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The advancement of two-dimensional polar metals tends to be limited by the incompatibility between electric polarity and metallicity as well as dimension reduction. Here, we report polar and metallic Janus monolayers of the MoSi2N4family by breaking the out-of-plane structural symmetry through Z (P/As) substitution of N. Despite the semiconducting nature of MoSi2X4(X = N/P/As), four Janus MoSi2NxZ4−xmonolayers are found to be polar metals owing to the weak coupling between the conducting electrons and electric polarity. The metallicity is originated from the Z substitution induced delocalization of occupied electrons in Mo-d orbitals. The out-of-plane electric polarizations around 1.5–15.7 pC m−1are determined by the asymmetric out-of-plane charge distribution due to the non-centrosymmetric Janus structure. The corresponding out-of-plane piezoelectricity is further revealed as high as 18.7–73.3 pC m−1and 0.05–0.25 pm V−1for the piezoelectric strain and stress coefficients, respectively. The results demonstrate polar metallicity and high out-of-plane piezoelectricity in Janus MoSi2NxZ4−xmonolayers and open new vistas for exploiting unusual coexisting properties in monolayers derived from the MoSi2N4family.

Magnetostrictive alloys are usually brittle materials with micromagnetic structures. Their structural reliability and durability depend on the complex micromagnetic-mechanical coupling at smaller length scales encompassing the evolution of micromagnetic structures. Herein we propose a micromagnetic-mechanically coupled phase-field model for fracture and fatigue behavior of magnetostrictive alloys with evolution of the micromagnetic structure. The thermodynamically-consistent model is derived from microforce theory, laws of thermodynamics, and Coleman–Noll analysis. The evolution of crack phase-field and magnetization-vector order parameters that are fully coupled is governed by history field dependent Allen–Cahn and Landau–Lifshitz–Gilbert equations, respectively. The model is extended to fatigue by introducing a degradation prefactor for the fracture energy as a function of positive elastic energy. One-dimensional analyses are then presented to anatomize the crack driving forces in terms of fully coupled micromagnetic-mechanical and pure mechanical driving force. We demonstrate the model capabilities by finite-element numerical studies on the micromagnetic domain evolution during the crack propagation and the influence of external magnetic field for type-I, type-II, and three-point bending fracture, as well as for the fracture of a single-edge notched specimen with an elliptical inclusion. The simulation result shows that depending on how micromagnetic domains are switched under micromagnetic-mechanical coupling, the magnetic field can enhance or decrease the critical load. In the presence of inclusion with larger fracture toughness, a crack is found to nucleate in the tri-junction of multi-domain micromagnetic structure owing to the high elastic strain around the tri-junction point. It is further found that a suitable magnetic field promoting magnetization-vector rotation around the crack tip could remarkably improve the fracturing load and fatigue life. The results demonstrate the model promising for the study of micromagnetic-mechanically coupled fracture and fatigue in magnetostrictive alloys. • A micromagnetic-mechanically coupled phase-field model for fracture and fatigue of magnetostrictive alloys is proposed. • Pure mechanical and micromagnetic-mechanical crack driving forces are clarified. • Micromagnetic domain switching under micromagnetic-mechanical coupling affects fracture properties. • Magnetic fields could improve the fatigue life. [ABSTRACT FROM AUTHOR]

The extraordinary properties of a heterostructure by stacking atom-thick van der Waals (vdW) magnets have been extensively studied. However, the magnetocaloric effect (MCE) of heterostructures that are based on monolayer magnets remains to be explored. Herein, we deliberate MCE of vdW heterostructure composed of a monolayer CrI3and metal atomic layers (Ag, Hf, Au, and Pb). It is revealed that heterostructure engineering by introducing metal substrate can improve MCE of CrI3, particularly boosting relative cooling power to 471.72 µJ m−2and adiabatic temperature change to 2.1 K at 5 T for CrI3/Hf. This improved MCE is ascribed to the enhancement of magnetic moment and intralayer exchange coupling in CrI3due to the CrI3/metal heterointerface induced charge transfer. Electric field is further found to tune MCE of CrI3in heterostructures and could shift the peak temperature by around 10 K in CrI3/Hf, thus manipulating the working temperature window of MCE. These theoretical results could enrich the research on low-dimensional magnetocaloric materials.

The nonlinear elasticity and strain-tunable magnetocaloric effect (MCE) of antiferromagnetic monolayer MnPS3are demonstrated by multiscale modeling and simulations. A continuum constitutive model expanding the elastic strain energy density by a Taylor series is formulated to describe the nonlinear and large-deformation elasticity of monolayer MnPS3. Fourteen independent elastic constants of the model are determined using the ab initioresults. The nonlinear behavior is found to initiate at about 7% strain and nearly isotropic before the rupture. MCE (adiabatic temperature change ΔTand isothermal entropy change −ΔS) of monolayer MnPS3is found anisotropic due to the anisotropy in the temperature derivative of magnetic susceptibility, i.e. stronger in-plane MCE. An in-plane biaxial strain could improve MCE by a factor of 1.5–2. The maximum −ΔSand ΔToccur at around 10 K and are evaluated as 2.8 μJ m−2K−1and 2.2 K, respectively, suggesting monolayer MnPS3as a potential candidate for low-temperature (¡ 20 K) magnetic refrigeration.