Atmospheric Pressure Synthesis of Ultrathin Monoclinic FeCr₂S₄ Crystals with Robust Antiferromagnetism

The synthesis of unconventional phases in two-dimensional (2D) materials can unlock unique properties not readily observed in their bulk counterparts. Recently, the naturally occurring monoclinic phase of FeCr₂S₄, which forms under extremely high pressure, has been discovered in meteorites. However, the properties of this unconventional phase have not yet been explored. Here, we have designed a phase-selective synthesis approach to grow 2D monoclinic FeCr₂S₄ crystals at atmospheric pressure under sulfur-deficient conditions, based on the sulfur content-dependent phase transition that we revealed. By combining theoretical calculations with transport measurements and variable-temperature polarized Raman spectroscopy, we revealed that the monoclinic phase of FeCr₂S₄ is an antiferromagnetic semiconductor with a thickness-independent Néel temperature (T_N) of ∼240 ± 10 K (determined from the inflection point in the resistance-temperature curve), due to the nonlayered structure inherent to FeCr₂S₄. The robust antiferromagnetism in 2D monoclinic FeCr₂S₄ crystals allowed us to construct ferrimagnetic/antiferromagnetic junctions using FeCr₂S₄ crystals with different phases, which exhibited an exchange bias field of up to 228 Oe at 50 K. Furthermore, a magnetic tunnel junction (MTJ) fabricated with 2D FeCr₂S₄ showed a storage field window of 925 Oe at 50 K, surpassing conventional 2D layered heterostructures. The high-T_N characteristics that are independent of thickness, coupled with a strong exchange bias effect, position 2D antiferromagnetic monoclinic FeCr₂S₄ crystals as a promising 2D component for future spintronic devices.