Molecular Spin Valve Research Articles

Large magnetoresistance and spin negative differential resistance in photoisomers-based molecular spin valves

By using non-equilibrium Green's functions in combination with density functional theory, the spin transport properties of photoisomers-based molecular spin valves constructed by salicylidene methylamine connected with two cobalt- dibenzotetraaza[14]annulene (Co-DBTAA) complex molecules via acetylene linkers are explored. A large magnetoresistance (MR) effect with a MR ratio larger than 104 can be observed in the molecular spin valves whether salicylidene methylamine is in the cis-enol form or the trans-keto form. Moreover, spin negative differential resistance (NDR) effects are also observed in the photoisomers-based molecular spin valves. The spin transport mechanisms are explored for MR and spin NDR effects observed in the molecular spin valves.

Interplay of structural chirality, electron spin and topological orbital in chiral molecular spin valves

Chirality has been a property of central importance in physics, chemistry and biology for more than a century. Recently, electrons were found to become spin polarized after transmitting through chiral molecules, crystals, and their hybrids. This phenomenon, called chirality-induced spin selectivity (CISS), presents broad application potentials and far-reaching fundamental implications involving intricate interplays among structural chirality, topological states, and electronic spin and orbitals. However, the microscopic picture of how chiral geometry influences electronic spin remains elusive, given the negligible spin-orbit coupling (SOC) in organic molecules. In this work, we address this issue via a direct comparison of magnetoconductance (MC) measurements on magnetic semiconductor-based chiral molecular spin valves with normal metal electrodes of contrasting SOC strengths. The experiment reveals that a heavy-metal electrode provides SOC to convert the orbital polarization induced by the chiral molecular structure to spin polarization. Our results illustrate the essential role of SOC in the metal electrode for the CISS spin valve effect. A tunneling model with a magnetochiral modulation of the potential barrier is shown to quantitatively account for the unusual transport behavior.

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

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.

Nearly perfect spin-filtering effect with tunable spin-polarization direction and huge magnetoresistance in molecular spin valves

Spin-transport properties of molecular spin valves constructed by two chromium-salophene (CrSal) molecules connected with boron nitride (BN) atomic chains are investigated by applying a nonequiblium Green's function method with density functional theory. Nearly perfect spin-filtering effect can be found when the molecular spin valves are set in parallel magnetic configuration. In particular, the direction of the detected spin-polarization signal is reversible by tuning the types of BN atomic chains. Moreover, obvious magnetoresistance (MR) effects are also found in the molecular spin valves. More importantly, the MR ratio can be changed from about 359% to about 1.31×105% by adjusting the types of BN atomic chains. Our work provides a new avenue to realize high-performance multifunctional molecular spin valves.

Length-induced large magnetoresistance in polyacene molecular spin valves

With the help of first-principles method, an unusual length effect in polyacene molecular spin valves is revealed. It is found that beyond a critical length, the magnitude of tunneling magnetoresistance is significantly enhanced from 10% to 2500%. Theoretical analysis demonstrates that such significant increase of magnetoresistance is related to the nonmagnetic-antiferromagnetic phase transition of polyacene molecules, which is determined by the relative magnetization orientation of the ferromagnetic electrodes. The molecular gap shows distinct length dependence with the change of molecular magnetic state, which keeps decreasing with length in the nonmagnetic state while a fine gap is obtained in the antiferromagnetic state. This work indicates a valid way to obtain considerable tunneling magnetoresistance in long molecular spin valves.

Electronic statistics on demand: Bunching, antibunching, positive, and negative correlations in a molecular spin valve

One of the long-standing goals of quantum transport is to use the noise, rather than the average current, for information processing. However, achieving this requires on-demand control of quantum fluctuations in the electric current. In this paper, we demonstrate theoretically that transport through a molecular spin-valve provides access to many different statistics of electron tunneling events. Simply by changing highly tunable parameters, such as electrode spin-polarization, magnetization angle, and voltage, one is able to switch between Poisson behavior, bunching and anti-bunching of electron tunnelings, and positive and negative temporal correlations. The molecular spin-valve is modeled by a single spin-degenerate molecular orbital with local electronic repulsion coupled to two ferromagnetic leads with magnetization orientations allowed to rotate relative to each other. The electron transport is described via Born-Markov master equation and fluctuations are studied with higher-order waiting time distributions. For highly magnetized parallel-aligned electrodes, we find that strong positive temporal correlations emerge in the voltage range where the second transport channel is partially open. These are caused by a spin-induced electron-bunching, which does not manifest in the stationary current alone.

Chemical Switching of the Magnetic Coupling in a MnPc Dimer by Means of Chemisorption and Axial Ligands

We present an ab initio density functional theory study of the magnetic properties of manganese phthalocyanine dimers, where we focus on the magnetic coupling between the Mn centers and on how it is affected by external factors like chemisorption or atomic axial ligands. We have studied several different configurations for the gas phase dimers, which resulted in ferromagnetic couplings of different magnitudes. For the bare dimer we find a significant ferromagnetic coupling between the Mn centers, which decreases by about 20% when a H atom is adsorbed on one of the Mn atoms and is reduced to about 7% when a Cl atom is adsorbed. The magnetic coupling is almost fully quenched when the dimer, bare or with the H ligand, is deposited on the ferromagnetic substrate Co(001). Our calculations indicate that the coupling between the two Mn atoms principally occurs via a superexchange interaction along two possible paths within a Mn–N–Mn–N four-atom loop. When these electrons get involved in chemical bonding outside the dimer itself, an appreciable alteration of the overlap between Mn and N molecular orbitals along the loop occurs, and consequently, the magnetic interaction between the Mn centers varies. We show that this is reflected by the electronic structure of the dimer in various configurations and is also visible in the structure of the atomic loop. The chemical tuning of the magnetic coupling is highly relevant for the design of nanodevices like molecular spin valves, where the molecules need to be anchored to a support.

Giant anisotropic magnetoresistance through a tilted molecular π -orbital

Anisotropic magnetoresistance (AMR), originating from spin-orbit coupling (SOC), is the sensitivity of the electrical resistance in magnetic systems to the direction of spin magnetization. Although this phenomenon has been experimentally reported for several nanoscale junctions, a clear understanding of the physical mechanism behind it is still elusive. Here we discuss a novel concept based on orbital symmetry considerations to attain a significant AMR of up to 95\% for a broad class of $\pi$-type molecular spin-valves. It is illustrated at the benzene-dithiolate molecule connected between two monoatomic nickel electrodes. We find that SOC opens, via spin-flip events at the ferromagnet-molecule interface, a new conduction channel, which is fully blocked by symmetry without SOC. Importantly, the interplay between main and new transport channels turns out to depend strongly on the magnetization direction in the nickel electrodes due to the tilting of molecular orbital. Moreover, due to multi-band quantum interference, appearing at the band edge of nickel electrodes, a transmission drop is observed just above the Fermi energy. Altogether, these effects lead to a significant AMR around the Fermi level, which even changes a sign. Our theoretical understanding, corroborated in terms of \textit{ab initio} calculations and simplified analytical models, reveals the general principles for an efficient realization of AMR in molecule-based spintronic devices.

Room-Temperature Operation of a p-Type Molecular Spin Photovoltaic Device on a Transparent Substrate.

The coupling of diverse degrees of freedom opens the door to physical effects that go beyond each of them individually, making multifunctionality a much sought-after attribute for high-performance devices. Here, the multifunctional operation of a single-layer p-type organic device, displaying both spin transport and photovoltaic effect at the room temperature on a transparent substrate, is shown. The generated photovoltage is almost three times larger than the applied bias to the device which facilitates the modulation of the magnetic response of the device with both bias and light. The device shows an increase in power conversion efficiency under magnetic field, an ability to invert the current with magnetic field and under certain conditions it can act as a spin photodetector with zero power consumption in the standby mode. The room-temperature exploitation of the interplay among light, bias, and magnetic field in the single device with a p-type molecule opens a way toward the development of efficient high-performance spin photovoltaic cells.

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.

Spintronic study based on molecular spin valves

Because of their extremely weak spin-orbit coupling and hence very long spin relaxation time, molecular semiconductors have considered to be very promising materials in spintronics. Since the first observation of spin transport in molecular semiconductors in 2002, molecular spintronics, as an emerging subject for studying spin-correlated phenomenon of electrons in molecular matrix, has attracted considerable research interests in the previous decade. Molecular spin valve is the most prototypical device in molecular spintronic, which should be the best test bed to study spintronic processes in molecules, including spin injection, spinterfacial effect, spin transport, as well as novel spintronic functions. Thus, to achieve highly reproducible molecular spin valves with high performances is of significant importance to the field of molecular spintronics, in which the efficient spin injection-transport process is the necessary precondition. However, according to current studies, there are several unexpected challenges for enhancing spin injection in spin valves, such as top electrode diffusion into molecular layer during device fabrication and the bad energy level alignment between ferromagnetic electrodes and molecules. To get away from these problems and improve device performance, considerable contributions have already been made to enhance spin injection from perspective of creating novel fabrication methods, optimizing device structure as well as various interfacial engineering methods. Besides, the spinterfacial effects at the interface between ferromagnetic electrodes and molecules are also considered to be crucial for spin injection. In latest studies, hybridized interface state (HIS) was proposed to characterize interaction between ferromagnetic electrode and the first molecular layer, and its spin-density of state at Fermi level of ferromagnetic electrode was changed. Therefore, spin polarization was influenced; as well its electronic structure at HIS was rebuilt to affect spin injection. In this review, research advances regarding spin transport mechanisms are also summarized, including tunneling transport and hopping transport separately. According to few recent reports, it is found that the hopping spin transport mode may avail longer spin transport distance especially at room temperature. Moreover, functional spintronic devices, integrated the spin transport and molecular functions (such as optical, electrical or spinterfacial) in one chip, are briefly introduced, which attracted considerable research interest very recently due to the great potential for applications in future. To summarize, the plentiful contributions regarding spintronic device, including performance optimization as well as mechanism study, have greatly facilitated the development of molecular spintronics in the past decade. However, molecular spintronics is an interdisciplinary filed of chemistry, physics and materials, thus exploring novel molecules that are suitable for spintronic applications must be one of the most important issues in future development of molecular spintronics.

The effect of electrodes on 11 acene molecular spin valve: Semi-empirical study

A new revolution in electronics is molecular spintronics, with the contemporary evolution of the two novel disciplines of spintronics and molecular electronics. The key point is the creation of molecular spin valve which consists of a diamagnetic molecule in between two magnetic leads. In this paper, non-equilibrium Green’s function (NEGF) combined with Extended Huckel Theory (EHT); a semi-empirical approach is used to analyse the electron transport characteristics of 11 acene molecular spin valve. We examine the spin-dependence transport on 11 acene molecular junction with various semi-infinite electrodes as Iron, Cobalt and Nickel. To analyse the spin-dependence transport properties the left and right electrodes are joined to the central region in parallel and anti-parallel configurations. We computed spin polarised device density of states, projected device density of states of carbon and the electrode element, and transmission of these devices. The results demonstrate that the effect of electrodes modifying the spin-dependence behaviours of these systems in a controlled way. In Parallel and anti-parallel configuration the separation of spin up and spin down is lager in the case of iron electrode than nickel and cobalt electrodes. It shows that iron is the best electrode for 11 acene spin valve device. Our theoretical results are reasonably impressive and trigger our motivation for comprehending the transport properties of these molecular-sized contacts.

Active Morphology Control for Concomitant Long Distance Spin Transport and Photoresponse in a Single Organic Device.

Long distance spin transport and photoresponse are demonstrated in a single F16 CuPc spin valve. By introducing a low-temperature strategy for controlling the morphology of the organic layer during the fabrication of a molecular spin valve, a large spin-diffusion length up to 180 nm is achieved at room temperature. Magnetoresistive and photoresponsive signals are simultaneously observed even in an air atmosphere.

Highly efficient spin polarizer based on individual heterometallic cubane single-molecule magnets

The spin-polarized transport across a single-molecule magnet [Mn3Zn(hmp)3O(N3)3(C3H5O2)3]·2CHCl3 has been investigated using a density functional theory combined with Keldysh non-equilibrium Green’s function formalism. It is shown that this single-molecule magnet has perfect spin filter behaviour. By adsorbing Ni3 cluster onto non-magnetic Au electrode, a large magnetoresistance exceeding 172% is found displaying molecular spin valve feature. Due to the tunneling via discrete quantum-mechanical states, the I–V curve has a stepwise character and negative differential resistance behaviour.

Spin‐Polarized Hopping Transport in Magnetically Tunable Rare‐Earth Quinolines

Molecular spin-valves based on magnetically tunable rare-earth quinolines are presented. Electrical characterization of the devices reveals genuine spin injection and transport across the molecules, supported by the coexistence of thermally activated hopping transport and a robust magnetoresistance effect at room temperature. It is found that both injection and spin-polarized transport are governed by the molecular ligand states, as there is no substantial effect from the molecular magnetic ions.

Magnetic interaction between a radical spin and a single-molecule magnet in a molecular spin-valve.

Molecular spintronics using single molecule magnets (SMMs) is a fast growing field of nanoscience that proposes to manipulate the magnetic and quantum information stored in these molecules. Herein we report evidence of a strong magnetic coupling between a metallic ion and a radical spin in one of the most extensively studied SMMs: the bis(phtalocyaninato)terbium(III) complex (TbPc2). For that we use an original multiterminal device comprising a carbon nanotube laterally coupled to the SMMs. The current through the device, sensitive to magnetic interactions, is used to probe the magnetization of a single Tb ion. Combining this electronic read-out with the transverse field technique has allowed us to measure the interaction between the terbium ion, its nuclear spin, and a single electron located on the phtalocyanine ligands. We show that the coupling between the Tb and this radical is strong enough to give extra resonances in the hysteresis loop that are not observed in the anionic form of the complex. The experimental results are then modeled by diagonalization of a three-spins Hamiltonian. This strong coupling offers perspectives for implementing nuclear and electron spin resonance techniques to perform basic quantum operations in TbPc2.

Effects of Molecular Conformation on the Spin-Dependent Transport Through the FM/Biphenyl/FM Junction

Effects of conformation on the spin-dependent transport in a molecular spin-valve, as FM/biphenyl/FM model junction, in which a biphenyl molecule attached to ferromagnetic (FM) electrodes are numerically investigated. The work is based on a tight-binding Hamiltonian model within the framework of a generalized Green’s function method and relies on the Landauer–Buttiker formalism as the basis for studying the spin-dependent transport properties of this system. We have found that transport characteristics, including the spin-dependent transmission function, current and tunnel magnetoresistance (TMR) are strongly influenced by the variation of twist angle between two phenyl rings. It is shown that the twist angle can alter the sign of the TMR from a positive to a negative value. We think that the results of the present report could be useful for designing the future molecular spintronic devices.

Spin transport properties of n-polyacene molecules (n = 1-15) connected to Ni surface electrodes: theoretical analysis.

Using non-equilibrium Green function formalism in conjunction with density functional theory, we explore the spin-polarized transport characteristics of several planar n-acene molecules suspended between two semi-infinite Ni electrodes via the thiol group. We examine the spin-dependence transport on Ni-n-acenes-Ni junctions, while the number of fused benzene rings varies between 1 and 15. Intriguingly, the induced magnetic moments of small acene molecules are higher than that of longer acene rings. The augmentation of fused benzene rings affects both the magnetic and transport features, such as the transmission function and conductance owing to their coupling to the Ni surface contacts via the anchoring group. The interplay between the spin-polarized transport properties, structural configuration and molecular electronic is a fortiori essential in these attractive molecular devices. Thus, this can conduct to the engineering of the electron spin transport in atomistic and molecular junctions. These prominent molecules convincingly infer that the molecular spin valves can conduct to thriving molecular devices.

Direct measurement of dysprosium(III)···dysprosium(III) interactions in a single-molecule magnet.

Lanthanide compounds show much higher energy barriers to magnetic relaxation than 3d-block compounds, and this has led to speculation that they could be used in molecular spintronic devices. Prototype molecular spin valves and molecular transistors have been reported, with remarkable experiments showing the influence of nuclear hyperfine coupling on transport properties. Modelling magnetic data measured on lanthanides is always complicated due to the strong spin-orbit coupling and subtle crystal field effects observed for the 4f-ions; this problem becomes still more challenging when interactions between lanthanide ions are also important. Such interactions have been shown to hinder and enhance magnetic relaxation in different examples, hence understanding their nature is vital. Here we are able to measure directly the interaction between two dysprosium(III) ions through multi-frequency electron paramagnetic resonance spectroscopy and other techniques, and explain how this influences the dynamic magnetic behaviour of the system.

Giant amplification of tunnel magnetoresistance in a molecular junction: Molecular spin-valve transistor

Amplification of tunnel magnetoresistance by gate field in a molecular junction is the most important requirement for the development of a molecular spin valve transistor. Herein, we predict a giant amplification of tunnel magnetoresistance in a single molecular spin valve junction, which consists of Ru-bis-terpyridine molecule as a spacer between two ferromagnetic nickel contacts. Based on the first-principles quantum transport approach, we show that a modest change in the gate field that is experimentally accessible can lead to a substantial amplification (320%) of tunnel magnetoresistance. The origin of such large amplification is attributed to the spin dependent modification of orbitals at the molecule-lead interface and the resultant Stark effect induced shift in channel position with respect to the Fermi energy.