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Unveiling complex magnetic structures of the $4 f − 4 d$ exchange coupled antiferromagnets $B a_{3} R R u_{2} O_{9}$ $(R = Tb, Er)$

Deepak Garg ^1,*, C. Ritter ², M. Skoulatos ³, S. Nandi ^4,5, and Yixi Su ¹

¹Jülich Centre for Neutron Science (JCNS) at Heinz Maier-Leibnitz Zentrum (MLZ), Forschungszentrum Jülich GmbH, Lichtenbergstrasse 1, D-85748, Garching, Germany
²Institut Laue-Langevin, Boite Postale 156, 38042 Grenoble Cedex, France
³Heinz Maier-Leibnitz Zentrum (MLZ), Technische Universität München, Lichtenbergstrasse 1, D-85748 Garching, Germany
⁴Forschungszentrum Jülich GmbH, Jülich Centre for Neutron Science (JCNS-2), 52425 Jülich, Germany
⁵RWTH Aachen, Lehrstuhl für Experimentalphysik IVc, Jülich-Aachen Research Alliance (JARA-FIT), 52074 Aachen, Germany

^*Contact author: deepak1.phy@gmail.com

Phys. Rev. B 113, 144435 – Published 28 April, 2026

DOI: https://doi.org/10.1103/pbtn-g5wz

Abstract

The magnetic structures of $B a_{3} R R u_{2} O_{9}$ $(R = Tb, Er)$ have been investigated using high-intensity neutron powder diffraction (NPD) supported by dc and ac susceptibility studies. The NPD data reveal antiferromagnetic (AFM) ground states below $T_{N} \sim 9.8$ and 6 K in $B a_{3} TbR u_{2} O_{9}$ and $B a_{3} ErR u_{2} O_{9}$ , respectively, consistent with the magnetic transitions observed in dc and ac susceptibility measurements. In $B a_{3} TbR u_{2} O_{9}$ , the magnetic structure is characterized by a propagation vector, $k = (0, 0, 0)$ , where Tb and Ru order ferromagnetically in the $ab$ plane. Neighboring planes of Tb are coupled antiferromagnetically along the $c$ direction as are neighboring planes of Ru, which leads to an AFM order between Tb and Ru as well. In contrast, the magnetic structure of $B a_{3} ErR u_{2} O_{9}$ is characterized by $k = (0.5, 0, 0)$ , with Er and Ru moments exhibiting a canted AFM ordering. Interestingly, the collinear (noncollinear) AFM order of Tb (Er) accompanied by the collinear (noncollinear) magnetic order of Ru in $B a_{3} TbR u_{2} O_{9}$ ( $B a_{3} ErR u_{2} O_{9}$ ) along with the concurrent ordering of the rare earth and Ru in both compounds suggests a strong coupling between $4 f$ and $4 d$ electrons. Both compounds show weak magnetic ordering above $T_{N}$ similar to that reported for $B a_{3} HoR u_{2} O_{9}$ . We have understood this intriguing feature by preparing a $B a_{3} ErR u_{2} O_{9}$ sample with 3% excess $Ru O_{2}$ . Our analysis suggests that the weak magnetic ordering above $T_{N}$ is strongly dependent on the excess $Ru O_{2}$ ; however, and interestingly, the magnetic structure below $T_{N}$ is robust against this $Ru O_{2}$ surplus. Our detailed study provides insights into the magnetic ground states of these two compounds, thereby enhancing the understanding of magnetic ordering in this fascinating family.

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Phys. Rev. B 113, 144435– Published 28 April, 2026

Vol. 113, Iss. 14 — 1 April 2026

Received 8 December 2025
Revised 19 February 2026
Accepted 31 March 2026

DOI: https://doi.org/10.1103/pbtn-g5wz

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

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Unveiling complex magnetic structures of the $4 f − 4 d$ exchange coupled antiferromagnets $B a_{3} R R u_{2} O_{9}$ $(R = Tb, Er)$

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Unveiling complex magnetic structures of the 4f−4d exchange coupled antiferromagnets Ba3RRu2O9 (R=Tb,Er)

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Unveiling complex magnetic structures of the $4 f − 4 d$ exchange coupled antiferromagnets $B a_{3} R R u_{2} O_{9}$ $(R = Tb, Er)$