Temperature-dependent local atomic structure in the colossal magnetoresistive manganese oxide lanthanum strontium manganese oxide

Extended X-ray Absorption Fine Structure (XAFS) measurements at the Mn, Sr, and La K-edges of the colossal magnetoresistive (CMR) manganese oxide La_1.2Sr_1.8 Mn₂O₇ show that the local atomic structure is coupled to the metal-insulator and paramagnetic-ferromagnetic transitions. These polarized and temperature dependent XAFS measurements are the first performed on a layered CMR manganite in an effort to further elucidate the strong interplay between charge, spin, and lattice degrees of freedom in these hole doped oxides. The polarization dependence of the XAFS indicates that the local distortions accompanying this electronic and magnetic phase transition are associated with Jahn-Teller type distortions to the apical (lying perpendicular to the perovskite MnO planes) oxygen distribution around the MN cations. The metal-insulator transition is accompanied by a sudden increase in the number of Jahn-Teller distorted Mn sites as well as anomalous increases in the thermal Debye-Waller factor indicating increased fluctuations of the local atomic arrangements. A possibility for the La and Sr cations to influence the electronic and magnetic transitions is suggested by their tendency to stabilize differing local arrangements of the nearest neighbor oxygen atoms. The contrasting temperature dependence of the La-oxygen and Sr-oxygen distributions shows that the temperature dependent local atomic distortions observed in the apical oxygen distribution around the Mn ions are primarily associated with Sr atoms. The observed differences in the local atomic structures surrounding Sr and La are supported by similar results in the non-CMR oxide LaSrMnO₄. This element specific interaction suggests the possibility that the local dopant concentration could enhance or induce electronic, magnetic, and structural phase separation. Thus, the XAFS experiments documented here have supported the general view that local lattice distortions can play a role in the MI transition and the CMR effect in the bilayer manganite La_1.2 Sr_1.8Mn₂O ₇ while also indicating that the effects of the local chemical environment of the Mn ions may play a role in determining the nature of the phase transitions.