Magnetoelectric Excitations in the Polar Antiferromagnets Ni-based Tellurates A3-xBxTeO6 (A, B = Ni, Mn, Co, x=1-2)

Abstract

Multiferroics (MFs) are materials that can combine at least two primary ferroic properties: ferromagnetism, ferroelectricity and ferroelasticity.[1] In the case of magnetoelectric (ME) MFs, coupling between ferroelectricity and ferromagnetism occurs. Throughout the history of MFs, the static ME coupling has peaked the interest of the MF community. However, there is great significance in what happens away from the static regime in MF systems; hence the popular movement toward the exploration of the dynamical ME effect in recent years.[2] Both cases share the same quantum-level microscopic mechanisms, but in the latter case the magnetization and polarization order parameters are not static, but oscillatory. This oscillating behaviour of polarization and magnetization manifests in the hybrid elementary excitations known as electromagnons, which very often lie in the terahertz (THz) or far infrared (IR) region of the spectrum. As the very word indicates, the electromagnons are electro-active magnons,[3] namely collective spin and lattice excitations that couple the dielectric and magnetic properties. Therefore, both the electric and magnetic component of the electromagnetic radiation can be brought into play to excite and detect such dynamical behaviour.The polar antiferromagnet Ni3TeO6 transcends the ME performance of most single-phase MFs, exhibiting non-hysteretic colossal ME coupling.[4] A collinear antiferromagnetic (AFM) order appears below TN = 53 K, inducing a tremendous increase in the existing - but negligible - polarization of the R3 polar structure (up to 3,280 μC m-2 at 2 K). The highest ME coupling value occurs at the spin-flop transition at 8 T. The presence of dynamical ME coupling in Ni3TeO6 was reported in 2017 by Skiadopoulou et al..[5] Two spin excitations were observed simultaneously in Raman and time-domain THz spectra below the Néel temperature TN=53 K. These magnons showed high tunability in external magnetic field. Due to symmetry considerations, these excitations were assigned to electromagnons. Since Ni3TeO6 presents a collinear AFM spin structure, the symmetric exchange striction is most probably responsible for the emergent dynamical ME effect.The quest for compounds with enhanced ME coupling led to the investigation of a series of compounds of the form A3-xBxTeO6 (A, B = Ni, Mn, Co, x=1-2), inspired by the highly praised ME behaviour of Ni3TeO6. In Ni2MnTeO6, although the spin order and magnon spectra closely resemble Ni3TeO6,[6] partial substitution of Ni by Mn shows increased TN by 20 K, while preserving the non-centrosymmetric structure R3.[6],[7] In addition, the spin-flop transition takes place at an external magnetic field of 5 T, which is 3 T lower than in Ni3TeO6. The increase in Neel temperature and decrease of the critical magnetic field which may trigger a colossal ME effect are in favour of possible ME applications.More recently, Ni2CoTeO6 and NiCo2TeO6 compounds were synthesised for the first time, in the form of single crystals and ceramics.[8] The ME properties are preserved by Co-substitution. The non-centrosymmetric R3 space group is maintained at least up to RT for both Ni2CoTeO6 and NiCo2TeO6. The AFM ordering appears at TN = 54 and 49 K for Ni2CoTeO6 and NiCo2TeO6, respectively (Fig. 1). As anticipated, both compounds evidence an interplay of magnetic and dielectric properties, indicated by the dielectric anomaly seen at TN (Fig. 1).In contrast to the magnetic ordering in the previously studied Ni3TeO6 and Ni2MnTeO6, which are collinear along the c-axis, both Co-based compounds present a helical non-collinear structure with the spins in the ab-plane. The magnetic structures of both compounds are quite similar, with a small difference between the magnetic propagation vectors: k1=(0, 0, 1.299(4)) and k2=(0, 0, 1.2109(1)), for Ni2CoTeO6 and NiCo2TeO6, respectively. Hysteretic spin-flop transitions were observed near 8 T (Ni2CoTeO6) and 4 T (NiCo2TeO6), unlike Ni3TeO6 and Ni2MnTeO6, which did not show any hysteretic behavior.Below TN both compounds demonstrate numerous spin-excitations in the THz range, which are considerably influenced by external magnetic field, with clear modifications at the spin-flop transition of ∼4 T for the case of NiCo2TeO6 (Fig. 2). Six magnons strongly dependent on magnetic field are present in the THz spectra of the Co-richer compound NiCo2TeO6. Both compounds present at least one magnetic excitation simultaneously seen by Raman spectroscopy, possibly revealing the magnetoelectric character of the magnon. **