Thermal transport of crystals in alter-magnets. The left part, which includes the balls, arrows and spin density isosurfaces, represents a typical altermagnet. When a temperature gradient field is applied, the charge and thermal currents are induced in a perpendicular direction, illustrating the thermal transport of the crystals, as shown in the right part. Credit: Zhou et al/Physical Examination Letters. DOI: 10.1103/PhysRevLett.132.056701.
In a new study, scientists investigated the newly discovered class of altermagnetic materials for their thermal properties, providing insight into the distinctive nature of altermagnets for spin caloritron applications.
Magnetism is an old and well-studied subject, lending itself to many applications, such as motors and transformers. However, new materials and magnetic phenomena are being studied and discovered, including alter-magnets.
Altermagnets exhibit a unique blend of magnetic characteristics, setting them apart from conventional magnetic materials like ferromagnets and antiferromagnets. These materials exhibit properties observed in both ferromagnets and antiferromagnets, making their study attractive.
The current research, published in Physical Examination Lettersexplores the thermal properties of alter-magnets and was led by Professors Wanxiang Feng and Yugui Yao of the Beijing Institute of Technology.
Speaking about their motivation behind exploring altermagnets, Professor Feng told Phys.org: “Magnetism is an ancient and fascinating topic in solid-state physics. While exploring non-collinear magnets over the past decades, we encountered a new type of collinear magnet, the alter-magnet.”
Professor Yao added: “With a dual nature resembling both ferromagnets and antiferromagnets, altermagnets have intrigued us with the potential for new physical effects. Our motivation stems from the desire to understand and discover 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 arranged in a parallel or antiparallel manner, forming the most common ferromagnets and antiferromagnets, respectively, which have been studied for more than a century ” Professor Feng explained.
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, which simultaneously results in zero net magnetization similar to antiferromagnets and ferromagnetic spin dynamics.
This unique behavior emerges from the complex interaction of atoms within the crystal structure. For example, ruthenium dioxide, the subject of this research, exhibits spin degeneracy induced by non-magnetic oxygen atoms, breaking spatial and temporal symmetries. This leads to the unique magnetic properties of the material.
Additionally, altermagnets exhibit a unique spin polarization. The term “spin polarization” means that a preponderance of electronic spins tend to align in a particular direction.
Spin polarization is notable 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 taken into account.
Nernst and Hall effects
The researchers focused on studying the emergence of crystal Nernst and crystal thermal Hall effects in rubidium dioxide (RuO2), chosen as a representative showcase of altermagnetism.
The crystal Nernst effect (CNE) observed in altermagnets is a result of their distinctive magnetic nature. Simply put, when the material experiences a temperature difference in 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 magnetic properties of the material influence its response to temperature changes, providing insight into the complex link between the thermal and magnetic behaviors of 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 additional layer of complexity to the thermal response.
Likewise, the crystalline thermal Hall effect (CTHE) sheds light on how heat moves in alter-magnets. Like the traditional thermal Hall effect, it occurs perpendicular to the temperature gradient and magnetic field. In alter-magnets, the CTHE shows a significant variation depending on the direction of the Néel vector. This anisotropy is a central factor in understanding the thermal transport behavior specific to altermagnetic materials.
Thermal properties of RuO2
The research methodology used a dual strategy, combining symmetry analysis and cutting-edge first-principles calculations, to uncover the thermal transport properties of RuO.2. Symmetry analysis played a crucial role in discovering the fundamental reasons for the emergence of altermagnetism.
Through two symmetry operations involving space inversion, time inversion and lattice translation, the study highlighted the complex interaction of atoms within the crystal structure, demonstrating how oxygen atoms non-magnetic induced non-relativistic spin splitting in the energy bands.
This process resulted in the breakdown of crystal time-reversal symmetry, giving rise to distinct thermal transport properties of the crystals.
“Through detailed analysis, we identified three physical mechanisms contributing to the thermal transport of crystals: Weyl pseudo-nodal lines, altermagnetic pseudo-nodal planes and altermagnetic ladder transitions,” said Professor Yao.
In simple terms, pseudo-nodal Weyl lines are pathways that guide heat within the material, altermagnetic pseudo-nodal planes can be imaged as designated areas influencing heat flow, and transitions of Alternagnetic ladders can be thought of as how material climbs over a layer of heat. ladder.
These findings are exciting because they play an important role in how heat moves within altermagnets.
Researchers discovered an extended Wiedemann-Franz law in RuO2, linking the unusual thermal and electrical transport characteristics of the material. Contrary to conventional expectations, this extended law operates over a wider temperature range, extending beyond 150 Kelvin.
Spin caloritronics
Researchers believe altermagnets could play a central role in spin caloritronics, a field of research that explores the interaction between spin and heat flow, something not possible with ferromagnets or antiferromagnets. . This area has potential applications in the development of new information processing and storage technologies.
“Alternagnetic materials with collinear antiparallel magnetic order exhibit faster spin dynamics and lower sensitivity to stray magnetic fields compared to ferromagnetic materials. This makes them promising for achieving higher storage density and higher spin caloritron devices fast,” explained Professor Feng.
The researchers also intend to study 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 higher-order thermal transport of crystals and higher-order magneto-optical effects in alter-magnets compared to antiferromagnets or ferromagnetic. We are in the early stages of this technology, and there is a long way to go before it becomes practically feasible. »
More information:
Xiaodong Zhou et al, Crystalline thermal transport in altermagnetic RuO2, Physical Examination Letters (2024). DOI: 10.1103/PhysRevLett.132.056701. On arXiv: DOI: 10.48550/arxiv.2305.01410
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