Reversible spin storage in metal oxide-fullerene heterojunctions

Exploiting the Synergism of a Carbon-Catalyst Interface to Achieve Magneto-Electrocatalytic Overall Water Splitting at 2.197 V

The desire to electrolyze water at low energy and high kinetics for achieving rapid H2 production forms the holy grail for the paradigm shift to a sustainable H2-driven economy. While alkaline electrolysis is preferred due to the use of earth-abundant catalysts, its sluggish kinetics and high overpotential are the persistent challenges. Addressing this, we demonstrate the coupling of an externally applied magnetic field (Hext) to a synergistically designed interface of nanostructured carbon floret with antiferromagnetic NiO nanoflakes that act in unison to achieve rapid hydrogen generation (6.3 N m3 h-1 W-1) that is comparable with existing technologies. Specifically, the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) overpotentials are simultaneously reduced by 10 and 7%, respectively, under the influence of a weak fridge magnet (Hext = 200 mT). Consequently, ∼11% improvement in the energy efficiency is observed with a 21% reduced cell voltage for overall water splitting. The stability of the system is demonstrated over a prolonged lifetime of ∼95 h. This performance enhancement with Hext for both HER and OER is explained in terms of improved kinetic facility for the reaction and lower resistance of charge transfer pathway. Moreover, the electrocatalyst is seen to retain the improved performance for prolonged usage (∼3 h) even after the removal of the Hext, and hence, it provides an energy-efficient hydrogen and oxygen generation pathway.

Molecular Compounds in Spintronic Devices: An Intricate Marriage of Chemistry and Physics

Spintronics, a flourishing new field of microelectronics, uses the electron spin for reading and writing information in modern computers and other spintronic devices with a low power consumption and high reliability. In a quest to increase the productivity of such devices, the use of molecular materials as a spacer layer allowed them to perform equally well or even better than conventional all-inorganic heterostructures from metals, alloys, or inorganic semiconductors. In this review, we survey various classes of chemical compounds that have already been tested for this purpose─from organic compounds and coordination complexes to organic-inorganic hybrid materials─since the creation of the first molecule-based spintronic device in 2002. Although each class has its advantages, drawbacks, and applications in molecular spintronics, together they allowed nonchemists to gain insights into spin-related effects and to propose new concepts in the design and fabrication of highly efficient spintronic devices. Other molecular compounds that chemistry could offer in great numbers may soon emerge as suitable spacers or even electrodes in flexible magnetic field sensors, nonvolatile memories, and multifunctional spintronic devices.

Revealing intrinsic spin coupling in transition metal-doped graphene

Graphene materials offer attractive possibilities in spintronics due to their unique atomic and electronic structures, which is in contrast to their limited applications in the design of sophisticated spintronic devices. This should be attributed to the lack of knowledge about the intrinsic characteristics of graphene materials, especially the diverse correlations between sites within the materials and their roles in spin-signal generation and propagation. This work comprehensively studies the spin couplings between transition metal atoms doped on graphene and reveals their potential application in spintronic device design through the realization of various logic gates. In addition, the effects of the distance between doped metal atoms and the number of carbon layers on the logic gate implementation further verify that the spin-coupling effect can exhibit a certain distance dependence and space propagation. The achievements in this work uncover the potential value of graphene materials and are expected to open up new avenues for exploring their application in the design of sophisticated spintronic devices.

Combined Role of Biaxial Strain and Nonstoichiometry for the Electronic, Magnetic, and Redox Properties of Lithiated Metal-Oxide Films: The LiMn2O4 Case

Understanding the interplay between strain and nonstoichiometry for the electronic, magnetic, and redox properties of LiMn₂O₄ films is essential for their development as Li-ion battery (LIB) cathodes, photoelectrodes, and systems for sustainable spintronics applications as well as for emerging applications that combine these technologies. Here, density functional theory (DFT) simulations suggest that compressive strain increases the reduction drive of (111) LiMn₂O₄ films by inducing >1 eV upshift of the valence band edge. The DFT results indicate that, regardless of the crystallographic orientation for the LiMn₂O₄ film, biaxial expansion increases the magnetic moments of the Mn atoms. Conversely, biaxial compression reduces them. For ferromagnetic films, these changes can be substantial and as large as over 4 Bohr magnetons per unit cell over the simulated range of strain (from -6 to +3%). The DFT simulations also uncover a compensation mechanism whereby strain induces opposite changes in the magnetic moment of the Mn and O atoms, leading to an overall constant magnetic moment for the ferromagnetic films. The calculated strain-induced changes in atomic magnetic moments reflect modifications in the local electronic hybridization of both the Mn and O atoms, which in turn suggests strain-tunable, local chemical, and electrochemical reactivity. Several energy-favored (110) and (111) ferromagnetic surfaces turn out to be half-metallic with minority-spin band gaps as large as 3.2 eV and compatible with spin-dependent electron-transport and possible spin-dependent electrochemical and electrocatalytic properties. The resilience of the ferromagnetic, half-metallic states to surface nonstoichiometry and compositional changes invites exploration of the potential of LiMn₂O₄ thin films for sustainable spintronic applications beyond state-of-the-art, rare-earth metal-based, ferromagnetic half-metallic oxides.

Tuning the Magnetic Coupling of a Molecular Spin Interface via Electron Doping

Mastering the magnetic response of molecular spin interfaces by tuning the occupancy of the molecular orbitals, which carry the spin magnetic moment, can be accomplished by electron doping. We propose a viable route to control the magnetization direction and magnitude of a molecular spin network, in a graphene-mediated architecture, achieved via alkali doping of manganese phthalocyanine (MnPc) molecules assembled on cobalt intercalated under a graphene membrane. The antiparallel magnetic alignment of the MnPc molecules with the underlying Co layer can be switched to a ferromagnetic state by electron doping. Multiplet calculations unveil an enhanced magnetic state of the Mn centers with a 3/2 to 5/2 spin transition induced by alkali doping, as confirmed by the steepening of the hysteresis loops, with higher saturation magnetization values. This new molecular spin configuration can be aligned by an external field, almost independently from the hard-magnet substrate effectively behaving as a free magnetic layer.