Molecular Hybridization Induced Antidamping and Sizable Enhanced Spin-to-Charge Conversion in Co20Fe60B20/β\beta-W/C60 Heterostructures

Development of power efficient spintronics devices has been the compelling need in the post-CMOS technology era. The effective tunability of spin-orbit-coupling (SOC) in bulk and at the interfaces of hybrid materials stacking is a prerequisite for scaling down the dimension and power consumption of these devices. In this work, we demonstrate the strong chemisorption of C60 molecules when grown on the high SOC $\beta$-W layer. The parent CFB/$\beta$-W bilayer exhibits large spin-to-charge interconversion efficiency, which can be ascribed to the interfacial SOC observed at the Ferromagnet/Heavy metal interface. Further, the adsorption of C60 molecules on $\beta$-W reduces the effective Gilbert damping by $\sim$15% in the CFB/$\beta$-W/C60 heterostructures. The anti-damping is accompanied by a gigantic $\sim$115% enhancement in the spin-pumping induced output voltage owing to the molecular hybridization. The non-collinear Density Functional Theory calculations confirm the long-range enhancement of SOC of $\beta$-W upon the chemisorption of C60 molecules, which in turn can also enhance the SOC at the CFB/$\beta$-W interface in CFB/$\beta$-W/C60 heterostructures. The combined amplification of bulk as well interfacial SOC upon molecular hybridization stabilizes the anti-damping and enhanced spin-to-charge conversion, which can pave the way for the fabrication of power efficient spintronics devices

Magnetic exchange coupling of a synthetic Co(II)-complex to a ferromagnetic Ni substrate

On-surface assembly of a spin-bearing and non-aromatic porphyrin-related synthetic Co(II)-complex on a ferromagnetic Ni thin film substrate and subsequent magnetic exchange interaction across the interface were studied by scanning tunnelling microscopy (STM), X-ray absorption spectroscopy (XAS), X-ray magnetic circular dichroism (XMCD) and density functional theory +U (DFT + U) calculations

Broken Edge Spin-Symmetry Induces Spin-polarized Current in Graphene Nanoribbon

Zig-zag graphene nanoribbons (ZGNRs) are known to possess spin moments at the hydrogen- terminated edge carbon atoms, thus the spin-polarized electron transmission is expected while the current is longitudinally passed through the ZGNRs. However, in pristine ZGNRs, the spin polarized transmission is not observed due to symmetric anti-parallel distributions of the spin densities between the edges. Here, the hypothesis is, any physical or chemical process that breaks such anti-parallel spin-symmetry can induce spin-polarized transmission in the ZGNRs. In this work, we have established this proof-of-concept by depositing the trimethylenemethane (TMM) radical on 6ZGNRH and investigating the quantum transport properties by employing the density functional theory in conjunction with nonequilibrium Green’s function (DFT-NEGF) method. Although TMM has a high magnetic moment (2 µB ), it does not induce magnetization in 6ZGNRH when TMM is physisorbed. But, during the chemisorption of TMM, it forms the π − π bond with the 6ZGNRH in a particular geometric configuration where the pz orbitals of carbon atoms of TMM have maximum overlap with the pz orbitals of carbon atoms of 6ZGNRH. The chemisorption of TMM transfers the spin moment to 6ZGNRH, which breaks the edge spin-symmetry of pristine 6ZGNRH. The adsorption of TMM radical results in transmission dips in the transmission spectra due to interference between localized states of TMM and 6ZGNRH states. This induces spin-polarized transmission with 60% spin-filtering efficiency (SFE) at zero bias, which can further be enhanced up to 92% by applying the bias voltage of 1.0 V

First Principle Investigations of Long-range Magnetic Exchange Interactions via Polyacene Coupler

The electronic and magnetic properties of polyacenes become quite fascinating as the number of linearly conjugated benzene rings increases. Higher-order conjugated polyacenes develop radicaloid characters due to the transition of electronic structures from closed-shell to the open-shell system. Here we have investigated the role of such polyacenes as the magnetic coupler when placed between the two spin-sources based on nitroxy radicals. To do so, the magnetic exchange interactions (2J) are computed employing electronic structure theories, i.e. broken-symmetry (BS) approach within the density functional theory (DFT) as well as symmetry-adopted wave function based multi-configurational methods. In the former approach, various genre of exchange-correlation (XC) functionals such as generalized gradient approximation (GGA), meta-GGA, hybrid functional, constrained spin density (i.e. CDFT) and on-site Coulomb correlation corrected GGA+U functionals are adopted. All DFT based calculations estimate an exponential increase in 2J values with the length of the couplers, especially for the higher-order acenes. This is indeed an unexpected observation and also there is no experimental report available in support of the DFT calculations. The complexity in the electronic structure enhances with the increasing number of benzene rings due to an increase in near-degenerate or quasi-degenerate molecular orbitals (MOs) and also the reduction of the energy gap with the low-lying excited states. Consequently, it invokes a severe challenge in the computations of the magnetic exchange interactions in DFT. As an alternative approach, the wave function based multi-reference calculations, e.g. CASSCF/NEVPT2 methods are also adopted. In the later calculations, it has been realized that the π-orbitals of the couplers play a crucial role in the exchange interactions. For larger polyacenes (i.e. hexacene to decacene) such calculations become prohibitively expensive and rigorous as the number of π-orbitals increases, thus expanding the active space enormously. The limited active spaces calculations indicate quite strong ferromagnetic exchange interactions, thus in principle, reinforcing long-range magnetic exchange interactions.</div

Understanding the Role of R266K Mutation in Cystathionine β-Synthase (CBS) Enzyme: An in Silico Study

Human cystathionine β-synthase (hCBS) is a Heme containing unique pyridoxal 5’-phosphate (PLP) dependent enzyme that catalyzes the bio-chemical condensation reactions in the transsulfuration pathway. The role of Heme in the catalytic activities of enzyme has not yet been understood completely, even though various experimental studies have indicated its participation in the bi-directional electronic communication with the PLP center. Most probably Heme acts as the electron density reservoir for the catalytic reaction center but not as a redox electron source. Here, in this work, we investigated In Silico dynamical aspects of the bi-directional communications by performing classical molecular dynamics (MD) simulations upon developing the necessary force field parameters for the cysteine and histidine bound hexa-coordinated Heme. The comparative aspects of electron density overlap across the communicating pathways are also explored adopting the density functional theory (DFT) in conjunction with the hybrid exchange-correlation functional for the CSBWT (wild-type) and CBSR266K (mutated) case. The atomistic MD simulations and subsequent explorations of the electronic structures not only confirm the reported observations but provide an in-depth mechanistic understating of how the non-covalent hydrogen bonding interactions with Cys52 control such long-distance communication. Our study also provides a convincing answer to the reduced enzymatic activities in the R266K hCBS in comparison to the wild-type enzymes. We further realized that the difference in hydrogen-bonding patterns, as well as salt-bridge interactions, play a pivotal role in such long distant bi-directional communications.<br /

Single Molecule Magnetism in Linear Fe(I) Complexes with Aufbau and non-Aufbau ground-state

With the ongoing efforts on synthesizing mono-nuclear single-ion magnets (SIMs) with promising applications in high-density data storage and spintronics devices, the linear Fe(I) complexes emerge as the enticing candidates possessing large unquenched angular momentum. Herein, we have studied five experimentally synthesized linear Fe(I) complexes to uncover the origin of single-molecule magnetic behavior of these complexes. To begin with, we benchmarked our methodology on the experimentally and theoretically well-studied complex, [Fe{C(SiMe3)}3]−1] (1) (SiMe3 = trimethylsilyl) which is characterized with large spin-reversal barrier of 226 cm−1 [Nat. Chem. 2013, 5, 577–581]. Further, the two Fe(I) complexes, i.e., [Fe(cyIDep)2]+1 (2) ((cyIDep= 1,3-bis(20,60-diethylphenyl)-4,5-(CH2)4-imidazol-2-ylidene) and [Fe(sIDep)2]+1] (3) (sIDep = 1,3-bis(20,60-diethylphenyl)-imidazolin-2-ylidene) are studied that do not possess SIM behavior under ac or dc magnetic fields, however, they are reported to exhibit large opposite axial zero field splitting (-62.4 and +34.0 cm−1 respectively) from ab initio calculations. Employing state-of-the-art ab initio calculations, we have unwrapped the origin of this contrasting observation between experiment and theory by probing their magnetic relaxation pathways and the pattern of d-orbitals splitting. Additionally, the two experimentally synthesized Fe(I) complexes, i.e., [(η6-C6H6)FeAr*-3,5-Pri 2] (4) (Ar*-3,5-Pri 2 = C6H-2,6-(C6H2-2,4,6-Pri 3)2-3,5-Pri 2) and [(CAAC)2Fe]+1 (5) (CAAC = cyclic (alkyl)(amino)carbene) are investigated for SIM behavior, since there is no report on their magnetic properties. To this end, complex 4 presents itself as the potential candidate for SIM

Anti-ohmic Nanoconductors: Myth, Reality and Promise

The recent accomplishments in the design of molecular nanowires characterised by an increasing conductance with length has embarked the origin of extraordinary new family of molecular junctions referred to as "anti-ohmic" wires. Herein, this highly desirable, non-classical behavior, has been examined for the longer enough molecules exhibiting pronounced diradical character in their ground state within the unrestricted DFT formalism with spin and spatial symmetry breaking. We demonstrate that highly conjugated acenes signals higher resistance in open-shell singlet (OSS) configuration as compared to their closed-shell counterparts. This anomaly has been further put to proof for experimentally certified cumulene wires, which reveals phenomenal modulation in the transport characteristics such that an increasing conductance is observed in closed-shell limit, while higher cumulenes in OSS ground state yields a regular decay of conductance

Can Iron-porphyrins behave as Single-molecule Magnets?

Spin-bearing metal ions encapsulated by macrocyclic ligands in porphyrin complexes make them an ideal component in molecular spintronic devices where there is scope for the logical manipulation of various magnetic properties of the molecular system. Adding an axial ligand to the planar porphyrin complexes is established to be a very useful route to alter different aspects of the magnetochemistry of the resulting complex. Magnetic anisotropy is a lesser-known avenue in this context. For a series of high-spin pentacoordinate $d^5$ Fe(III) and $d^6$ Fe(II) porphyrin complexes with varying axial ligands, the magnetic anisotropy parameters are obtained from the spin Hamiltonian formalism. The $d^5$ high spin complexes, having a net zero orbital angular momentum, possess an almost isotropic magnetic environment. The small positive zero-field splitting (2-7 $cm^{-1}$) for these complexes arises due to near proximity of the quartet excited states. This ZFS is found to increase down the group for the halide ligands owing to the decrease of the sextet-quartet energy gap. {On the other hand, possessing a triaxial anisotropic magnetic environment, the sign and the magnitude of the ZFS parameters of the $d^6$ complexes are mostly dependent on the axial ligand itself. Amongst the considered complexes, H2O and NH3 exhibit positive ZFS while the Imidazole-based ligands possess a negative sign to the ZFS parameter