Magnetic evolution, phase transitions, and electronic band structure of the ferrotoroidic candidate BaCrS

Zhao, G.*; Li, J.*; Zhang, J.*; 小嶋 健児*; Cai, Y.*; 伊藤 孝   ; Yoon, S. W.*; Wang, X.*; 前川 禎通*; Su, G.*; Gu, B.*; Ziman, T.*; Jin, C.*; 植村 泰朋*

Zhao, G.*; Li, J.*; Zhang, J.*; Kojima, Kenji M*; Cai, Y.*; Ito, Takashi; Yoon, S. W.*; Wang, X.*; Maekawa, Sadamichi*; Su, G.*; Gu, B.*; Ziman, T.*; Jin, C.*; Uemura, Yasutomo*

Ferrotoroidic materials, which break both time- and space-reversal symmetries to enhance magnetoelectric responses, are of great interest for applications. BaCrS, a recently synthesized quasi-one-dimensional ferrotoroidic candidate, combines polarization, magnetization, and toroidal moment, yet its electronic band structure, magnetic evolution, and phase transitions remain incompletely understood to date. Here, we investigate BaCrS, using electrical transport, direct current magnetization, alternating current susceptibility, specific heat, and muon spin rotation (SR) measurements, along with various first-principles calculations. Electrical transport measurements and density functional theory confirm a narrow band gap (0.707~eV, experimentally; 0.680~eV, theoretically). SR measurements reveal that static magnetism dominates down to 2~K, with an ordered volume fraction of 89% at this temperature. Two distinct transitions are identified: one around the Nel temperature [~K] and another corresponding to a phase transition near 30~K, as consistently indicated by our complementary experimental techniques. The absence of dynamic critical behaviors and the lack of a -type specific heat anomaly are consistent with a first-order-like transition at . Based on various complementary experimental results and theoretical calculations, we propose a hypothesis that the 30~K broad feature arises from the complex interplay between crystal structure distortion and magnetic coupling, which is potentially directly linked to ferrotoroidicity. As the first study integrating SR with complementary experiments and first-principles calculations to explore ferrotoroidicity, this work provides key insights into the underlying physics of BaCrS and related compounds in this area.