Size Dependence of Metal–Insulator Transition in Stoichiometric Fe₃O₄ Nanocrystals

Magnetite (Fe₃O₄) is one of the most actively studied materials with a famous metal–insulator transition (MIT), so-called the Verwey transition at around 123 K. Despite the recent progress in synthesis and characterization of Fe₃O₄ nanocrystals (NCs), it is still an open question how the Verwey transition changes on a nanometer scale. We herein report the systematic studies on size dependence of the Verwey transition of stoichiometric Fe₃O₄ NCs. We have successfully synthesized stoichiometric and uniform-sized Fe₃O₄ NCs with sizes ranging from 5 to 100 nm. These stoichiometric Fe₃O₄ NCs show the Verwey transition when they are characterized by conductance, magnetization, cryo-XRD, and heat capacity measurements. The Verwey transition is weakly size-dependent and becomes suppressed in NCs smaller than 20 nm before disappearing completely for less than 6 nm, which is a clear, yet highly interesting indication of a size effect of this well-known phenomena. Our current work will shed new light on this ages-old problem of Verwey transition

Magnetically Separable Microporous Fe–Porphyrin Networks for Catalytic Carbene Insertion into N–H Bonds

Microporous organic networks (MONs) are a new class of porous materials. This work shows the application of MON chemistry for the preparation of magnetically separable catalytic systems. By the Sonogashira coupling of Fe^III–tetrakis­(4-ethynylphenyl)­porphyrin and 1,4-diiodobenzene, Fe₃O₄ nanoparticles were coated successfully with Fe–porphyrin networks. The average thickness of the homogeneous coating was ∼17 nm. According to the powder X-ray diffraction and N₂ isotherm analyses, the Fe–porphyrin network coating exhibited amorphous and microporous characteristics. The microporous Fe–porphyrin networks on the Fe₃O₄ nanoparticles showed good catalytic performance for carbene insertion into the N–H bond of amines. The catalytic systems were easily recycled from the reaction mixture by magnetic separation. We believe that the synthetic strategy in this work can be extended to the various catalytic systems

Ising-Type Magnetic Ordering in Atomically Thin FePS₃

Magnetism in two-dimensional materials is not only of fundamental scientific interest but also a promising candidate for numerous applications. However, studies so far, especially the experimental ones, have been mostly limited to the magnetism arising from defects, vacancies, edges, or chemical dopants which are all extrinsic effects. Here, we report on the observation of <i>intrinsic</i> antiferromagnetic ordering in the two-dimensional limit. By monitoring the Raman peaks that arise from zone folding due to antiferromagnetic ordering at the transition temperature, we demonstrate that FePS₃ exhibits an Ising-type antiferromagnetic ordering down to the monolayer limit, in good agreement with the Onsager solution for two-dimensional order–disorder transition. The transition temperature remains almost independent of the thickness from bulk to the monolayer limit with <i>T</i>_N ∼ 118 K, indicating that the weak interlayer interaction has little effect on the antiferromagnetic ordering

Microscopic States and the Verwey Transition of Magnetite Nanocrystals Investigated by Nuclear Magnetic Resonance

⁵⁷Fe nuclear magnetic resonance (NMR) of magnetite nanocrystals ranging in size from 7 nm to 7 μm is measured. The line width of the NMR spectra changes drastically around 120 K, showing microscopic evidence of the Verwey transition. In the region above the transition temperature, the line width of the spectrum increases and the spin–spin relaxation time decreases as the nanocrystal size decreases. The line-width broadening indicates the significant deformation of magnetic structure and reduction of charge order compared to bulk crystals, even when the structural distortion is unobservable. The reduction of the spin–spin relaxation time is attributed to the suppressed polaron hopping conductivity in ferromagnetic metals, which is a consequence of the enhanced electron–phonon coupling in the quantum-confinement regime. Our results show that the magnetic distortion occurs in the entire nanocrystal and does not comply with the simple model of the core–shell binary structure with a sharp boundary

Emergence of a Metal–Insulator Transition and High-Temperature Charge-Density Waves in VSe₂ at the Monolayer Limit

Emergent phenomena driven by electronic reconstructions in oxide heterostructures have been intensively discussed. However, the role of these phenomena in shaping the electronic properties in van der Waals heterointerfaces has hitherto not been established. By reducing the material thickness and forming a heterointerface, we find two types of charge-ordering transitions in monolayer VSe₂ on graphene substrates. Angle-resolved photoemission spectroscopy (ARPES) uncovers that Fermi-surface nesting becomes perfect in ML VSe₂. Renormalization-group analysis confirms that imperfect nesting in three dimensions universally flows into perfect nesting in two dimensions. As a result, the charge-density wave-transition temperature is dramatically enhanced to a value of 350 K compared to the 105 K in bulk VSe₂. More interestingly, ARPES and scanning tunneling microscopy measurements confirm an unexpected metal–insulator transition at 135 K that is driven by lattice distortions. The heterointerface plays an important role in driving this novel metal–insulator transition in the family of monolayer transition-metal dichalcogenides