Gigantic terahertz magnetochromism via electromagnons in hexaferrite magnet Ba$_2$Mg$_2$Fe$_{12}$O$_{22}$

TL;DR

This study demonstrates a giant terahertz magnetochromism in a hexaferrite magnet driven by electromagnons, with absorption changes exceeding 500% under magnetic fields, highlighting the critical role of conical spin structures.

Contribution

It reveals the strong dependence of electromagnon spectral intensity on magnetic structure and identifies maximal magnetochromism near a conical angle of 45°, advancing understanding of magnetically controllable terahertz phenomena.

Findings

Magnetochromic change exceeds 500% at 0.6 T.

Electromagnon intensity depends critically on conical spin structure.

Maximal magnetochromism occurs around a conical angle of 45°.

Abstract

Effects of temperature (6--225 K) and magnetic field (0--7 T) on the low-energy (1.2--5 meV) electrodynamics of the electromagnon, the magnetic resonance driven by the light electric field, have been investigated for a hexaferrite magnet Ba$_2$Mg$_2$Fe$_{12}$O$_{22}$ by using terahertz time-domain spectroscopy. We find the gigantic terahertz magnetochromism via electromagnons; the magnetochromic change, as defined by the difference of the absorption intensity with and without magnetic field, exceeds 500% even at 0.6 T. The results arise from the fact that the spectral intensity of the electromagnon critically depends on the magnetic structure. With changing the conical spin structures in terms of the conical angle $\theta$ from the proper screw ($\theta=0^\circ$) to the ferrimagnetic ($\theta=90^\circ$) through the conical spin-ordered phases ($0^\circ<\theta<90^\circ$) by external magnetic fields, we identify the maximal magnetochromism around $\theta\approx45^\circ$. On the contrary, there is no remarkable signature of the electromagnon in the proper screw and spin-collinear (ferrimagnetic) phases, clearly indicating the important role of the conical spin order to produce the magnetically-controllable electromagnons. The possible origin of this electromagnon is argued in terms of the exchange-striction mechanism.