Engineering Robust Molecular Qubits with High Magnetic Anisotropy

Abstract:
Organic diradical dications, due to reduced intermolecular interactions, exhibit a greater tendency to adopt high spin states in the solid phase compared to their neutral diradical counterparts. This characteristic makes them promising candidates for applications involving organic electronics. We present a theoretical study of a recently synthesized sulfur-based diradical dication, a unique system exhibiting a robust triplet ground state. Using a number of density functional theory (DFT)-based methods (e.g., standard broken-symmetry DFT, constrained DFT, spin-flip TDDFT) and wave function-based multireference CASSCF+NEVPT2 methods, we investigate its magnetic properties and explore the influence of chalcogen substitution on magnetic exchange coupling. An active space scanning method was adopted to overcome the difficulties in choosing the correct active space for multireference calculation. Our findings highlight the critical role of multireference methods in accurately capturing the magnetic behavior of highly π-conjugated systems. The study reveals a surprising variation in magnetic properties among sulfur, selenium, and tellurium-based diradical dications despite being elements of the same group. These results offer valuable insights into the design and tuning of magnetic properties in organic diradical dications.

Abstract:
Among lanthanide-based single-molecule magnets (SMMs), erbium(III) is a Kramers ion, apart from dysprosium(III), which provides magnetic bistability in the presence of a suitable coordination environment. However, Er-based SMMs exhibit significantly less magnetic anisotropy than Dy because their prolate electronic density necessitates equatorially correlated ligands to minimize the charge contact with the Er atom. Here, in this work, we have computationally investigated the heteroleptic organometallic complexes with an Er(III) atom sandwiched between two distinct cyclic rings (five- and eight-membered) with the aim of tuning the magnetic anisotropy via exploiting the ligand field. The ligand field is manipulated by substituting one of the C atoms from the five-membered ring with heteroatoms (groups 14 and 15), while the other (eight-membered) ring remains intact. The electronic and magnetic properties have been investigated using first-principles-based ab initio approaches. The distortion in the planarity of the five-membered ring generated by the larger heteroatom affects the bonding with magnetic Er and consequently the electronic structure. This is observed to modify the ligand field and the magnetic axis, thereby improving the magnetic relaxation barrier.

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:
Single-molecule magnets (SMMs) with a large magnetization reversal barrier are predominated by the lanthanide systems due to their strong spin–orbit coupling (SOC). However, the transition metals have also emerged as potential contenders and the largest magnetic anisotropy has been identified for a cobalt system among any d-series-based SMMs (Bunting et al. Science 2018, 362, eaat7319). In this work, we have explored the magnetic anisotropy in highly axial ligand field systems of metallocene, having different d-subshell (3d4, 4d4, and 5d4). The wave function-based multireference methods including static and dynamic electron correlations have been employed to investigate the zero-field splitting (ZFS) parameters. Here, we report exceptionally large magnetic anisotropy for a 5d complex of [WCp2]0 with the highest energy barrier that is nearly twice as high as the previous record value for the Co complex. We have also observed that the axial ZFS parameter (D) is increasing down the group in the order of 3d < 4d < 5d, pertaining to a large SOC.

Abstract:

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.