The van der Waals (vdW) antiferromagnets with a large uniaxial magnetocrystalline anisotropy (UMA) have compelling advantages for terahertz spintronics applications. Although the temperature dependence of UMA is essential for ensuring device endurance, it has scarcely been explored. Here, we unveil an unconventional temperature dependence of UMA in vdW antiferromagnet ${\mathrm{FePS}}_3$. The easy-magnetization axis exhibits an anomalous transition from in-plane to out-of-plane above 110 K under a low magnetic field, with this behavior persisting even beyond the Néel temperature of 118 K. Temperature-dependent x-ray diffraction measurements combined with density functional theory calculations reveal that the switching originates from an anomalous transition from an antiferromagnetic to a quasiferromagnetic state. This transition is governed by the evolving competition between significantly strengthened intra-subchain ferromagnetic coupling and drastically diminished inter-subchain antiferromagnetic coupling upon heating; thereby, the latter becomes ultimately suppressed as a consequence of substantial lattice thermal expansion and the intrinsic antiparallel zigzag configuration of the ferromagnetic subchains. Our findings offer not only the basic understanding of the temperature dependence of UMA in vdW antiferromagnetic materials but also the low magnetic field- and temperature-tunable magnetic states by considering the cooperation between lattice and antiferromagnetic exchange coupling for terahertz spintronics applications.

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• Antiferromagnetism
• Magnetic anisotropy
• Magnetic susceptibility
• Antiferromagnets
• Magnetic semiconductors
• Density functional theory
• Electron spin resonance
• Magnetization measurements
• Susceptibility measurements
• X-ray diffraction