March 1, 2024

A New Chapter in Magnetism and Thermal Science

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Thermal transport of crystals in altermagnets. The left part, which includes the balls, arrows, and spin density isosurfaces, represents a typical altermagnet. When a temperature gradient field is applied, charge and thermal currents are induced in a perpendicular direction, illustrating the thermal transport of the crystal, as shown in the right part. Credit: Zhou et al/Physical Review Letters. DOI: 10.1103/PhysRevLett.132.056701.

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Thermal transport of crystals in altermagnets. The left part, which includes the balls, arrows, and spin density isosurfaces, represents a typical altermagnet. When a temperature gradient field is applied, charge and thermal currents are induced in a perpendicular direction, illustrating the thermal transport of the crystal, as shown in the right part. Credit: Zhou et al/Physical Review Letters. DOI: 10.1103/PhysRevLett.132.056701.

In a new study, scientists investigated the newly discovered class of altermagnetic materials for their thermal properties, offering insights into the distinctive nature of altermagnets for spin-caloritronic applications.

Magnetism is an old and well-researched topic, lending itself to many applications, such as motors and transformers. However, new magnetic materials and phenomena are being studied and discovered, one of which is altermagnets.

Altermagnets exhibit a unique blend of magnetic characteristics, differentiating them from conventional magnetic materials such as ferromagnets and antiferromagnets. These materials exhibit properties observed in both ferromagnets and antiferromagnets, making their study attractive.

The current research, published in Physical Review Lettersexplores the thermal properties of altermagnets and was led by Prof. Wanxiang Feng and Prof. Yugui Yao of the Beijing Institute of Technology.

Speaking about his motivation behind exploring altermagnets, Prof. Feng told Phys.org: “Magnetism is an old and fascinating topic in solid-state physics. While exploring non-collinear magnets over the past few decades, we have found a new type of collinear magnet, the altermagnet.”

Professor Yao added: “With a dual nature reminiscent of ferromagnets and antiferromagnets, altermagnets intrigued us with the potential for new physical effects. Our motivation stemmed from the desire to understand and unlock the unique properties of these magnetic materials.”

The emergence of magnetism

Magnetic properties emerge from the behavior of atoms, particularly the arrangement and movement of electrons within a material.

“In magnetic materials, due to the exchange interaction between atoms, the spin magnetic moments are organized parallel or antiparallel, forming the most common ferromagnets and antiferromagnets, respectively, which have been studied for more than a century”, explained Prof.

Altermagnets defy conventional norms by embodying a dual nature – resembling antiferromagnets with zero net magnetization and ferromagnets with non-relativistic spin splitting.

In altermagnets, collinear antiparallel magnetic order combines with non-relativistic spin splitting, resulting in antiferromagnet-like net zero magnetization and ferromagnetic spin dynamics simultaneously.

This unique behavior emerges from the intricate interaction of atoms within the crystal structure. For example, ruthenium dioxide, the subject of this research, presents spin degeneration induced by non-magnetic oxygen atoms, breaking spatial and temporal symmetries. This leads to the material’s unique magnetic properties.

Furthermore, altermagnets exhibit a unique spin polarization. The term “spin polarization” means that a preponderance of electron spins tends to align in a specific direction.

Spin polarization is noteworthy in altermagnets because it occurs in the physical arrangement of atoms (real space) and in momentum space, where the distribution of electron spins in the material is considered.

Nernst and Hall effects

The researchers focused on studying the emergence of the Nernst crystal and the thermal effects of the Hall crystal on rubidium dioxide (RuOtwo), chosen as a representative showcase of altermagnetism.

The crystal Nernst effect (CNE) observed in altermagnets is a result of their distinct magnetic nature. In simple terms, as the material experiences a temperature difference across its dimensions, this leads to the emergence of a voltage perpendicular to both the temperature gradient and the magnetic field. This phenomenon reveals that the material’s magnetic properties influence its response to temperature changes, providing insights into the intricate connection between thermal and magnetic behaviors in altermagnets.

In altermagnets, this effect is significantly influenced by the direction of the Néel vector, which represents the direction in which neighboring magnetic moments align. This adds an extra layer of complexity to the thermal response.

Similarly, the crystal thermal Hall effect (CTHE) clarifies how heat moves in altermagnets. Like the traditional thermal Hall effect, it occurs perpendicular to the temperature gradient and magnetic field. In altermagnets, the CTHE presents significant variation depending on the direction of the Néel vector. This anisotropy is a central factor in understanding the unique thermal transport behavior of altermagnetic materials.

Thermal properties of RuOtwo

The research methodology employed a dual strategy, combining symmetry analysis and cutting-edge first-principles calculations, to unravel the thermal transport properties of RuOtwo. Symmetry analysis played a crucial role in unraveling the fundamental reasons behind the emergence of altermagnetism.

Through two symmetry operations involving spatial inversion, time reversal and lattice translation, the study showed the intricate interaction of atoms within the crystal lattice, demonstrating how non-magnetic oxygen atoms induced non-relativistic spin splitting into energy bands. .

This process resulted in the breaking of crystalline time reversal symmetry, giving rise to distinct thermal transport properties of the crystal.

“Through detailed analyses, we identified three physical mechanisms that contribute to crystal thermal transport: pseudo-nodal Weyl lines, pseudo-nodal altermagnetic planes, and altermagnetic ladder transitions,” said Prof.

In simple terms, Weyl pseudo-nodal lines are paths that guide heat within the material, altermagnetic pseudo-nodal planes can be imaged as designated zones that influence heat flow, and altermagnetic ladder transitions can be thought of as the way the material rises in a heat wave. ladder.

These findings are exciting because they play a significant role in how heat travels inside altermagnets.

Researchers discovered an extended Wiedemann-Franz law in RuOtwo, linking the material’s unusual thermal and electrical transport characteristics. Contrary to conventional expectations, this extended law operates over a wider temperature range, extending beyond 150 Kelvin.

Spin caloritronics

Researchers believe that altermagnets could play a key role in spin caloritronics, a field of research that explores the interaction between spin and heat flow, which is not achievable with ferromagnets or antiferromagnets. This field has potential applications in developing new technologies for processing and storing information.

“Altromagnetic materials with collinear antiparallel magnetic order exhibit faster rotation dynamics and lower sensitivity to stray magnetic fields compared to ferromagnetic materials. This makes them promising for achieving higher storage density and faster rotating caloritronic devices,” explained Prof.

The researchers also intend to investigate the thermal transport of higher-order crystals and magneto-optical effects in the future.

Speaking about this, Professor Yao said, “We are curious about the differences in the thermal transport of higher-order crystals and the higher-order magneto-optical effects in altermagnets compared to antiferromagnets or ferromagnets. We are in the early stages of this technology, and there are a long journey ahead before this becomes practically achievable.”

More information:
Xiaodong Zhou et al, Crystal Thermal Transport in Altermagnetic RuO2, Physical Review Letters (2024). DOI: 10.1103/PhysRevLett.132.056701. About arXiv: DOI: 10.48550/arxiv.2305.01410

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