Magnetic localization in mixed-valence manganites

TLDR

The metal‑insulator transition in mixed‑valence manganites such as La₀.₇Ca₀.₃MnO₃ is driven by changes in the spin‑dependent potential J_H·S at the Curie temperature, with electrons above T_C localized by random spin potentials and conducting via variable‑range hopping. Here J_H is the on‑site Hund's‑rule exchange coupling of an e_g electron (s = ½) with the t₂g core spin S = 3/2. The resistivity follows ln(ρ/ρ∞) = [T₀(1 – (M/Ms)²)/T]¹⁄⁴, and the molecular‑field theory of magnetization accurately reproduces its temperature and field dependence across a wide range.

Abstract

The metal-insulator transition is mixed-valence manganites of the (${\mathrm{La}}_{0.7}$${\mathrm{Ca}}_{0.3}$)${\mathrm{MnO}}_{3}$ type is ascribed to a modification of the spin-dependent potential ${\mathrm{J}}_{\mathrm{H}}$s\ensuremath{\cdot}S associated with the onset of magnetic order at ${\mathrm{T}}_{\mathrm{C}}$. Here ${\mathrm{J}}_{\mathrm{H}}$ is the on-site Hund's-rule exchange coupling of an ${\mathrm{e}}_{\mathrm{g}}$ electron with s=1/2 to the ${\mathrm{t}}_{2\mathrm{g}}$ ion core with S=3/2. Above ${\mathrm{T}}_{\mathrm{C}}$, the ${\mathrm{e}}_{\mathrm{g}}$ electrons are localized by the random spin-dependent potential and conduction is by variable-range hopping. Over the whole temperature range, the resistivity varies as ln(\ensuremath{\rho}/${\mathrm{\ensuremath{\rho}}}_{\mathrm{\ensuremath{\infty}}}$)=[${\mathrm{T}}_{0}${1-(M/${\mathrm{M}}_{\mathrm{S}}$${)}^{2}$}/T${]}^{1\mathrm{/}4}$, where M/${\mathrm{M}}_{\mathrm{S}}$ is the reduced magnetization. The temperature and field dependence of the resistivity deduced from the molecular-field theory of the magnetization reproduces the experimental data over a wide range of temperature and field.