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abstracts
In the process of the growth of the magnetically frustrated pyrochlore holmium titanate, Ho2Ti2O7, fluctuations in the pressure of the optical floating zone image furnace were observed. Upon removal, banding from colour centres was noted. These bands were removed by annealing the material at 600 degrees Celsius for 12 hours. Based on this observation, a series of growths was performed to determine the effects of small pressure fluctuations on the lattice spacing of a material grown by the optical floating zone method. Multiple crystals of holmium titanate were prepared from the same precursor materials, pre-reacted in an identical way and grown under variable overpressures between 2 and 8 atmospheres in both argon and oxygen. Initial measurements using single crystal x-ray diffraction showed that there were differences in the crystallographic lattice spacings that were significantly outside of instrumental error. These differences generally corresponded to a correlation of decreased spacing with increase pressure. This talk will present our recent results of a significantly larger sample set of this material, analyzed using powder x-ray diffraction methods on ground up portions of the grown single crystal.
abstracts
B-site ordered double perovskites of the type AA'BB'O6 have attracted attention recently due to unusual and unpredictable modes of magnetic order. Of particular interest are materials where AA' = Ba, Sr or La and B' = Mo,Ru,Os and Re and B is a diamagnetic ion such as Li+,Na+,Ca2+, Mg2+, Zn2+ or Cd2+.Crystal symmetries encountered are Fm-3m, I4/m and P21/n as a function of the Goldschmidt tolerance factor which in turn impose local symmetries at the magnetic B' site of m-3m, 4/m and -1, respectively. The magnetic ions have electronic configurations 4d1, 4d3,5d1, 5d2 and 5d3 with nominal spins, S = 1/2,1 and 3/2. Spin orbit coupling is very large, reaching ~ 0.5eV for the 5d ions, and plays a more important role than in the more familiar 3d-based magnetic materials. Geometric frustration, the B' ion sublattice is f.c.c., based on edge sharing tetrahedra, also is of critical importance. Observed magnetic ground states range from spin glasses (without apparent disorder), collective spin singlets, ferromagnets and antiferromagnets. Often, ordered moments are a small fraction of a Bohr magneton. Systematic behavior is elusive. For example Ba2YReO6 and Ba2CaOsO6 (both Fm-3m) have the same unit cell constant to within .01 % and both are based on 5d2 ions but the former is a highly unconventional spin glass and the latter orders antiferromagnetically below 50K. Recent results on this class of magnetic materials will be described in the context of existing theories.
