Neumeier, John

Measurements of magnetization versus temperature and field have been conducted on La1-xCaxMnO3 compounds which for some values of x exhibit colossal magnetoresistence (CMR). Recent theories propose that polaron forming double exchange interactions and Jahn-Teller distortions create a strong interaction amongst charge, spin and lattice degrees of freedom which is responsible for the unusually large magnotoresistence. In this work the enhancement of paramagnetic susceptibility is studied in order to investigate the possible existence of magnetic polarons. Enhancements of the effective moments are found in the range 0.15 < x < 0.94, which are qualitatively consistent with the existence of magnetic polarons. The findings are discussed within the framework of recent experimental and theoretical findings.

Magnetic and electrical properties for La(1-x)Gd(x)0.7Ca(0.3)MnO3 were studied at ambient and high pressure. For x > 0.1 the ferromagnetic saturation moment M is smaller than that expected for complete ferromagnetic alignment of the Mn moments. Gd substitution lowers both the paramagnetic-ferromagnetic transition temperature T(C), the metal-insulator transition temperature T(MI) and dramatically increases the electrical resistivity rho. T(MI) is observed to be significantly lower than T(C) for x > 0.1. The disagreement of T(C) and T(MI) is a result of the A-site ion size difference which causes the coexistence of ferromagnetic La-Ca-Mn-O and antiferromagnetic Gd-Ca-Mn-O phases and leads to the anomalous electrical transport properties. Samples with the largest average A-site ionic radius <rA> show the highest T(C) or T(MI) and are least sensitive to pressure.

A high sensitivity capacitance dilatometer constructed solely of fused quartz was designed to study phase transitions of sub-millimeter sized samples from 4 to 300 K. The minimal thermal expansion contribution from quartz reduced the overall magnitude of the background by a factor of 10. As a consequence, the increased resolution of the data provided greater sensitivity than has been capable with metallic capacitance dilatometers. A one millimeter long single crystal of La1.67Sr0.33NiO 4 was analyzed from 80 to 300 K. Our highly reproducible measurements reveal phase transitions at similar temperatures to those observed with magnetization and specific heat measurement techniques.

Early reports of antiferromagnetism in CaRuO3 will be discussed within the framework of our recent experiments. We have observed that slight Ru deficiency (∼4%) leads to a weak feature in the magnetic susceptibility. Temperature-dependant time-of-flight (TOF) neutron powder diffraction data revealed no long-range magnetic order in the Ruthenium deficient CaRu 0.96O3 sample. More careful analysis of magnetization data indicates that the feature in chi(T) can be attributed to a weak ferromagnetic phase which forms below an ordering temperature of T = 147 K. This phase is destroyed with Lanthanum doping (∼2%) and annealing in reducing conditions dramatically shifts this feature. Measurements of the magnetic susceptibility versus temperature of the series Ca1-xLaxRu 0.96O3 (0 ≤ x ≤ 0.12) led to further study of CaRuO 3 and SrRuO3. The structural parameters of CaRuO 3 and SrRuO3 powder samples, obtained from neutron time-of-flight data analysis via Rietveld refinement, will be compared.