Citation: Prosnikov, M.A.; Barilo, S.N.; Liubochko, N.A.; Pisarev, R.V.; Christianen, P.C.M. High-Field Raman Scattering in an Antiferromagnet Fe 3 BO 6 . Magnetochemistry 2022, 8, 77. https://doi.org/ 10.3390/magnetochemistry8080077 Academic Editor: Alexander S. Krylov, Mikhail Prosnikov Received: 2 July 2022 Accepted: 20 July 2022 Published: 24 July 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). magnetochemistry Article High-Field Raman Scattering in an Antiferromagnet Fe 3 BO 6 Mikhail A. Prosnikov 1,2,3 * , Sergei N. Barilo 4 , Nadzeya A. Liubochko 4 , Roman V. Pisarev 1 and Peter C. M. Christianen 2,3 1 Ioffe Institute, Russian Academy of Sciences, 194021 St. Petersburg, Russia; pisarev@mail.ioffe.ru 2 High Field Magnet Laboratory (HFML–EMFL), Radboud University, Toernooiveld 7, 6525 ED Nijmegen, The Netherlands; peter.christianen@ru.nl 3 Institute for Molecules and Materials, Radboud University, Heyendaalseweg 135, 6525 AJ Nijmegen, The Netherlands 4 Institute of Solid State and Semiconductor Physics, National Academy of Sciences, 220072 Minsk, Belarus; sergeibarilo@yahoo.com (S.N.B.); n_lubochko@mail.ru (N.A.L.) * Correspondence: prosnikov@mail.ioffe.ru Abstract: The detailed Raman scattering investigation of the lattice and spin dynamics of a single crystal of the Fe 3 BO 6 antiferromagnet is reported. Azimuthally resolved polarization measurements provided an unambiguous determination of the symmetry of observed excitations. Low-temperature experiments at T = 4.2 K allowed us to reduce anharmonic contribution and deconvolute several overlapping phonon modes. Low-frequency measurements have made it possible to find two excita- tions at 13.1 and 16.6 cm −1 , which were assigned to quasi-ferromagnetic and quasi-antiferromagnetic magnon excitations, respectively. The magnetic field applied along the hard-magnetization axis causes energy shifts of these excitations, but no spin-flip transition was observed up to B = 30 T. Keywords: antiferromagnet; weak ferromagnetism; Fe 3 BO 6 ; high magnetic field; Raman scattering; lattice dynamics; spin dynamics; spin waves; magnons 1. Introduction Several boron oxides were proposed as alternative and promising cathode materials for batteries over traditional lithium ones [1], and among those are several iron–boron oxides. One of them, namely Fe 3 BO 6 , has been synthesized in different forms by using various technologies and demonstrated potentially interesting electrochemical properties [2–6]. However, the interest in Fe 3 BO 6 lies beyond potential practical applications, e.g., due to its chemical similarity to a well-known and thoroughly studied easy-plane antiferromagnet iron borate FeBO 3 [7]. Magnetic properties of both these materials are defined by Fe 3+ ions in distorted octahedral coordination with the 3d 5 electronic shell and the spin S = 5/2 and L = 0 in the ground electronic and magnetic state. Fe 3 BO 6 is an antiferromagnet with a weak ferromagnetic moment and a high Néel temperature T N = 503 K, which has been investigated using different techniques, mainly focusing on magnetic and magneto-optical properties. Surprisingly, it was found that the magneto-optical Faraday rotation in antiferromagnetic Fe 3 BO 6 is of the same order of magnitude as that in a model ferrimagnetic material yttrium iron garnet (YIG), even though the net magnetic moment in Fe 3 BO 6 is approximately two orders of magnitude smaller than in YIG [8]. Compared to the FeBO 3 , the local inversion symmetry on the iron ions in Fe 3 BO 6 is broken, thus strongly enhancing the intensity of d −→ d crystal field transitions due to parity selection rules violation, as was suggested in optical absorption studies [9]. The difference in the local symmetry of iron ions in FeBO 3 and Fe 3 BO 6 results in notably different colors of single crystals of these two iron borates—while the former is of green color, the latter is black and has dark red coloring only in thin layers. The investigation of the magnon sidebands of an optical exciton in the absorption spectrum of Fe 3 BO 6 in a magnetic field applied along the c-axis ([001]) showed that there is a spin-flop transition at Magnetochemistry 2022, 8, 77. https://doi.org/10.3390/magnetochemistry8080077 https://www.mdpi.com/journal/magnetochemistry