Date of Completion

8-16-2013

Embargo Period

8-16-2013

Major Advisor

Dr. Menka Jain

Associate Advisor

Dr. Ramamurthy Ramprasad

Associate Advisor

Dr. Pu-Xian Gao

Field of Study

Physics

Degree

Doctor of Philosophy

Open Access

Open Access

Abstract

This work details the studies of several perovskite rare earth manganite (RMnO3) systems, focusing on the effects of both A-site and B-site doping of TbMnO3 and the magnetotransport behavior of La1-xSrxMnO3:insulator composites.

In pure TbMnO3 synthesized by solution route, it was revealed that the magnetic properties could be modulated by structural modifications resulting from different synthesis methods. In bulk Tb0.67Ho0.33MnO3, signatures of a weak ferromagnetic moment were detected in the magnetization measurements, and an anomaly due to Ho3+-Mn3+ exchange striction was observed in the polarization measurement. These results signify that both the ionic radius and the magnetic properties of the dopant affect the multiferroic properties in this system. Signatures of both long-range magnetic ordering as well as spin-glass-like behavior were observed in the DC and AC susceptibility data of Tb1-xMxMnO3 (M = Ca, Sr). This suggests the importance of the A-site ionic-size mismatch in determining the magnetic interactions. In bulk TbMn1-xCrxO3, the DC and AC susceptibility measurements and neutron diffraction revealed competition between several different spin configurations. A magnetic phase diagram for TbMn1-xCrxO3 is proposed.

Composite thin films of La0.67Sr0.33MnO3:ZnO and La0.67Sr0.33MnO3:MgO were grown using solution routes. A shift in the ferromagnetic transition to lower temperatures in the composite films indicates strain effects or doping at the interfaces. Disorder at the interfaces is presumed to result in spin-polarized tunneling at the grain boundaries. A significant enhancement of magnetoresistance, particularly at low applied magnetic fields (≤ 0.5 T), was observed in all composite films, making these promising for magnetic field sensing applications.

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