Evidence for a Purely Double-Exchange Mechanism for the Anisotropic Conductivities in Layered La-Sr-Mn-O Single Crystals Below TC

TL;DR

This study provides experimental evidence that the double-exchange mechanism fully explains the anisotropic conductivities in layered La-Sr-Mn-O single crystals below the Curie temperature, linking magnetic order to electrical transport.

Contribution

It demonstrates that the double-exchange mechanism accounts for both in-plane and c-axis conductivity, supported by data aligning with spin-wave and neutron scattering results.

Findings

Conductivity proportional to magnetization squared below TC

Double-exchange ratio close to conductance anisotropy of ~300

Prediction of a mixed antiferromagnetic and spin-flop state

Abstract

Experimental evidence supports the double-exchange (DE) mechanism for both in-plane and c-axis conductivity in the colossal-magnetoresistive (CMR) layered manganite La1.4Sr1.6Mn2O7. Below TC, the data determine both the DE and antiferromagnetic (AF) superexchange between bilayers. These agree with recent spin-wave data, such that the ratio of intra- to inter-bilayer DE constants is very close to the spin-independent conductance anisotropy of ~300. The conductivity is shown to be proportional to the square of the measured magnetization over a wide range of fields and for temperatures below the in-plane ferromagnetic ordering temperature, TC. This dependence is shown to be consistent with DE coupling and earlier zero-field studies of La1.2Sr1.8Mn2O7 near TC, which used neutron scattering for the local magnetization. A mixed AF and spin-flop state (similar to the intermediate state of type-I superconductors) is a rigorous prediction of the data and modeling.