These are manganese oxide based ceramics that exhibit a metal - insulator and ferromagnetic - paramagnetic phase transition as a function of temperature, as shown in the figure below. The application of a magnetic field to the system drastically alters the behavior of the system, such that if we are sitting near the transition temperature Tc, there will be a large drop in resistance. In the plot at left this drop (a negative magnetoresistance) is about 2 orders of magnitude, while in certain materials it may be up to an 11 order of magnitude effect. These materials offer promise as sensors, e.g. as read heads in magnetic storage devices, and they are also challenging us to find an understanding for the exciting and novel effects.
We have been studying the electronic structure and phase transitions of these systems using high resolution angle-resolved photoemission spectroscopy and X-ray absorption spectroscopy. Important results include the finding of a strongly temperature-dependent pseudogap and a "ghost" Fermi Surface.