Beating the Stoner criterion using molecular interfaces

Abstract

It is accepted that only three elements are ferromagnetic at room temperature, the transition metals iron, cobalt and nickel. The Stoner criterion explains why, for example, iron is ferromagnetic but manganese is not, even though both elements have an unfilled 3d shell and are adjacent in the periodic table: the product of the density of states with the exchange integral must be greater than unity for spontaneous ordering to emerge.1,2 Here, we demonstrate that it is possible to alter the electronic states of nonferromagnetic materials, such as diamagnetic copper and paramagnetic manganese, in 2 order to drive them ferromagnetic at room temperature. This remarkable effect is achieved via interfaces between metallic thin films and C 60 molecular layers. The emergent ferromagnetic state can exist over several layers of the metal before being quenched at large sample thicknesses by the material's bulk properties. While the induced magnetisation is easily measurable by magnetometry, low energy muon spin spectroscopy 3 provides insight into its magnetic distribution by studying the depolarisation process of low energy muons implanted in the sample. This technique indicates localized spin-ordered states at and close to the metallo-molecular interface.Density functional theory simulations suggest a mechanism based on magnetic hardening of the metal atoms due to electron transfer. 4,5 This opens a path for the exploitation of molecular coupling to design magnetic metamaterials using abundant, non-toxic elements such as organic semiconductors. Charge transfer at molecular interfaces can then be used to control spin polarisation or magnetisation, with far reaching consequences in the design of devices for electronic, power or computing applications. 6,7 Multifunctional materials with the spin degree of freedom such as multiferroics, magnetic semiconductors and molecular magnets have all aroused huge interest as potentially transformative components in quantum technologies. [8][9][10][11][12] Strategies used to bring magnetic ordering to these materials typically rely on the inclusion of magnetic transition metals, heavy elements with a large atomic moment or rare earths. In thin film structures, proximity effects and coupling at interfaces play an essential role. 13,14 This is especially the case for molecular spintronics, 15,16 where organic thin films grown on copper have demonstrated spin filtering. 17The organic magnetic coupling can propagate for long distances in systems such as nanoscale vortex-like configurations or nanoskyrmion lattices. 183We choose C 60 as a model molecule due to its structural simplicity and robustness as well as its high electron affinity. C 60 /transition metal complexes exhibit strong interfacial coupling between metal 3d z electrons and molecular π-bonded p electrons. The potential created by the mismatch of molecular and metal work functions leads to a partial filling of the interface states. [19][20][21] Other molecules with close electron affinity and the potential for 3d z /p coupling ...

Interface-Induced Spin Polarization in Graphene on Chromia

“…The effect has been reported for a variety of molecules on ferromagnetic substrates as well as for graphene [Dedkov 2008, Dzemiantsova 2011, Mandal 2014, Marchenko 2015, Weser 2010, Weser 2011, Usachov 2015, also on ferromagnetic substrates. This trend towards an induced polarization in the adlayer may be rationalized in terms of the Stoner criterion [Ma'Mari 2015], and induced polarization from proximity effects is well described by mean field arguments [Dowben 1991, Mathon 1986, Schwenk 1998]. …”

Emergent magnetism at transition-metal–nanocarbon interfaces

“…Charge transfer and d(metal)-π(carbon) orbital coupling at the interface may change the density of states, spin population, and exchange of metallocarbon interfaces (4,15,16). The interaction between the molecule and the metal depends strongly on the morphology and specific molecular geometry (17, 18).…”

“…In particular, the fine-tuning of spin polarization at metallo-organic interfaces opens a realm of possibilities, from the direct applications in molecular spintronics and thin-film magnetism to biomedical imaging or quantum computing. This interaction at the surface can control the spin polarization in magnetic field sensors, generate magnetization spin-filtering effects in nonmagnetic electrodes, or even give rise to a spontaneous spin ordering in nonmagnetic elements such as diamagnetic copper and paramagnetic manganese (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11).…”

Recent progress in organic spintronics

“…This approach, which has been coined "spinterface science" [57,88], has become a very active field of research. It has been shown that, due to orbital hybridization, the spin polarization at ferromagnetic surfaces may be amplified or inverted [57,89], spin-filtering tunneling may occur at molecular adsorbate sites [90], and non-magnetic metallic layers may even become ferromagnetic upon interface formation with carbon-based molecules [91]. In addition, interfacial magnetoresistance due to a single ferromagnetic/molecular interface has been demonstrated [92].…”

“…Very recently, it was demonstrated that even for nonmagnetic metals, Cu and Mn, magnetism may emerge due to interfaces with C 60 molecules [91]. Magnetic hysteresis was observed at room temperature in C 60 /Cu and C 60 /Mn stacks using magnetometry.…”