Internal dipolar field and soft magnons in periodic nanocomposite magnets

TL;DR

This paper investigates how dipolar interactions influence spin wave excitations in a 3D nanocomposite magnet composed of hard and soft magnetic phases, revealing how increasing soft phase content softens spin waves and alters the ground state.

Contribution

It provides a detailed analysis of the effects of dipolar interactions and soft phase content on spin wave spectra and ground state configurations in nanocomposite magnets.

Findings

Dipolar interactions split spin wave energies into upper and lower branches.

Increasing soft phase content softens low-lying spin excitations.

The system transitions to a different ground state with magnetization mismatch.

Abstract

We study spin wave excitations in a three-dimensional nanocomposite magnet of exchange coupled hard (SmCo$_5$) and soft (FeCo) phases. The dipolar interaction splits the spin wave energies into the upper and lower branches of the spin wave manifold. When the amount of the soft phase is increased the energy of low-lying spin excitations is considerably softened due to two reasons: (i) the low- lying mode locked into the soft phase region with a spin wave gap at ${\bf k}= 0$ which scales approximately proportional to the anisotropy constant of the soft phase and (ii) the internal dipolar field which comes from magnetic charges forming at hard-soft boundaries with normals parallel to the magnetization displaces the spin wave manifold toward the lower energies. With adding more soft phase the spin wave gap closes and the system moves to another ground state characterized by the magnetization mismatch between spins of the hard and soft phases.