Interplay between optical emission and magnetism in the van der Waals magnetic semiconductor CrSBr in the two-dimensional limit

The Van der Waals semiconductor metamagnet CrSBr offers an ideal platform for studying the interplay between optical and magnetic properties in the two-dimensional limit. Here, we carried out an exhaustive optical characterization of this material by means of temperature and magnetic field dependent photoluminescence (PL) on flakes of different thicknesses down to the monolayer. We found a characteristic emission peak that is quenched upon switching the ferromagnetic layers from an antiparallel to a parallel configuration and exhibits a different temperature dependence from that of the peaks commonly ascribed to excitons. The contribution of this peak to the PL is boosted around 30-40 K, coinciding with the hidden order magnetic transition temperature. Our findings reveal the connection between the optical and magnetic properties via the ionization of magnetic donor vacancies. This behavior enables a useful tool for the optical reading of the magnetic states in atomically thin layers of CrSBr and shows the potential of the design of two-dimensional heterostructures with magnetic and excitonic properties.Comment: 20 pages, 5 figure

Interplay between optical emission and magnetism in the van der Waals magnetic semiconductor CrSBr in the two-dimensional limit

The van der Waals semiconductor metamagnet CrSBr offers an ideal platform for studying the interplay between optical and magnetic properties in the two-dimensional limit. Here, we carried out an exhaustive optical characterization of this material by means of temperature- and magnetic-field-dependent photoluminescence (PL) on flakes of different thicknesses down to the monolayer. We found a characteristic emission peak that is quenched upon switching the ferromagnetic layers from an antiparallel to a parallel configuration and exhibits a temperature dependence different from that of the peaks commonly ascribed to excitons. The contribution of this peak to the PL is boosted around 30-40 K, coinciding with the hidden order magnetic transition temperature. Our findings reveal the connection between the optical and magnetic properties via the ionization of magnetic donor vacancies. This behavior enables a useful tool for the optical reading of the magnetic states in atomically thin layers of CrSBr and shows the potential of the design of 2D heterostructures with magnetic and excitonic properties

Out-of-Plane Transport of 1T-TaS2/Graphene-Based van der Waals Heterostructures

Due to their anisotropy, layered materials are excellent candidates for studying the interplay between the in-plane and out-of-plane entanglement in strongly correlated systems. A relevant example is provided by 1T-TaS2, which exhibits a multifaceted electronic and magnetic scenario due to the existence of several charge density wave (CDW) configurations. It includes quantum hidden phases, superconductivity and exotic quantum spin liquid (QSL) states, which are highly dependent on the out-of-plane stacking of the CDW. In this system, the interlayer stacking of the CDW is crucial for interpreting the underlying electronic and magnetic phase diagram. Here, atomically thin-layers of 1T-TaS2 are integrated in vertical van der Waals heterostructures based on few-layers graphene contacts and their electrical transport properties are measured. Different activation energies in the conductance and a gap at the Fermi level are clearly observed. Our experimental findings are supported by fully self-consistent DFT+U calculations, which evidence the presence of an energy gap in the few-layer limit, not necessarily coming from the formation of out-of-plane spin-paired bilayers at low temperatures, as previously proposed for the bulk. These results highlight dimensionality as a key effect for understanding quantum materials as 1T-TaS2, enabling the possible experimental realization of low-dimensional QSLs

Multistep magnetization switching in orthogonally twisted ferromagnetic monolayers

The advent of twist-engineering in two-dimensional (2D) crystals enables the design of van der Waals (vdW) heterostructures exhibiting emergent properties. In the case of magnets, this approach can afford artificial antiferromagnets with tailored spin arrangements. Here, we fabricate an orthogonally-twisted bilayer by twisting 90 degrees two CrSBr ferromagnetic monolayers with an easy-axis in-plane anisotropy. The magneto-transport properties reveal multistep magnetization switching with a magnetic hysteresis opening, that is absent in the pristine case. By tuning the magnetic field, we modulate the remanent state and coercivity and select between hysteretic and non-hysteretic magneto-resistance scenarios. This complexity pinpoints spin anisotropy as a key aspect in twisted magnetic superlattices. Our results highlight the control over the magnetic properties in vdW heterostructures, leading to a variety of field-induced phenomena and opening a fruitful playground for creating desired magnetic symmetries and manipulating non-collinear magnetic configurations.Comment: Main Text + Supplementary Informatio

Probing the spin dimensionality in single-layer CrSBr van der Waals heterostructures by magneto-transport measurements

Two-dimensional (2D) magnetic materials offer unprecedented opportunities both in terms of fundamental concepts and applied devices with special relevance in the fields of spintronics and magnonics. Beyond the pioneering studies on CrI3 and Cr2Ge2Te6, the family of 2D magnets has expanded to layered antiferromagnets with different spin anisotropies and spin textures. However, all these compounds are highly insulating, thus limiting their possibilities for being integrated into devices. Of particular interest is the case of the layered metamagnet CrSBr, a 2D semiconductor formed by ferromagnetic layers (Tc ~ 150 K) coupled antiferromagnetically between them. This material exhibits a rich physical scenario, including thermal spin dimensionality crossovers and low-temperature hidden order (T* ~ 40 K). Here, we inspect the magneto-transport properties of monolayer, bilayer and trilayer CrSBr integrated into vertical van der Waals heterostructures. Our results in the monolayer limit demonstrate (1) the marked low dimensional character of the ferromagnetic layer, with short-range correlations extending at temperatures well above Tc, (2) a spin anisotropy, with the spins spontaneously aligned along the easy axis (b) of the plane, (3) a reorientation of these spins along a and c upon applying a moderate magnetic field in these directions, and (4) the appearance of field-induced phases in these two directions below ca. 30-40 K due to a cooperative freezing of the spins. In the multilayer case, a spin valve-like behavior is also observed characterized by a negative MR strongly enhanced below T*. Overall, the present results, supported by first-principles calculations, show that the monolayer and bilayer of CrSBr capture most of the physics present in the bulk, offering new insights into the physics of 2D magnets and the integration of these layers into vertical spintronic devices.Comment: 4 figure

Strain Switching in van der Waals Heterostructures Triggered by a Spin-Crossover Metal-Organic Framework

Van der Waals heterostructures (vdWHs) provide the possibility of engineering new materials with emergent functionalities that are not accessible in another way. These heterostructures are formed by assembling layers of different materials used as building blocks. Beyond inorganic 2D crystals, layered molecular materials remain still rather unexplored, with only few examples regarding their isolation as atomically thin layers. Here, the family of van der Waals heterostructures is enlarged by introducing a molecular building block able to produce strain: the so-called spin-crossover (SCO). In these metal-organic materials, a spin transition can be induced by applying external stimuli like light, temperature, pressure, or an electric field. In particular, smart vdWHs are prepared in which the electronic and optical properties of the 2D material (graphene and WSe2) are clearly switched by the strain concomitant to the spin transition. These molecular/inorganic vdWHs represent the deterministic incorporation of bistable molecular layers with other 2D crystals of interest in the emergent fields of straintronics and band engineering in low-dimensional materials

Ultrafast coherent THz lattice dynamics coupled to spins in the van der Waals antiferromagnet FePS3

Coherent THz optical lattice and hybridized phonon-magnon modes are triggered by femtosecond laser pulses in the antiferromagnetic van der Waals semiconductor FePS3. The laser-driven lattice and spin dynamics are investigated in a bulk crystal as well as in a 380 nm-thick exfoliated flake as a function of the excitation photon energy, sample temperature and applied magnetic field. The pump-probe magneto-optical measurements reveal that the amplitude of a coherent phonon mode oscillating at 3.2 THz decreases as the sample is heated up to the Néel temperature. This signal eventually vanishes as the phase transition to the paramagnetic phase occurs, thus revealing its connection to the long-range magnetic order. In the presence of an external magnetic field, the optically triggered 3.2 THz phonon hybridizes with a magnon mode, which is utilized to excite the hybridized phonon- magnon mode optically. These findings open a pathway toward the optical control of coherent THz photo-magnonic dynamics in a van der Waals antiferromagnet, which can be scaled down to the 2D limit

Magnetic imaging and domain nucleation in CrSBr down to the 2D limit

Recent advancements in two-dimensional (2D) materials have revealed the potential of van der Waals magnets, and specifically of their magnetic anisotropy that allows applications down to the two-dimensional limit. Among these materials, CrSBr has emerged as a promising candidate, because its intriguing magnetic and electronic properties have appeal for both fundamental and applied research in spintronics or magnonics. Here, we use nano SQUID-on-tip (SOT) microscopy to obtain direct magnetic imaging of CrSBr flakes with thicknesses ranging from monolayer (N = 1) to few-layer (N = 5). The ferromagnetic order is preserved down to the monolayer, while the antiferromagnetic coupling of the layers starts from the bilayer case. For odd layers, at zero applied magnetic field, we directly image the stray field resulting from the uncompensated layer. We also measure the progressive spin reorientation along the out-of-plane direction (hard axis) with a finite applied magnetic field, allowing us to evaluate the anisotropy constant, which remains stable down to the monolayer and is close to the bulk value. Finally, by controlling the size of the applied magnetic field, we can observe the formation of Néel magnetic domain walls down to the single layer limit

Optomechanical methodology for characterizing the thermal properties of 2D materials

Heat transport in two dimensions is fundamentally different from that in three dimensions. As a consequence, the thermal properties of 2D materials are of great interest, from both scientific and application points of view. However, few techniques are available for the accurate determination of these properties in ultrathin suspended membranes. Here, we present an optomechanical methodology for extracting the thermal expansion coefficient, specific heat, and thermal conductivity of ultrathin membranes made of 2H-TaS2, FePS3, polycrystalline silicon, MoS2, and WSe2. The obtained thermal properties are in good agreement with the values reported in the literature for the same materials. Our work provides an optomechanical method for determining the thermal properties of ultrathin suspended membranes, which are difficult to measure otherwise. It provides a route toward improving our understanding of heat transport in the 2D limit and facilitates engineering of 2D structures with a dedicated thermal performance

Enhanced sensitivity and tunability of thermomechanical resonance near the buckling bifurcation

The high susceptibility of ultrathin two-dimensional (2D) material resonators to force and temperature makes them ideal systems for sensing applications and exploring thermomechanical coupling. Although the dynamics of these systems at high stress has been thoroughly investigated, their behavior near the buckling transition has received less attention. Here, we demonstrate that the force sensitivity and frequency tunability of 2D material resonators are significantly enhanced near the buckling bifurcation. This bifurcation is triggered by compressive displacement that we induce via thermal expansion of the devices, while measuring their dynamics via an optomechanical technique. We understand the frequency tuning of the devices through a mechanical buckling model, which allows to extract the central deflection and boundary compressive displacement of the membrane. Surprisingly, we obtain a remarkable enhancement of up to 14× the vibration amplitude attributed to a very low stiffness of the membrane at the buckling transition, as well as a high frequency tunability by temperature of more than 4.02$\%$ K−1. The presented results provide insights into the effects of buckling on the dynamics of free-standing 2D materials and thereby open up opportunities for the realization of 2D resonant sensors with buckling-enhanced sensitivity.Dynamics of Micro and Nano SystemsQN/van der Zant La