Spin-Charge-Lattice Couplings in the Mn Oxide Systems

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The substituted Lanthanum manganites (quasicubic and layered compounds) provide an unusual opportunity to investigate spin-charge-lattice correlations at the local structure level and connect the correlations with macroscopic phenomena such as Colossal Magnetoresistance (CMR) and charge ordering (CO). Local probe studies (Extended X-ray Absorption Fine Structure (EXAFS) and neutron Pair Distribution Function (PDF) analysis) show that there is a significant, polaron-induced, structural change as the temperature is increased through the ferromagnetic transition temperature, Tc, in zero field. In addition, there is a large and anomalous magnetostriction that is connected with the CMR behavior. This individual investigator award supports a project to determine for the first time the field-induced changes in the local structure as a function of magnetic field and extend the EXAFS measurements to several of the layered compounds. The high field measurements will probe how the magnetization initially develops just below Tc. The measurements on the layered materials will determine the anisotropy of the local distortions. These are important results for developing better models of ferromagnetism and electrical transport in these systems. The proposed project will provide the thesis topic for a graduate student as well as several senior thesis topics for undergraduates. The students will work as a team under the PI. They will gain the experience of performing research at a national facility.Some of the substituted the Lanthanum manganites (with part of the La replaced with Ca or Sr) show an unusual decrease in the local atomic distortions of the Mn-O bond as the sample becomes magnetized at low temperatures. For temperatures near the magnetic transition temperature there is also a large change in the resistivity of the sample and a large anomalous magnetostriction (a change in sample size when a magnetic field is applied). This individual investigator award supports a project to investigate how the local distortion changes in a large applied magnetic field and to correlate these results with the sample magnetization, magnetostriction and electrical resistivity. These results should also determine how the initial stages of the magnetization process take place in these materials. Similar measurements will be carried out on a layered manganite compound - but here the distortions in the layers and perpendicular to the layers need to be determined separately. The proposed project will provide the thesis topic for a graduate student as well as several senior thesis topics for undergraduates. The students will work as a team under the PI. They will gain the experience of performing research at a national facility.