Abstract
The magnetism of small (only a few nanometers in diameter) Co nanoparticles (NPs) grown on Au(111) was investigated by means of spin-dependent scanning tunneling microscopy and spectroscopy in a broad energy range. Direct evidence is provided for the existence of localized d-states of minority and majority character that govern the spin polarization of the NPs below the Fermi level. On the other hand, the discrete electronic states resulting from the spatially confined sp-like Co surface state electrons above the Fermi level are found to be of majority character. This confirms the theoretically predicted spin-polarized character of the delocalized surface state electrons of Co NPs on Au(111).
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GENERAL SCIENTIFIC SUMMARY Introduction and background. Research on nanomagnetism is of direct relevance for nanoelectronics, e.g., magnetic data storage and spintronics. Whereas bulk materials can be adequately investigated by standard magnetometry and magnetic imaging techniques, study of the spin-dependent properties of their nanosized counterpart requires the use of local probes. Here, we have probed the magnetism of Co nanoparticles (NPs), which self-organize on a Au(111) surface, by means of spin-dependent scanning tunneling microscopy (STM) and spectroscopy (STS) in a broad energy range. Although Co NPs on Au(111) have already been predicted to exhibit a spin-polarized character, detailed experimental investigations of their spin-resolved electronic structure are still lacking.
Main results. The 'herringbone' reconstructed Au(111) surface was used as a template for the growth of bilayer Co NPs that have a somewhat monodisperse size distribution and are equidistantly arranged on the Au(111) surface. Our measurements provide direct evidence for the existence of localized d-states of minority and majority character that govern the spin polarization of the NPs below the Fermi level. Furthermore, the discrete electronic states resulting from the spatially confined sp-like Co surface state electrons above the Fermi level are found to be of majority character.
Wider implications. Co NPs can be considered as potential building blocks for future magnetic data storage devices. Whereas their electronic properties are dominated by complex quantum confinement phenomena, our measurements reveal that these particles still exhibit a net magnetization at low temperatures, which is governed by the effective spin polarization of their quantized electronic states.
Figure. A 50 × 50 nm2 image of self-organized Co nanomagnets on Au(111). Blue and red color depicts either the upward or downward oriented magnetization of the nanoparticles.