Relay-Like Exchange Mechanism through a Spin Radical between TbPc2 Molecules and Graphene/Ni(111) Substrates

An approach for patterned molecular adsorption on ferromagnets, achieved via Moiré superstructures

We have used a scanning tunneling microscope operated under ultrahigh vacuum conditions to investigate an oxo-vanadium-salen complex V(O)salen, that has potential applications as qubits in future quantum-based technologies. The adsorption and self-assembly of V(O)salen on a range of single crystal metal surfaces and nanoislands and the influence of substrate morphology and reactivity has been measured. On the close-packed flat Ag(111) and Cu(111) surfaces, the molecules adsorb isolated or form small clusters arranged randomly on the surface, whereas structured adsorption occurs on two types of Co nanoislands; Co grown on Ag(111) and Ag capped Co islands grown on Cu(111), both forming a Moiré pattern at the surface. The adsorption configuration can by scanning tunneling spectroscopy be linked to the geometric and electronic properties of the substrates and traced back to a Co d-related surface state, illustrating how the modulated reactivity can be used to engineer a pattern of adsorbed molecules on the nanoscale.

Zero-field slow magnetic relaxation behavior of Zn2Dy in a family of trinuclear near-linear Zn2Ln complexes: synthesis, experimental and theoretical investigations

We hereby report a series of near-linear trinuclear [Zn₂Ln^III(HL)₄(CH₃COO)]·(NO₃)₂ (where Ln^III = La (1-La), Ce (2-Ce), Nd (3-Nd), Sm (4-Sm), Tb (5-Tb), and Dy (6-Dy)) complexes with Schiff base ligand (H2L). Magnetization relaxation dynamic studies on complexes 2-Ce, 5-Tb, and 6-Dy reveal the existence of well resolved frequency dependent zero-field out-of-phase χ''_M signals, which is an indicator of a typical single-ion magnet behavior observed only for complex 6-Dy with U_eff = 43.7 K (τ₀ = 2.42 × 10^-6 s). The presence of two Zn(II) ions near the coordination geometry of Dy(III) ion in 6-Dy is likely to keep the first excited m_J levels significantly away from the ground state m_J level and is responsible for the observation of zero field slow magnetic relaxation behavior. The data collected in the presence of a magnetic field of H_dc = 2 kOe enhances the energy barrier by two-fold (88.63 K, τ₀ = 1.36 × 10^-7 s) in 6-Dy, suggesting the presence of QTM at zero field along with other under barrier relaxations, such as the Raman process. On the other hand, complex 2-Ce shows field induced slow relaxation of magnetization behavior with an effective energy barrier of 12.24 K (τ₀ = 1.89 × 10^-4 s). The CASSCF/SO-RASSI/SINGLE_ANISO based ab initio calculations using MOLCAS 8.0 code further rationalized our experimentally observed magnetization dynamics.

Rare-earth based tetrapyrrolic sandwiches: chemistry, materials and applications

This review summarises advances in chemistry of tetrapyrrole sandwiches with rare earth elements and highlights the current state of their use in single-molecule magnetism, organic field-effect transistors, conducting materials and nonlinear optics.

Substrate mediated interaction of terbium(III) double-deckers with the TiO2(110) surface

A terbium(iii)-bis(phthalocyaninato) neutral complex was deposited on the rutile TiO2(110) surface, and their interaction was studied by Scanning Tunneling Microscopy (STM) and X-ray Photoelectron Spectroscopy (XPS). It was found that the TiO2 rutile surface favours the adsorption of isolated molecules adopting a lying down configuration with the phthalocyanine planes tilted by about 30° when they lie in the first layer. The electronic and chemical properties of the molecules on the surface were studied by XPS as a function of the TiO2(110) substrate preparation. This study evidences that strong molecule-substrate interactions are present and a charge transfer process occurs from the molecule to the surface.

Nature of Hyperfine Interactions in TbPc2 Single-Molecule Magnets: Multiconfigurational Ab Initio Study

Lanthanide-based single-ion magnetic molecules can have large magnetic hyperfine interactions as well as large magnetic anisotropy. Recent experimental studies reported tunability of these properties by changes of chemical environments or by application of external stimuli for device applications. In order to provide insight onto the origin and mechanism of such tunability, here we investigate the magnetic hyperfine and nuclear quadrupole interactions for a ¹⁵⁹Tb nucleus in TbPc₂ (Pc = phthalocyanine) single-molecule magnets using multiconfigurational ab initio methods including spin-orbit interaction. Since the electronic ground and first-excited (quasi)doublets are well separated in energy, the microscopic Hamiltonian can be mapped onto an effective Hamiltonian with an electronic pseudospin S = 1/2. From the ab initio calculated parameters, we find that the magnetic hyperfine coupling is dominated by the interaction of the Tb nuclear spin with electronic orbital angular momentum. The asymmetric 4f-like electronic charge distribution leads to a strong nuclear quadrupole interaction with significant transverse terms for the molecule with low symmetry. The ab initio calculated electronic-nuclear spectrum including the magnetic hyperfine and quadrupole interactions is in excellent agreement with the experiment. We further find that the transverse quadrupole interactions significantly influence the avoided level crossings in magnetization dynamics and that the molecular distortions affect mostly the Fermi contact terms as well as the transverse quadrupole interactions.

Screening the 4f-electron spin of TbPc2 single-molecule magnets on metal substrates by ligand channeling

Bis(phthalocyaninato)lanthanide (LnPc₂) double-decker-based devices have recently attracted a great deal of interest for data encoding purposes. Although the 4f-electrons of lanthanide ions play a key role in the experimental methodology, their localized character, deeper in energy compared to the 3d electrons of transition metals, hampers a detailed investigation. Here, our approach consists of the follow-up of the entanglement process with other molecules and with the substrate electrons by means of space-resolved detection of the Kondo resonance by scanning tunneling spectroscopy (STS), using different substrates (from weak to strong interaction). It is found that TbPc₂ molecules firstly interact with their environment by means of the π-radicals of the ligand. The radical spin of TbPc₂ can be identified by STS on a weakly interacting substrate such as Au(111). In the case of a Ag(111) substrate, we are able to analyze the effect of an electron transfer on the molecule (pairing-up of the radical spin) and the subsequent quenching of the Kondo resonance. Finally, on a strongly interacting substrate such as Cu(111), a significant rearrangement of electrons takes place and the Kondo screening of the 4f electrons of the Tb ion of TbPc₂ is observed. By comparative STS measurements on YPc₂, that has empty 4d and 4f shells, we prove that the Kondo resonance measured in the center of the TbPc₂ molecule indeed stems from the 4f-electrons. At the same time, we provide evidence for the hybridization of the 4f states with the π electron.

Understanding the Superior Stability of Single-Molecule Magnets on an Oxide Film

The stability of magnetic information stored in surface adsorbed single-molecule magnets is of critical interest for applications in nanoscale data storage or quantum computing. The present study combines X-ray magnetic circular dichroism, density functional theory and magnetization dynamics calculations to gain deep insight into the substrate dependent relevant magnetization relaxation mechanisms. X-ray magnetic circular dichroism reveals the opening of a butterfly-shaped magnetic hysteresis of DyPc2 molecules on magnesium oxide and a closed loop on the bare silver substrate, while density functional theory shows that the molecules are only weakly adsorbed in both cases of magnesium oxide and silver. The enhanced magnetic stability of DyPc2 on the oxide film, in conjunction with previous experiments on the TbPc2 analogue, points to a general validity of the magnesium oxide induced stabilization effect. Magnetization dynamics calculations reveal that the enhanced magnetic stability of DyPc2 and TbPc2 on the oxide film is due to the suppression of two-phonon Raman relaxation processes. The results suggest that substrates with low phonon density of states are beneficial for the design of spintronics devices based on single-molecule magnets.

Single-ion anisotropy and exchange coupling in cobalt(ii)-radical complexes: insights from magnetic and ab initio studies

The concurrent effects of single-ion anisotropy and exchange interactions on the electronic structure and magnetization dynamics have been analyzed for a cobalt(ii)-semiquinonate complex. Analogs containing diamagnetic catecholate and tropolonate ligands were employed for comparison of the magnetic behavior and zinc congeners assisted with the spectroscopic characterization and assessment of intermolecular interactions in the cobalt(ii) compounds. Low temperature X-band (ν ≈ 9.4 GHz) and W-Band (ν ≈ 94 GHz) electron paramagnetic resonance spectroscopy and static and dynamic magnetic measurements have been used to elucidate the electronic structure of the high spin cobalt(ii) ion in [Co(Me3tpa)(Br4cat)] (1; Me3tpa = tris[(6-methyl-2-pyridyl)methyl]amine, Br4cat2- = tetrabromocatecholate) and [Co(Me3tpa)(trop)](PF6) (2(PF6); trop- = tropolonate), which show slow relaxation of the magnetization in applied field. The cobalt(ii)-semiquinonate exchange interaction in [Co(Me3tpa)(dbsq)](PF6)·tol (3(PF6)·tol; dbsq- = 3,5-di-tert-butylsemiquinonate, tol = toluene) has been determined using an anisotropic exchange Hamiltonian in conjunction with multistate restricted active space self-consistent field ab initio modeling and wavefunction analysis, with comparison to magnetic and inelastic neutron scattering data. Our results demonstrate dominant ferromagnetic exchange for 3+ that is of similar magnitude to the anisotropy parameters of the cobalt(ii) ion and contains a significant contribution from spin-orbit coupling. The nature of the exchange coupling between octahedral high spin cobalt(ii) and semiquinonate ligands is a longstanding question; answering this question for the specific case of 3+ has confirmed the considerable sensitivity of the exchange to the molecular structure. The methodology employed will be generally applicable for elucidating exchange coupling between orbitally-degenerate metal ions and radical ligands and relevant to the development of bistable molecules and their integration into devices.

Multireference Ab Initio Studies of Magnetic Properties of Terbium-Based Single-Molecule Magnets

We investigate how different chemical environments influence magnetic properties of terbium(III) (Tb)-based single-molecule magnets (SMMs), using first-principles relativistic multireference methods. Recent experiments showed that Tb-based SMMs can have exceptionally large magnetic anisotropy and that they can be used for experimental realization of quantum information applications, with a judicious choice of chemical environment. Here, we perform complete active space self-consistent field calculations including relativistic spin-orbit interaction for representative Tb-based SMMs such as TbPc₂ and TbPcNc in three charge states. We calculate the low-energy electronic structure from which we compute the Tb crystal-field (CF) parameters and construct an effective pseudospin Hamiltonian. Our calculations show that the ligand type and fine points of molecular geometry do not affect the gap between the ground-state and first-excited doublets, whereas the latter varies weakly with oxidation number. On the other hand, higher-energy levels have a strong dependence on all these characteristics. For neutral TbPc₂ and TbPcNc molecules, the Tb magnetic moment and ligand spin are parallel to each other and the coupling strength between them does not depend much on the ligand type and details of the atomic structure. However, ligand distortion and molecular symmetry play a crucial role in transverse CF parameters which lead to tunnel splitting. The tunnel splitting induces quantum tunneling of magnetization by itself or by combining with other processes. Our results provide insights into the mechanisms of magnetization relaxation in the representative Tb-based SMMs.

On-surface structural and electronic properties of spontaneously formed Tb2Pc3 single molecule magnets

The single molecule magnet (SMM) bis(phthalocyaninato)terbium(iii) (TbPc2) has received significant and increasing attention as an exemplar system for realizing molecule-based spin electronics. Attaining higher nuclearity via multi-decker TbPc systems has remained an outstanding challenge, as known examples of Tb2Pc3 systems are only those containing Pc rings with substituents (e.g. alkyl, alkoxyl). Here we report on the spontaneous formation of Tb2Pc3 species from TbPc2 precursors via sublimation in ultrahigh vacuum (UHV) onto an Ag(111) surface. The presence of Tb2Pc3 molecules on the surface are inspected using scanning probe microscopy with submolecular resolution supported by density functional theory (DFT) calculations and additional chemical analysis. We observe the selective presence of a Kondo resonance (30 K) in the Tb2Pc3 species, that we attribute to differences in the orientation of the internal molecular ligands. Formation of triple-decker complexes offers new possibilities to study and control magnetic interactions not accessible with standard TbPc2 molecules.

Magnetic bistability of a TbPc2 submonolayer on a graphene/SiC(0001) conductive electrode

The alteration of the properties of single-molecule magnets (SMMs) due to the interaction with metallic electrodes is detrimental to their employment in spintronic devices. Conversely, herein we show that the terbium(iii) bis-phthalocyaninato complex, TbPc₂, maintains its SMM behavior up to 9 K on a graphene/SiC(0001) substrate, making this alternative conductive layer highly promising for molecular spintronic applications.

Self-Assembly of TbPc2 Single-Molecule Magnets on Surface through Multiple Hydrogen Bonding

The complexation between 2-ureido-4[1H]-pyrimidinone (UPy) and 2,7-diamido-1,8-naphthyridine (NaPy) is used to promote the mild chemisorption of a UPy-functionalized terbium(III) double decker system on a silicon surface. The adopted strategy allows the single-molecule magnet behavior of the system to be maintained unaltered on the surface.

Probing magnetic coupling between LnPc2 (Ln = Tb, Er) molecules and the graphene/Ni (111) substrate with and without Au-intercalation: role of the dipolar field

Lanthanides (Ln) bis-phthalocyanine (Pc), the so-called LnPc₂double decker, are a promising class of molecules with a well-defined magnetic anisotropy. In this work, we investigate the magnetic properties of LnPc₂ molecules UHV-deposited on a graphene/Ni(111) substrate and how they modify when an Au layer is intercalated between Ni and graphene. X-ray absorption spectroscopy (XAS), and linear and magnetic circular dichroism (XLD and XMCD) were used to characterize the systems and probe the magnetic coupling between LnPc₂ molecules and the Ni substrate through graphene, both gold-intercalated and not. Two types of LnPc₂ molecules (Ln = Tb, Er) with a different magnetic anisotropy (easy-axis for Tb, easy-plane for Er) were considered. XMCD shows an antiferromagnetic coupling between Ln and Ni(111) even in the presence of the graphene interlayer. Au intercalation causes the vanishing of the interaction between Tb and Ni(111). In contrast, in the case of ErPc₂, we found that the gold intercalation does not perturb the magnetic coupling. These results, combined with the magnetic anisotropy of the systems, suggest the possible importance of the magnetic dipolar field contribution for determining the magnetic behaviour.

Designing a molecular magnetic button based on 4d and 5d transition-metal phthalocyanines

The field of molecular spintronics exploits the properties of organic molecules possessing a magnetic moment, either native in the form of radicals or induced by the insertion of transition metal magnetic ions. To realize logic or storage molecular spin-tronics devices, molecules with stable different magnetic states should be deposited on a substrate, and switching between the states controllably achieved. By means of a first-principles calculations, we have devised a functional molecule exhibiting different magnetic states upon structural changes induced by current injection. We investigate the prototypical case of non-planar M-Phthalocyanine (MPc), where M is a transition-metal ion belonging to the 4d and 5d series. We find that for ZrPc and HfPc deposited on a graphene decorated Ni(111) substrate, two different structural conformations could be stabilized, for which the molecules attain different magnetic states depending on the position of the M ion - whether above the Pc or between the Pc and the substrate -, acting therefore as molecular magnetic button. Our work indicates an intuitive way to engineer a magnetic molecular switch with tailored properties, starting from the knowledge of the basic atomic properties of elements and surfaces.