Organic Semiconductors for Room-Temperature Spin Valves

A IO N Spintronics, or the possibility of performing electronics with the spin of the electron, has been fundamental for the exponential growth of digital data storage which has occurred in the last decades. Indeed, hard-disk drives read-heads are the maximum exponent of what is currently being called fi rstgeneration spintronic devices. Current read-heads, although technologically very complex, are scientifi cally based simply on the tunnel magnetoresistance effect (TMR; magnetoresistance being the change in electrical resistance of a device under the application of an external magnetic fi eld). A tunnel magnetoresistive vertical spin valve is composed of two ferromagnetic layers separated by a thin (around 1 nm) insulating layer, and the resistance of the structure can be switched between two different values upon the application of a magnetic fi eld capable of rotating the magnetization vector of the ferromagnetic layers from parallel to antiparallel. [ 1 ] For the eventual success of a second-generation of spintronic devices, more complex mechanisms than the nanometre-distance spin transport in metallic or insulating materials have to be obtained. In particular, coherent spin transport at distances above a few nm and spin manipulation are unavoidable requirements for the production of sophisticated prototypes of, for example, spin transistors or spin light-emitting diodes. [ 2 , 3 ] Organic semiconductors (OS) have emerged as promising materials for advanced spintronics applications. Their spin relaxation mechanisms, mainly represented by spin orbit interaction and hyperfi ne interaction with protons, [ 4 ] are very small, and long spin lifetimes have been consistently detected. [ 5 ] Moreover, in spite of the relatively low carrier mobility of these materials, organic vertical spin valves with semiconducting channels thicker than 100 nm have been demonstrated. [ 6–11 ] In parallel, OS ultrathin layers perform successfully as spin tunnel junctions, and extremely high ( > 300%) magnetoresistance (MR) values have been obtained at low temperatures. [ 12 ] Regarding possible applications of spin transport in OS, a basic operational requirement is the room temperature (RT) operation of the devices. So far, only organic spin tunnel junctions have shown any signifi cant MR effect at RT. [ 13–17 ] By contrast, most devices employing thicker organic layer ( > 15 nm) show a clear decay of the MR well below RT. [ 6–11 , 18 ]

http://absimage.aps.org/image/MAR11/MWS_MAR11-2010-000398.pdf

Room Temperature Spin Transport in C$_{60}$-based spin valves.

Figure 1 . a) Device structure and b) schematic view of the energy levels of V[TCNE] x ( x ≈ 2). Spintronics is the new paradigm of electronics and utilizes the spin degree of freedom of the electron. [ 1 ] In addition to metalbased spintronic devices, which already have wide applications (e.g., read heads of hard disk drives), semiconductor spintronics provides the possibility to combine logic, communication, and storage operation using hybrid devices. [ 2 ] Recently, spintronic devices based on organic materials have attracted much attention because of the potential long spin lifetime due to low spin-orbit coupling and weak hyperfi ne interaction, as well as the ability to fabricate low-cost, light-weight, and mechanically fl exible devices. [ 3–9 ] Organic semiconductors have been used as the spacers in spin valve devices with ferromagnetic metal or transitional metal oxide contacts. One fundamental obstacle for spin injection from a ferromagnetic metal into a semiconductor is the conductivity mismatch. [ 10 ] The development of fully spinpolarized magnetic semiconductors offers a promising route to circumvent this problem. Organic-based magnets such as V[TCNE] x ( x ≈ 2, TCNE: tetracyanoethylene) provide advantages including high magnetic ordering temperature, fully spin-polarized semiconducting electronic structure, chemical tunability, and low-temperature processing. [ 11 ] Here, we show results for spin injection and detection in an all-organic-based spin valve using the organic magnetic semiconductor V[TCNE] x as both the spin injector and analyzer. We observed unusual inverted spin valve effect and propose that the negative magnetoresistance (MR) originates from the spin-dependent tunneling between highly spin-polarized bands split by Coulomb interaction. V[TCNE] x is the fi rst reported room-temperature moleculebased magnet with magnetic ordering temperature T C ≈ 400 K. [ 12 ] The magnetic order originates from the antiferromagnetic coupling between the three unpaired electrons of the V 2 +

http://absimage.aps.org/image/MAR12/MWS_MAR12-2011-001929.pdf

Magnetoresistance in an all-organic-based spin valve

An anionic radical through the charge transfer process of hybridization on the spinterface was found in a doped P3HT spin valve, which may effectively enhance the device performance. 

/pdf/molecular-spinterface-in-f-sub-4-sub-tcnq-doped-polymer-spin-2bmq3ase.pdf

Molecular spinterface in F<sub>4</sub>TCNQ-doped polymer spin valves

The molecular spinterface is an ideal platform to realize multistate storage in organic spin valve (OSV) devices. However, as an effective strategy to enhance the conductivity and modify the spinterface, molecular doping has rarely been reported in vertical OSV devices, and the effect of artificial doping induced spinterface is not clear yet. Herein, we fabricated a F4TCNQ-doped polymer spin valve with a stacking structure of La2/3Sr1/3MO3 (LSMO)/poly(3-hexylthiophene-2,5-diyl) (P3HT)/tetrafluoro-tetracyanoquinodimethane (F4TCNQ)/Co/Au, and found a significant improvement in conductivity and the magnetoresistance (MR) ratio. According to the characterization of interfacial states and investigation of half spin valves, it was determined that F4TCNQ was doped in the form of free radicals and created a spin-dependent hybrid interfacial state (SDHIS). Such a SDHIS can exhibit an extra interface magnetoresistance (IMR) effect imposed on the standard giant magnetoresistance (GMR) effect. This interfacial doping strategy shows great potential for application in future multistate molecular spintronic devices and provides a new insight into the mechanism of radical-induced molecular spinterface.

Molecular spinterface in F4TCNQ-doped polymer spin valves

Preparation and assessment of reliable organic spin valves

Incomplete Hetero-Structure Molecular Layer Enhancing Room-Temperature Spin-Photovoltaic Performances

The explosive growth of the information era has put forward urgent requirements for ultra-high-speed and extremely efficient computations. In direct contrary to charge-based computations, spintronics aims to use spins as information carriers for data storage, transmission, and decoding, to help fully realize electronic device miniaturization and high integration for next-generation computing technologies. Currently, many novel spintronic materials have been developed with unique properties and multi-functionalities, including organic semiconductors (OSCs), organic-inorganic hybrid perovskites (OIHPs), and two-dimensional materials (2DMs). These materials are useful to fulfil the demand for developing diverse and advanced spintronic devices. Herein, we systematically reviewed these promising materials for advanced spintronic applications. Due to the distinct chemical and physical structures of OSCs, OIHPs, and 2DMs, their spintronic properties (spin transport and spin manipulation) were discussed separately. In addition, some multifunctionalities due to photoelectric and chiral-induced spin selectivity (CISS) were overviewed, including the spin-filter effect, spin-photovoltaics, spin-light emitting devices, and spin-transistor functions. Subsequently, we presented challenges and future perspectives of using these multifunctional materials for the development of advanced spintronics. This article is protected by copyright. All rights reserved.

Emerging Spintronic Materials and Functionalities.

Abundant spin-related phenomena that originate from interfaces between ferromagnetic electrodes and molecular semiconductors have greatly enriched research in spintronics, and they are considered promising for realizing novel spintronic functionalities in the future. However, despite great efforts, the interfacial effect cannot be precisely controlled to achieve steady and predictable functions, especially at room temperature, and this has gradually become a significant bottleneck in the development of molecular spintronics. In this study, an innovative spin-filtering-competition mechanism was first proposed to continuously modulate the interfacial effect in molecular spin valves at room temperature. To form this novel mechanism, the original spin-filtering effect from pure Co competed with the newly generated one, which was induced by the bonding effect between Co and LiF. Subsequently, by precisely controlling competition through LiF coverage on the Co surface, continuous modulation of the spin-injection process could be successfully achieved at room temperature. Spin polarization of the injected current and magnetoresistance effect could be actively controlled or their sign could be completely reversed through this novel mechanism. This study provided an innovative approach and theory for precisely controlling the spin-related interfacial effects, which may further promote the scientific and technological development of spintronics. This article is protected by copyright. All rights reserved.

Shunhua Hu

Papers

Preparation and assessment of reliable organic spin valves

Incomplete Hetero-Structure Molecular Layer Enhancing Room-Temperature Spin-Photovoltaic Performances

Emerging Spintronic Materials and Functionalities.

Continuous room-temperature spin-injection modulation achieved by spin-filtering competition in molecular spin valves.