In solid-state compounds, the valence of europium can sometimes be mixed -- which is especially favored in structures with several positions for the europium atoms. In this work, we study the Eu-based intermetallic noncentrosymmetric system Eu10Hg55 which has 65 atoms per unit cell and 4 distinct crystallographic positions for europium and 17 positions for mercury. Our detailed analysis of magnetism of large single crystals suggests that europium in Eu10Hg55 might be present in two valence states, resulting in a fragile magnetic ground state. Due to the cage-like structure with a large distance between the Eu atoms, those atoms are weakly ferromagnetically coupled and Eu10Hg55 orders at low temperatures, below T1=5.5 K, with a subsequent spin re-orientation at T2=4.3 K. There is no sign of magnetic frustration. Interestingly, the magnetic ordering of europium sub-lattices results in a magnetization pole reversal with a weak ferrimagnetic ground state. Additional magnetic phases can be induced by application of a modest external magnetic field.
@article{arxiv.2509.07589,
title = {Unusual magnetic order in Eu$_{10}$Hg$_{55}$},
author = {Rachel Nixon and Nazar Zaremba and Samuel A. Adegboyega and Andreas Leithe-Jasper and Mitja Krnel and Yurii Prots and Ulrich Burkhardt and Jörg Sichelschmidt and Lucia Amidani and Fabio La Mattina and Michael Shatruk and Alexander Shengelaya and Manuel Brando and Eteri Svanidze},
journal= {arXiv preprint arXiv:2509.07589},
year = {2025}
}