Perovskite Manganese Oxides popularly known as “CMR Manganites” [1] are doped rare earth manganese oxides of the general formula R1-xAxMnO 3 where R is a trivalent rare earth element (La, Nd, Pr) and A is a divalent alkaline earth element (Ca, Sr, Ba or Pb). The charge imbalance created by this substitution results in a mixed valence state of Mn (Mn3+and Mn4+) which promotes ferromagnetism and metallicity through electron transfer between these two valence states (via the so-called “double exchange” mechanism) and leads to colossal magnetoresistance. In addition to the double-exchange phenomenon, the behavior of these materials is also subtly governed by the interplay between structure and electronic properties, brought about the tendency for Mn3+ ion to induce Jahn-Teller distortions of the Manganese-Oxygen octahedra [2]. The close coupling between structure, electron transport and magnetic ordering results in a very rich compositional phase diagram exhibiting a range of electronic phenomena including metal-insulator transitions, ferromagnetism, colossal magnetoresistance and charge /orbital ordering. These phenomena are interesting in terms of the rich materials physics underlying the complex interplay of structural electronic and magnetic degrees of freedom. Several of these phenomena also make these materials promising candidates for technological applications such as magnetic sensors, bolometric infrared detectors etc.

Recent research has lead to the understanding that many of the observed phenomena are strongly influenced by an intrinsic tendency of these materials to allow the co-existence of ferromagnetic metallic phases with insulating phases which could be either charge-ordered or charge disordered [3], the phase co-existence arising due to several competing electronic ground states. The propensity for phase separation varies between different material systems and is controlled by the average rare earth ionic size (example, difference between the Low Tc manganites such as Nd 2/3 Sr 1/3 MnO3 vs high Tc manganites such as La2/3Sr1/3MnO3 local strains induced by random variation of the average ionic size (example, PryLa(1-x-y)CaxMnO3). Multi-phase coexistence results in a percolative first order insulator-metal transition and a suppression of the transition to lower temperatures. In contrast, the metal-insulator transition in higher Tc systems which is found to have second order characteristics related to the development of long range ferromagnetic order. It has also been found that the 1/f electrical noise in ceramics and single crystals of PryLa(1-x-y)CaxMnO3 shows orders of magnitude enhancement near the metal-insulator transition compared to the high Tc manganites. Noise characteristics in the low temperature phase indicate the presence of charge ordered domains even in the metallic state, suggesting that the percolative transition does not involve the charge ordered phase, but a charge disordered one[4].

Epitaxial thin films of the two-phase system PryLa(1-x-y)CaxMnO3 have been found to behave quite differently from the bulk in that the transport characteristics do not follow the expected behavior, according to the effective medium theory of a binary mixture of insulating and metallic domains near the transition temperature[5]. The behavior of noise in thin films have been found to be different from that of the bulk in previous studies of the high Tc manganite systems. This suggests that a detailed investigation of the multiphase behavior in thin films is important for a complete understanding of the phase separation phenomena, which is the objective of this proposal. In addition to studying the basic electrical and magneto-transport and detailed investigation of 1/f noise characteristics we will also study the photo response, i.e. effect of visible radiation of varying intensities on the transport and noise behavior. The charge ordered phase has been shown to be sensitive to radiation over a wide range of wavelengths. It will be interesting to investigate the radiation sensitivity of the multi-phase system, details of which would contribute to the understanding of the nature of the transition and the role of the charge ordered domains. Such a study in thin films for the visible range is feasible in a relatively simple experimental set up. Besides furthering the fundamental understanding of the multi-phase behavior, the study proposed here also has relevance to the IR detectors, which is a focus of my on-going research. Percolative systems offer the advantage of very steep metal-insulator transitions desirable for a bolometric detector. However, the accompanying large noise is a concern. Understanding the noise behavior and investigating possibilities for optimization of the coupled behavior of resistivity and noise with some potential trade-offs, is therefore relevant.

Finally, the proposed work is synergistic with Towson’s University’s emphasis on high quality undergraduate education through active student participation. The proposed work will involve 2 students each summer for 2 summers. These students will also participate in the research during the academic year via independent research course offerings. The proposal involves setting up a noise measurement system which will expose the students to sensitive electrical measurement techniques with the potential for this measurement to be included in our advanced lab curriculum.