A comprehensive study of piezomagnetic response in CrPS4 monolayer: mechanical, electronic properties and magnetic ordering under strains

Very high-frequency, gate-tunable CrPS4 nanomechanical resonator with single mode

Two-dimensional (2D) antiferromagnetic semiconductor chromium thiophosphate (CrPS₄) has gradually become a major candidate material for low-dimensional nanoelectromechanical devices due to its remarkable structural, photoelectric characteristics and potentially magnetic properties. Here, we report the experimental study of a new few-layer CrPS₄ nanomechanical resonator demonstrating excellent vibration characteristics through the laser interferometry system, including the uniqueness of resonant mode, the ability to work at the very high frequency, and gate tuning. In addition, we demonstrate that the magnetic phase transition of CrPS₄ strips can be effectively detected by temperature-regulated resonant frequencies, which proves the coupling between magnetic phase and mechanical vibration. We believe that our findings will promote the further research and applications of the resonator for 2D magnetic materials in the field of optical/mechanical signal sensing and precision measurement.

Two-dimensional chalcogenide-based ferromagnetic semiconductors

Two-dimensional (2D) magnetic materials draw an enormous amount of attention due to their novel physical properties and potential spintronics device applications. Room-temperature ferromagnetic (FM) semiconductors have long been pursued in 2D magnetic materials, which show a long range magnetic order down to atomic-layer thickness. The intrinsic ferromagnetism has been predicted in a series of 2D materials and verified in experiments and the magnetism can be modulated by multiple physical fields, exhibiting promising application prospects. In this review, we overview several types of 2D chalcogenide-based FM semiconductors discovered in recent years. We summary and compare their basic physical properties, including the crystal structures, electronic structures, and mechanical stability. The 2D magnetism can be described by several physical models. We also focus on the recent progresses about theoretical prediction of FM semiconductors and experimental observation of external-field regulation. Most of investigations have shown that 2D chalcogenide-based FM semiconductors have relatively high Curie temperature (Tc) and structural stability. These materials are promising to realize the room-temperature ferromagnetism in atomic-layer thickness, which is significant to design spintronics devices.

The Magnetic Genome of Two-Dimensional van der Waals Materials

Magnetism in two-dimensional (2D) van der Waals (vdW) materials has recently emerged as one of the most promising areas in condensed matter research, with many exciting emerging properties and significant potential for applications ranging from topological magnonics to low-power spintronics, quantum computing, and optical communications. In the brief time after their discovery, 2D magnets have blossomed into a rich area for investigation, where fundamental concepts in magnetism are challenged by the behavior of spins that can develop at the single layer limit. However, much effort is still needed in multiple fronts before 2D magnets can be routinely used for practical implementations. In this comprehensive review, prominent authors with expertise in complementary fields of 2D magnetism (i.e., synthesis, device engineering, magneto-optics, imaging, transport, mechanics, spin excitations, and theory and simulations) have joined together to provide a genome of current knowledge and a guideline for future developments in 2D magnetic materials research.

Air-Stable and Layer-Dependent Ferromagnetism in Atomically Thin van der Waals CrPS4

Ferromagnetism in two-dimensional materials presents a promising platform for the development of ultrathin spintronic devices with advanced functionalities. Recently discovered ferromagnetic van der Waals crystals such as CrI₃, readily isolated two-dimensional crystals, are highly tunable through external fields or structural modifications. However, there remains a challenge because of material instability under air exposure. Here, we report the observation of an air-stable and layer-dependent ferromagnetic (FM) van der Waals crystal, CrPS₄, using magneto-optic Kerr effect microscopy. In contrast to the antiferromagnetic (AFM) bulk, the FM out-of-plane spin orientation is found in the monolayer crystal. Furthermore, alternating AFM and FM properties observed in even and odd layers suggest robust antiferromagnetic exchange interactions between layers. The observed ferromagnetism in these crystals remains resilient even after the air exposure of about a day, providing possibilities for the practical applications of van der Waals spintronics.

Magnetic Structure and Metamagnetic Transitions in the van der Waals Antiferromagnet CrPS4

In 2D magnets, interlayer exchange coupling is generally weak due to the van der Waals layered structure but it still plays a vital role in stabilizing the long-range magnetic ordering and determining the magnetic properties. Using complementary neutron diffraction, magnetic, and torque measurements, the complete magnetic phase diagram of CrPS₄ crystals is determined. CrPS₄ shows an antiferromagnetic ground state (A-type) formed by out-of-plane ferromagnetic monolayers with interlayer antiferromagnetic coupling along the c axis below T_N = 38 K. Due to small magnetic anisotropy energy and weak interlayer coupling, the low-field metamagnetic transitions in CrPS_4, that is, a spin-flop transition at ≈0.7 T and a spin-flip transition from antiferromagnetic to ferromagnetic under a relatively low field of 8 T, can be realized for H∥c. Intriguingly, with an inherent in-plane lattice anisotropy, spin-flop-induced moment realignment in CrPS₄ for H∥c is parallel to the quasi-1D chains of CrS₆ octahedra. The peculiar metamagnetic transitions and in-plane anisotropy make few-layer CrPS₄ flakes a fascinating platform for studying 2D magnetism and for exploring prototype device applications in spintronics and optoelectronics.

Exfoliated CrPS4 with Promising Photoconductivity

Layered semiconductors have attracted significant attention due to their diverse physical properties controlled by composition and the number of stacked layers. Herein, large crystals of the ternary layered semiconductor chromium thiophosphate (CrPS₄ ) are prepared by a vapor transport synthesis. Optical properties are determined using photoconduction, absorption, photoreflectance, and photoacoustic spectroscopy exposing the semiconducting properties of the material. A simple, one-step protocol for mechanical exfoliation onto a transmission electron microscope grid is developed, and multiple layers are characterized by advanced electron microscopy methods, including atomic resolution elemental mapping confirming the structure by directly showing the positions of the columns of different elements' atoms. CrPS₄ is also liquid exfoliated, and in combination with colloidal graphene, an ink-jet-printed photodetector is created. This all-printed graphene/CrPS₄ /graphene heterostructure detector demonstrates a specific detectivity of 8.3 × 10⁸ (D*). This study shows a potential application of both bulk crystal and individual flakes of CrPS₄ as active components in light detection, when introduced as ink-printable moieties with a large benefit for manufacturing.

From Paramagnetic to Superparamagnetic Ionic Liquid/Poly(ionic liquid): The Effect of π-π Stacking Interaction

Magnetic ionic liquids (MILs) and poly(magnetic ionic liquids) (PMILs) with FeCl₄^- as anions usually show weak magnetism, such as paramagnetism or antiferromagnetism, at room temperature. Inspired by the natural inorganic ferromagnet with ordered crystal structures, a soft superparamagnetic ionic liquid (TMBBDI[FeCl₄]) and corresponding poly(ionic liquid) (PTMBBDI[FeCl₄]) were prepared by introducing π-π stacking biphenyl groups into the organic cations. Both of the compounds exhibited superparamagnetism from 100 to 300 K, while a ferromagnetic hysteresis loop was found at 300 K. Ferromagnetic interactions were observed from zero field cooling and field cooling studies for both TMBBDI[FeCl₄] and PTMBBDI[FeCl₄]. However, the MIL and PMIL without π-π stacking interaction were paramagnetic without ferromagnetic interaction. The superparamagnetism of the TMBBDI[FeCl₄] and PTMBBDI[FeCl₄] was ascribed to the π-π stacking interactions between biphenyl groups, which not only shortened the Fe-Fe distance to the ferromagnetic interaction range but also increased the order degree of the molecules.