Advances in Magnetic Two Dimensional Materials

Since the discovery of graphene in 2004, two-dimensional (2D) van der Waals (vdW) layered materials have attracted extensive attention for their great potential for application in the electronic, optoelectronic, and electrochemical fields. Recently, a new research highlight has appeared in the form of 2D materials: 2D vdW magnetic materials that debunk the Mermin–Wagner theorem that isotropic 2D materials cannot show long-range magnetism due to thermal fluctuation. A long-range magnetic order was first observed in 2D FePS₃, Cr₂Ge₂Te₆, and CrI₃ crystals. Due to their ultrathin thickness and the combination of vdW forces between layers, their magnetism can be effectively controlled by the electric field, chemical modification, thickness, and so on. At the same time, some interesting physical phenomena have been revealed in these materials, such as the magnetic proximity effect, chiral spin state, exchange bias effect, and quantum anomalous Hall effect.

This Special Issue of Magnetochemistry on “Advances in Magnetic Two Dimensional Materials” features a total of nine contributions reflecting the interdisciplinary nature of this research field and demonstrating the intersection and integration of magnetic physics, chemistry, condensed matter physics, and other scientific disciplines. Let us briefly overview these insightful contributions.

The first paper, “Bipolar Nb₃Cl₈ Field Effect Transistors” by Yixiang Lu from the Dalian University of Technology (China) and Kai Zhao, Tongyao Zhang, and Baojuan Dong from the Institute of Opto-Electronics, Shanxi University (China), reports a study on few-layered Nb₃Cl₈, which exhibits N-type semiconducting behavior with its electrical signal strength increasing as thickness grows [1]. The use of an ionic liquid gate (EDL) significantly enhances its FET properties, increasing the on–off ratio by approximately 100 times and enabling bipolar modulation of the transfer curve.

The second paper, authored by Shan Wang, Chuanwu Cao, and Jian-Hao Chen from the School of Physics, Peking University (China), explores “In Situ Tuning of Magnetism in Fe₃GeTe₂ via Argon Ion Irradiation” [2]. The study demonstrates that the magnetization (M) and coercive field (H_C) of Fe₃GeTe₂ can be significantly weakened through defects introduced by argon-ion irradiation, providing insights into the tunability of magnetic properties in Fe₃GeTe₂. A reduction in M and H_C by up to 40% and 62.4%, respectively, was also observed at the base temperature. This study showcases in situ transport modification as a powerful tool for tailoring magnetic properties in two-dimensional materials.

The contribution by Lingling Ren and Baojuan Dong from the Institute of Opto-Electronics, Shanxi University (China) explores “Ferroelectric Polarization in an h-BN-Encapsulated 30-Twisted Bilayer–Graphene Heterostructure”, a robust hysteretic resistance was detected at the top interface between h-BN and the TBLG from room temperature down to 40 mK in their incommensurate moiré superlattice [3]. This hysteretic phenomenon can be understood via the extra carrier induced by the interfacial 2D ferroelectric polarization, which is estimated to be around 0.7 pC/m.

The paper “An Optical Spectroscopic Study of Air—Degradation of van der Waals Magnetic Semiconductor Cr₂Ge₂Te₆” by Woye Pei, Zhiren Xiong, Xingguang Wu, Zheng Vitto Han, and Tongyao Zhang from the Institute of Opto-Electronics, Shanxi University (China), and Yingjia Liu and Siwen Zhao from the Institute of Metal Research, Chinese Academy of Sciences (China), presents an optical spectroscopic study of the van der Waals magnetic semiconductor Cr₂Ge₂Te₆ as it undergoes degradation [4]. The results show that the magneto-optic Kerr hysteresis loop of Cr₂Ge₂Te₆ can remain largely unchanged below the Curie temperature despite a significant Raman redshift and the splitting of the Eg peak in a partially degenerated state at room temperature.

In a paper entitled “Mechanical Detection of Magnetic Phase Transition in Suspended CrOCl Heterostructures” by Xiaoxi Li, Xuanzhe Sha, Ning Yan, and Tongyao Zhang from the Institute of Opto-Electronics, Shanxi University (China), the mechanical properties of the few-layered antiferromagnetic material CrOCl were investigated [5]. By using a static detection method, the achieved Young’s modulus of the CrOCl resonator was ~137.29 GPa. The mechanical resonance was found to strongly depend on magnetic fields with an enormous blueshift of ~3.1% in the magnetic field-induced phase transition with ~1.5 × 10⁻³ strain variation when reducing the temperature to below the transition temperature.

The sixth article, “Phototransistors Based on hBN-Encapsulated NiPS₃”, by Yingjia Liu and Xingdan Sun from the Institute of Metal Research, Chinese Academy of Sciences (China), investigates the photoelectric response characteristic in hexagonal boron nitride (hBN)-encapsulated NiPS₃ field-effect transistors fabricated using the dry-transfer technique [6]. The results show a broad photoresponse from near-infrared to ultraviolet radiation at the liquid nitrogen temperature in typical NiPS₃ devices, and the minimum rise time can reach 30 ms under a wavelength of 405 nm. This work successfully extends the antiferromagnetic material to the photodetector system.

The study on “Spin-Filter Magnetic Tunnel Junctions Based on A-Type Antiferromagnetic CrSBr with Giant Tunnel Magnetoresistance” establishes magnetic tunnel junction devices based on graphene–CrSBr–graphene structures [7]. Transmission properties calculated via density functional theory (DFT) combined with the nonequilibrium Green’s function approach reveal tunnel magnetoresistance (TMR) values of up to 330%, 2 × 10⁷%, and 10⁵% with two-, four- and six-layer CrSBr at zero bias, respectively. The underlying mechanism of the TMR effect in the sf-MTJs is elaborated through systematic analysis of the transmission spectra, transmission eigenstates, electrostatic potentials, band structures, and the local density of states. The work was developed by Hao Liu, Yue-Yang Liu, Hongyu Wen, Haibin Wu, Yixin Zong, Jianbai Xia, and Zhongming Wei from the Institute of Semiconductors, Chinese Academy of Sciences (China).

The eighth contribution, entitled “Anomalous Hall Effect and Magneto-Optic Kerr Effect in Pt/Co/Pt Heterostructure”, by Yiming Sun, Liangwei Wu, Mengmeng Yang, Mengjia Xia, Wei Gao, Dongxiang Luo, Nengjie Huo, and Jingbo Li from the Institute of Semiconductors, South China Normal University (China), reports on high-quality Pt/Co/Pt heterostructures with strong perpendicular magnetic anisotropy, exhibiting a large anomalous Hall effect (AHE) with squared Hall loops, which is rooted in external skew scattering (SS) and side jump (SJ) [8]. The magneto-optical Kerr effect (MOKE) observations, using polar MOKE microscopy, reveal obviously circular magnetic domains that respond to the out-of-plane magnetization reversal of the entire film.

The final contribution to this Special Issue is a review entitled “Two-Dimensional Doped Materials” by Junchi Liu, Bo Li, and Qiuqiu Li from Hunan University (China) that presents a tutorial review of 2D doped materials (except graphene) [9]. A comprehensive overview of various types of 2D doped materials, along with their theoretical calculations, was first conducted and detailed discussions on the preparation and characterization methods were further provided. The article highlights the diverse functional properties of these materials, including optical, electronic, magnetic, and electrochemical characteristics, which are engineered for a wide array of practical applications. Finally, the current challenges and future opportunities in the development of 2D doped materials are also summarized at the end of this review.

This Special Issue provides valuable insights into the rapidly evolving landscape of current research on 2D magnetic and magnetoelectric materials and related studies. We would like to sincerely thank all of the authors who contributed to this Special Issue for their dedicated efforts and the outstanding quality of their submissions. Finally, we would like to express our gratitude for the unwavering support and commitment of the editorial team at Magnetochemistry, whose assistance has played a crucial role in preparing this Special Issue.