From pure to mixed: Altermagnets as intrinsic symmetry-breaking indicators

We investigate the impact of the presence of altermagnetism on physical observables in the presence of explicit or spontaneous symmetry-breaking fields. We focus on unconventional superconductivity as a potential source of spontaneous symmetry-breaking fields and derive the symmetry considerations for the transmutation of altermagnets from pure to mixed, which can give rise to unusual phenomena. Based on this analysis, we put forward scenarios that could explain the apparent onset of time-reversal symmetry breaking at the superconducting critical temperature in Sr${}_2$RuO${}_4$, and the hidden magnetic phase and magnetic memory in 4Hb-TaS${}_2$.