Kyoto, Japan -- 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.
"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 the mean-field predictions, and 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. To the team's knowledge, this is the first insulating ferrimagnet in which dipolar interactions have been shown to drive mean-field criticality. The result extends a phenomenon previously established in insulating ferromagnets to a more complex magnetic class, filling a gap in the universality of magnetic phase transitions.
"Ferrimagnets combine a net magnetization with internal antiferromagnetic correlations," Nambu adds. "Our research closes an important gap between ferromagnets and the still-unresolved antiferromagnetic case."
These findings establish Eu2MnSi2O7 as a platform for studying long-range interactions in complex insulating magnets and provide a framework for predicting the behavior of other materials. Such understanding could contribute to the development of future magnetic and spintronics-based technologies.
