Determining the Nature of Magnetism in Altermagnetic Candidate RuO₂ Using Thermodynamic Probes

The topic of altermagnetism has gained significant attention in recent years. Altermagnetic materials have compensated magnetization, like antiferromagnets, but show properties associated with time-reversal symmetry breaking, like ferromagnets. These unique features are rooted in specific crystallographic and magnetic symmetries as well as an alternating spin polarization in both real and reciprocal space. Since the seminal work on altermagnetism, RuO₂ was put forward as the prototypical model of an altermagnetic material. However, the magnetic state in this system is still controversial and heavily debated. In single crystals, heat capacity, electrical transport, magnetic susceptibility, and μSR measurements suggest RuO₂ is a Pauli paramagnet without a local magnetic moment. On the other hand, resonant x-ray scattering and neutron diffraction have shown a small local magnetization consistent with collinear antiferromagnetic order. These conflicting results, coupled with differences between thin films and bulk crystals, have led to ambiguity in the magnetic phase of RuO₂. It is imperative to resolve this debate with precise thermodynamic measurements to determine if bulk RuO₂ is an appropriate system to explore altermagnetism and establish a firmer understanding of RuO₂ for potential applications. Thus, we present a combination of thermodynamic measurements under various temperatures and magnetic fields to reveal the nature of magnetism in bulk RuO₂ single crystals.