This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Close to a phase transition, very different materials can follow the same mathematical rules. The concept of universality, which groups seemingly distinct systems based on their common properties, was developed to describe this phenomenon.
In magnetic systems, ferromagnets, ferrimagnets and antiferromagnets fall into the same universality class when short-range interactions dominate and their spatial and spin dimensionalities coincide. However, this universality has not been established when long-range interactions dominate. In insulating magnets, long-range coupling may originate from dipole-dipole interactions.
However, dipolar-driven mean-field criticality has only been firmly established in ferromagnets, leaving the ferrimagnetic and antiferromagnetic cases unexplored. A collaborative team of researchers from Kyoto University, Tohoku University and the Australian Nuclear Science and Technology Organisation (ANSTO) set out to bridge this gap. The work was published in the journal Physical Review Letters.
"Near a phase transition, the microscopically strongest interaction is not always the one that sets the critical rules," explains corresponding author Yusuke Nambu. "Exchange interactions build the ferrimagnetic state, but because dipolar interactions reach much farther, they determine how the material approaches the transition." The team focused on the melilite-type compound Eu2MnSi2O7, which previous studies have suggested adopts a ferrimagnetic structure. Ferrimagnets contain magnetic sublattices that point mainly in opposite directions but do not cancel out because their moments have different sizes.
In Eu2MnSi2O7, the large spin-only moments of Eu2+ and Mn2+ make the compound a particularly clean platform for testing how long-range interactions affect critical behavior. The researchers synthesized polycrystalline Eu2MnSi2O7 and combined magnetization measurements with neutron powder diffraction at ANSTO's Echidna and Wombat instruments. They then performed three complementary magnetization analyses that yielded a transition temperature and critical exponents close to mean-field predictions.
An independent analysis of the temperature-dependent neutron magnetic reflection supported the same conclusion. Finally, neutron diffraction revealed that Eu2+ and Mn2+ order simultaneously in a tilted ferrimagnetic structure, consistent with the crystal's lack of inversion symmetry. This study demonstrates that the insulating ferrimagnet Eu2MnSi2O7 follows rules close to mean-field theory because of long-range magnetic dipole-dipole interactions.
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