Anomalous Magnetoresistance in the Lanthanide Manganites and Its Relation to High-Tc Superconductivity.
Abstract
A theory of electron transport is formulated from the polaronic and magnetic exchange character of mixed-valence transition metal oxides. Where magnetic ordering is established at low temperatures, the conduction is mainly by polarons. Above the curve temperature Tc, electron hopping by thermal activation becomes dominant. The conventional relations for mobility-activated semiconduction anti the molecular field theory of ferromagnetism are adapted for interpretation of electrical resistivity p data of the lanthanitle (rare-earth-RE) manganites (RE(3+)1-xA(2+)x)MnO3 as functions of temperature and external magnetic field. Computed results are consistent with the original resistivity data and agree quantitatively with the more recently observed magnetoresistance in film versions of this oxide system. The anomalies in electrical behavior are related to the superexchange couplings derived from sigma and pi bonding of the various 3d orbital states among Mn25+(3d5), Mn3+(3d4), and Mn4+(3d3) ions in 180 metal-oxygen-metal configurations. The resistivity maximum occurs at the susceptibility peak slightly above Tc, and its magnitude is determined principally by the factor ((l-x)Ix) (kTc) exp(hop/kTc) where the activation energy E(hop) is the sum of a polaron trap energy Ehop0(approx. 5meV) and a spin polarization-dependent component Ehopex(1-Bs)T,H)), and Bs is the Brillouin function. For T >= Tc, ferromagnetic spin alignment becomes disordered and Ehop increases to a value approx. 0.1 eV equal to the decrease in stabilization energy of the transfer electrons caused by the transition from spin alignment to disorder, i.e., Bs = 0. If E(hop) is independent of temperature above Tc, the maximum sensitivity dp/dH, which occurs at T = Tc, may be exclusively a function of Tc through the function (1/kTc) exp(E1hop/kTc).
Document Details
- Document Type
- Technical Report
- Publication Date
- May 01, 1996
- Accession Number
- ADA309080
Entities
People
- G. F. Dionne
Organizations
- Massachusetts Institute of Technology