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Beating the Stoner criterion using molecular interfaces

Abstract

Only three elements are ferromagnetic at room temperature: the transition metals iron, cobalt and nickel. The Stoner criterion explains why iron is ferromagnetic but manganese, for example, is not, even though both elements have an unfilled 3d shell and are adjacent in the periodic table: according to this criterion, the product of the density of states and the exchange integral must be greater than unity for spontaneous spin ordering to emerge1,2. Here we demonstrate that it is possible to alter the electronic states of non-ferromagnetic materials, such as diamagnetic copper and paramagnetic manganese, to overcome the Stoner criterion and make them ferromagnetic at room temperature. This effect is achieved via interfaces between metallic thin films and C60 molecular layers. The emergent ferromagnetic state exists over several layers of the metal before being quenched at large sample thicknesses by the material’s bulk properties. Although the induced magnetization is easily measurable by magnetometry, low-energy muon spin spectroscopy3 provides insight into its distribution by studying the depolarization process of low-energy muons implanted in the sample. This technique indicates localized spin-ordered states at, and close to, the metal–molecule interface. Density functional theory simulations suggest a mechanism based on magnetic hardening of the metal atoms, owing to electron transfer4,5. This mechanism might allow for the exploitation of molecular coupling to design magnetic metamaterials using abundant, non-toxic components such as organic semiconductors. Charge transfer at molecular interfaces may thus be used to control spin polarization or magnetization, with consequences for the design of devices for electronic, power or computing applications (see, for example, refs 6 and 7).

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Figure 1: Effect of molecular interfaces.
Figure 2: Room-temperature magnetization for Cu and Mn films.
Figure 3: Muon spin rotation (μSR) spectroscopy at 250 K.
Figure 4: DFT simulations and metamagnetic modellling.

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Acknowledgements

This work was supported by the Engineering and Physical Sciences Research Council through grants EP/K00512X/1, EP/K036408/1, EP/J01060X/1 and EP/I004483/1. Use of the N8 POLARIS (EPSRC EP/K000225/1), ARCHER (via the UKCP Consortium, EP/K013610/1), and the High Performance Computing (HPC) Wales facilities is acknowledged. Use of the National Synchrotron Light Source, Brookhaven National Laboratory, was supported by the US Department of Energy, Office of Science, Office of Basic Energy Sciences, under contract number DE-AC02-98CH10886.

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Contributions

F.A.M. and T.M. grew and characterized the samples, conducted the magnetometry and μSR, and contributed to the data analysis; G.T. performed and analysed the DFT simulations; W.D. grew and characterized the Cu–C60 multilayers; T.P., H.L., S.L. and M.F. contributed to the design, measurement and analysis of the μSR experiments; D.A.M. contributed to the TEM images and structural analysis; G.E.S. and D.A.A. performed the X-ray magnetic circular dichroism and X-ray absorption spectroscopy measurements; M.A., M.C.W., G.B. and B.J.H. contributed to the sample structure and measurement setup; and O.C. designed the study, analysed the data and wrote the manuscript. All authors discussed the results and commented on the manuscript.

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Correspondence to Oscar Cespedes.

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The authors declare no competing financial interests.

Additional information

The data presented here are available at http://dx.doi.org/10.5518/6.

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Supplementary Information

This file contains Supplementary Information sections 1-6, including Supplementary Figures 1-27, Supplementary Tables 1-9 and Supplementary References. (PDF 5170 kb)

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Ma’Mari, F., Moorsom, T., Teobaldi, G. et al. Beating the Stoner criterion using molecular interfaces. Nature 524, 69–73 (2015). https://doi.org/10.1038/nature14621

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