Hanle effect missing in a prototypical organic spintronic device

Riminucci, Alberto; Prezioso, M.; Pernechele, C.; Graziosi, Patrizio; Bergenti, I.; Cecchini, Raimondo; Calbucci, Marco; Solzi, M.; Dediu, Alek

doi:10.1063/1.4794408

Cited by 54 publications

(56 citation statements)

References 52 publications

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“…Thus it is important to investigate both remanent states in detail. In a recent publication 31 , data has already been presented for magnetic field sweeps perpendicular to the sample plane, however, starting the sweep from perpendicular saturation. In our case the use of a 3D vector magnet allows us to prepare both remanent states individually by running a field sweep from parallel saturation to the desired state and then reducing the in-plane field to zero.…”

Section: B Experiments In the Perpendicular Geometry: Alq 3 Osvsmentioning

confidence: 99%

Vertical organic spin valves in perpendicular magnetic fields

et al. 2013

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We report the results of magnetoresistance measurements in vertical organic spin valves with the magnetic field oriented perpendicular to the layer stack. The magnetoresistance measurements were performed after carefully preparing either parallel or antiparallel in-plane magnetization states of the magnetic electrodes in order to observe traces of Hanle precession. Due to the low mobility in organic semiconductors the transit time of spin polarized carriers should allow for precession of the spins in perpendicular fields which in statistical average would quench the magnetoresistance.However, in none of the experiments we do observe any change in resistance while sweeping the perpendicular field, up to the point where the electrode's magnetization starts to reorient. This absence of Hanle type effects indicates that the magnetoresistance is not based on the injection of spin polarized electrons into the organic semiconductor but rather on tunneling through pinholes superimposed with tunneling anisotropic magnetoresistance.

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Section: B Experiments In the Perpendicular Geometry: Alq 3 Osvsmentioning

confidence: 99%

Vertical organic spin valves in perpendicular magnetic fields

et al. 2013

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“…2,4,5,12,13,17,18 We use a similar structure with sputtered MgO as a barrier between the Alq3 and the top electrode. The active device area is defined by a small window in an insulating layer which acts as a kind of nanosized shadow mask.…”

Section: Nanosized Vertical Organic Spin-valvesmentioning

confidence: 99%

Nanosized perpendicular organic spin-valves

Göckeritz

Homonnay

Müller

et al. 2015

Applied Physics Letters

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A fabrication process for vertical organic spin-valve devices has been developed which offers the possibility to achieve active device areas of less than 500x500 nm² and is flexible in terms of material choice for the active layers. Characterization of the resulting devices shows a large magnetoresistance of sometimes more than 100 %, however with equally large variation from device to device. Comparison with large-area spin-valves indicates that the magnetoresistance of both, large and small devices most likely originates from tunneling through pinholes and tunneling magnetoresistance.Organic spin-valve devices are promising candidates for low cost non-volatile electronics, for example for RF-ID tags.1 Nevertheless, a deeper understanding of the underlying device physics is still necessary, especially of the charge transport mechanisms through the organic material and the origin of the magnetoresistance. In vertical organic spin-valves (OSV) an organic semiconductor (OSC) thin film is sandwiched between two ferromagnetic electrodes and the device is operated by a current flow perpendicular through the layers.2 Similar to giant magnetoresistance (GMR) 2-5 and tunneling magnetoresistance (TMR) 6-9 the device resistance depends on the relative magnetization of the two electrodes. However, in contrast to GMR in all-metal structures or TMR in metal/oxide structures it is still an open question whether the origin of the magnetoresistance (MR) effects in OSVs published so far is dominated by spin polarized charge transport through the organic spacer layer or by tunneling processes. 10,11 There is, however, a possible approach to gain deeper insight into this problem which takes into account the scaling of the MR effect with device size. In the case of spin injection and charge transport (GMR) the complete device area is expected to contribute to the current path through the OSC. Reducing the size of the device in this case should keep the resistance area product and the relative magnetoresistance constant. Tunneling processes, however, most likely occur in just a small part of the active area.12 Most probable candidates for tunneling sites would be pinholes at which the thickness of the OSC is reduced to such an extent that tunneling and thus TMR becomes possible. [8][9][10][11][12][13] Pinholes, however, follow certain statistics resulting in a different resistance change upon downscaling. They vary in size and also exhibit a varying thickness of the remaining OSC layer. Each pinhole thus contributes to the total device characteristics by a different resistance and MR contribution. In order to appreciate the necessary density of pinholes we can use results published by Barraud and coworkers.14 They create artificial pinholes in an OSC layer using conducting tip atomic force microscopy (AFM) and study the magnetoresistance after the pinhole is filled up with a ferromagnetic metal. The resistance of the pinholes is typically in the range of 100 MΩ while the magnetoresistance can be as high as 300%. In addition, they ob...

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“…The low electron-or hole-polaron mobility in the organic suggests the Hanle effect be seen under a transverse magnetic field as small as B c ¼ 10 À 6 mT 17,18 . However, no Hanle effect is detected up to B ¼ 10 mT.…”

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confidence: 99%

Impurity-band transport in organic spin valves

2014

Nat Commun

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The central phenomenon in the field of organic spintronics is the large magnetoresistance in thick organic spin valves. A prerequisite for understanding the magnetoresistance is a reliable description of the device resistance, or the I-V characteristics. Here I show that the observed I-V characteristics in the organic spin valves is incompatible with charge injection into the organic's lowest unoccupied molecular orbital or highest occupied molecular orbital but can be explained by electrons tunnelling into a broad impurity band located in the gap between these molecular orbitals. Voltage drop takes place mainly across depletion layers at the two electrode/organic interfaces, giving rise to electrode-limited charge transport. Spin-dependent electron tunnelling into the impurity band from the ferromagnetic electrodes results in spin accumulations inside the organic, which rapidly diffuses through the organic primarily via the exchange between impurity-band electrons. This picture explains the major magnetoresistance features and predicts enhanced capacitance in these devices.

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Hanle effect missing in a prototypical organic spintronic device

Cited by 54 publications

References 52 publications

Vertical organic spin valves in perpendicular magnetic fields

Vertical organic spin valves in perpendicular magnetic fields

Nanosized perpendicular organic spin-valves

Impurity-band transport in organic spin valves

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