Real space and momentum space X-ray spectroscopy of phase separated manganites [electronic resource]
Keywords: Applied Physics

Mixed-valence manganites have been the subject of intense study since the discovery of colossal magnetoresistance (CMR), whereby an applied magnetic field of a few tesla leads to an decrease in resistivity that varies from about a factor of two to 10 orders of magnitude, depending on the exact composition of the material. That a magnetic field would reduce resistivity was not itself surprising, as it was well-known that metallic manganites are also ferromagnetic. This link could not explain the magnitude of the effect, though. Soon thereafter, manganites were shown to exhibit phase separation, where incompatible phases coexisted. Since its discovery, phase separation has been strongly linked to CMR, although the exact mechanism is not clear. Also a mystery is the reason that phase separation happens in the first place: it happens over a large range of compositions, so it cannot be the result of a the free energy being perfectly balanced between the competing phases. In this dissertation, I discuss work on La0.35Pr0.275Ca0.375MnO3 (LPCMO), which is the ideal material for studying phase separation in manganites. At low temperatures, it is a ferromagnetic metal, and at the Curie temperature it becomes charge and orbitally ordered, antiferromagnetic, and insulating. The phase transition between the two is first order, with a thermal hysteresis of more than 30 K. I performed resonant elastic soft x-ray scattering measurements, which are sensitive to orbital order and antiferromagnetism, on LPCMO to study the temperature dependence of those phases. To study the ferromagnetic metallic phase, I took images using a x-ray photoemission electron microscope (PEEM), which measures x-ray absorption spectroscopy at high spatial resolution. The two datasets show strong evidence for glassy behavior over most of the temperature range where there is electronic order. A glass is a system that does not reach its equilibrium state in an "experimental" timescale, so this leads to the conclusion that a state with phase separation is not the ground state. Below the Curie temperature, the magnetization has an unconventional temperature dependence, which is consistent with a battle between the two types of order over control of the boundaries between them. The band structure of mixed-valence manganites contains important information, so there have been a series of angle-resolved photoemission measurements on bi-layered La2-2xSr1+2xMn2O7. This is the only manganite system that is currently possible to measure using that technique, as it can be cleaved in vacuum to create a surface that has never been exposed to air and has a high enough conductivity to avoid space charge. Unfortunately, different studies have produced conflicting data. One theory is that the samples contain "stacking faults, " which are inclusions of n-layered manganites, n > 2. The PEEM is sensitive to chemical inhomogeneities, so I used it to image La1.2Sr1.8Mn2O7. My results confirmed that there are indeed stacking faults, and that they are responsible for the magnetic anomalies that had been observed in bi-layered systems above the nominal Curie temperature.