We investigate spin precession (Hanle effect) in the prototypical organic spintronic giant magnetoresistance (GMR) device La 0.7 Sr 0.3 MnO 3 (LSMO)/tris(8-hydroxyquinoline)(Alq3)/AlOx/Co. The Hanle effect is not observed in measurements taken by sweeping a magnetic field at different angles from the plane of the device. As possible explanations we discuss the tilting out of plane of the magnetization of the electrodes, exceptionally high mobility or hot spots. Our results call for a greater understanding of spin injection and transport in such devices.
merging classical and quantum behavior in a nanosized object. [1][2][3][4] From the applicative point of view, the use of magnetic molecules in spintronics can offer several advantages due to different aspects. On one side, molecular magnets have the functionality of carrying magnetic information down to the molecular size. This encourages the race toward miniaturization of magnetic devices as well as the exploitation of magnetic molecules for quantum computing, [5] although this solution is still hindered by the low working temperature. [6][7][8] On the other side, nonmagnetic molecular materials like organic semiconductors (OSCs) have been considered rather extensively during the past decade in the search for optimal combinations for prototypical devices in the area of spintronics technology, i.e., spin-valves. [9] The magnetoresistance in such devices, which consist of ferromagnetic metal electrodes sandwiching a semiconducting material, depends on the injection and transport of the spin through the semiconductor spacer. OSCs typically possess weak spin-orbit coupling and, because of this, they guarantee longer spin coherence time compared to both inorganic semiconductors and metals. In this context, different organic materials, such as pentacene [10] and tris(8-hydroxyquinoline) aluminium(III) (Alq 3 ) [11,12] and the gallium(III) analogue (Gaq 3 ), [13] have been employed in combination with several ferromagnetic metals, such as Fe and Co:TiO 2[10] or Co and La 0.7 Sr 0.3 MnO 3 (briefly termed LSMO). [11,12] The changes of physical properties of both the metal and organic molecule at the interface have also attracted great scientific interests, so that the ad hoc term spinterface [14,15] has been advanced to describe the topic. From the organic side the spinterface describes the spin filtering effects caused by the spin-dependent hybridization of the organic and metallic orbitals, leading to different interfacial broadening (and hence transmissivity) of the localized organic states for the two spin channels. [14] On the other hand, it has been shown both experimentally [16] and theoretically [17][18][19] that some organic molecules can affect the magnetic properties of the underlying magnetic surface, in terms of magnitude and direction of magnetic moments and spin polarization as well as of strength of exchange interactions. Moreover, coupling between the spins of transition-metal based molecular magnets and magnetic surfaces can be obtained promoting A novel functionalization of a ferromagnetic electrode employed in spintronic devices is reported. Self-assembling monolayer technique has been used to chemisorb a paramagnetic phosphonate functionalized nitronyl-nitroxide radical (NitPO) on the ferromagnetic La 0.7 Sr 0.3 MnO 3 (LSMO) manganite surface. This interfacial layer causes clearly detectable modifications of the behavior in prototypical LSMO/NitPO/Gaq 3 /AlOx/Co vertical spintronic devices at temperatures below the ferromagnetic alignment (estimated by density functional theory) of the magnetic mome...
Vertical crossbar devices based on manganite and cobalt injecting electrodes and a metal-quinoline molecular transport layer are known to manifest both magnetoresistance (MR) and electrical bistability. The two effects are strongly interwoven, inspiring new device applications such as electrical control of the MR and magnetic modulation of bistability. To explain the device functionality, we identify the mechanism responsible for electrical switching by associating the electrical conductivity and the impedance behavior with the chemical states of buried layers obtained by in operando photoelectron spectroscopy. These measurements revealed that a significant fraction of oxygen ions migrate under voltage application, resulting in a modification of the electronic properties of the organic material and of the oxidation state of the interfacial layer with the ferromagnetic contacts. Variable oxygen doping of the organic molecules represents the key element for correlating bistability and MR, and our measurements provide the first experimental evidence in favor of the impurity-driven model describing the spin transport in organic semiconductors in similar devices.
The understanding of spin injection and transport in organic spintronic devices is still incomplete, with some experiments showing magnetoresistance and others not detecting it. We have investigated the transport properties of a large number of tris-(8-hydroxyquinoline)aluminum-based organic spintronic devices with an electrical resistance greater than 5 MΩ that did not show magnetoresistance. Their transport properties could be described satisfactorily by known models for organic semiconductors. At high voltages (>2 V), the results followed the model of space charge limited current with a Poole-Frenkel mobility. At low voltages (∼0.1 V), that are those at which the spin valve behavior is usually observed, the charge transport was modelled by nearest neighbor hopping in intra-gap impurity levels, with a charge carrier density of n0 = (1.44 ± 0.21) × 1015 cm−3 at room temperature. Such a low carrier density can explain why no magnetoresistance was observed.
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