Scanning tunnelling microscopy of bilayer manganites

This thesis describes experimental work carried out on bilayer manganites with the general composition R_{2-2x}A_{1+2x}Mn_2O_7, where R is a trivalent rare earth cation and A is a divalent alkaline-earth cation. Experiments have been carried out primarily using Scanning Tunnelling Microscopy (STM) and Spectroscopy (STS); bulk electrical transport, MPMS and LEED measurements have also been made. The primary results are obtained from single crystal samples of PrSr_2Mn_2O_7. This compound provides a surface suitable for STM study when cleaved at low temperature in ultra-high vacuum: atomic resolution can be readily achieved. The expected square lattice is observed, together with a larger scale surface modulation which is not presently explained. In some areas of the PrSr_2Mn_2O_7 surface a population of adatoms and surface vacancies is observed. STS data indicate that adatoms carry a negative charge compared to the rest of the surface, and vacancies a positive charge: the adatoms and vacancies are interpreted as oxygen adatoms and oxygen vacancies. A detailed study is made of the oxygen adatoms and vacancies: this is believed to be the firrst such study made on a manganite surface. Oxygen adatoms on the PrSr_2Mn_2O_7 surface are found to be mobile: hopping and adatom-vacancy recombination are observed. Additional results are reported on the layered manganite compound La_{2-2x}Sr_{1+2x}Mn_2O_7 at a range of cation doping x. For the LaSr_2Mn_2O_7 compound (x = 0.5) spectroscopic variation has been identi_ed in a variable-temperature STS survey. This indicates the coexistence of two surface electronic phases, possibly the charge ordered and antiferromagnetic phases

90° Rotation of orbital stripes in bilayer manganite PrCa2Mn2O7 studied by in situ transmission electron microscopy

We present an in situ transmission electron microscopy study on the half-doped bilayer manganite PrCa2Mn2O7 to reveal the rotation process of the orbital stripes. Between the reported initial and final ordering phases, we identified an intermediate state with two sets of satellite spots to bridge the 90° rotation of the orbital stripes. Furthermore, we determined that the rotation of the orbital stripes does not always occur. Some restricted conditions for the orbital rotation to occur were found and reasons are discussed. © 2013, Elsevier Inc