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Magnetic exchange force microscopy with atomic resolution

Abstract

The ordering of neighbouring atomic magnetic moments (spins) leads to important collective phenomena such as ferromagnetism and antiferromagnetism. A full understanding of magnetism on the nanometre scale therefore calls for information on the arrangement of spins in real space and with atomic resolution. Spin-polarized scanning tunnelling microscopy accomplishes this1 but can probe only conducting materials. Force microscopy can be used on any sample independent of its conductivity. In particular, magnetic force microscopy2 is well suited to exploring ferromagnetic domain structures. However, atomic resolution cannot be achieved because data acquisition involves the sensing of long-range magnetostatic forces between tip and sample. Magnetic exchange force microscopy has been proposed3 for overcoming this limitation: by using an atomic force microscope4 with a magnetic tip, it should be possible to detect the short-range magnetic exchange force between tip and sample spins. Here we show for a prototypical antiferromagnetic insulator, the (001) surface of nickel oxide, that magnetic exchange force microscopy can indeed reveal the arrangement of both surface atoms and their spins simultaneously. In contrast with previous attempts to implement this method5,6,7,8, we use an external magnetic field to align the magnetic polarization at the tip apex so as to optimize the interaction between tip and sample spins. This allows us to observe the direct magnetic exchange coupling between the spins of the tip atom and sample atom that are closest to each other, and thereby demonstrate the potential of magnetic exchange force microscopy for investigations of inter-spin interactions at the atomic level.

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Figure 1: Concept of MExFM on the insulating antiferromagnet nickel oxide.
Figure 2: Chemical and magnetic exchange contrast with atomic resolution on NiO(001).
Figure 3: Quantification of the chemical contrast and the MExFM contrast after unit cell averaging.

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Acknowledgements

This work was supported by a grant from the Sonderforschungsbereich Magnetismus vom Einzelatom zur Nanostruktur of the Deutsche Forschungsgemeinschaft.

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Correspondence to Alexander Schwarz.

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Supplementary information

Supplementary Information

This file contains Supplementary Figures S1-S5 with Legends and Supplementary Discussion. The Supplementary Information contains eight sections. They describe the AFM set-up (section S1) and the methods employed to characterise the tip (section S2 and S3), which in turn allows to identify the two atomic species on the nickel oxide surface (section S4). The role of the magnetic polarisation at the tip apex and its controlled manipulation in a favourable direction is explained in section S5. More details regarding the exclusion of an oscillatory noise source and a tip artefact are given in section S6 and S7, respectively. Finally, section S8 refers to previous attempts to perform MExFM. (PDF 342 kb)

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Kaiser, U., Schwarz, A. & Wiesendanger, R. Magnetic exchange force microscopy with atomic resolution. Nature 446, 522–525 (2007). https://doi.org/10.1038/nature05617

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