Citation: Masunaga, S.H.; Barbeta, V.B.; Abud, F.; Torikachvili, M.S.; Jardim, R.F. Anomalous Ferromagnetic Phase in the Gd 1−x Er x B 4 Series: Crystal Growth, Thermal, and Magnetic Properties. Crystals 2023, 13, 1137. https:// doi.org/10.3390/cryst13071137 Academic Editor: Sergey L. Bud’ko Received: 5 July 2023 Revised: 17 July 2023 Accepted: 18 July 2023 Published: 21 July 2023 Copyright: © 2023 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/). crystals Article Anomalous Ferromagnetic Phase in the Gd 1-x Er x B 4 Series: Crystal Growth, Thermal, and Magnetic Properties Sueli H. Masunaga 1,2, * , Vagner B. Barbeta 1 ,Fábio Abud 2,3 , Milton S. Torikachvili 4 and Renato F. Jardim 2 1 Departamento de Física, Centro Universitário FEI, São Bernardo do Campo 09850-901, SP, Brazil; vbarbeta@fei.edu.br 2 Instituto de Física, Universidade de São Paulo, São Paulo 05315-970, SP, Brazil; fabio.abud@usp.br (F.A.); rjardim@if.usp.br (R.F.J.) 3 Escola de Engenharia de Lorena, Departamento de Engenharia de Materiais, Universidade de São Paulo, Lorena 12612-550, SP, Brazil 4 Department of Physics, San Diego State University, San Diego, CA 92182, USA; miltont@sdsu.edu * Correspondence: sueli.masunaga@gmail.com Abstract: Rare-earth tetraborides RB 4 are of great interest due to the occurrence of geometric magnetic frustration and corresponding unusual magnetic properties. While the Gd 3+ spins in GdB 4 align along the ab plane, Er 3+ spins in the isomorphic ErB 4 are confined to the c–axis. The magnetization in the latter exhibits a plateau at the midpoint of the saturation magnetization. Therefore, solid solutions of (Gd, Er)B 4 provide an excellent playground for exploring the intricate magnetic behavior in these compounds. Single crystals of Gd 1−x Er x B 4 (x = 0, 0.2, and 0.4) were grown in aluminum flux. X-ray diffraction scans revealed single-phase materials, and a drop in the unit cell volume with increasing Er content, suggesting the partial substitution of Er at the Gd sites. Heat capacity measurements indicated a systematic decrease of the Néel temperature (T N ) with increasing Er content. The effective magnetic moment determined from the magnetization measurement agreed with the calculated free ion values for Gd 3+ and Er 3+ , providing further evidence for the successful substitution of Er for Gd. The partial substitution resulted in an anomalous ferromagnetic phase below T N , exhibiting significant anisotropy, predominantly along the c-axis. This intriguing behavior merits further studies of the magnetism in the Gd 1−x Er x B 4 borides. Keywords: tetraborides; susceptibility; specific heat; geometric magnetic frustration; antiferromagnetism; flux method; Shastry–Sutherland; induced ferromagnetism 1. Introduction Motivated by their interesting magnetic properties, the rare-earth tetraborides with general formula RB 4 (R = rare earth) have been studied for many years [1–3]. These compounds are metallic conductors and show antiferromagnetic (AF) ordering, except for R = Pr, which is ferromagnetic (FM) [2]. The indirect coupling between the magnetic ions is of the Ruderman–Kittel–Kasuya–Yosida type (RKKY) [2]. The crystal structure is tetragonal belonging to the symmetry group P4/mbm. Due to the nature of the crystal structure, these compounds exhibit strongly anisotropic magnetic and electrical properties [4,5]. The magnetic sublattice of R ions in RB 4 consists of 2d orthogonal R–R dimers in the ab-plane, forming squares and triangles [6]. The bond length between the rare earth nearest-neighbor dimmer (NN) is very close to the next-nearest neighbor (NNN). Therefore, one can presume that the corresponding magnetic interactions J 1 and J 2 , as shown in Figure 1, are also close to each other. If the magnetic interaction between the rare-earth ions is antiferromagnetic, it is likely that the system should exhibit geometrically frustrated magnetic interactions, consistently with the theoretical approach described in the Shastry– Sutherland lattice (SSL) [6–8]. Crystals 2023, 13, 1137. https://doi.org/10.3390/cryst13071137 https://www.mdpi.com/journal/crystals