Materials and their interaction with electromagnetic fields have long been a fundamental topic in physics, providing insights into the underlying properties of materials and their potential applications. One of the key aspects of this study revolves around the dielectric properties of materials, which describe how a material responds to an external electric field. The Lyddane-Sachs-Teller (LST) relation, introduced in 1941, is a well-established physics construct that links a material’s static dielectric constant (the response when no external field is applied) with its dynamic dielectric constant (the response in the presence of an external electric field) in relation to the material’s vibrational modes, or resonant frequencies, of the crystal lattice. This relation has been invaluable in the study of solid-state physics, helping researchers better understand and predict material properties and thereby enabling the development of new electronic devices.
However, the understanding of how materials respond to external magnetic fields, rather than electric fields, has not always been as straightforward. Recently, a groundbreaking study from researchers at Lund University has extended the principles of the Lyddane-Sachs-Teller relation to the domain of magnetism. This extension, known as the Magnetic Lyddane-Sachs-Teller (MLST) relation, unveils a connection between a material’s static permeability (its response to a static magnetic field) and its dynamic response, or magnetic resonance frequencies, in the presence of oscillatory magnetic fields.
The concept for the Magnetic Lyddane-Sachs-Teller relation was inspired by prior work conducted by Professor Mathias Schubert, who had already explored the interaction of electric fields with phonons (quantized lattice vibrations). Schubert suspected that a similar relationship might exist for magnetic fields and materials, an idea that ultimately led to the development of the MLST relation. This breakthrough in the study of materials opens up exciting new possibilities in the field of magnetism, particularly in the understanding of magnetic materials’ dynamic properties.
In the paper published in Physical Review Letters, the researchers, led by Viktor Rindert, used a highly specialized tool known as a terahertz ellipsometer, developed in their lab, to measure the polarization response of materials. This allowed them to test whether the MLST relation could hold true in a similar way to the electric version of the relation. According to Rindert, “The opportunity to investigate this came with our development of a terahertz ellipsometer capable of capturing the polarization response. With this tool, we rigorously tested whether such a relation exists, leading to the discovery of the Magnetic Lyddane-Sachs-Teller relation.”
The MLST relation mirrors the structure of the LST relation, but instead of relating the static and dynamic dielectric constants for electric fields, it connects the static and dynamic responses of materials in relation to magnetic fields. Specifically, the MLST relation links a material’s magnetic resonance frequencies (essentially the frequencies at which magnetic excitations or spin excitations occur) to its static permeability, which is the material’s response to a direct, non-oscillatory magnetic field. By comparing this to well-established techniques, the researchers were able to confirm that the MLST relation indeed holds true.
To validate this new relation, the team measured the magnetic resonance frequencies of an iron-doped gallium nitride (GaN) semiconductor. GaN is an important material in the field of electronics, particularly for optoelectronic applications such as light-emitting diodes (LEDs) and laser diodes. The team used their newly developed measurement technique, THz-EPR-GSE (terahertz electron paramagnetic resonance with global spin ensemble), to measure the resonance frequencies of the iron-doped GaN semiconductor. This method proved effective in capturing the precise magnetic resonance data required to demonstrate the existence of the Magnetic Lyddane-Sachs-Teller relation. Their measurements were cross-referenced with results obtained from SQUID (Superconducting Quantum Interference Device) magnetometry, a standard and precise method of measuring magnetic properties, further confirming the validity of their findings.
The discovery of the MLST relation holds significant potential for the study and application of magnetic materials. One of the main areas of interest is how this new relation can be used to study magnetic excitations in various materials, especially in semiconductors and other magnetic materials that are crucial to modern electronics. Materials with magnetic properties, such as antiferromagnetic, ferromagnetic, and altermagnetic materials, are used in many electronic devices, and understanding their magnetic behavior at a deeper level is essential for improving performance and developing new technologies.
“The study provides a new fundamental relation in magneto-optics, particularly relevant for researchers working on antiferromagnetic and altermagnetic materials,” said Rindert. These materials are of growing interest in modern electronics due to their unique magnetic properties, which differ from conventional ferromagnetic materials. Antiferromagnetic materials, for example, have opposite magnetic moments that cancel out, while altermagnetic materials have properties that are intermediate between antiferromagnetism and ferromagnetism. Understanding the magnetic resonance frequencies of such materials, as revealed by the MLST relation, can help researchers design better components for future electronic applications.
One of the most promising applications of this discovery is in the realm of power electronics, where materials with wide band gaps, such as GaN, are key to improving the efficiency and performance of power devices. Wide band gap semiconductors can operate at higher voltages and temperatures, making them ideal for use in power electronics, electric vehicles, renewable energy systems, and other high-performance technologies. The new MLST relation may provide researchers with the tools needed to study and optimize these materials for specific applications, ultimately contributing to the advancement of the field.
Rindert and his team are particularly focused on using the THz-GSE-EPR technique to study paramagnetic point defects in ultrawide band gap semiconductors. Paramagnetic defects are imperfections in the crystal lattice of a semiconductor that can have significant effects on its electrical and magnetic properties. Understanding how these defects influence the material’s magnetic response could provide valuable insights into how to improve the efficiency and performance of semiconductors in electronic devices.
While the immediate focus of this research is on semiconductors and their applications in power electronics, the implications of the Magnetic Lyddane-Sachs-Teller relation extend far beyond this field. The relation could contribute to the understanding of a wide variety of magnetic materials, from those used in data storage devices to advanced magnetic sensors and quantum computing technologies. By unlocking new ways of measuring and understanding magnetic excitations in materials, this discovery opens the door to the development of novel materials and devices with unprecedented performance characteristics.
More information: Viktor Rindert et al, Magnetic Lyddane-Sachs-Teller Relation, Physical Review Letters (2025). DOI: 10.1103/PhysRevLett.134.086703.