Full Bulk Spin Polarization and Intrinsic Tunnel Barriers at the Surface of Layered Manganites

TL;DR

This study reveals that the surface of layered manganites can naturally form an insulating, non-magnetic layer while the underlying layers retain full spin polarization, enabling potential applications in magnetic tunnel junctions.

Contribution

It demonstrates that the outermost bilayer of layered manganites becomes an insulator without ferromagnetic order, while the next bilayer maintains full spin polarization, highlighting a natural self-assembly process for tunnel junction components.

Findings

Surface bilayer is a 1-nm insulator with no ferromagnetic order.

Next bilayer exhibits full bulk spin polarization.

Surface effects are localized due to layered structure and reconstructed surface.

Abstract

Transmission of information using the spin of the electron as well as its charge requires a high degree of spin polarization at surfaces. At surfaces however this degree of polarization can be quenched by competing interactions. Using a combination of surface sensitive x-ray and tunneling probes, we show for the quasi-two-dimensional bilayer manganites that the outermost Mn-O bilayer, alone, is affected: it is a 1-nm thick insulator that exhibits no long-range ferromagnetic order while 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 while 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 demonstrates the ability to naturally self-assemble two of the most demanding components of an ideal magnetic tunnel junction.