Researchers from the Hefei Institutes of Physical Science (HFIPS) of the Chinese Academy of Sciences (CAS) have developed a unified framework for establishing symmetry criteria and classifying unconventional magnets.
According to the researchers, the new framework provides a practical route for discovering and designing unconventional magnetic materials by replacing fragmented, case-by-case rules with a predictive and exhaustive symmetry principle.
Published in Newton on Nov. 12, the research was led by associate researcher Luo Xunjiang from the High Magnetic Field Laboratory of HFIPS, in collaboration with professor K.T. Law from Hong Kong University of Science and Technology.
Two magnetic families, separate rules
Unconventional magnets combine the compensated magnetization of antiferromagnets with ferromagnet-like spin splitting, making them attractive for next-generation spintronics.
Depending on the parity of their spin splitting under momentum inversion, they fall into two categories: even-parity and odd-parity magnets. Until now, however, the two families have been described by separate empirical rules, with no unified principle for establishing their symmetry criteria.
Six operations unify the symmetry criteria
To establish unified symmetry criteria for both categories of magnets, the researchers started from the spin space group and identified two decisive subsets of symmetry operations: those that preserve momentum and those that reverse it. Together, these subsets contain only six key operations.
Checking their presence or absence provides a necessary and sufficient criterion for determining whether the momentum-space spin texture is collinear, coplanar or noncoplanar, and whether the nonrelativistic spin splitting has even or odd parity.
This straightforward checklist yields a complete taxonomy of six spin-splitting types arising from 15 distinct symmetry-driven mechanisms.
It also challenges several prevailing paradigms. Odd-parity magnets require neither coplanar magnetic order nor the widely assumed time-reversal-translation symmetry. Meanwhile, collinear even-parity spin splitting can arise in coplanar magnets through a mechanism fundamentally distinct from altermagnetism, broadening the range of possible unconventional magnetic states.
Database search identifies 93 candidate magnets
Applying the criteria to MAGNDATA, a digital database that catalogs hundreds of published magnetic structures, the researchers identified 33 candidate odd-parity magnets and 60 candidate even-parity magnets, excluding known altermagnets.
