Vacancy-driven non-cubic local structure and magnetic anisotropy tailoring in Fe$_x$O-Fe$_{3-\delta}$O$_4$ nanocrystals

TL;DR

This study reveals how vacancy-driven local structural distortions in Fe_xO-Fe_{3- extdelta}O_4 nanocrystals influence magnetic anisotropy, offering a new approach to tailor magnetic properties at the nanoscale.

Contribution

It demonstrates that cationic vacancies in iron-oxide nanocrystals can be used to control local structure and magnetic anisotropy, advancing defect engineering in nanomaterials.

Findings

Vacancies induce tetragonal distortion in local structure.

Vacancies produce exchange-anisotropy fields enhancing magnetization.

Defect control improves hyperthermia performance.

Abstract

In contrast to bulk materials, nanoscale crystal growth is critically influenced by size- and shape-dependent properties. However, it is challenging to decipher how stoichiometry, in the realm of mixed-valence elements, can act to control physical properties, especially when complex bonding is implicated by short and long-range ordering of structural defects. Here, solution-grown iron-oxide nanocrystals (NCs) of the pilot wustite system are found to convert into iron-deficient rock-salt and ferro-spinel sub-domains, but attain a surprising tetragonally distorted local structure. Cationic vacancies within chemically uniform NCs are portrayed as the parameter to tweak the underlying properties. These lattice imperfections are shown to produce local exchange-anisotropy fields that reinforce the nanoparticles magnetization and overcome the influence of finite-size effects. The concept of atomic-scale defect control in subcritical size NCs, aspires to become a pathway to tailor-made properties with improved performance for hyperthermia heating over defect-free NCs.