New Quantum Model Sheds Light on Mysterious Giant Magnetoresistance Materials

Quantum double-exchange ferromagnets

Published on Quantum Server Networks | July 2025

A new theoretical breakthrough is reshaping our understanding of materials exhibiting giant magnetoresistance (GMR), a phenomenon that revolutionized data storage technologies and led to a Nobel Prize in Physics in 2007. A team led by Jacek Herbrych at the Wrocław University of Science and Technology in Poland has shown that two key properties of quantum double-exchange ferromagnets arise purely from quantum spin effects and multiorbital physics—without the need for lattice vibrations, previously believed to play a central role.

The Legacy of Giant Magnetoresistance

Discovered in the late 1980s by Albert Fert and Peter Grünberg, GMR describes a sharp change in a material’s electrical resistance in response to an external magnetic field. It became the basis for sensitive magnetic field sensors used in hard drives, boosting data storage capacities worldwide. But despite its transformative impact, the exact quantum mechanics behind GMR materials—particularly in complex ferromagnets—have remained elusive.

Double-Exchange Ferromagnets: A Complex Playground

The materials under study belong to a class called quantum double-exchange ferromagnets. These are not simple magnets: electrons in these materials occupy multiple orbitals, and their mobility is closely tied to spin alignment. This interplay of charge, spin, and orbital degrees of freedom makes theoretical modeling extremely challenging.

Physicists typically distinguish two spin alignment mechanisms: the Goodenough-Kanamori rules for insulators, and the double-exchange mechanism for metallic ferromagnets. The latter is the focus of Herbrych’s study. In these systems, electron motion and spin alignment are inherently linked, especially in the presence of multiple orbitals—a situation common in transition-metal oxides.

A Simpler Model With Quantum Depth

Herbrych and his team employed two simplified but realistic quantum models: the two-orbital Hubbard-Kanamori model and the Kondo lattice model with interactions. These allowed the researchers to isolate the core effects of quantum interactions in one-dimensional systems.

Their study uncovered two hallmark behaviors of these ferromagnets: magnon mode softening and magnon damping. Magnons, which are quasiparticles representing collective spin oscillations, usually exhibit a well-defined energy vs. momentum relationship. However, in these systems, magnons lose that predictability at short wavelengths, becoming nearly dispersionless. This suggests that short- and long-range spin dynamics differ significantly.

The second feature, magnon damping, involves the loss of coherence among magnons, meaning that the usual picture of spin waves moving through a lattice breaks down. Previously, these behaviors were attributed to Jahn-Teller phonons—lattice vibrations thought to disrupt spin coherence. Herbrych's work overturns that assumption, showing that these effects can emerge purely from quantum spin correlations and multiorbital interactions.

Implications for Correlated Materials and Magnetism

This finding not only deepens our understanding of GMR materials, but also redefines how physicists approach strongly correlated systems more broadly. The authors suggest that their framework could extend to two- and three-dimensional systems, though doing so will involve major computational challenges.

Nonetheless, the models already offer a solid conceptual basis for studying competing phenomena in correlated materials, including ferromagnetism, orbital ordering, and even superconductivity. Such insights could guide the design of next-generation spintronic devices, magnetic memories, and quantum materials.

To read the original article and explore the full findings, visit: https://physicsworld.com/a/new-mechanism-explains-behaviour-of-materials-exhibiting-giant-magnetoresistance/

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#Magnetoresistance #QuantumMaterials #DoubleExchange #Spintronics #Ferromagnets #MultiorbitalPhysics #Magnons #MaterialsScience #CondensedMatter #QuantumServerNetworks #PWmat

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