Magnetic structure and thermal conductivity of FeVO4 multiferroic

Multiferroics are new class of materials, attracting attention because of simultaneous presence of more than one ferroic orders, especially magnetic and ferroelectric ordering. Recently, FeVO₄ has been identified a type II multiferroic, where onset of electric polarization is due to the non-col-linear spin structure at T_N2 = 15 K, breaking the spatial inversion symmetry. We synthesized phase pure stoichiometric FeVO₄ bulk sample using solid state reaction route, as explained in Ref. [2]. The structural and microstructural characterizations of the sample were carried out using x-ray diffraction, scanning electron microscopy and x-ray photoelectron spectroscopic measurements, discussed in detail elsewhere. In brief, these measurements confirmed the single phase of FeVO₄ with right valence state for Fe, V and O elements. The grain size of synthesized bulk FeVO₄, estimated from XRD measurements, varies in the range of 1-2 mm. We carried out temperature-dependent ac magnetic measurements at different frequencies, as shown in Fig. 1. We observed two antiferromagnetic transitions at T_N1~ 22 K and T_N2~ 15 K, marked by the dashed lines in Fig. 1 for eye view, which consistent with the earlier reports. The magnetic structure at and below 15 K antiferromagnetic transition is complex non-collinear magnetic structure, responsible for breaking spatial inversion symmetry in FeVO₄. Moreover, we do not observe any frequency dependence for both antiferromagnetic transitions, suggesting the absence of any magnetic clustering or impurities in FeVO₄, substantiating x-ray diffraction results. In order to understand the pho-non-magnon coupling in the system, we also measured the temperature-dependent thermal conductivity, k(T) on bulk FeVO₄ pellet, as plotted in Fig. 2a. We found that the value of k decreases quite linearly with decreasing temperature until 70 K, and then starts - o increase below 70 K to reach a peak value of about ~ 0.8 Wm^-1K^-1 at around 30 K followed by the deep fall below 30 K. This feature is commonly seen in crystalline solids, and the phonon peak usually appears at the temperature when the phonon mean free path is approximately equal to the crystal site distance, attributed to the phonon-phonon Umklapp process. However, its temperature-dependence above 70 K is somewhat different from that of the normal insulators, which usually follows 1/T-dependence at high temperatures due to the Umklapp scattering. We also noticed that there is a small slope change at 20 K near to the value of T_N1, as indicated in inset of Fig. 2a. The thermal conductivity data was analyzed using the combined Debye-Einstein phonon model with heat conduction from both acoustic (k_aph) and optical phonon (k_oph), and contribution from different phonons is plotted in Fig 2(b) with their sum as the total thermal conductivity. We observed a good fit for the low-temperature data, and the main contribution to the thermal transport is coming from the acoustic phonons below 50 K, whereas optical phonons starts to contribute significantly to heat conduction at relatively higher temperatures above 50 K. Further details about thermal transport and its mechanisms in FeVO₄ will be presented and discussed in the manuscript. Additionally, the observed variation in thermal conductivity near T_N1 ~ 22 K, suggest strong coupling between magnetic ordering and phonons. These results suggest that the phonons are strongly coupled with magnetic ordering in FeVO₄ bulk system and may provide a mechanism for the charge-lattice-spin interactions in this multiferroic material.