Session P32: Focus Session: Spin Dependent Physics in Organic Materials

Organic Magnetoresistance

In recent years a broad range of magnetoresistance phenomena have been reported for organic-based semiconductors, conductors and magnets. Organic systems illustrating magnetoresistance, include molecular- and polymer-based nonmagnetic semiconductors[1], organic-based spin polarized magnetic semiconductors,[2] nonmagnetic conducting polymers, and ferromagnet/organic semiconductor/ferromagnet heterojunctions. Examples of each of these organic-based systems will be presented together with a discussion of the roles of magnetotransport mechanisms including interconversion of singlets and triplets, compression of the electronic wavefunction in presence of a magnetic field, quantum interference phenomena, effects of a ``Coulomb gap'' in $\pi $* subbands of organic magnetic semiconductors with resulting near complete spin polarization in conduction and valence bands of magnetic organic semiconductors.[2,3] Opportunities for magnetotransport in Ferromagnet/Organic Semiconductor/Ferromagnet heterojunctions will be discussed.[4] \\[4pt] [1] V.N. Prigodin \textit{et al.}, Synth. Met. \textbf{156}, 757 (2006); J.D. Bergeson \textit{et al.}, Phys. Rev. Lett. \textbf{100}, 067201 (2008) \\[0pt] [2] V.N. Prigodin \textit{et al}., Adv. Mater. \textbf{14}, 1230 (2002. \\[0pt] [3] J.B. Kortright \textit{et al}., Phys. Rev. Lett., \textbf{100}, 257204 (2008). \\[0pt] [4] J.D. Bergeson, \textit{et al.}, Appl. Phys. Lett. \textbf{93}, 172505 (2008).   

Magnetic and magnetotransport properties of organic trilayers of alkanedithiol self-assembled monolayers sandwiched between ferromagnetic thin films

Magnetic and magnetotransport properties of organic spin valve structures have been studied. The organic trilayer structure consists of a self-assembled monolayer (SAM) of alkanedithiol sandwiched between two ferromagnetic metal contacts, a Ni film as the bottom contact and a Co film as the top contact. The SAM was formed using novel methods on the Ni surface on the bottom of the vertical structure. Two alternative designs have been developed, one uses an additional conducting polymer layer for electrical isolation during thermal evaporation of the top Co contact and another uses nanotransfer printing to directly apply the top Co contact. Each trilayer was examined in vacuum using 4-terminal transport measurements. Both designs have indicated tunneling as the transport mechanism between contacts. Magnetooptic Kerr Effect (MOKE) measurements show independent switching of the ferromagnetic layers at approximately 50 and 100 Gauss. Magnetotransport measurements were carried out as a function of bias voltage, temperature and field, in order to explore spin-dependent transport through the organic interlayer.   

Effects of interface microstructure on magnetotransport in organic spin valve structures

Organic semiconductors hold promise for spintronics because of their potentially long spin diffusion length. We have studied Fe/Alq$_3$ (tris(8-hydroxyquinoline) aluminum)/Co multilayer films with Alq$_3$ thickness in the range 50 to 150 nm.[1] Similar to previously reported results, we found considerable variability in the magnetotransport properties for cross junctions made in nominally identical conditions. To explore the sources of these effects, we studied the microstructure of such multilayer films by X-ray reflectometry and polarized neutron reflectometry (PNR). We found that the films show well-defined layers with limited chemical intermixing (3-5 nm) at the Alq$_3$/ferromagnet (FM) interfaces. However, larger magnetoresistance (MR) is associated with sharper Alq$_3$/FM interfaces, and with a magnetically dead Fe-rich region at the Alq$_3$/Fe interface, which may potentially circumvent the resistivity mismatch problem. The PNR data also show that the Co layer on top of the Alq$_3$ can adopt a multi-domain magnetic structure at low field and a perfect anti-parallel state is not obtained. [1] Y. Liu et al., arXiv:0810.0289v1.   

Enhanced Magnetoresistance in Alq3-based spin valve using buffer-layer assisted growth

In the field of organic spintronics, interfacial diffusion between magnetic electrodes and organic spacer layers is a serious problem for both understanding the underlying mechanism and achieving high magnetoresistance. Using buffer layer assist growth, we have successfully fabricated vertical organic spin valves with much sharper interface. Spin valves prepared by this method exhibit considerably larger magnetoresistance. The spacer layer thickness-dependent magnetoresistance suggests that field-dependent interfacial barrier plays the crucial role for the observed magnetoresistance.   

Spin and Charge Injection and Transport in Ferromagnet/Organic Semiconductor/Ferromagnet Heterejunction

We studied the spin injection and transport in organic semiconductor by employing LSMO and Fe as an anode and cathode in hybrid spin valve structure. Using thin layer ($t$ $<$ 10 nm) of rubrene as a spacer, our device displays MR as high as 50 $\% $ at low temperature and at low bias voltage. The charge injection into organic spacer in our devices is injection limited. At high applied voltage ($V$ $>$ $V_{th}$) field-driven drift current prevails in current density through the organic semiconductor. At low bias $V$, inelastic hopping followed by thermionic emission is dominant at high $T$, which decreases significantly as $T$ lowered. And eventually the current density through the device becomes purely tunneling at low $T$ and $V_b$. The spin and charge injection, effects of inclusion of tunneling barrier, and the effects of crystallinity of organic layer will be discussed.   

Spin filtering of Photo-excited charge from Organic Nanostructures

In organic materials, coupling between the incident photon and the electron spin is very weak. Here we demonstrate that spin filtering materials can be used to induce intersystem crossing, and allow the spin polarized triplet excitonic states to be probed. A thin layer of EuS was deposited at the interface formed between a single-wall nanotube and an aluminum contact. EuS is a ``spin filtering material,'' a ferromagnetic insulator with large spin-orbit coupling, allowing preferential tunneling by electrons of a preferred spin direction. A small magnetic field is applied to align the electron spin in the EuS with the carbon nanotube. The enhanced spin-orbit interaction allows for observation of a low-energy peak in the capacitive photocurrent scan. The energy spacing between the two peaks matches closely with the theoretical predictions for the S$_{1}$--T$_{1}$ spacing in nanotubes. Further measurements of the triplet peak reveal that its magnitude depends on the orientation of the B-field with maximum peak height occurring when the spins in EuS line up with the nanotube axis. These measurements suggest that introduction of a spin filtering layer could be used to study the triplet formation in organic solar cell materials.   

Spin and charge transport study in single crystal organic semiconductors

Spin transport studies in amorphous rubrene films have shown exciting and promising results [1]. A large spin diffusion length in these amorphous films has increased the motivation to perform spin transport study in high purity single crystal rubrene. This will provide the fundamental understanding on the spin transport behavior in OS; not influenced by defects or traps. We will present work on small channel single crystal rubrene FET device with magnetic electrodes. For example, our preliminary studies have show mobility for FET with Co electrode to be 0.014cm$^{2}$/V-s. A study on the spin and charge transport properties in single crystals of OS with magnetic electrodes is being done and the results will be reported. The influence of gate voltage and applied magnetic field on the transport properties will be discussed. [1] J.H. Shim et al., PRL \textbf{100}, 226603 (2008)   

Inter-Molecular Spin-Orbital Coupling Effects on Magnetoresistance and Spin-Dependent Excited Processes in Organic Semiconductors

A low magnetic field can change electrical current and electroluminescence in organic semiconductors, leading to magnetoresistance and magnetic field effects due to magnetic field-dependent singlet/triplet ratio involved in charge transport and excited states. In general, an external magnetic field can change singlet and triplet ratios through two major pathways: spin-dependent electron-hole pairing and field-dependent intersystem crossing. We found that tuning inter-molecular spin-orbital coupling leads to a significant change in magnetoresistance, electro-fluorescence, and electro-phosphorescence. These experimental findings indicate that (i) inter-molecular and intra-molecular electron-hole pairs account for magnetoresistance and magnetic field effects, respectively, (ii) spin mixing occurs in inter-molecular excited states, and (iii) spin-mixing is a function of both spin-orbital coupling and singlet-triplet energy difference. This presentation will discuss the effects of magnetic field on both spin-dependent electron-hole pairing and spin mixing in magnetoresistance and magnetic field effects in organic semiconductors.   

Magnetoresistance in the High Magnetic Field Regime for Organic Semiconductors

While there has been much study of the low field (of order 100 Oe) magnetoresistance in organic semiconductor (OSC) materials, very little has been done in high fields of order 10kOe. Magnetoresistance studies in the high field were conducted on the OSC materials tris-(8-hydroxyquinoline) aluminum (Alq$_{3})$ and alpha sexithiophene ($\alpha $-6T). The high field shows a different response from that of the low field and displays several features suggesting that multiple mechanisms are at work. In addition, the two materials demonstrate behaviors that differ from one another, suggesting different classes of OSC. The experimental results are discussed in the context of the MIST model [1], which attributes magnetoresistance to the recombination of electron-hole pairs with interconversion of singlets and triplets. \\[0pt] [1] J.D. Bergeson, \textit{et al.}, PRL \textbf{100}, 067201 (2008).   

Iron Nanoparticle Driven Spin-valve Behavior in Aligned Carbon Nanotube Arrays*

Spin-valve structures have been constructed from aligned arrays of carbon nanotubes, yielding a magnetoresistance reaching 25 {\%}$^{1}$. In addition to including vertically aligned carbon nanotube arrays, iron catalyst nanoparticles that form the array function as the second ferromagnetic electrode. Reversal of the magnetization of the electrode in an applied magnetic field results in a clear peak in the resistance of the device. A spin scattering length in excess of 9 $\mu $m shows excellent spin transport through the nanotube array. The effect of oxide barriers and device patternability are explored. 1. Bergeson, et al., Appl. Phys. Lett. \textbf{93}, 172505 (2008) *This work is supported in part by DOE Grant Nos. DE-FG02-86ER45271 and DE-FG02-01ER45931, and AFOSR Grant No. FA9550-06-1-0175 and FA9550-06-1-0384, NSF Grant No. CMS-0609077, and IMR Grant Nos. FG0004 and FG 0036. The support of the Materials and Manufacturing Directorate of the Air Force Research Laboratory is gratefully acknowledged.   

Electrically controlled $g$-factor and magnetism in conjugated metallorganic molecules

Conjugated metallorganic molecules have localized spins at the central transition-metal ions and mobile $\pi$-electrons in the surrounding ligands. Here we construct model Hamiltonians based on first-principles calculations to describe spins at the ions and $\pi$-electrons in the ligands. It is shown that the $g$-factor and magnetic susceptibility in such a molecule can be tuned to a great extent by an electrical voltage across one of the ligands. The underlying physics is that the voltage modifies the charge distribution of the ligand, which in turn changes the interplay of the the ion's spin-orbit coupling and the energy splitting among its $d$ orbitals. The capability of controlling the $g$-factor and magnetism at the molecular level has great implications in quantum information storage and processing.   

Magnetic-Field-Driven Ising Quantum Criticality of Two-Dimensional Square-Lattice Antiferromagnet Cr(dien)(O$_{2})_{2}$.H$_{2}$O

We report on a systematic study of magnetically driven quantum phase transition in a new compound based on Cr(IV). The compound, Cr(dien)(O$_{2})_{2}$.H$_{2}$O, is a low dimensional antiferromagnet with a Neel temperature T$_{N}$ of 2.55 K in zero field. We have used torque magnetometry, heat capacity and magnetocaloric-effect measurements down to 200 mK, to obtain a complete magnetic phase diagram. A detailed analysis of the dependence of T$_{N}$ on magnetic field using the power law T$_{N}$ $\sim $ (H$_{c}$-H)$^{\alpha }$ yielded the critical exponent $\alpha $ = 2.01$\pm $0.02, with H$_{c}$ = 12.392$\pm $0.003 T, indicating that this system behaves like a 3-d Ising magnet at low temperatures.