Magnetism in metallic films and interfaces has been intensively studied since the discovery of Giant MagnetoResistance (GMR) in the late 1980s. This effect enabled fabrication of high sensitivity magnetic field sensors for the read heads in magnetic hard disks, revolutionizing magnetic recording. GMR occurs in structures where an ultra-thin "non-magnetic" film is sandwiched between two magnets, Fe / Cr / Fe being a popular example. Although often ignored, the weak magnetism of the Cr film is fascinating in its own right. As in all magnets, this magnetism originates from the arrangement of the spin magnetic moments of the electrons. Cr has one of the most extraordinary spin arrangements that can be found in the entire periodic table. The spins form a structure that is referred to as an antiferromagnetic Incommensurate Spin Density Wave. As shown in the figure, this involves a periodic modulation of the Cr spin, with a wavelength that does not match (i.e. is incommensurate with) the atomic periodicity. IRG3 postdoc Jeff Parker, working with graduate student Lan Wang, undergraduate Kim Steiner, and Professors Leighton and Crowell have used a well-known effect in thin film magnetism to directly probe this spin structure even in very thin films where other techniques cannot be applied. The work is based on the concept of interfacial exchange coupling between neighboring layers, using an effect called exchange bias. Exchange bias is an easily measurable perturbation of the properties of a ferromagnetic film due to proximity to an antiferromagnet like Cr. Its great sensitivity to the interface spins in the antiferromagnet means that in Fe/Cr thin film layers one can use the exchange bias in the Fe to probe the spin arrangement in the Cr. In particular, the wavelength of the Cr spin density wave varies with temperature, leading to an oscillation in the Cr surface spin magnitude, and therefore an oscillation in the strength of the coupling to the Fe, with temperature. By measuring these oscillations they were able to determine, in a unique way, important and detailed information on the spin density wave, such as the extent of the wavelength change with temperature, and the critical temperatures at which the nature of the spin density wave changes. The experiment required not only the fabrication of high crystalline perfection epitaxial Fe/Cr films (with very smooth interfaces) but also measurement of the exchange coupling strength to very high accuracy due to the small magnitude of the oscillations.
