Physics Department - Super-Altermagnetism  

Abstract
Altermagnetism is a theoretically proposed class of magnetic states characterized by collinear antiferromagnetic spins combined with alternating local structural environments, arranged so that the symmetry permits ferromagnet-like behaviors—such as spin-split electronic bands—even in the absence of spin-orbit coupling (SOC)¹². To facilitate experimental studies, we define altermagnets more broadly: as spin-compensated magnets with broken combined parity and time-reversal (PT) symmetry, allowing spin-split bands even in the presence of SOC.
These altermagnets can be categorized into four types:
M-type: with a net orbital “M”agnetization (analogous to compensated ferrimagnets),
S-type: showing “S”ymmetric spin splitting due to broken time-reversal symmetry (T),
A-type: exhibiting “A”ntisymmetric spin splitting due to broken spatial inversion (P),
S/A-type: displaying both symmetric and antisymmetric spin splitting.
We can also examine spin splitting in the absence of SOC by invoking spin-rotation symmetry rather than using full spin group theory. Altermagnets that exhibit spin splitting without SOC are referred to as strong altermagnets.
In M-type altermagnets, the net magnetization (M) typically arises from SOC and may be small. However, external magnetic fields can flip M, switching the entire Néel vector. The dynamics of these systems are governed by the Néel vector and can operate at THz frequencies. In certain M-type altermagnets, the M direction aligns with the non-relativistic spin splitting direction. This alignment enables the non-relativistic spin splitting to influence properties such as the anomalous Hall effect, anomalous Nernst effect, magneto-optical Kerr effect, and Faraday effect.
These particularly responsive M-type altermagnets are termed super-strong M-type altermagnets, or simply super-altermagnets. We will discuss various material realizations of super-altermagnetism and their versatile functionalities. Super-altermagnetism offers exciting new frontiers in both fundamental science and technological applications.

References
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