Spin-Interface

Abstract:
Controlling the spin orientation of magnetic systems is crucial for various quantum technologies. In this work, applying first-principle studies, we have investigated how interfacial interactions between a single-molecule magnet (SMM), chromocene (CrCp2), and substrates (magnetic and nonmagnetic) could control the magnetization of the SMM. Due to magnetic anisotropy, an isolated chromocene molecule exhibits the magnetization direction aligned parallel to the principal molecular axis. Typically, molecule-substrate interactions quench the magnetic anisotropy in the SMMs. In the current work, we observed a switch in the magnetization direction from out-of-plane to in-plane direction while CrCp2 is deposited on Au(111), graphene, hexagonal closed packed–Co(0001) and face-centered cubic–Co(111) substrates. The resulting interface formations induce structural and electronic structure changes in the SMMs. We realized that only the structural change enhanced the SMM behaviors of CrCp2 compared to the isolated systems. Unfortunately, this is inseparable from the electronic structural changes at the interface. The charge density redistribution at the interfaced molecule modulates the magnetic anisotropy of SMMs and switches their magnetization directions. Finally, we explored the complex nature of the magnetic interactions between the SMM and magnetic substrate, accounting for the Dzyaloshinski-Moriya and symmetric/antisymmetric exchange interactions.

Abstract:
The development of power-efficient spintronic devices has been a compelling need in the post-CMOS technology era. The effective tuneability of spin-orbit coupling (SOC) in the bulk and at the interfaces of hybrid material stacks is a prerequisite for scaling down the dimensions and power consumption of these devices. In this work, we demonstrate the strong chemisorption of C60 (fullerene) molecules when grown on the high-SOC 𝛽-W layer. The parent Co20⁢Fe60⁢B20/𝛽-W (CFB/⁢𝛽-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 𝛽-W reduces the effective Gilbert damping by  ∼15% in CFB/⁢𝛽-W/C60 heterostructures. The antidamping is accompanied by a gigantic  ∼115% enhancement in the spin-pumping-induced output voltage owing to molecular hybridization. The noncollinear density-functional-theory calculations confirm the long-range enhancement of the SOC of 𝛽-W upon the chemisorption of C60 molecules, which in turn can also enhance the SOC at the CFB/⁢𝛽-W interface in CFB/⁢𝛽-W/C60 heterostructures. The combined amplification of the bulk as well as the interfacial SOC upon molecular hybridization stabilizes the antidamping and enhanced spin-to-charge conversion, which can pave the way for the fabrication of power-efficient spintronic devices.

Abstract:

Abstract:
Buckminsterfullerene (C₆₀) can exhibit ferromagnetism at the interface (called a spinterface) when it is placed next to a ferromagnet (FM). The formation of a spinterface occurs due to orbital hybridization and spin polarized charge transfer at the interface. The spinterface can influence the domain size and magnetization dynamics of the organic/ferromagnetic heterostructure. Here, we have performed magnetic domain imaging and studied the relaxation dynamics in Pt/Co/C₆₀/Pt systems which exhibit perpendicular anisotropy. The obtained results have been compared with a reference Pt/Co/Pt system. It is observed that the presence of C₆₀ in the Pt/Co/Pt system leads to an increase in perpendicular magnetic anisotropy and a decrease in the size of bubble domains. Furthermore, the switching time of the Pt/Co/C₆₀/Pt system is faster than its reference Pt/Co/Pt system. Spin polarized density functional theory (DFT) calculations have been performed to understand the underlying mechanism. The DFT results show the formation of a spin polarized spinterface which leads to an enhancement in anisotropy.

Abstract:
The magnetic properties of adsorbed metalloporphyrin molecules can be altered or tuned by the substrate, additional axial ligands, or changes to the molecules’ macrocycle. These modifications influence the electronic configuration of the fourfold-coordinated central metal ion that is responsible for the metalloporphyrins’ magnetic properties. We report a substantial increase in the effective spin moment obtained from sum-rule analysis of X-ray magnetic circular dichroism for an iron metalloporphyrin molecule on Au(111) through its conversion from iron(II)-octaethylporphyrin to iron(II)-tetrabenzoporphyrin in a surface-assisted ring-closure ligand reaction. Density functional theory calculations with additional strong Coulomb correlation (DFT+U) show that the on-surface reaction alters the conformation of the molecule, increasing its planarity and the ion–surface distance. A spin-Hamiltonian fit of the magnetization as a function of field reveals a substantial increase in the intra-atomic magnetic dipole term (T_z) and a decrease in the magnitude of the easy-plane anisotropy upon ring closure. This consequence of the ring closure demonstrates how new magnetic properties can be obtained from on-surface reactions, resulting here in significant modifications to the magnetic anisotropy of the Fe ion, and sheds light onto the molecule–substrate interaction in these systems.

Abstract:
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.

Abstract:
The electronic structures, spin-states, and geometrical parameters of tetra-, penta-, and hexa-coordinated iron-porphyrins are investigated applying density functional theory (DFT) based calculations, utilizing the plane-wave pseudopotential as well as localized basis set approaches. The splitting of the spin multiplet energies are investigated applying various functionals including recently developed hybrid meta-GGA (M06 family) functionals. Almost all of the hybrid functionals accurately reproduce the experimental ground state spins of the investigated Fe-porphyrins. However, the energetic ordering of the spin-states and the energies between them are still an issue. The widely used B3LYP provides consistent results for all chosen systems. The GGA+U functionals are found to be equally competent. After assessing the performance of various functionals in spin-state calculations, the potential energy surfaces of the oxygen binding process by heme is investigated. This reveals a “double spin-crossover” feature for the lowest energy reaction path that is consistent with previous CASPT2 calculations but predicting a lowest energy singlet state. The calculations have hence captured the spin-crossover as well as spin-flip processes. These are driven by the intra-atomic orbital polarization on the central metal atom due to the atomic and orbitals rearrangements. The nature of the chemical bonding and a molecular orbital analysis are also performed for the geometrically simple but electronic structurally complicated system tetra-coordinated planar Fe porphyrin in comparison to the penta-coordinated systems. This analysis explains the observed paradoxical appearance of certain peaks in the local density of states (DOS).

Abstract:
We report, on the basis of density-functional theory+U (DFT+U) calculations that metalloporphyrins can adsorb on ferromagnetic metal surfaces in two distinct configurations. Two separate adsorption minima are obtained for manganese porphyrin (MnP) on Co from our DFT+U total energy calculations, which correspond to strong and weak adsorption strengths, respectively. By steering the nature of adsorption, we find that distinct chemical interactions as well as magnetic exchange interactions between the metalloporphyrin and the metal surface can be realized. We furthermore show that a switching of the MnP molecule’s spin state can occur even for the weakly adsorbed case. This new discovery opens up prospects for engineering the chemical and magnetic exchange interaction in new functionalized spintronic materials.

Abstract:
We demonstrate that an antiferromagnetic coupling between paramagnetic Fe-porphyrin molecules and ultrathin Co and Ni magnetic films on Cu(100) substrates can be established by an intermediate layer of atomic oxygen. The coupling energies have been determined from the temperature dependence of x-ray magnetic circular dichroism measurements. By density functional $\text{theory}+U$ calculations the coupling mechanism is shown to be superexchange between the Fe center of the molecules and Co surface-atoms, mediated by oxygen.