Positive magnetoresistance (MR) has been observed to increase linearly up to 32
T in the magnetically ordered state of organic-based ferrimagnetic semiconductor V(TCNE)
x
films (
x
∼
2; TCNE
=
...tetracyanoethylene) with
T
c
above room temperature (
>
350
K). In this material conductivity takes place via electrons activated from 3d level of V
2+ to upper
π
* subband of TCNE
−
. We show an unusual MR behavior without any sign of saturation up to a magnetic field of 32
T. For temperatures less than
T
c
MR exhibits a linear behavior in the entire field range and above
T
c
it has a quadratic dependence at low fields. Temperature and field dependent behaviors of MR in this material are explained on the basis of spin polarizations in V
2+ 3d level and the upper
π
* subband of TCNE
−
formed by Coulomb repulsion.
Recent observation of a large “field effect” for conducting polymers is in conflict with a fact that the electric field cannot penetrate in a conductor further than the molecular scale Debye radius. ...We review the experimental results for gate voltage control of conductivity in a FET structure. We propose that the field effect is related to inhomogeneous structure and remnant ions. The free space within the polymer network enables ions to move in and out. The electronic screening at the dielectric/conducting polymer boundary transforms the electric potential into the shift of chemical potential for ions inside leading to ionically charging/discharging of the polymer film. The ionic charge is compensated by the variation of the electron concentration. In the linear response to the gate voltage, the charging capacity is
c
=
ε
ε
0
(
r
i
2
+
r
e
2
)
, where r
i,e is the Debye radius for ionic and electronic subsystems. For low concentration of ions (r
i
≫
r
e) the charging response increases exponentially with applied voltage and approaches saturation c
0
=
e
2N(E
F) that is a few hundred F/cm
3. Surprisingly, the experimental “off state” is reached with only a few percent of compensation. Therefore we propose that the main effect of the ionic invasion is suppression of the charge conduction between metallic grains of the inhomogeneous polymers.
Positive MR in (a) dense polyaniline nanofiber networks and (b) polyaniline nanofiber micro-clusters. Negative MR in (c) dense nanofiber networks (d) nanofiber micro-clusters for different ...temperatures, up to 8T. SEM images of corresponding nanofiber sample are given as inset. (e) Magnetoresistance of polyaniline nanofiber networks for different applied currents at 50K and magnetic field up to 8T. (f) IV behavior of nanofiber network at 50K. Each number on the figures corresponds to an MR curve and the related applied current value. Inset-left Current dependency of MR. Inset-right SEM image of the dense nanofiber network. Display omitted
•Charge transport strongly depends on the density of nanofibers.•Conductivity peak at 240K diminishes as interfiber contacts decrease.•Conductivity enhances with reduced interfiber contacts.•Both positive and negative MR decreases with reduced interfiber contacts.•Positive MR decreases with increased applied electric field in dense networks.
We report the unusual charge transport phenomena in polyaniline nanofiber networks and its dependencies on temperature, applied magnetic and electric field, and on the type of fiber morphology. The conductivity of nanofiber networks follows quasi 1-D variable range hopping (VRH) and demonstrates a peak at ∼240K in samples with high density of interfiber intersections. This anomalous peak of conductivity is attributed to change in the interfiber contact resistance with temperature. In all polyaniline nanofiber networks, positive and negative magnetoresistances (MRs) are observed and accounted for by shrinkage of localized electron wavefunctions and suppression of quantum interference among possible tunneling paths by magnetic field. We found that the magnitude of both positive and negative MR gets smaller with decreasing density of interfiber intersections and can be explained by decreasing of hopping barriers as well as decreasing of number of current pathways within VRH network. We also detected that the magnitude of MR in polyaniline nanofiber networks is affected by the applied electric field which we propose is due to field-induced suppression of the hopping activation energies.
The 2D layered organic-based magnet Fe(TCNE)(NCMe)2FeCl4 (TCNE=tetracyanoethylene) exhibits a unique macroscopic magnetic bistability between the field-cooled and zero-field-cooled states, which ...cannot be explained by either superparamagnetic behavior or spin freezing due to spin glass order. This magnetic bistability is described through consideration of the ensemble of uncoupled 2D Ising layers and their magnetization reversal initiated by a field-induced nucleation of magnetic bubbles in individual layers. The bubble nucleation rate strongly depends on the external field and temperature resulting in anomalous magnetic relaxation.
The existence of spin polarized sub‐bands, a half‐semiconducting state in which the electron spins in “valence” and “conduction” bands are oppositely polarized, has been derived for the ...vanadium‐organyl‐based “soft” ferrimagnet V(TCNE)x by measurements of its electrical and magnetic resistance at different temperatures. The Figure shows a schematic of a level diagram for the system investigated.
New advances in organic spintronics Prigodin, V N; Yoo, J W; Jang, H W ...
Journal of physics. Conference series,
04/2011, Letnik:
292, Številka:
1
Journal Article
Recenzirano
Odprti dostop
The basic components of spintronic devices are spin polarized ferromagnets and spin transporting non-magnetic spacers. Exploiting carbon-based materials for these components promises to extend ...functionality of information storage and processing as well as to improve device integration and fabrication. Here we present the magnetoresistance of organic semiconductor rubrene (C42H28) used as a spacer in La2/3Sr1/3MnO3 (LSMO)/organic semiconductor (OSC)/Fe heterojunctions. Efficient spin polarized tunneling through the thin layer of rubrene spacer (5 nm) was observed. As the thickness of rubrene layers is increased, device current is strongly limited by carrier injection resulting in strong temperature dependent device resistance. The carrier injection is described with thermionic field emission at the metal/OSC interface. As a next step toward organic spintronics we used an organic based magnet vanadium-tetracyanoethylene (V(TCNE)x, x∼2) in tandem with LSMO in a spin- valve with 5 nm rubrene spacer. V(TCNE)x is the earliest developed room temperature molecule-based magnet (Tc ∼ 400 K). Due to strong on-site Coulomb interaction and weak intermolecule overlapping their magnetic state can be described with a model of half-semiconductor in which valence and conduction bands are spin polarized. The magnetoresistance data for bulk V(TCNE)x is in agreement with the model of spin polarized valence and conduction bands. We demonstrated that an organic-based magnetic semiconductor V(TCNE)x functions very well as an electron spin polarizer in the standard spintronic device geometry.
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