Date of Award

Spring 2003

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Applied Physics - (Ph.D.)

Department

Federated Physics Department

First Advisor

Trevor Tyson

Second Advisor

Gordon A. Thomas

Third Advisor

Ken K. Chin

Fourth Advisor

John Francis Federici

Fifth Advisor

Zhen Wu

Abstract

Pressure effects on the electronic, magnetic properties and structure of several typical colossal magnetoresistive manganites, La0.60Y0.07Ca0.33MnO3, Pr1-xCaxMn03 (X = 0.25, 0.30, 0.35), Nd1-xSrxMnO3 (x = 0.45, 0.50), were explored through high pressure resistivity and structure measurements. It was shown that pressure up to ~7 GPa induces more complicated charge, spin and lattice state changes than in the low pressure range explored previously. In La0.60Y0.07Ca0.33MnO3, pressure induces a local atomic structure transformation at a critical point P*, and hence, a non-monotonic change in metal insulator (MI) transition temperature (Tmi) and spin state. In Pr0.75Ca0.25MnO3, with pressure increase, Tmi, increases and Tc decreases below P* and the trend is reversed above P*. In Pr0.7Ca0.3MnO3, pressure induces reentrant electronic and magnetic states: between ~0.8-5 GPa, TmI, and Tc are coupled and have a behavior similar to La0.60Y0.07Ca0.33MnO3, outside of this range, TmI and Tc are decoupled and at low and high pressure the material is insulating. In all three Pr1-xCaxMnO3 compounds, charge ordering is suppressed below P*. Above P*, an insulating state with unknown conducting mechanism is induced. In Nd1-xSrxMnO3, at x = 0.45, in addition to the effect on TmI, pressure possibly induces an A-type antiferromagnetic phase. For x = 0.5, the charge ordering transition temperature is increased, which is different from Pr1-xCaxMnO3 system. The effects of chemical doping (bandwidth) and pressure are not equivalent in the high pressure range. This is unlike the results in the low pressure range acquired by other groups previously. A universal P* exists for samples with metal-insulator transitions.

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