The surfaces of crystalline materials have, in general, a different structure from that found within the bulk of the same material. The main reason for this is that, in forming a surface, bonds must be broken, which leaves electrons "dangling" out of the surface. These will redistribute themselves to create the lowest energy surface, which inevitably results in the atoms also rearranging themselves to a greater or lesser extent.
This surface rearrangement (called a "relaxation" if the movements are only in or out of the surface, and "reconstructions" if lateral movements in the plane of the surface are also present) can have fundamental influences on the physical properties of the material in this surface region. This is especially true for so-called "strongly correlated electron systems", where even the subtlest structural changes can bring about very large changes in the physical and electronic properties.
One problem in identifying how structural changes effect the physical properties is that it is very difficult to quantitatively see what the shifts in the atomic positions are. Even movements of as little as 1 picometer (1 millionth of 1 millionth of a meter), or an angular tilt in bond directions of less than 1 degree can be crucially important. Essentially, there is only one technique capable of providing the necessary accuracy, and this is Surface X-Ray Diffraction (SXRD).
In this project, we have studied the surface and interface structures of ultrathin films of La-Sr-Mn-O (LSMO) using SXRD. LSMO changes its resistivity by several orders of magnitude, depending on the strength of any external field it may be in, and therefore very interesting as a data storage medium. In this project, we want to see if there is a minimum thickness of thin film, below which this effect vanishes, and how this is related to changes in the thin film and interface structure.