Possible evidence for electromagnons in multiferroic manganites

Abstract

M agnetodielectric materials are characterized by a strong coupling of the magnetic and dielectric properties and, in rare cases, simultaneously show both magnetic and polar order. Among other multiferroics, TbMnO 3 and GdMnO 3 reveal a strong magneto-dielectric coupling and as a consequence fundamentally different spin excitations exist: electro-active magnons (or electromagnons), spin waves that can be excited by a.c. electric fields. Here we provide evidence that these excitations appear in the phase with an incommensurate magnetic structure of the manganese spins. In external magnetic fields this incommensurate structure can be suppressed and the electromagnons wiped out, thereby inducing considerable changes in the index of refraction from d.c. up to terahertz frequencies. Hence, besides adding a creature to the zoo of fundamental excitations, the refractive index can be tuned by moderate magnetic fields, which enables the design of the next generation of optical switches and optoelectronic devices.Enormous progress has been made in the field of multiferroics and the discovery of new classes of ferroelectromagnets (FEMs) with the simultaneous occurrence of magnetic and polar order [1][2][3][4][5] has triggered a revival 6,7 of this old field of magneto-dielectric effects and the electrodynamics of multiferroic media 8 . As well as promising applications of FEMs in the field of modern electronics, for example, as multiple-state memory devices with mutual magnetic or electric control or as magnetically switchable optical devices, fascinating new problems can be tackled in basic research, such as the search for magneto-dielectric excitations.

Infrared phonon anomaly and magnetic excitations in single-crystal Cu3Bi(SeO3)2OMiller1, Stephens2, Martin3 et al. 2012Phys. Rev. B

“…[26][27][28] Electromagnons, which have been extensively studied in a number of rare-earth manganites, are observed to possess optical conductivities of at least a factor of 10 larger. 9,29 Although this method of comparison does not yield objective certainty, we can further support its conclusion by employing optical sum rule analysis on our measured reflectance data. Since an electromagnon contributes to the dielectric constant, it must gain spectral weight from a dipole active excitation, the main candidates being domain relaxations, phonons, or electronic transitions.…”

“…Electromagnons were first proposed as strongly renormalized spin waves with dipolar momentum. 9 Since their discovery, electromagnons have been extensively studied in rare earth manganite compounds, namely in TbMnO 3 , where a large body of recent literature (Raman 23 , neutron 24 , and infrared 25 ) has tied the excitation to the lattice itself. The work ) 7K 15K 30K 50K 70K 85K 100K 110K 120K 130K 140K 150K 200K 250K 300K 0 100 200 300 Temperature (K) 318 has resulted in a generalized hybrid magnon-phonon mode picture of electromagnons.…”

“…6,7 Furthermore, the a.c. electric (magnetic) field of the infrared light can interact with ordered moments and excite an electromagnon (magnon) excitation, which in turn can provide information about the symmetry and nature of the magnetic order. 8,9 Here we present our infrared studies on single crystal Cu 3 Bi(SeO 3 ) 2 O 2 Cl. We observe 16 new modes in the phonon spectra originating below 115 K. Strikingly, our subsequent powder x-ray diffraction measurements reveal the same 300 K structure existing at 85 K. Preliminary Raman measurements suggest that a loss of inversion symmetry is likely.…”

Emergent functions of quantum materials

“…The lower-lying sharp peak at 1 meV represents the spin-current electromagnon mode 47 , while the higher-lying broad peak is assigned to the E ω -active but H ω -inactive magnon (located at the magnetic zone edge); the latter was the first-reported electromagnon for RMnO 3 (ref. 48). A large directional dichroism of up to ∼50% has been observed for the lower-lying electromagnon modes with spincurrent origin.…”

Exploring nanomagnetism with soft X-ray microscopy

“…Typical examples are e.g 4 multiferroic systems which hold the promise to switch the magnetization e.g. with electrical fields [17][18][19][20].…”