Sensing Spin Order

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Science  12 Jul 2013:
Vol. 341, Issue 6142, pp. 111
DOI: 10.1126/science.341.6142.111-a

Noncontact atomic force microscopy (ncAFM) can be used to image the spin structure of surfaces when the scanning probe (tip) is fabricated from a ferromagnetic material, such as iron. One target for imaging is the superexchange interaction between oxygen atoms on the surface of the antiferromagnetic nickel oxide, a Mott insulator. For the (001) surface of this oxide, the exchange interaction creates a (2 × 1) spin pattern. This spin structure could be imaged at cryogenic temperatures via magnetic exchange force microscopy with iron tips when a 5-tesla magnetic field was applied to stabilize the tip's magnetization. Pielmeier and Giessibl have imaged the spin structure of this surface with ncAFM without applying magnetic fields at 4.4 kelvin. They used an iron tip in a frequency modulation mode; the tip oscillated at constant amplitudes that were below 0.5 angstrom. They found that with samariumcobalt alloy tips, the spin of the imaging atom of the tip was much more stable, and the spin signal was enhanced by a factor of 3 to 10. With this tip, a height contrast of 0.05 angstrom was seen for different oxygen atoms in the lattice that was the result of indirect superexchange interactions between the tip and subsurface nickel atoms.

Phys. Rev. Lett. 110, 266101 (2013).

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