Magnetic field sensors based on organic thin-film materials have attracted considerable interest in recent years as they can be manufactured at very low cost and on flexible substrates. However, the technological relevance of such magnetoresistive sensors is limited owing to their narrow magnetic field ranges (∼30 mT) and the continuous calibration required to compensate temperature fluctuations and material degradation. Conversely, magnetic resonance (MR)-based sensors, which utilize fundamental physical relationships for extremely precise measurements of fields, are usually large and expensive. Here we demonstrate an organic magnetic resonance-based magnetometer, employing spin-dependent electronic transitions in an organic diode, which combines the low-cost thin-film fabrication and integration properties of organic electronics with the precision of a MR-based sensor. We show that the device never requires calibration, operates over large temperature and magnetic field ranges, is robust against materials degradation and allows for absolute sensitivities of <50 nT Hz−1/2.
Spin-dependent processes play a crucial role in organic electronic devices. Spin coherence can give rise to spin mixing due to a number of processes such as hyperfine coupling, and leads to a range of magnetic field effects. However, it is not straightforward to differentiate between pure single-carrier spin-dependent transport processes which control the current and therefore the electroluminescence, and spin-dependent electron-hole recombination which determines the electroluminescence yield and in turn modulates the current. We therefore investigate the correlation between the dynamics of spin-dependent electric current and spin-dependent electroluminescence in two derivatives of the conjugated polymer poly(phenylene-vinylene) using simultaneously measured pulsed electrically detected (pEDMR) and optically detected (pODMR) magnetic resonance spectroscopy. This experimental approach requires careful analysis of the transient response functions under optical and electrical detection. At room temperature and under bipolar charge-carrier injection conditions, a correlation of the pEDMR and the pODMR signals is observed, consistent with the hypothesis that the recombination currents involve spin-dependent electronic transitions. This observation is inconsistent with the hypothesis that these signals are caused by spin-dependent charge carrier transport. These results therefore provide no evidence that supports earlier claims that spindependent transport plays a role for room temperature magnetoresistance effects. At low temperatures, however, the correlation between pEDMR and pODMR is weakened, demonstrating that more than one spin-dependent process influences the optoelectronic materials properties. This conclusion is consistent with prior studies of half-field resonances that were attributed to spin-dependent triplet exciton recombination which becomes significant at low temperatures when the triplet lifetime increases. PACS
Li[TCNE] (TCNE = tetracyanoethylene) magnetically orders as a weak ferromagnet (canted antiferromagnet) below 21.0 ± 0.1 K, as observed from the bifurcation of the field-cooled and zero-field-cooled magnetizations, as well as remnant magnetization. The structure, determined ab initio from synchrotron X-ray powder diffraction data, consists of a planar μ4-[TCNE](•-) bound to four tetrahedral Li(+) ions. The structure consists of two interpenetrating diamondoid sublattices, with closest interlattice distances of 3.43 and 3.48 Å. At 5 K this magnetic state is characterized by a coercivity of ~30 Oe, a remnant magnetization of 10 emu·Oe/mol, and a canting angle of 0.5°.
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Large surface to volume ratios of semiconductor nanocrystals cause susceptibility to charge trapping, which can modify luminescence yields and induce single-particle blinking. Optical spectroscopies cannot differentiate between bulk and surface traps in contrast to spin-resonance techniques, which in principle avail chemical information on such trap sites. Magnetic resonance detection via spin-controlled photoluminescence enables the direct observation of interactions between emissive excitons and trapped charges. This approach allows the discrimination of two functionally different trap states in CdSe/CdS nanocrystals underlying the fluorescence quenching and thus blinking mechanisms: a spin-dependent Auger process in charged particles; and a charge-separated state pair process, which leaves the particle neutral. The paramagnetic trap centers offer control of energy transfer from the wide-gap CdS to the narrow-gap CdSe, i.e. light harvesting within the heterostructure. Coherent spin motion within the trap states of the CdS arms of * Corresponding authors. 2 | P a g e nanocrystal tetrapods is reflected by spatially remote luminescence from CdSe cores with surprisingly long coherence times of >300 ns at 3.5 K. Substantial control over the chemistry of semiconductor nanocrystals has been demonstrated in recent years while pursuing novel optoelectronic device schemes 1,2,3 . Shortcomings in the performance of these materials are routinely attributed to ill-defined "trap" states competing with the quantum-confined primary exciton 4 . While frequently implicated in explaining device inefficiencies 2 , photoluminescence (PL) blinking 5-9 and delayed PL dynamics 4,10 , little is known about the underlying chemical nature of these deleterious states. Despite the wealth of structural and electronic information accessible in optical spectroscopy, the spin degree of freedom has received only marginal consideration as a complementary probe of semiconductor nanocrystals. Approaches pursued previously include isolation of paramagnetic centers in doped dilute magnetic semiconductor nanoparticles 11-12 ; resolving the exciton fine structure by fluorescence spectral line narrowing 13 , time-resolved Faraday rotation 12,14 or photon-echo techniques 15 ; and continuous-wave optically-detected magnetic resonance (ODMR), where the fluorescence is modulated under spin-resonant excitation in a magnetic field 16-19 . The latter requires stable paramagnetic centers, where the carrier's spin and energy are maintained on long timescales compared to the oscillation period of the resonantly driven spin manifold, i.e. for tens of nanoseconds under excitation in the 10 GHz (~0.3 T) range.The persistence of spin states in bulk materials comprising heavy atoms such as cadmium is largely determined by mixing due to spin-orbit coupling.the high degree of carrier localization since coherence information remains unperturbed upon change of environment (i.e. addition of the core to form the tetrapod heterostructure). This result also demonstrates the ...
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