Giant magnetoelectric effect induced by intrinsic surface stress in ferroic nanorods

TL;DR

This paper develops a phenomenological theory showing that intrinsic surface stress in ferroic nanorods induces a giant magnetoelectric effect, significantly altering phase transitions and properties at nanoscale.

Contribution

It introduces a comprehensive model incorporating surface stress, magneto- and electrostriction, and piezo effects to explain magnetoelectric phenomena in ferroic nanorods, highlighting the impact of nanorod radius.

Findings

Intrinsic surface stress shifts transition temperatures and induces built-in fields.

Quadratic magnetoelectric coupling increases dramatically as nanorod radius decreases.

Phase diagrams are significantly altered, with possible change from second to first order transitions.

Abstract

The general approach for the consideration of the magnetoelectric effects in ferroic nanorods is proposed in the framework of the phenomenological theory. The intrinsic surface stress, magneto- and electrostriction as well as piezoelectric and piezomagnetic effects are included into the free energy. The intrinsic surface stress under the curved nanoparticle surface is shown to play an important role in the shift of ferroelectric and ferromagnetic transition temperatures and built-in magnetic and electric fields appearance, which are inversely proportional to the nanorod radius. We consider the case of quadratic and linear magnetoelectric coupling coefficients. The linear coupling coefficient is radius independent, whereas the quadratic ones include terms inversely proportional to the nanorod radius and thus strongly increase with decrease of the radius. The predicted giant relative dielectric tunability in the vicinity of ferromagnetic and ferroelectric phase transition points induced by quadratic magnetoelectric coupling increases by 2-50 times. The quadratic magnetoelectric coupling dramatically changes the phase diagrams of ferroic nanorods when the radius decreases. In particular the second order phase transition may become a first one, the triple point state characterized by continuous set of order parameters appears at zero external electric and magnetic fields and the tricritical points appear under external fields.