The new paradigm of electronics, ‘spintronics’, promises to extend the functionality of information storage and processing in conventional electronics. The principal spintronics device, the ‘spin ...valve’, consists of two magnetic layers decoupled by a spin-transporting spacer, which allows parallel (on) and antiparallel (off) alignment of the magnetizations (spins) of the two magnetic layers. The device resistance then depends on the spin alignment controlled by the external magnetic field. In pursuit of semiconductor spintronics, there have been intensive efforts devoted to develop room-temperature magnetic semiconductors and also to incorporate both inorganic semiconductors and carbon-based materials as the spin-transporting channels. Molecule/organic-based magnets, which allow chemical tuning of electronic and magnetic properties, are a promising new class of magnetic materials for future spintronic applications. Here, we report the realization of an organic-based magnet as an electron spin polarizer in the standard spintronics device geometry. A thin non-magnetic organic semiconductor layer and an epitaxial ferromagnetic oxide film were employed to form a hybrid magnetic tunnel junction. The results demonstrate the spin-polarizing nature of the organic-based magnetic semiconductor, vanadium(TCNE: tetracyanoethylene)x (x∼2; Tc∼400 K), and its function as a spin injector/detector in hybrid magnetic multilayer devices.
We report that organic semiconductors such as alpha-sexithiophene (alpha-6T) have magnetoresistance (MR) with unexpected sign changes; depending on applied voltage, temperature, and layer thickness, ...the resistance may either increase or decrease upon application of a small magnetic field (<100 mT). We propose that MR and the inversion of MR are due to the role of hyperfine interaction in a magnetic field, as illustrated by the recombination-limited regime.
We report the substantial change in the large room temperature (∼8% at 100
Oe, up to 15% at 1000
Oe) magnetoresistance of thin organic semiconductor films of tris-(8-hydroxyquinoline) aluminum (Alq
...3) upon doping with PtOEP and Ir(ppy)
3 complexes. The origin of magnetic field effects on charge transport properties of organic semiconductors until now has remained obscure. We propose a model for the anomalous magnetoresistance and its change with doping based on the charge transport in these semiconductors being electron–hole (e–h) recombination limited. The process of e–h recombination includes formation of correlated e–h pairs and the subsequent annihilation of e–h pairs with different rates for the singlet and triplet spin states. The e–h pairs may also dissociate back into free charge carriers. We suggest that a magnetic field controls spin interconversion of e–h pairs. In the absence of field the singlet mixes with the entire triplet manifold by hyperfine interaction. The magnetic field lifts the triplet degeneracy, and for strong field, the mixing remains only between the singlet and the
T
0 component of the triplet, thus changing the e–h recombination rate and hence the current. The experimental results are consistent with the model.
Highly conducting polymers such as polyaniline and polypyrrole in a metallic state have unusual frequency dependent conductivity, including multiple zero crossings of the dielectric function. A low ...frequency electromagnetic response, when be analyzed by the Drude theory of metals, is provided by an extremely small fraction of the total number electrons ∼0.1%, but with extremely high mobility or anomalously long scattering time ∼10
−12
s. We show that a network of metallic grains connected by resonance quantum tunneling has a Drude type response for both the high and low frequency regimes and behaves as a dielectric at intermediate frequency in agreement with experimental observations. The metallic grains in polymers represent crystalline domains of well-packed chains with delocalized electrons embedded in the amorphous media of poor chain order. Intergrain resonance tunneling occurs through the strongly localized states in amorphous media. The small concentration of electrons participating in dc-transport is assigned to the low density of resonance states, and the long relaxation time is related to the narrow width of energy levels in resonance.
Organic semiconductor (OSC) devices have been shown to have a large magnetoresistance (MR) response at room temperature for relatively small-applied magnetic fields of 0.1–100
milli-Tesla (mT). This ...large MR is not limited to one class of organics, but is seen in small molecules, oligomers, conjugated polymers, and non-conjugated polymers. In this paper, data is presented on the MR effect for the poly(phenylene vinylene) (PPV) derivative “Super Yellow,” for poly(vinylenecarbazole) (PVK), for alpha-sexithiophene (α-6T), and for tris(8-hydroxyquinoline) aluminum (Alq
3). The data is analyzed in the context of the Magnetoresistance by the Interconversion of Singlets and Triplets (MIST) model. The MR data of Alq
3 for magnetic fields of less than 1
mT are fitted to a polynomial expansion, and an estimate for the hyperfine interaction constant, which is consistent with values for small molecules, is extracted from the fitting parameters. Curve fits at fields in the 100
mT range are also presented and they show that there exist two kinds of magnetic field behavior, inverse square root, and inverse even orders. Furthermore, the scaling factor at this range is one order of magnitude larger than that found in the 3
mT range.
We studied magnetoresistance (MR) in La
2/3Sr
1/3MnO
3 (LSMO)/organic semiconductor (OSC)/Fe heterojunction devices using rubrene (C
42H
28) as an organic semiconductor. Efficient spin polarized ...tunneling using a hybrid barrier (oxide (1.2
nm)/rubrene (5
nm)) was observed. Devices with a thin layer of rubrene as the barrier may have magnetic clusters and/or pinholes in the barrier, which could explain significant variations of MR among devices. As the thickness of the rubrene layer is increased, device current becomes strongly limited by carrier injection resulting in strong temperature and bias dependent device resistance. The carrier injection in these devices can be described with thermionic field emission at the metal/OSC interface and is analyzed with both empirical and theoretical models. The effect of carrier transport through the spacer on the magnetoresistance for organic-based spin valve is discussed. The observed giant magnetoresistance (GMR) in 20
nm rubrene device demonstrates the spin polarized carrier injection and transport through the rubrene OSC layer.
We report large magnetoresistance (up to 55% at
T
=
2.5
K and
H
=
8
T) for polyaniline nanofiber networks. Magneto-transport behavior has been studied at low and high magnetic and electric fields for ...temperatures 2–250
K. A crossover from positive magnetoresistance (MR) to negative MR is observed at
∼
87.5
K. Two competing mechanisms which account for the MR behavior are shrinkage of localized electron wavefunctions and destruction of the quantum interference of wavefunctions of electrons propagating along various paths during charge hopping processes in the applied magnetic field. We also observed that the magnitude of positive magnetoresistance decreases with increasing electric field which we propose is due to decrease in the activation energies in hopping process.
Highly conducting polymers such as polyaniline and polypyrrole in a dielectrical state have unusually strong temperature of dependence of dynamic conductivity. We argue that this strong temperature ...dependence is due to the low dimensionality of charge transport in the polymers. The hopping network is formed by fast one-dimensional diffusion along the chains and rare interchain hops. Depending on the chain morphology the network is modeled by a quasi-one-dimensional system of randomly linked chains or by a quasi-one-dimensional chain fractal with dimensionality 1+
s, where
s≪1. We point out the experimental data for dielectric constant and ac-conductivity that supports these two models.
Highly conducting polymers such as polyaniline and polypyrrole in a metallic state have unusual frequency-dependent conductivity, including multiple zero crossing of the dielectric function. A ...low-frequency electromagnetic response in terms of a Drude metal is provided by an extremely small fraction of the total number of conduction electrons ∼0.1% but with extremely high mobility or anomalously long scattering time ∼10
−13
s. We show that a network of metallic grains connected by resonance quantum tunneling has a Drude-type response for both the high- and low-frequency regimes and behaves as a dielectric at intermediate frequency in agreement with experimental observations. The metallic grains in polymers represent crystalline domains of well-packed chains with delocalized electrons embedded in the amorphous media of poor chain order. Intergrain resonance tunneling occurs through the strongly localized states in the amorphous media.
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