Unusual ferromagnetism in nanoparticles of doped oxides and manganites

TL;DR

This paper explains the unusually large ferromagnetism observed in doped oxide and manganite nanoparticles through a microscopic charge transfer model, highlighting the role of surface effects and defect states.

Contribution

It introduces a microscopic calculation framework that accounts for high ferromagnetic responses in nanoparticles, extending the phenomenological charge transfer model.

Findings

Charge transfer between defect bands and dopants explains ferromagnetism.

Surface imperfections and charges enhance double exchange in manganites.

Nanoparticles exhibit significantly increased ferromagnetic tendencies.

Abstract

The observation of unusually large ferromagnetism in the nanoparticles of doped oxides and enhanced ferromagnetic tendencies in manganite nanoparticles have been in focus recently. For the transition metal-doped oxide nanoparticles a phenomenological `charge transfer ferromagnetism' model is recently proposed by Coey et al. From a microscopic calculation with charge transfer between the defect band and mixed valent dopants, acting as reservoir, we show how the unusually high ferromagnetic response develops. The puzzle of nanosize-induced ferromagnetic tendencies in manganites is also addressed within the same framework where lattice imperfections and uncompensated charges at the surface of the nanoparticle are shown to reorganize the surface electronic structures with enhanced double exchange.