Suppressed Magnetization at the Surfaces and Interfaces of Ferromagnetic Metallic Manganites

TL;DR

This paper investigates how ferromagnetism is suppressed at the surfaces and interfaces of manganites, revealing that surface magnetic order can differ significantly from the bulk due to competing electronic degrees of freedom.

Contribution

It provides experimental evidence of suppressed or absent ferromagnetic order at manganite surfaces and interfaces using advanced surface probes, highlighting the role of intrinsic surface layers and interlayer coupling.

Findings

Surface of bilayer manganite exhibits an insulating, non-ferromagnetic layer.

Interface magnetization in 3D manganite is less than half of the bulk.

Surface magnetic properties vary with material dimensionality and temperature.

Abstract

What happens to ferromagnetism at the surfaces and interfaces of manganites? With the competition between charge, spin, and orbital degrees of freedom, it is not surprising that the surface behavior may be profoundly different than that of the bulk. Using a powerful combination of two surface probes, tunneling and polarized x-ray interactions, this paper reviews our work on the nature of the electronic and magnetic states at manganite surfaces and interfaces. The general observation is that ferromagnetism is not the lowest energy state at the surface or interface, which results in a suppression or even loss of ferromagnetic order at the surface. Two cases will be discussed ranging from the surface of the quasi-2D bilayer manganite (La$_{2-2x}$Sr$_{1+2x}$Mn$_2$O$_7$) to the 3D Perovskite (La$_{2/3}$Sr$_{1/3}$MnO$_3$)/SrTiO$_3$ interface. For the bilayer manganite, that is, ferromagnetic and conducting in the bulk, these probes present clear evidence for an intrinsic insulating non-ferromagnetic surface layer atop adjacent subsurface layers that display the full bulk magnetization. This abrupt intrinsic magnetic interface is attributed to the weak inter-bilayer coupling native to these quasi-two-dimensional materials. This is in marked contrast to the non-layered manganite system (La$_{2/3}$Sr$_{1/3}$MnO$_3$/SrTiO$_3$), whose magnetization near the interface is less than half the bulk value at low temperatures and decreases with increasing temperature at a faster rate than the bulk.