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 (TN) 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-TN 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.