A team of international researchers has for the first time directly observed altermagnetism deep within a bulk material, settling a key question about this recently identified class of magnets. Using a specialized form of high-resolution X-ray imaging, the scientists successfully mapped the distinct magnetic domains inside a crystal of manganese telluride, or MnTe. This breakthrough confirms that the unique magnetic properties are an intrinsic characteristic of the material itself, not just a phenomenon confined to thin films or surfaces. The findings establish a powerful new method for exploring these materials and open pathways for developing next-generation spintronic devices that could be smaller and more efficient.
Altermagnetism represents a third primary class of magnetic order, joining the familiar ferromagnetism and antiferromagnetism. Unlike ferromagnets, which have a strong net external magnetic field, and antiferromagnets, which have no net field due to their perfectly alternating internal magnetic moments, altermagnets present a hybrid set of properties. They possess an alternating arrangement of magnetic moments similar to antiferromagnets, resulting in a net-zero magnetization, which is a highly desirable trait for dense data storage applications. However, due to a unique crystal structure, altermagnets also exhibit a strong interaction with electron spins, a feature previously associated only with ferromagnets. This combination of traits—being externally non-magnetic while having strong internal spin-related effects—has generated significant interest for its potential applications in everything from data processing to superconductivity research.
A New Class of Magnetic Material
The theoretical prediction of altermagnetism has spurred a global search for materials that exhibit its unique characteristics. This magnetic phase is defined by a compensated antiferromagnetic order combined with an anisotropic crystal field, a combination that results in time-reversal symmetry breaking. This symmetry breaking is a crucial feature typically found in ferromagnets and is responsible for many of their useful properties, but it comes without the stray magnetic fields that make ferromagnetic components difficult to scale down. The absence of a net magnetization makes altermagnets robust against interference from neighboring components, allowing for the potential creation of much more densely packed electronic devices.
Among the many candidates predicted by theorists, manganese telluride has emerged as one of the most promising systems for studying this new magnetic phenomenon. Previous experimental work provided growing evidence for altermagnetic effects in MnTe. However, a critical question remained unanswered, as the majority of these measurements were performed on thin-film samples or were sensitive only to the material’s surface. It was unclear whether the observed properties were intrinsic to the material or an effect induced by interactions with a substrate or by surface distortions. Confirming that altermagnetism exists within the bulk of a material, independent of such external factors, was a critical step for verifying its fundamental nature and technological promise.
Advanced Imaging Techniques
Probing the Nanoscale
To investigate the internal magnetic structure of MnTe, the research team employed a sophisticated method known as scanning transmission X-ray microscopy, or STXM. This technique involves focusing a beam of circularly polarized X-rays to a very fine point and scanning it across a sample to build a high-resolution image. The researchers combined this with X-ray magnetic circular dichroism (XMCD), a method that makes the instrument sensitive to the direction of magnetic moments in a material. The result is a powerful nano-imaging tool capable of mapping out the magnetic domains—regions where the magnetic moments are aligned in a specific direction—with nanoscale spatial resolution.
Preparing the Sample
The experiment, led by Claire Donnelly at the Max Planck Institute for Chemical Physics of Solids, required a meticulously prepared sample. Marcus Schmidt, a scientist at the institute, first grew a high-quality single crystal of MnTe. From this large crystal, the team then extracted a very thin slice, called a lamella, with a thickness ranging from 150 to 200 nanometers. This lamella was thin enough for the X-ray beam to pass directly through it, enabling the scientists to image the magnetic structure in transmission. This transmission setup was crucial for ensuring the measurement probed the entire volume of the sample, thereby providing a true picture of the bulk magnetic order.
Observing Altermagnetic Domains
The nanoscale X-ray imaging experiment was a success, revealing complex magnetic textures and distinct domains within the MnTe lamella. In the XMCD projection images, the team observed clear regions of dark and bright contrast, indicating areas with different altermagnetic ordering. These observations were the first direct evidence of altermagnetic domains within a bulk sample. Prior to this study, such domains had only been seen in thin film systems, where their properties could have been influenced by the underlying substrate.
To confirm their findings, the researchers performed a quantitative analysis of the XMCD signal from the images. The results showed excellent agreement with theoretical predictions for an altermagnetic material. This close match established that the observed magnetic order exists throughout the full thickness of the lamella, confirming the intrinsic, bulk nature of the state in manganese telluride. The experiment definitively proved that altermagnetism is not merely a surface effect but a fundamental property of the material’s volume.
Implications for Future Technology
This study provides the first conclusive experimental evidence for the bulk nature of altermagnetism in manganese telluride. More than just confirming a theory, it establishes transmission X-ray nano-imaging as a robust and reliable method for identifying and characterizing altermagnetic order. The technique can now be readily applied to the many other materials that theorists are predicting as potential altermagnets, accelerating the pace of discovery in the field. As scientist Marcus Schmidt noted, the ability to grow these materials and directly probe their altermagnetic order on the nanoscale is very exciting.
The verification of bulk altermagnetism opens the door to exploring a wide range of potential applications. The unique combination of properties could offer significant advantages for energy generation and data processing. For the field of spintronics, which aims to use the spin of electrons to carry information, altermagnets offer a promising platform. Their lack of external magnetic fields would allow for the creation of ultra-dense and highly scalable digital and neuromorphic computing devices. The demonstrated ability to detect and characterize altermagnetic spin configurations is a critical step toward realizing these future technologies and exploring the interplay of altermagnetism with other exotic phases of matter, such as superconductivity and topological insulators.
