Physics & Astronomy

Colossal magnetoresistance (CMR) and short-range structural correlations

The CMR manganites are mixed-valence materials containing both Mn3+ and Mn4+ ions, so that the extra d electrons in the eg orbitals associated with Mn3+ may hop amongst the Mn sites. The eg electrons influence the magnetic exchange interactions and also interact strongly with the lattice via the Jahn-Teller mechanism (JT). Charge-carriers that have been "self-trapped" by the local JT lattice distortions that they induce are called electron-lattice polarons. Thus, the charge, orbital, magnetic, and lattice degrees become intimately coupled, giving rise to a great variety of magnetic, electronic, and structural phases. At Mn4+ concentrations near 30% some systems exhibit a pronounced transition from the paramagnetic insulating (PI) state to a ferromagnetic-metallic (FM) ground state that can be triggered by an applied magnetic field.

Our recent single-crystal diffuse x-ray and neutron scattering measurements on the bilayered La2-2xSr1+2xMn2O7 system (0.3 < x < 0.4) have conclusively demonstrated the existence of electron-lattice polarons. Anisotropic strain-field scattering distributions show that the polaronic orbitals exhibit a high degree of orientational order, and broad satellite maxima further reveal spatially-correlated polarons on the 1 to 3 nanometer length scale, that are indicative of Jahn-Teller-coupled charge-density-waves. The development of these polaronic structures above TC tracks the rise of the resistivity in the paramagnetic-insulating state, and their sudden disappearance at TC marks a precipitous resistivity drop. The onset of ferromagnetic order at TC appears to destabilize the polarons, allowing the eg electrons to delocalize, and the insulator-metal transition that results is the defining characteristic of CMR.

Our single-crystal diffuse scattering work has opened a new window on the world of CMR phenomena, where the influence of nanoscale structural features on the macroscale transport properties can now be studied in exquisite detail. New experiments are currently underway to further explore this relationship in a variety of magnetoresistive materials, and instruments and methods are being developed that will also aid the study of other interesting systems that possess a high degree of local structure, such as ferroelectric relaxors and zeolite catalysts.