What are the key differences between altermagnetism and non-collinear antiferromagnetism in terms of symmetry, spin structure, and electronic properties?
I am looking for a clear comparison of these two phases, particularly in terms of their magnetic ordering, symmetry operations, and how they affect the electronic band structure and potential spintronic applications.
Non-Colliniear Antiferromagnet is any antiferromagnetic material where spins are not perfectly aligned, including materials with complex spin spirals, skyrmions, or other non-collinear arrangements.
Altermagnets are non-collinear antiferromagnets, not all non-collinear antiferromagnets are altermagnets. The defining characteristic of altermagnetism is the broken PT (Parity and Time) symmetry, which leads to unique spin-dependent electronic properties like electric polarization.
Altermagnetic structures are collinear and crystal-symmetry compensated, resulting in zero net magnetisation. Unlike ordinary collinear antiferromagnets, electronic states in altermagnets are not spin-degenerated (Kramer's degeneracy).
A classification of magnetic materials including altermagnets can be found in: 10.1103/PhysRevX.12.040002.
The existence of this type of order has been experimentally confirmed for RuO2, MnTe, and CrSb.
There are newer reports of non-collinear magnetic order with full compensation of magnetic moments but still spin-polarized electronic states, which was named altermagnetic order. I think this is still debated.
Compared to Non colinear antiferromagnet (AFM) in Altermagnet (ALM) crystal symmetry is reduced (Reference:10.1103/PhysRevX.12.040002). There exists only rotational symmetry in ALM.
Non colinear AFM has some net bulk magnetization which can give rise to time reversal symmetry breaking phenomenon anomalous hall effect (AHE). ALM has colinear AFM structure with net zero bulk magnetization. ALM like RuO2 depicts AHE which is due to spin splitting (Kramer's degeneracy) in momentum space (Reference: https://doi.org/10.1038/s41928-022-00866-z).
Spin splitting in ALM is non-relativistic (not coming from spin orbit coupling), can be experimentally probed through spin resolved ARPES (Reference: 10.1103/PhysRevLett.133.206401). People are trying to understand the origin of unconventional spin splitting in ALM.
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