Defect-induced magnetic phase transition in 2D semiconductor CrSBr
Date: 2024/10/18 - 2024/10/18
Academic Seminar: Defect-induced magnetic phase transition in 2D semiconductor CrSBr
Speaker: Shengqiang Zhou, department leader of “Semiconductor Materials” at Helmholtz-Zentrum Dresden-Rossendorf, Germany
Time: 2:00 p.m., October 18, 2024 (Beijing Time)
Location: Room 503, JI Long Bin Building
Abstract
CrSBr is rapidly gaining attention as a prominent candidate within the family of van der Waals magnetic semiconductors [1-3]. Below the Néel temperature of 132 K, the material is supposed to exhibit prototypical A-type antiferromagnetic order, as predicted by mean-field theory years ago. Recently, however, several groups reported the observation of unusual magnetic signatures indicating a second magnetic phase transition at temperatures around 40 K. In this contribution, we are going to discuss: (1) whether these signatures reflect an intrinsic property of the material or are caused by extrinsic influences; (2) if not the former, whether one can tailor the magnetic properties after growth.
We start with CrSBr single crystals synthesized by chemical vapor transport. Surprisingly, by extensive magnetization measurement utilizing SQUID magnetometry, we cannot detect the second, 40 K magnetic phase transition [4]. Our magnetometry measurements confirm the theoretically predicted magnetic phase diagram and thus demonstrate that the 40 K phase observed by other groups is not an intrinsic element of the magnetic phase diagram of CrSBr. The pure antiferromagnetic CrSBr crystals and flakes were then subjected to non-magnetic ion irradiation, which produces structural defects in the crystals in a controllable way. We observe a transition from antiferromagnetic to ferromagnetic behavior in CrSBr [5, 6]. Already at moderate fluences, ion irradiation induces a remanent magnetization with hysteresis adapting to the easy-axis anisotropy of the pristine magnetic order up to a critical temperature of 110 K. Structure analysis of the irradiated crystals in conjunction with density functional theory calculations suggest that the displacement of constituent atoms due to collisions with ions and the formation of interstitials favor ferromagnetic order between the layers. Increasing irradiation fluences gradually lowers the Curie temperature, reflecting the impact of crystalline degradation. This suggests that by finely tuning the irradiation parameters and employing precise lithography techniques, it is possible to selectively modulate induced ferromagnetism in CrSBr in terms of magnetization strength, critical temperature, and spatial distribution. However, in our opinion, the origin and nature of the second phase with a transition temperature around 40 K reported in pristine CrSBr samples by various studies still remains elusive.
Moreover, I will also introduce ion irradiation as a versatile tool for defect engineering in crystalline materials. HZDR is running a user facility, ion beam center (https://www.hzdr.de/db/Cms?pNid=1984&pOid=40983), which is open for research community world widely.
Biography
Shengqiang Zhou is the department leader of “Semiconductor Materials” at Helmholtz-Zentrum Dresden-Rossendorf, Germany. He received a Bachelor degree in Physics in 1999 and Master degree in Nuclear and Particle physics in 2002, both from Peking University in China. He obtained his PhD in Physics in 2008 from the Technical University of Dresden. From 2008 to 2010 he was a postdoc at the Research-Center Dresden-Rossendorf focusing on the ion implantation into Ge. In 2010 he returned to Peking University as a research professor. From 2011 to 2016, he was leading a Helmholtz Young Investigator Group at Helmholtz-Zentrum Dresden-Rossendorf. Since 2018, he became the department leader of “Semiconductor Materials” at Helmholtz-Zentrum Dresden-Rossendorf. He is working on semiconductors (mainly GaAs, Si, Ge and SiC) processed by ion implantation and by milli-second flash lamp or nano-second pulsed laser.