Magnetic excitations in multiferroic pyroxene NaFeSi₂O₆

The natural mineral aegirine NaFeSi₂O₆, which exhibits multiferroic properties, is of interest for both applied and fundamental physics. The material displays incommensurate antiferromagnetic spin density wave order at 8.2 K. Spiral magnetic order sets in below 6 K making the sample ferroelectric. Understanding the magnetic interactions that lead to the stabilization of coupled magnetic and ferroelectric order is crucial to understand the origins of this effect. Through comprehensive inelastic neutron scattering measurements, we have studied the magnetic excitations within and above the multiferroic phase of NaFeSi₂O₆. The material reveals an unusual excitation spectrum, featuring a coexistence of sharp spin waves and broad continuum excitations. Based on the fitted magnetic excitation spectra, we propose a magnetic exchange Hamiltonian that provides microscopic insight on the magnetic and ferroelectric behavior of NaFeSi₂O₆.