On the nature of symmetry breaking in NiBr₂

Type-II multiferroics have gained significant attention due to their potential to elucidate the physics of intertwined orders in condensed matter and their promise for applications in magneto-electric devices. Among these materials, NiBr₂ has emerged as a prototypical van der Waals multiferroic. NiBr₂ exhibits a paramagnetic phase at room temperature, followed by two sequential phase transitions to an antiferromagnetic state (T_N=50K) and a helimagnetic phase (T_C=23K). Below T_C, the formation of a cycloidal magnetic order leads to the breaking of inversion symmetry within the crystal, resulting in the generation of a finite electric polarization. To explore the multiple phase transitions shown by NiBr₂, here we employ a suite of non-invasive, symmetry-sensitive optical techniques in and out of thermodynamic equilibrium. We will discuss the emergence of anomalously strong order-parameter-like signals across the different transition temperatures and describe their implications for understanding the magnetic and multiferroic properties of NiBr₂.