Spin waves in paramagnetic BCC iron: spin dynamics simulations

TL;DR

This study uses large-scale spin dynamics simulations to demonstrate the existence of propagating and over-damped spin waves in paramagnetic BCC iron well above the Curie temperature, resolving a longstanding controversy.

Contribution

It provides the first detailed simulation evidence of spin waves persisting in paramagnetic BCC iron above T_c, challenging previous assumptions.

Findings

Well-defined peaks in the dynamic structure factor persist above T_c

Propagating spin waves exist at large wave vectors

Over-damped spin waves are observed at small wave vectors

Abstract

Large scale computer simulations are used to elucidate a longstanding controversy regarding the existence, or otherwise, of spin waves in paramagnetic BCC iron. Spin dynamics simulations of the dynamic structure factor of a Heisenberg model of Fe with first principles interactions reveal that well defined peaks persist far above Curie temperature T_c. At large wave vectors these peaks can be ascribed to propagating spin waves, at small wave vectors the peaks correspond to over-damped spin waves. Paradoxically, spin wave excitations exist despite only limited magnetic short-range order at and above T_c.