Magnetoresistance In Organic Spin Valves Research Articles

Magnetoresistance in Organic Spin Valves Based on Acid‐Exfoliated 2D Covalent Organic Frameworks Thin Films

AbstractCovalent organic frameworks (COFs), as a burgeoning class of crystalline porous materials, have made significant progress in their application to optoelectronic devices such as field‐effect transistors, memristors, and photodetectors. However, the insoluble features of microcrystalline two‐dimensional (2D) COF powders limit development of their thin film devices. Additionally, the exploration of spin transport properties in this category of π‐conjugated skeleton materials remains vacant thus far. Herein, an imine‐linked 2D Py‐Np COF nanocrystalline powder was synthesized by Schiff base condensation of 4,4′,4′′,4′′′‐(pyrene‐1,3,6,8‐tetrayl)tetraaniline and naphthalene‐2,6‐dicarbaldehyde. Then, we prepared a large‐scale free‐standing Py‐Np COF film via a top‐down strategy of chemically assisted acid exfoliation. Moreover, high‐quality COF films acted as active layers were transferred onto ferromagnetic La0.67Sr0.33MnO3 (LSMO) electrodes for the first attempt to fabricate organic spin valves (OSVs) based on 2D COF materials. This COF‐based OSV device with a configuration of LSMO/Py‐Np COF/Co/Au demonstrated a remarkable magnetoresistance (MR) value up to −26.5 % at 30 K. Meanwhile, the MR behavior of the COF‐based OSVs exhibited a highly temperature dependence and operational stability. This work highlights the enormous application prospects of 2D COFs in organic spintronics and provides a promising approach for developing electronic and spintronic devices based on acid‐exfoliated COF thin films.

Magnetoresistance in Organic Spin Valves Based on Acid-Exfoliated 2D Covalent Organic Frameworks Thin Films.

Covalent organic frameworks (COFs), as a burgeoning class of crystalline porous materials, have made significant progress in their application to optoelectronic devices such as field-effect transistors, memristors, and photodetectors. However, the insoluble features of microcrystalline two-dimensional (2D) COF powders limit development of their thin film devices. Additionally, the exploration of spin transport properties in this category of π-conjugated skeleton materials remains vacant thus far. Herein, an imine-linked 2D Py-Np COF nanocrystalline powder was synthesized by Schiff base condensation of 4,4',4'',4'''-(pyrene-1,3,6,8-tetrayl)tetraaniline and naphthalene-2,6-dicarbaldehyde. Then, we prepared a large-scale free-standing Py-Np COF film via a top-down strategy of chemically assisted acid exfoliation. Moreover, high-quality COF films acted as active layers were transferred onto ferromagnetic La0.67 Sr0.33 MnO3 (LSMO) electrodes for the first attempt to fabricate organic spin valves (OSVs) based on 2D COF materials. This COF-based OSV device with a configuration of LSMO/Py-Np COF/Co/Au demonstrated a remarkable magnetoresistance (MR) value up to -26.5 % at 30 K. Meanwhile, the MR behavior of the COF-based OSVs exhibited a highly temperature dependence and operational stability. This work highlights the enormous application prospects of 2D COFs in organic spintronics and provides a promising approach for developing electronic and spintronic devices based on acid-exfoliated COF thin films.

Molecular and Interfacial Adjustment of Magnetoresistance in Organic Spin Valves Using Isoindigo-Based Polymers

Molecular and Interfacial Adjustment of Magnetoresistance in Organic Spin Valves Using Isoindigo-Based Polymers

Length dependence of magnetoresistance in organic spin valves

With the Su–Schrieffer–Heeger model and Green's function method, the length dependence of magnetoresistance in organic spin valves is calculated in the frame of tunneling transport. Based on different energy level alignments between the molecule and the electrodes, the length effect on the magnetoresistance is investigated in three transport schemes, barrier tunneling, resonant tunneling, and transition between them. In the first scheme, a length-induced exponential or linear decline of the magnetoresistance is obtained. An oscillation and nonlinear dependence are observed in the second and third schemes. The mechanism is explained by investigating the transmission spectra at different lengths, where the evolution of the efficient transmission in the bias window with length differs in different schemes. The results agree with many experimental measurements qualitatively.

Angular dependent magnetoresistance in organic spin valves

Vertical organic spin valve (OSV)-based organic spintronic devices with additional degree of freedom to utilize and control the magnetoresistance (MR) by spin of electrons, have attracted a lot of interests for both foundation science and future functional device applications. Herein the effects of temperature, bias voltage and direction of magnetic field on the MR of the OSV are investigated to disclose the mechanisms. Specifically, the MR shows angular dependence with the value tuned from negative to positive by rotating the field direction from in-plane to out-of-plane, corresponding to the angular dependent spin moment. A domain-switch model is proposed to simulate the resistance changes in OSV device. The research provides a new route to tune the MR in organic spintronic devices, which is significant for future functional device applications.

Achieving large and nonvolatile tunable magnetoresistance in organic spin valves using electronic phase separated manganites

Tailoring molecular spinterface between novel magnetic materials and organic semiconductors offers promise to achieve high spin injection efficiency. Yet it has been challenging to achieve simultaneously a high and nonvolatile control of magnetoresistance effect in organic spintronic devices. To date, the largest magnetoresistance (~300% at T = 10 K) has been reached in tris-(8-hydroxyquinoline) aluminum (Alq3)-based organic spin valves (OSVs) using La0.67Sr0.33MnO3 as a magnetic electrode. Here we demonstrate that one type of perovskite manganites, i.e., a (La2/3Pr1/3)5/8Ca3/8MnO3 thin film with pronounced electronic phase separation (EPS), can be used in Alq3-based OSVs to achieve a large magnetoresistance (MR) up to 440% at T = 10 K and a typical electrical Hanle effect as the Hallmark of the spin injection. The contactless magnetic field-controlled EPS enables us to achieve a nonvolatile tunable MR response persisting up to 120 K. Our study suggests a new route to design high performance multifunctional OSV devices using electronic phase separated manganites.

Controlling Magnetoresistance by Oxygen Impurities in Mq3-Based Molecular Spin Valves.

The understanding of magnetoresistance (MR) in organic spin valves (OSVs) based on molecular semiconductors is still incomplete after its demonstration more than a decade ago. Although carrier concentration may play an essential role in spin transport in these devices, direct experimental evidence of its importance is lacking. We probed the role of the charge carrier concentration by studying the interplay between MR and multilevel resistive switching in OSVs. The present work demonstrates that all salient features of these devices, particularly the intimate correlation between MR and resistance, can be accounted for by the impurity band model, based on oxygen migration. Finally, we highlight the critical importance of the carrier concentration in determining spin transport and MR in OSVs and the role of interface-mediated oxygen migration in controlling the OSV response.

Oxygen Impurities Link Bistability and Magnetoresistance in Organic Spin Valves.

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.

A review on organic spintronic materials and devices: II. Magnetoresistance in organic spin valves and spin organic light emitting diodes

In the preceding review paper, Paper I [Journal of Science: Advanced Materials and Devices 1 (2016) 128–140], we showed the major experimental and theoretical studies on the first organic spintronic subject, namely organic magnetoresistance (OMAR) in organic light emitting diodes (OLEDs). The topic has recently been of renewed interest as a result of a demonstration of the magneto-conductance (MC) that exceeds 1000% at room temperature using a certain type of organic compounds and device operating condition. In this report, we will review two additional organic spintronic devices, namely organic spin valves (OSVs) where only spin polarized holes exist to cause magnetoresistance (MR), and spin organic light emitting diodes (spin-OLEDs) where both spin polarized holes and electrons are injected into the organic emissive layer to form a magneto-electroluminescence (MEL) hysteretic loop. First, we outline the major advances in OSV studies for understanding the underlying physics of the spin transport mechanism in organic semiconductors (OSCs) and the spin injection/detection at the organic/ferromagnet interface (spinterface). We also highlight some of outstanding challenges in this promising research field. Second, the first successful demonstration of spin-OLEDs is reviewed. We also discuss challenges to achieve the high performance devices. Finally, we suggest an outlook on the future of organic spintronics by using organic single crystals and aligned polymers for the spin transport layer, and a self-assembled monolayer to achieve more controllability for the spinterface.

From spin-polarized interfaces to giant magnetoresistance in organic spin valves

We calculate the spin-polarized electronic transport through a molecular bilayer spin valve from first principles, and establish the link between the magnetoresistance and the spin-dependent interactions at the metal-molecule interfaces. The magnetoresistance of a Fe$|$bilayer-C${}_{70}|$Fe spin valve attains a high value of 70% in the linear-response regime, but it drops sharply as a function of the applied bias. The current polarization has a value of 80% in linear response and also decreases as a function of bias. Both these trends can be modeled in terms of prominent spin-dependent Fe$|$${\text{C}}_{70}$ interface states close to the Fermi level, unfolding the potential of spinterface science to control and optimize spin currents.

Strong asymmetrical bias dependence of magnetoresistance in organic spin valves: the role of ferromagnetic/organic interfaces

We report a highly asymmetric magnetoresistance (MR) bias dependence, with the inverse MR peaking at a negative bias and a sign reversal occurring at a positive bias in prototypical La0.7Sr0.3MnO3 (LSMO)/Alq3/Co organic spin valve (OSV) with a tunnel barrier between LSMO and Alq3. This behavior is in strong contrast with the commonly found inverse MR in entire bias range for LSMO/Alq3/Co OSVs. The MR bias voltage dependence is independent on the type of the tunnel barrier, either SrTiO3 or Al2O3. Together with first-principle calculations, we demonstrate that the strongly hybridized Co d-states with Alq3 molecules at the interface are responsible for the efficient d-states spin injection and the observed MR bias dependence is originated from the energy dependent density of states of Co d-states. These findings open up new possibilities to engineer interfacial bonding between ferromagnetic materials and a wide variety of molecule selections for the desired spin transport properties.

Tunnel magnetoresistance in organic spin valves in the regime of multistep tunneling

A model of a spin valve in which electron transport between the magnetized electrodes is due to multistep tunneling is analyzed. Motivated by recent experiments on organic spin valves, we assume that spin memory loss in the course of transport is due to random hyperfine fields acting on electron while it waits for the next tunneling step. Amazingly, we identify the three-step configurations of sites, for which the tunnel magnetoresistance (TMR) is negative in the absence of interfacial effects, suggesting that the resistance for antiparallel magnetizations of the electrodes is smaller than for parallel magnetizations. We analyze the phase volume of these configurations with respect to magnitudes and relative orientations of the on-site hyperfine fields. The effect of sign reversal of TMR is exclusively due to interference of the spin-flip amplitudes on each site; it does not emerge within commonly accepted probabilistic description of spin transport. Another feature specific to multistep inelastic tunneling is bouncing of electron between nearest neighbors while awaiting a ``hard'' hop. We demonstrate that this bouncing, being absolutely insignificant for conduction of current, can strongly affect the spin memory loss. This effect is also of interference origin.

Room-temperature magnetoresistance in organic spin-valves based on a Co2MnSi Heusler alloy

We report room-temperature magnetoresistance in sandwich structures consisting of Co2MnSi/organic spacer/Co. We have prepared the epitaxial films of a Heusler alloy Co2MnSi on MgO (001) by pulsed laser deposition, followed by thermal depositions of organic and Co layers. The spin-valve with an organic layer of N,N-bis (3-methylphenyl)-N,N-diphenylbenzidine exhibited a positive magnetoresistance ratio of up to 7.8%, which was the highest value among organic spin-valves at room temperature.

Investigating the effect of buffer layer on magnetoresistance in organic spin-valves

According to the magnetoresistance (MR) experiments in the Co/Al2O3/Alq3/LSMO device, we investigate spin injection and MR of this organic device. Taking into account the spin-related resistance and partial voltage of the Al2O3 buffer layer, we obtain an apparent adjustable MR of the organic device. A large MR is predicated with an optimized buffer layer thickness. In addition, the effects of different buffer layer materials on the MR are discussed to give a suggestion on experimental investigations.

The effect of dimerization on the magnetoresistance in organic spin valves

The effect of lattice dimerization on the magnetoresistance (MR) in organic spin valves is investigated based on the Su—Schrieffer—Heeger (SSH) model and the Green's function method. By comparing with the results for a uniform chain, we find that the dimerization of the molecular chain modifies the monotonic dependence of the MR on the bias to an oscillatory one. A sign inversion of the MR is observed when the amplitude of the dimerization is adjusted. The results also show that at a low bias, the MR through a dimerized chain decreases with the increasing bias as well as the increasing chain length, which is consistent with the experimental reports. A further understanding can be achieved by analyzing the electronic states and the spin-dependent transmission spectrum with the parallel and antiparallel magnetization orientations of the two ferromagnetic electrodes.

Magneto-resistance of organic spin valves due to spin-polarized tunnel injection and extraction of charge carriers

Spin-polarized tunnel injection and extraction of charge carriers can give rise to magneto-resistance in organic spin valves. To describe this magneto-resistance, the tunneling process is modeled as a transfer of electrons through a thin insulating layer between a ferromagnetic contact and an organic semiconductor. Transition rates between extended states in the metal and model “molecular” orbitals localized at the semiconductor/insulator interface are calculated based on a transfer Hamiltonian. The transition rates are then used in a rate equation model to calculate the injected current for the two spin types and the associated magneto-resistance of organic spin valves. Consistent with experimental data, it is found that the magneto-resistance can be of either sign and its magnitude strongly decreases with the applied bias.

Resonant magnetoresistance in organic spin valves (invited)

We investigate theoretically the effects of surface states over the magnetoresistance of Ni-based organic spin valves. In particular, we perform ab initio electronic transport calculations for a benzene-thiolate molecule chemically attached to a Ni [001] surface and contacted either by Te to another Ni [001] surface or terminated by a thiol group and probed by a Ni scanning tunnel microscope (STM) tip. In the case of S- and Te-bonded molecules we find a large asymmetry in the spin currents as a function of the bias, although the I-V is rather symmetric. This leads to a smooth although not monotonic dependence of the magnetoresistance over the bias. In contrast, in the case of a STM-type geometry we demonstrate that the spin current and the magnetoresistance can be drastically changed with bias. This is the result of a resonance between a spin-polarized surface state of the substrate and the d-shell band edge of the tip.

WITHDRAWN: Structural characterization and magnetoresistance in organic spin valves

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Giant magnetoresistance in organic spin-valves.

A spin valve is a layered structure of magnetic and non-magnetic (spacer) materials whose electrical resistance depends on the spin state of electrons passing through the device and so can be controlled by an external magnetic field. The discoveries of giant magnetoresistance and tunnelling magnetoresistance in metallic spin valves have revolutionized applications such as magnetic recording and memory, and launched the new field of spin electronics--'spintronics'. Intense research efforts are now devoted to extending these spin-dependent effects to semiconductor materials. But while there have been noteworthy advances in spin injection and detection using inorganic semiconductors, spin-valve devices with semiconducting spacers have not yet been demonstrated. pi-conjugated organic semiconductors may offer a promising alternative approach to semiconductor spintronics, by virtue of their relatively strong electron-phonon coupling and large spin coherence. Here we report the injection, transport and detection of spin-polarized carriers using an organic semiconductor as the spacer layer in a spin-valve structure, yielding low-temperature giant magnetoresistance effects as large as 40 per cent.