  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.
