Magneto-electronic phase separation in strontium doped perovskite cobaltites.

摘要

Magneto-electronic phase separation refers to the presence of multiple magnetic and electronic phases within a material that is chemically homogeneous. It is very common in oxides and has been related to their most exciting properties such as superconductivity and colossal magnetoresistance (CMR). In this work, phase separation in perovskite La1-xSr xCoO3 was investigated comprehensively by conventional magnetometry and electrical measurements as well as Small Angle Neutron Scattering (SANS), Nuclear Magnetic Resonance (NMR), and Transmission Electronic Microscopy (TEM). All the work leads to a consistent picture based on short range ferromagnetic ordering and intrinsic magnetic phase separation. When the doping is less than 18%, the doped cobaltites separate into ferromagnetic (FM) metallic clusters embedded in the non-FM insulating matrix. With increasing x, the ferromagnetic clusters become more populous, and coalesce with each other, forming a long-range ferromagnetic network. The metal insulator transition occurs at the same x value, which can be described by a simple percolation transition. This phase separation occurs without experiencing any structural changes or chemical inhomogeniety by TEM analysis. Unlike their manganite counterparts, where the phase separation often occurs on the mesoscopic length scale, the phase separation in cobaltites happens at the nm scale, of the order 10-30 A.;The direct evidence of phase separation is proven by NMR and small angle neutron scattering. Both 59Co NMR and high field 139 La NMR have established clearly the inhomogeneity with ferromagnetic phase, spin---glass phase and paramagnetic phase coexisting in La1--x SrxCoO3 over the entire doping concentration and temperature range. SANS confirm that La1--xSrxCoO 3 phase separates into ferromagnetic metallic clusters embedded in a non-ferromagnetic semiconducting matrix. On the insulating side of the MIT, the ferromagnetic clusters are isolated, while on the metallic side of the MI transition, there exists long range ferromagnetic ordering.;The consequences of this intrinsic nanoscale phase separation were probed. The formation of FM clusters in a non-FM matrix results in an intergranular giant magnetoresistance effect analogous to artificial heterostructures, the existence of glassy transport phenomena analogous to the relaxor ferroelectrics, and a simple percolation threshold at x = 0.18.