Vertical organic spin valves in perpendicular magnetic fields

Grünewald, M.; Göckeritz, Robert; Homonnay, N.; Würthner, Frank; Molenkamp, L. W.; Schmidt, G.

doi:10.1103/physrevb.88.085319

Cited by 54 publications

(57 citation statements)

References 42 publications

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“…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.…”

Section: Nanosized Vertical Organic Spin-valvesmentioning

confidence: 99%

“…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%

“…[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.…”

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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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Section: Nanosized Vertical Organic Spin-valvesmentioning

confidence: 99%

Section: Nanosized Vertical Organic Spin-valvesmentioning

confidence: 99%

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Nanosized perpendicular organic spin-valves

Göckeritz

Homonnay

Müller

et al. 2015

Applied Physics Letters

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“…As mentioned above, a similar picture has been invoked to explain memristor behavior in these spin valves [67]. Gruenewald et al investigated organic spin valves with Alq 3 (40-100 nm) and PTCDI-C4F7 (100-600 nm) spacers, and LSMO (bottom) and CoFe (top) electrodes [84]. Again, no Hanle effect could be observed.…”

Section: Organic Spin Valves: Where Are the Carrier Spins?mentioning

confidence: 96%

Recent progress in organic spintronics

Jong

2016

Open Physics

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Abstract:The field of organic spintronics deals with spin dependent phenomena occurring in organic semiconductors or hybrid inorganic/organic systems that may be exploited for future electronic applications. This includes magnetic field effects on charge transport and luminescence in organic semiconductors, spin valve action in devices comprising organic spacers, and magnetic effects that are unique to hybrid interfaces between (ferromagnetic) metals and organic molecules. A brief overview of the current state of affairs in the field is presented.

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“…Rn, 72.25.Dc, 75.40.Gb, Introduction. Organic spin valves (OSVs), being one of the most promising applications of organic spintronics, are actively studied experimentally [1][2][3][4][5][6][7][8][9] . The organic active layer of an OSV is sandwiched between two magnetized electrodes.…”

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Giant Fluctuations of Local Magnetoresistance of Organic Spin Valves and the Non-Hermitian 1D Anderson Model

et al. 2014

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Motivated by recent experiments, where the tunnel magnetoresitance (TMR) of a spin valve was measured locally, we theoretically study the distribution of TMR along the surface of magnetized electrodes. We show that, even in the absence of interfacial effects (like hybridization due to donor and acceptor molecules), this distribution is very broad, and the portion of area with negative TMR is appreciable even if on average the TMR is positive. The origin of the local sign reversal is quantum interference of subsequent spin-rotation amplitudes in course of incoherent transport of carriers between the source and the drain. We find the distribution of local TMR exactly by drawing upon formal similarity between evolution of spinors in time and of reflection coefficient along a 1D chain in the Anderson model. The results obtained are confirmed by the numerical simulations.PACS numbers: 72.15. Rn, 72.25.Dc, 75.40.Gb, Introduction. Organic spin valves (OSVs), being one of the most promising applications of organic spintronics, are actively studied experimentally [1][2][3][4][5][6][7][8][9] . The organic active layer of an OSV is sandwiched between two magnetized electrodes. Due to long spin-relaxation times of carriers in organic materials, the net resistance of OSV is sensitive to the relative magnetizations of the electrodes. Among many advantages that OSVs offer, is wide tunability due to e.g. chemical doping, and enormous flexibility. The processes that limit the performance of OSVs can be conventionally divided into two groups: (i) interfacial, which take place at the interfaces between the electrodes and active layer [11][12][13][14][15][16][17][18] , and (ii) intralayer, which exist even if the interfaces are ideal. 19,20 Due to the latter processes the injected polarized electrons, Fig. 1, lose memory about their initial spin orientation while traveling between the electrodes. One of the most prominent mechanisms of this spin-memory loss is the precession of a carrier spin in random hyperfine fields of hydrogen nuclei 5,19,20 . The effectiveness of the OSV performance is quantified by tunnel magnetoresistance (TMR) given by a so-called modified Julliere's formula 22 , see e.g. the review Ref. 21,where P 1 , P 2 stand for polarizations of the electrodes. The difference from the original Julliere's formula 22 is the exponential factor Q = exp(−d/λ s ) describing the spin-memory loss over the active layer of thickness, d. Processes (i) can be incorporated into Eq. (1) by appropriately modifying P 1 , P 2 . For example, in Ref. 11 replacement of P 1 , P 2 by "effective" spin polarizations reflects the relative position of the Fermi level with respect to interfacial donor (acceptor) level. In this way, the "effective" polarization depends on bias, which might explain the sign reversal of TMR [11][12][13][14][15][16][17][18] . Processes (ii), on the other hand, are reflected in Eq. (1) via the factor Q = exp(−d/λ s ), where λ s is the spin diffusion length. The meaning of Q is the polarization of electrons at that the...

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Vertical organic spin valves in perpendicular magnetic fields

Cited by 54 publications

References 42 publications

Nanosized perpendicular organic spin-valves

Nanosized perpendicular organic spin-valves

Recent progress in organic spintronics

Giant Fluctuations of Local Magnetoresistance of Organic Spin Valves and the Non-Hermitian 1D Anderson Model

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