Spin-Dependent Hybridization between Molecule and Metal at Room Temperature through Interlayer Exchange Coupling

The Role of Rare-Earth Atoms in the Anisotropy and Antiferromagnetic Exchange Coupling at a Hybrid Metal-Organic Interface

Magnetic anisotropy and magnetic exchange interactions are crucial parameters that characterize the hybrid metal-organic interface, a key component of an organic spintronic device. It is shown that the incorporation of 4f RE atoms to hybrid metal-organic interfaces of CuPc/REAu2 type (RE = Gd, Ho) constitutes a feasible approach toward on-demand magnetic properties and functionalities. The GdAu2 and HoAu2 substrates differ in their magnetic anisotropy behavior. Remarkably, the HoAu2 surface promotes the inherent out-of-plane anisotropy of CuPc, owing to the match between the anisotropy axis of substrate and molecule. Furthermore, the presence of RE atoms leads to a spontaneous antiferromagnetic exchange coupling at the interface, induced by the 3d-4f superexchange interaction between the unpaired 3d electron of CuPc and the 4f electrons of the RE atoms. It is shown that 4f RE atoms with unquenched quantum orbital momentum ( L $L$ ), as it is the case of Ho, induce an anisotropic interfacial exchange coupling.

Room-temperature-persistent magnetic interaction between coordination complexes and nanoparticles in maghemite-based nanohybrids

Maghemite nanoparticles functionalised with Co(II) coordination complexes at their surface show a significant increase of their magnetic anisotropy, leading to a doubling of the blocking temperature and a sixfold increase of the coercive field. Magnetometric studies suggest an enhancement that is not related to surface disordering, and point to a molecular effect involving magnetic exchange interactions mediated by the oxygen atoms at the interface as its source. Field- and temperature-dependent X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) studies show that the magnetic anisotropy enhancement is not limited to surface atoms and involves the core of the nanoparticle. These studies also point to a mechanism driven by anisotropic exchange and confirm the strength of the magnetic exchange interactions. The coupling between the complex and the nanoparticle persists at room temperature. Simulations based on the XMCD data give an effective exchange field value through the oxido coordination bridge between the Co(II) complex and the nanoparticle that is comparable to the exchange field between iron ions in bulk maghemite. Further evidence of the effectiveness of the oxido coordination bridge in mediating the magnetic interaction at the interface is given with the Ni(II) analog to the Co(II) surface-functionalised nanoparticles. A substrate-induced magnetic response is observed for the Ni(II) complexes, up to room temperature.

Observation of Exchange Interaction in Iron(II) Spin Crossover Molecules in Contact with Passivated Ferromagnetic Surface of Co/Au(111)

Spin crossover (SCO) complexes sensitively react on changes of the environment by a change in the spin of the central metallic ion making them ideal candidates for molecular spintronics. In particular, the composite of SCO complexes and ferromagnetic (FM) surfaces would allow spin-state switching of the molecules in combination with the magnetic exchange interaction to the magnetic substrate. Unfortunately, when depositing SCO complexes on ferromagnetic surfaces, spin-state switching is blocked by the relatively strong interaction between the adsorbed molecules and the surface. Here, the Fe(II) SCO complex [Fe^II (Pyrz)₂ ] (Pyrz = 3,5-dimethylpyrazolylborate) with sub-monolayer thickness in contact with a passivated FM film of Co on Au(111) is studied. In this case, the molecules preserve thermal spin crossover and at the same time the high-spin species show a sizable exchange interaction of > 0.9 T with the FM Co substrate. These observations provide a feasible design strategy in fabricating SCO-FM hybrid devices.

Recent Progress for Single-Molecule Magnets Based on Rare Earth Elements

Single-molecule magnets (SMMs) have attracted much attention due to their potential applications in molecular spintronic devices. Rare earth SMMs are considered to be the most promising for application owing to their large magnetic moment and strong magnetic anisotropy. In this review, the recent progress in rare earth SMMs represented by mononuclear and dinuclear complexes is highlighted, especially for the modulation of magnetic anisotropy, effective energy barrier (U_eff) and blocking temperature (T_B). The terbium- and dysprosium-based SMMs have a U_eff of 1541 cm^-1 and an increased T_B of 80 K. They break the boiling point temperature of liquid nitrogen. The development of the preparation technology of rare earth element SMMs is also summarized in an overview. This review has important implications and insights for the design and research of Ln-SMMs.

Organic/metal interface-modulated magnetism in [Fe/C60]3 multilayers and Fe-C60 composites

Because of the expected long spin-transport length of organic materials, the magnetic metal/organic interface is crucial to the application of organic spintronics. In this study, [Fe/C₆₀]₃ multilayers were fabricated for the investigation of C₆₀-mediated magnetic interlayer coupling. [Fe/C₆₀]₃ thin films were characterized using the magneto-optical Kerr effect, transmission electron microscopy, Raman spectroscopy, and x-ray photoelectron spectroscopy (XPS). The thin films revealed in-plane magnetic anisotropy, and the magnetic coercivity (H _c ) drastically decreased from 6-8 mT to 0.5 mT with the increase of C₆₀ thickness from 0.1 nm to 5 nm. The insertion of the C₆₀ layer considerably reduced H _c because a thickness greater than 1 nm of the C₆₀ layer is sufficient for blocking magnetic exchange coupling between Fe layers. In addition, post-annealing increased H _c because of Fe inter-diffusion, which promotes magnetic exchange coupling and further Fe-C bonding, as confirmed by a comparative study of XPS C-spectra. The thermally triggered inter-diffusion between Fe and C₆₀ layers turned the multilayers into a mixed composite film and thus caused magnetic variation. Annealing time and temperature can be used as control parameters for the tuning of magnetism in Fe-C₆₀ composites.

Giant spin signals in chemically functionalized multiwall carbon nanotubes

Transporting quantum information such as the spin information over micrometric or even millimetric distances is a strong requirement for the next-generation electronic circuits such as low-voltage spin-logic devices. This crucial step of transportation remains delicate in nontopologically protected systems because of the volatile nature of spin states. Here, a beneficial combination of different phenomena is used to approach this sought-after milestone for the beyond-Complementary Metal Oxide Semiconductor (CMOS) technology roadmap. First, a strongly spin-polarized charge current is injected using highly spin-polarized hybridized states emerging at the complex ferromagnetic metal/molecule interfaces. Second, the spin information is brought toward the conducting inner shells of a multiwall carbon nanotube used as a confined nanoguide benefiting from both weak spin-orbit and hyperfine interactions. The spin information is finally electrically converted because of a strong magnetoresistive effect. The experimental results are also supported by calculations qualitatively revealing exceptional spin transport properties of this system.

Reversible coordination-induced spin-state switching in complexes on metal surfaces

Molecular spin switches are attractive candidates for controlling the spin polarization developing at the interface between molecules and magnetic metal surfaces^1,2, which is relevant for molecular spintronics devices^3-5. However, so far, intrinsic spin switches such as spin-crossover complexes have suffered from fragmentation or loss of functionality following adsorption on metal surfaces, with rare exceptions^6-9. Robust metal-organic platforms, on the other hand, rely on external axial ligands to induce spin switching^10-14. Here we integrate a spin switching functionality into robust complexes, relying on the mechanical movement of an axial ligand strapped to the porphyrin ring. Reversible interlocked switching of spin and coordination, induced by electron injection, is demonstrated on Ag(111) for this class of compounds. The stability of the two spin and coordination states of the molecules exceeds days at 4 K. The potential applications of this switching concept go beyond the spin functionality, and may turn out to be useful for controlling the catalytic activity of surfaces¹⁵.

Tuning spinterface properties in iron/fullerene thin films

In ferromagnetic (FM) metal/organic semiconductor (OSC) heterostructures charge transfer can occur which leads to induction of magnetism in the non-magnetic OSC. This phenomenon has been described by the change in the density of states in the OSC which leads to a finite magnetic moment at the OSC interface and it is called the 'spinterface'. One of the main motivations in this field of organic spintronics is how to control the magnetic moment in the spinterface. In this regard, there are several open questions such as (i) which combination of FM and OSC can lead to more moment at the spinterface? (ii) Is the thickness of OSC also important? (iii) How does the spinterface moment vary with the FM thickness? (iv) Does the crystalline quality of the FM matter? (v) What is the effect of spinterface on magnetization reversal, domain structure and anisotropy? In this context, we have tried to answer the last four issues in this paper by studying Fe/C₆₀ bilayers of variable Fe thickness deposited on Si substrates. We find that both the induced moment and thickness of the spinterface vary proportionally with the Fe thickness. Such behavior is explained in terms of the growth quality of the Fe layer on the native oxide of the Si (100) substrate. The magnetization reversal, domain structure and anisotropy of these bilayer samples were studied and compared with their respective reference samples without the C₆₀ layer. It is observed that the formation of spinterface leads to a reduction in uniaxial anisotropy in Fe/C₆₀ on Si (100) in comparison to their reference samples.

Realization of Spin-dependent Functionality by Covering a Metal Surface with a Single Layer of Molecules

An interface of molecule and metal has attracted much attention in the research field of nanoelectronics because of their high degree of design freedom. Here, we demonstrate an efficient spin-to-charge current conversion at the metal surface covered by a single layer of molecules. Spin currents are injected into an interface between metal (Cu) and lead(II) phthalocyanine by means of the spin pumping method. An observed voltage signal is caused by the inverse Edelstein effect, i.e., spin-to-charge current conversion at the interface. The conversion coefficient, inverse Edelstein length, is estimated to be 0.40 ± 0.06 nm, comparable with the largest Rashba spin splitting of interfaces with heavy metals. Interestingly, the Edelstein length strongly depends on the thickness of the molecule and takes a maximum value when a single layer of molecules is formed on the Cu surface. Comparative analysis between scanning probe microscopy and first-principles calculations reveal that the formation of interface state with Rashba spin splitting causes the inverse Edelstein effect, whose magnitude is sensitive to the adsorption configuration of the molecules.

Iron Phthalocyanine and Ferromagnetic Thin Films: Magnetic Behavior of Single and Double Interfaces

Metal-phthalocyanines are quasi-planar heterocyclic macrocycle molecules with a highly conjugated structure. They can be engineered at the molecular scale (central atom, ligand) to tailor new properties for organic spintronics devices. In this study, we evaluated the magnetic behavior of FePc in a ∼1 nm molecular film sandwiched between two ferromagnetic films: cobalt (bottom) and nickel (top). In the single interface, FePc in contact with a Co film is magnetically coupled with the inorganic film magnetization, though the relatively small Fe(Pc) X-ray magnetic circular dichroism (XMCD) signal in remanence, with respect to that observed in applied field of 6 T, suggests that a fraction of molecules in the organometallic film have their magnetic moment not aligned or antiparallel with respect to Co. When in contact with two interfaces, Fe(Pc) XMCD doubles, indicating that part of the Fe(Pc) are now aligned with the Ni topmost layer, saturated at 1 T. We discussed the relevance of the finding in terms of understanding and developing hybrid organic/inorganic spin devices.

Linking Electronic Transport through a Spin Crossover Thin Film to the Molecular Spin State Using X-ray Absorption Spectroscopy Operando Techniques

One promising route toward encoding information is to utilize the two stable electronic states of a spin crossover molecule. Although this property is clearly manifested in transport across single molecule junctions, evidence linking charge transport across a solid-state device to the molecular film's spin state has thus far remained indirect. To establish this link, we deploy materials-centric and device-centric operando experiments involving X-ray absorption spectroscopy. We find a correlation between the temperature dependencies of the junction resistance and the Fe spin state within the device's [Fe(H₂B(pz)₂)₂(NH₂-phen)] molecular film. We also factually observe that the Fe molecular site mediates charge transport. Our dual operando studies reveal that transport involves a subset of molecules within an electronically heterogeneous spin crossover film. Our work confers an insight that substantially improves the state-of-the-art regarding spin crossover-based devices, thanks to a methodology that can benefit device studies of other next-generation molecular compounds.

Light-Induced Spin Crossover in an Fe(II) Low-Spin Complex Enabled by Surface Adsorption

Understanding and controlling the spin-crossover properties of molecular complexes can be of particular interest for potential applications in molecular spintronics. Using near-edge X-ray absorption fine structure spectroscopy, we investigated these properties for a new vacuum-evaporable Fe(II) complex, namely [Fe(pypyr(CF₃)₂)₂(phen)] (pypyr = 2-(2'-pyridyl)pyrrolide, phen = 1,10-phenanthroline). We find that the spin-transition temperature, well above room temperature for the bulk compound, is drastically lowered for molecules arranged in thin films. Furthermore, while within the experimentally accessible temperature range (2 K < T < 410 K) the bulk material shows indication of neither light-induced excited spin-state trapping nor soft X-ray-induced excited spin-state trapping, these effects are observed for molecules within thin films up to temperatures around 100 K. Thus, by arranging the molecules into thin films, a nominal low-spin complex is effectively transformed into a spin-crossover complex.

Robust and Selective Switching of an FeIII Spin-Crossover Compound on Cu2N/Cu(100) with Memristance Behavior

The switching between two spin states makes spin-crossover molecules on surfaces very attractive for potential applications in molecular spintronics. Using scanning tunneling microscopy, the successful deposition of [Fe(pap)₂]⁺ (pap = N-2-pyridylmethylidene-2-hydroxyphenylaminato) molecules on Cu₂N/Cu(100) surface is evidenced. The deposited Fe^III spin-crossover compound is controllably switched between three different states, each of them exhibiting a characteristic tunneling conductance. The conductance is therefore employed to readily read the state of the molecules. A comparison of the experimental data with the results of density functional theory calculations reveals that all Fe(pap)₂ molecules are initially in their high-spin state. The two other states are compatible with the low-spin state of the molecule but differ with respect to their coupling to the substrate. As a proof of concept, the reversible and selective nature of the switching is used to build a two-molecule memory.

Ligand-Induced Energy Shift and Localization of Kondo Resonances in Cobalt-Based Complexes on Cu(111)

Magnetic sandwich complexes are of particular interest for molecular spintronics. Using scanning tunneling microscopy, we evidence the successful deposition of 1,3,5-tris(η⁶-borabenzene-η⁵-cyclopentadienylcobalt) benzene, a molecule composed of three connected magnetic sandwich units, on Cu(111). Scanning tunneling spectra reveal two distinct spatial-dependent narrow resonances close to the Fermi level for the trimer molecules as well as for molecular fragments composed of one and two magnetic units. With the help of density functional theory, these resonances are interpreted as two Kondo resonances originating from two distinct nondegenerate d-like orbitals. These Kondo resonances are found to have defined spatial extents dictated by the hybridization of the involved orbitals with that of the ligands. These results opens promising perspectives for investigating complex Kondo systems composed of several "Kondo" orbitals.

Role of Metal Lattice Expansion and Molecular π-Conjugation for the Magnetic Hardening at Cu-Organics Interfaces

Magnetic hardening and generation of room-temperature ferromagnetism at the interface between originally nonmagnetic transition metals and π-conjugated organics is understood to be promoted by interplay between interfacial charge transfer and relaxation-induced distortion of the metal lattice. The relative importance of the two contributions for magnetic hardening of the metal remains unquantified. Here, we disentangle their role via density functional theory simulation of several models of interfaces between Cu and polymers of different steric hindrance, π-conjugation, and electron-accepting properties: polyethylene, polyacetylene, polyethylene terephthalate, and polyurethane. In the absence of charge transfer, expansion and compression of the Cu face-centered cubic lattice is computed to lead to magnetic hardening and softening, respectively. Contrary to expectations based on the extent of π-conjugation on the organic and resulting charge transfer, the computed magnetic hardening is largest for Cu interfaced with polyethylene and smallest for the Cu-polyacetylene systems as a result of a differently favorable rehybridization leading to different enhancement of exchange interactions and density of states at the Fermi level. It thus transpires that neither the presence of molecular π-conjugation nor substantial charge transfer may be strictly needed for magnetic hardening of Cu-substrates, widening the range of organics of potential interest for enhancement of emergent magnetism at metal-organic interfaces.

Emergent magnetism at transition-metal-nanocarbon interfaces

Charge transfer at metallo-molecular interfaces may be used to design multifunctional hybrids with an emergent magnetization that may offer an eco-friendly and tunable alternative to conventional magnets and devices. Here, we investigate the origin of the magnetism arising at these interfaces by using different techniques to probe 3d and 5d metal films such as Sc, Mn, Cu, and Pt in contact with fullerenes and rf-sputtered carbon layers. These systems exhibit small anisotropy and coercivity together with a high Curie point. Low-energy muon spin spectroscopy in Cu and Sc-C60 multilayers show a quick spin depolarization and oscillations attributed to nonuniform local magnetic fields close to the metallo-carbon interface. The hybridization state of the carbon layers plays a crucial role, and we observe an increased magnetization as sp3 orbitals are annealed into sp2-π graphitic states in sputtered carbon/copper multilayers. X-ray magnetic circular dichroism (XMCD) measurements at the carbon K edge of C60 layers in contact with Sc films show spin polarization in the lowest unoccupied molecular orbital (LUMO) and higher π*-molecular levels, whereas the dichroism in the σ*-resonances is small or nonexistent. These results support the idea of an interaction mediated via charge transfer from the metal and dz-π hybridization. Thin-film carbon-based magnets may allow for the manipulation of spin ordering at metallic surfaces using electrooptical signals, with potential applications in computing, sensors, and other multifunctional magnetic devices.

Controlling the magnetism of adsorbed metal-organic molecules

Gaining control on the size or the direction of the magnetic moment of adsorbed metal-organic molecules constitutes an important step towards the realization of a surface-mounted molecular spin electronics. Such control can be gained by taking advantage of interactions of the molecule's magnetic moment with the environment. The paramagnetic moments of adsorbed metal-organic molecules, for example, can be controlled by the interaction with magnetically ordered substrates. Metalloporphyrins and -phthalocyanines display a quasi-planar geometry, allowing the central metal ion to interact with substrate electronic states. This can lead to magnetic coupling with a ferromagnetic or even antiferromagnetic substrate. The molecule-substrate coupling can be mediated and controlled by insertion layers such as oxygen atoms, graphene, or nonmagnetic metal layers. Control on the magnetic properties of adsorbed metalloporphyrins or -phthalocyanines can also be gained by on-surface chemical modification of the molecules. The magnetic moment or the magnetic coupling to ferromagnetic substrates can be changed by adsorption and thermal desorption of small molecules that interact with the fourfold-coordinated metal center via the remaining axial coordination site. Spin-crossover molecules, which possess a metastable spin state that can be switched by external stimuli such as temperature or light, are another promising class of candidates for control of magnetic properties. However, the immobilization of such molecules on a solid surface often results in a quench of the spin transition due to the interaction with the substrate. We present examples of Fe(II) spin-crossover complexes in direct contact with a solid surface that undergo a reversible spin-crossover transition as a function of temperature, by illumination with visible light, or can be switched by the tip of a scanning tunneling microscope.

High Spin Polarization at Ferromagnetic Metal-Organic Interfaces: A Generic Property

A high spin polarization of states around the Fermi level, EF, at room temperature has been measured in the past at the interface between a few molecular candidates and the ferromagnetic metal Co. Is this promising property for spintronics limited to these candidates? Previous reports suggested that certain conditions, such as strong ferromagnetism, i.e., a fully occupied spin-up d band of the ferromagnet, or the presence of π bonds on the molecule, i.e., molecular conjugation, needed to be met. What rules govern the presence of this property? We have performed spin-resolved photoemission spectroscopy measurements on a variety of such interfaces. We find that this property is robust against changes to the molecule and ferromagnetic metal's electronic properties, including the aforementioned conditions. This affirms the generality of highly spin-polarized states at the interface between a ferromagnetic metal and a molecule and augurs bright prospects toward integrating these interfaces within organic spintronic devices.

Exchange Coupling of Spin-Crossover Molecules to Ferromagnetic Co Islands

The properties of Fe(1,10-phenanthroline)2(NCS)2 (Fe-phen) molecules deposited on Co/Cu(111) are studied with scanning tunneling microscopy (STM) operated in ultrahigh vacuum at low temperature (4 K) and ab initio calculations. Both the experimental and theoretical results are used to identify the high-spin (HS) state of Fe-phen. Additionally, the calculations reveal a strong spin-polarization of the density of states (DOS) and is validated experimentally using the spin sensitivity of spin-polarized STM. Finally, it is shown that the magnetic moment of the Fe-ion within HS Fe-phen is strongly magnetically coupled to the underlying magnetic Co through the NCS groups. These findings enable promising spintronic perspectives.