Full bulk spin polarization and intrinsic tunnel barriers at the surface of layered manganites

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Abstract

Transmission of information using the spin of the electron as well as its charge requires a high degree of spin polarization at surfaces. However, at surfaces this degree of polarization can be quenched by competing interactions. Using a combination of surface-sensitive X-ray and tunnelling probes, we show for the quasi-two-dimensional bilayer manganites that only the outermost Mn–O bilayer is affected: it is a 1-nm-thick insulator that exhibits no long-range ferromagnetic order, whereas the next bilayer displays the full spin polarization of the bulk. Such an abrupt localization of the surface effects is due to the two-dimensional nature of the layered manganite, and the loss of ferromagnetism is attributed to weakened double exchange in the reconstructed surface bilayer and a resultant antiferromagnetic phase. The creation of a well-defined surface insulator atop a fully spin-polarized bulk demonstrates the ability of two of the most demanding components of an ideal magnetic tunnel junction to self-assemble naturally.

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Figure 1: Structure of the naturally bilayered manganite La2–2xSr1+2xMn2O7.
Figure 2: Polarization-dependent scattering and absorption data at the Mn L3 edge.
Figure 3: Magnetic profile determination and temperature dependence of the bulk-like sub-surface bilayer.
Figure 4: The log current–voltage characteristics of numerous high-resistance Au point-contacts (PT) taken at 4.2 K on a crystal x = 0.36, plotted together with representative STS data taken well above TC.
Figure 5: Field-induced metal–insulator transition for a layered manganite with x = 0.48.

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Acknowledgements

The research, including the use of the Advanced Photon Source, was supported by the US Department of Energy (DOE), Basic Energy Sciences, under contract W-31-109-ENG-38.

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Correspondence to J. W. Freeland.

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Freeland, J., Gray, K., Ozyuzer, L. et al. Full bulk spin polarization and intrinsic tunnel barriers at the surface of layered manganites. Nature Mater 4, 62–67 (2005) doi:10.1038/nmat1280

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