Molecular spintronics: the role of coordination chemistry

State of the art

Spin-based electronics (or spintronics) is an emerging branch of nanotechnology and the most active area within nanomagnetism. This area was born in the 80s as a result of the convergence between conventional electronics and magnetism. Its central theme is the use of the electron spin to process information, and one of the first achievements in this field was the discovery of giant magnetoresistance in metallic multilayers by Albert Fert and Peter Grünberg, Nobel Prize winners in Physics in 2007. This discovery had a strong scientific impact and opened the way to a completely new technology developed by IBM, which resulted in applications in hard drive information storage and in magnetic sensing.¹

So far, spintronics has been based on conventional materials like inorganic metals and semiconductors. However, with the contemporary evolution of molecular electronics and molecular magnetism and with the evolution of spintronics towards nanospintronics, a new field, namely Molecular Spintronics, which combines the ideas and concepts developed in spintronics with the unique possibilities offered by molecular systems to perform electronic functions, to form self-organized nanostructures and to exhibit quantum effects at the nanoscale, has emerged. The ultimate goals of Molecular Spintronics are the fabrication of new and cheaper spintronic devices using molecular materials and the reduction of device sizes down to one molecule or a few molecules in the race toward miniaturization.²

By analogy to what happens in molecular electronics, Molecular Spintronics can be divided in two major areas: molecular-based spintronics (or organic spintronics) and single-molecule spintronics (or molecular quantum spintronics). In the first sub-area – organic spintronics – the main goal has been that of integrating molecule-based materials into spintronic heterostructures to obtain second-generation spintronic devices, which may be compatible with plastic technology. The most popular devices to study the spin transport are the so-called spin valves and magnetic tunnel junctions. Typically, these involve sandwich trilayer heterostructures in which a non-magnetic material (metallic or insulating) acting as a spin collector is placed between two ferromagnetic electrodes. These layered structures are quite similar to those already developed in OLED technologies, and therefore, they will benefit from the advances produced in these areas. Still, the few attempts of incorporating molecules into these spintronic heterostructures have been limited to the use of molecular systems as spacers situated in between the two ferromagnetic electrodes. Interestingly, these initial attempts have already shown that novel properties, such as reversal of the spin polarization of a ferromagnetic metal, can emerge at the molecular/inorganic interface (spinterface), which is due to the electronic interactions between the interfacial molecules and the metal surface.³ This result is really important since it completely changes our perception of the molecules in these systems. Their role is not merely limited to the transport of the spin, but they are also able to tune and even to reverse the spin at the surface of the metal, a role which is very difficult to reach with inorganic materials. Another development in this sub-area has been propelled by the possibility of creating multifunctional devices by taking advantage of the multiple properties that a molecular material may exhibit. Thus, new types of hybrid devices are emerging in which magnetoresistive and photoresistive signals are simultaneously observed,⁴ or in which the light emission of an OLED can be tuned through a magnetic field.⁵

In the second sub-area – molecular quantum spintronics – the challenge is to condense the functionality of a spintronic device into a single molecule and to do so reliably and reproducibly. This aspect has seen very little development experimentally. Over the last few years, in molecular nanoelectronics, it has been demonstrated that it is possible to attach metallic contacts to individual organic molecules, triggering single-molecule studies of electrical transport and, in particular, its sensitivity to conformational changes and to external stimuli, such as temperature, magnetic field and light. Although transport through single molecules (or self-assembled molecular monolayers) has been extensively studied in molecular electronics, very few experiments have been reported so far in the case of magnetic molecules. Two methods are commonly used to perform such experiments: in one case, the molecule is connected between an STM tip and a conducting surface, and the other case involves a single-molecule transistor architecture, in which a break-junction setup traps the molecule between two electrodes while a third electrode acts as a gate. There are still many problems, which are intrinsic to the magnetic molecules, to be solved to obtain reliable results. A first concern deals with the stability of these molecules when they are deposited on a metallic surface. In fact, most of the molecules of interest (single-molecule magnets, SMMs, for example) are coordination metal complexes, which often undergo dramatic structural and chemical changes when they are transferred from the solution to the surface. A second concern involves the positioning of magnetic molecules in a controlled manner and with a given orientation on the surface (or in between the electrodes). Assuming that these chemical and processing problems can be solved, the next step is that of fabricating nanodevices in which the molecular spins can be addressed. However, very few functional single-molecule devices have been reported. In fact, only very recently, the first experimental steps in the manipulation of molecular spin instead of its charge have been undertaken, and the studies have revealed the potential for memory or quantum technologies.^6,7 This novel approach in the intersection of molecular electronics with molecular magnetism will enable the creation of electrically addressable magnetic qubits and single-molecule spin switches, opening the possibility to tailor and manipulate molecules down to the single spin. Finally, the control in the deposition of magnetic molecules on surfaces can be important to produce self-assembled ordered arrays of identical magnetic “dots” of nanometer size for their use in information storage devices. In this context, the use of the self-organization processes for patterning technologies will be particularly appealing thanks to their low cost and high production efficiency.

An appealing possibility arising from the quantum effects associated with the above spintronic nanodevices is that of using magnetic molecules not only for the storage of classical bits but also for the creation, manipulation and readout of quantum superpositions of two spin states. In other words, spin qubits would be realized. The possible advantages of molecular spin systems in the field of quantum technologies are the long quantum-coherence times they exhibit compared with semiconducting materials and their scalability either by arranging several qubits in a given molecule or via self-assembly processes of individual molecules. However, the coherence time of the magnetic molecules must be increased. Application of error correction protocols means that gate operations must be 10⁴ times faster than the rate at which qubits lose coherence (at present figure of merit of 10³ has been demonstrated). The couplings of the spins with the environment (phonons, nuclear spins, dipolar interactions) are important sources of decoherence, which are far from being understood and controlled. Still, via appropriate molecular design, some sources of decoherence (nuclear hyperfine and dipolar interactions) can be minimized.⁸ A second important issue to be addressed is the search for new spin qubit architectures and the schemes for applying quantum information processing tasks.⁹ In this context, the quantum behaviour of magnetic molecules is unique for the richness and variety of levels and states and for the wide possibilities to couple spins between them (entanglement) or with the external world (photons, electrons, nuclei, phonons). This issue involves strong theoretical and experimental efforts.

Role of coordination chemistry

Above, we presented the different aspects of the burgeoning field of Molecular Spintronics, and chemistry is playing a central role because it provides the variety molecules that are needed to build spintronic devices and spin qubits. Coordination chemistry is the major source of molecules in this context.

In spite of its name, organic spintronics is largely based on coordination complexes which act as molecular spacers and spin collectors. The term “organic” was first introduced by physicists to cover any kind of molecule independent of its chemical nature (purely organic, purely inorganic, and metal–organic). This is the case of the phthalocyanine and the Alq₃ families (tris-(8-hydroxyquinolinato)aluminium), which are two relevant examples. These are coordination compounds and not organic compounds. In fact, Alq₃ is an organometallic coordination complex in which a metal ion is surrounded by quinoline-type organic ligands; it has been extensively used in molecular optoelectronics and is the most used molecule in the construction of molecular spin valves.¹⁰ In general, these molecules must be thermally stable since their incorporation into a device usually involves evaporation onto a substrate using high-vacuum technologies. Hence, over the last few years, a current trend in this area has been to design new molecular complexes (magnetic or not) to be used as spin collectors in these molecular devices.¹¹

Molecular magnetic materials based coordination compounds have also been a source of inspiration in molecular spintronics. A key example was provided by combining a bimetallic coordination compound based on an oxalate ligand with an organic donor, i.e., BEDT-TTF.¹² This hybrid system provided the first multi-layered molecular material exhibiting both electrical conductivity and magnetism, opening the possibility for designing multifunctional molecular materials with interesting magneto-resistance properties. This possibility was first illustrated in a purely organic system formed by an organic radical directly bonded to an organic donor molecule, which exhibited giant negative magneto-resistance.¹³ More recently, such a behaviour has also observed in other magnetic materials based on coordination compounds. The double-decker type phthalocyaninato-Tb(III) SMM (TbPc₂) is a very good candidate for giant- and tunnelling-magnetoresistance (GMR and TMR, respectively). When TbPc₂ is evaporated onto a Co surface, 200% GMR was observed by using Cr tip. Moreover, when ferromagnetic Co and TbPc₂ were evaporated on an Au surface, double-butterfly type TMR of 350% was observed by using a Cr tip.

In the area of single-molecule spintronics coordination chemistry is ubiquitous. Thus, the magnetic molecules that are most often used are the so-called single-molecule magnets (SMMs), based either on magnetic clusters or, more recently, on coordination complexes containing a single metal ion.¹⁴ These molecules exhibit slow relaxation of the magnetization at low temperatures and marked quantum effects. A second class of magnetic molecules are the so-called spin-crossover compounds, which are almost exclusively based on iron coordination complexes. These molecules undergo a spin transition from low to high spin configurations upon the application of external stimuli, such as temperature, light or pressure. Thus, they provide unique examples of molecular switching and bistability. For more than 30 years, the community of molecular magnetism has been chemically designing and studying the properties of crystalline compounds based on these magnetic molecules, and hundreds of new SMMs based on mononuclear lanthanoid complexes have been reported in the last decade.

With the development of molecular spintronics, the challenge has become to study these chemical objects on the nanoscale. Thus, a significant effort has been devoted to the deposition and self-assembly of these molecules on surfaces in order to check the stability and properties of the individual molecules. A recent result worth mentioning in this context is the self-assembly of TbPc₂ SMMs on an insulating MgO thin layer.¹⁵ Thanks to the insertion of a tunnel barrier between the SMMs and the electrode, the magnetic remanence and hysteresis opening of these molecules outperform the properties of any other surface-adsorbed SMMs, as well as those of bulk samples of TbPc₂. In a second step, some nanodevices containing these molecules have started to appear in which the molecules have been placed between two electrodes or deposited on carbon nanotubes.¹⁶ As already pointed out, these initial steps have required, and still require, a large chemical effort in order to make the molecular complexes suitable for integration into devices. Thus, a current focus in this area is the design of robust molecular coordination complexes that survive the processing techniques required to fabricate the device and that are chemically stable when they are in direct contact with the device components (electrodes and substrates). In some cases, this will require the design of thermally stable molecular species and, in many other cases, functionalization of the molecule in order to tune its interactions with the device components and its chemical stability.

Recent examples of molecular spin qubits are also based on coordination complexes. In fact, some of the research on SMMs has now been re-directed towards their use in quantum computing. This is the case of magnetic polyoxometalates based on lanthanoids, which under some circumstances can either behave as SMMs or as molecular spin qubits.¹⁷ Still, as already noticed, a spin qubit has to exhibit a long quantum coherence time to be useful. To reach this goal, several approaches are being pursued in which several mononuclear transition metal complexes are being tested. Thus, playing with the rigidity of the lattice as well as removal of nuclear spins from the vicinity of the magnetic ion, impressive quantum coherences on the order of microseconds at room temperature have been obtained using transition metal complexes, such as [Cu(mnt)₂]²⁻ (mnt²⁻ = maleonitriledithiolate)¹⁸ and vanadyl phthalocyanine,¹⁹ reaching coherence values as long as milliseconds at low temperatures²⁰ for the complex [V(C₈S₈)₃]²⁻. These values are quite competitive with those reported for spin-qubits isolated in inorganic lattices, such as the diamond nitrogen-vacancy (NV) centres.

In this special issue, the major achievements, current trends and challenging goals in molecular spintronics are highlighted. We focus, in particular, on the key role that coordination chemistry is playing in this area by providing a variety of molecular species and molecular materials that can be used either as active components of second generation spintronic devices as well as magnetic units in molecular quantum spintronics and quantum technologies. Thus, we hope that the studies presented in this special issue will be important and useful to the variety of scientists if the field of Molecular Spintronics.

References

1. A. Fert, Rev. Mod. Phys., 2008, 80, 1517.
2. J. Camarero and E. Coronado, J. Mater. Chem., 2009, 19, 1678.
3. C. Barraud, et al. , Nat. Phys., 2010, 6, 615.
4. X. Sun, et al. , Adv. Mater., 2016, 28, 2609.
5. E. Coronado, J. P. Prieto-Ruiz and H. Prima, P201300083.
6. F. Prins, et al. , Adv. Mater., 2011, 23, 1545.
7. R. Vincent, et al. , Nature, 2012, 488, 357.
8. M. Shiddiq, et al. , Nature, 2016, 531, 348.
9. G. Aromi, et al. , Chem. Soc. Rev., 2012, 41, 537.
10. F. Wang and Z. V. Vardeni, J. Mater. Chem., 2009, 19, 1685.
11. A. Bedoya-Pinto, et al. , Adv. Electron. Mater., 2015, 1, 1500065.
12. E. Coronado, et al. , Nature, 2000, 408, 447.
13. T. Sugawara and M. M. Matsushita, J. Mater. Chem., 2009, 19, 1738.
14. L. Bogani and W. Wernsdorfer, Nat. Mater., 2008, 7, 179.
15. C. Wäckerlin, et al. , Adv. Mater., 2016, 28, 5195.
16. M. Urdampilleta, et al. , Nat. Mater., 2011, 10, 502.
17. J. M. Clemente-Juan, E. Coronado and A. Gaita-Ariño, Chem. Soc. Rev., 2012, 41, 7464.
18. K. Bader, et al. , Nat. Commun., 2014, 5, 5304.
19. M. Atzori, et al. , J. Am. Chem. Soc., 2016, 138, 2154.
20. J. M. Zadrozny, et al. , ACS Cent. Sci., 2015, 1, 488.

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