Harmon and M. E. Flatt\'e, PRL 108, 186602 (2012).\\[0pt] [2] N. J. Harmon and M. E. Flatt\'e, PRB 85, 075204 (2012).\\[0pt] [3] F. L. Bloom et al, PRL 99, 257201 (2007).\\[0pt] [4] T. D. Nguyen et al., Nature Materials 9, 345 (2010)\\[0pt] [5] J. A. Gomez et al., Synth. Met. 160, 317 (2010)\\[0pt] [6] W. Wagemans et al., JAP 103, 07F303 (2008).\\[0pt] [7] F. Wang et al., PRX 2, 021013 (2012).   

Spin-polarized organic light emitting diode based on a novel bipolar spin-valve

The spin-polarized organic light emitting diode (spin-OLED) has been long sought device within the field of organic spintronics. We designed, fabricated and studied a spin-OLED with ferromagnetic (FM) electrodes that acts as a bipolar organic spin valve (OSV), based on deuterated derivative of poly(phenylene-vinylene) with small hyperfine interaction [1]. In the double-injection limit the device shows $\sim$ 1{\%} spin-valve magneto-electroluminescence (MEL) response that follows the FM electrode coercive fields, which originates from the bipolar spin-polarized space charge limited current [2]. In stark contrast to the response properties of homopolar OSV devices, the MEL response in the double-injection device is practically bias voltage independent, and its temperature dependence follows that of the FM electrode magnetization. Our findings provide a pathway for organic displays controlled by external magnetic fields. \\[4pt] [1] T. D. Nguyen, G. Hukic-Markosian, F. Wang, L. Wojcik, Xiao-Guang Li, E. Ehrenfreund, Z. V. Vardeny, ``Isotope effect in spin response of $\pi $-conjugated polymer films and devices,'' Nature Materials 9, 345-352 (2010)\\[0pt] [2] T. D. Nguyen, E. Ehrenfreund and Z. V. Vardeny, ``Spin-polarized organic light emitting diode based on a novel bipolar spin-valve,'' Science 337, 204 (2012)   

Spin-orbit coupling in organic spintronics

I will talk about spin-orbit coupling (SOC) in $\pi$-conjugated organicmaterials and its effects on spin characteristics including the spin-relaxation time, spin-diffusion length, and $g$ factor [1]. While $\pi$ electrons are responsible for low-energy electrical and optical processes in $\pi$-conjugated organic solids, $\sigma$ electrons must be explicitly included to properly describe the SOC. The SOC mixes up- and down-spin states and, in the context of spintronics, can be quantified by an admixture parameter in the electron and hole polaron states in $\pi$-conjugated organics. Molecular geometry fluctuations such as ring torsion, which are common in soft organic materials and may depend on sample preparation, are found to have a strong effect on the spin mixing. The SOC-induced spin mixing leads to spin flips as polarons hop from one molecule to another, giving rise to spin relaxation and diffusion. The spin-relaxation rate is found to be proportional to the carrier hopping rate. The spin-diffusion length depends on the spin mixing and hopping distance but is insensitive to the carrier mobility. The SOC influences the $g$ factor of the polaron state and makes it deviate from the free-electron value. The SOC strengths in common organics are quantified based on first-principles calculations and their values in tris-(8-hydroxyquinoline) aluminum (Alq$_3$) and in copper phthalocyanine (CuPc) are particularly strong, due to the orthogonal arrangement of the three ligands in the former and Cu $3d$ orbitals in the latter. The theory quantitatively explains the recent measured spin-diffusion lengths in Alq$_3$ from muon spin rotation and in CuPc from spin-polarized two-photon photoemission. \\[4pt] [1] Z. G. Yu, Phys. Rev. Lett. {\bf 106}, 106602 (2011); Phys. Rev. B {\bf 85}, 115201 (2012).