A rare spin effect once limited to large crystals is now seen in ultrathin magnetic films, pointing to new ways of building faster, more stable, and miniaturized spin-based devices.
(Nanowerk Spotlight) The effort to build smaller, faster, and more stable computing devices has exposed a growing problem in materials science. Magnetic materials, which control information by aligning electron spins, often generate unwanted magnetic fields. These stray fields can disrupt nearby components, particularly in compact circuits where precision matters. Traditional ferromagnets work by aligning spins in the same direction to produce a strong magnetic signal, but this comes at the cost of interference and instability.
Antiferromagnets take a different approach. In these materials, spins are aligned in opposite directions that cancel each other out. This eliminates stray magnetic fields and creates a system that is stable and resistant to external magnetic noise. In principle, antiferromagnets should be ideal for spin-based devices. In practice, they have a critical weakness. Their internal symmetry often hides the very spin properties that designers want to use.
That constraint has led researchers to investigate a new class of materials known as altermagnets. These are antiferromagnets with a key difference. Although their overall magnetization is still zero, they break a subtle combination of spatial and time-reversal symmetry. As a result, their electronic bands split by spin depending on the momentum of the electrons. This spin splitting does not rely on relativistic effects or heavy atoms. Instead, it comes from the arrangement of atoms in the crystal, which creates a direct link between the shape of the lattice and the structure of the electronic states.
Theoretical models and experimental data from bulk crystals have confirmed that some altermagnets exhibit this momentum-dependent band splitting. For applications in spintronics, which aims to use spin rather than charge to process information, this behavior is especially promising. It offers the potential to combine fast, stable switching with minimal energy loss and no interference from magnetic fields.
But there is a problem. Devices are not made from bulk crystals. They are built from films just a few nanometers thick. Reducing a material to this scale can change its behavior in fundamental ways. The symmetry that protects the spin splitting may be disrupted by lattice strain, surface effects, or interactions at the interface with a substrate. Whether altermagnetism survives in this regime is a question that needs direct experimental evidence.
A new study in Advanced Materials (“Altermagnetic Band Splitting in 10 nm Epitaxial CrSb Thin Films”) begins to answer that question. The researchers focus on chromium antimonide, or CrSb, a material predicted to be a strong altermagnet in its bulk form. They investigate whether CrSb still displays its characteristic band structure when grown as an ultrathin film with a thickness of just 10 nanometers.
The study combines material synthesis with a set of structural, magnetic, and spectroscopic measurements. The team used molecular beam epitaxy to grow thin films of CrSb in the (0001) orientation on SrTiO₃ substrates. A thin layer of Sb₂Te₃ was added between the substrate and the film to improve lattice matching and promote uniform growth. The resulting films ranged in thickness from 10 to 100 nanometers.
Structural and magnetic characterization of ultrathin CrSb films grown on SrTiO₃. (a) The crystal structure of CrSb in the NiAs phase, showing two magnetic sublattices with spins pointing in opposite directions. This configuration produces no net magnetization but supports momentum-dependent spin splitting due to crystal symmetry. (b) Reflection high-energy electron diffraction (RHEED) patterns of the underlying Sb₂Te₃ buffer layer and the overlying CrSb film. The streaky patterns indicate smooth, well-ordered, epitaxial growth. (c) X-ray diffraction (XRD) scan confirming the film’s orientation along the (0001) direction, consistent with the NiAs structure. A sharp peak suggests high crystallinity. (d) In-plane XRD phi scan showing six-fold rotational symmetry in the CrSb layer, aligned with the three-fold symmetry of the SrTiO₃ substrate. This indicates good lattice alignment between film and substrate. (e) High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) cross-section image of the full film stack: substrate, buffer layer, and CrSb film. The inset highlights the atomic arrangement of chromium (blue) and antimony (gold), confirming the expected crystalline order. (f) Polarized neutron reflectometry (PNR) profile of a 100 nm CrSb film at room temperature, showing no net magnetization within the CrSb layer. This is consistent with antiferromagnetic order and supports the identification of CrSb as an altermagnet. (Image: Reprinted from DOI:10.1002/adma.202508977, CC BY) (click on image to enlarge)
To assess the quality of the films, the researchers used techniques including X-ray diffraction, electron microscopy, and atomic force microscopy. These measurements confirmed that the films were smooth, well-ordered, and free of major defects. The crystal structure matched the NiAs-type phase of CrSb, which is known to support antiferromagnetic order. The atomic ratio of chromium to antimony was close to one to one, consistent with the ideal stoichiometry.
The magnetic behavior of the films was tested using polarized neutron reflectometry. This technique can detect magnetization even when it is confined to thin layers or buried beneath the surface. The results showed no measurable net magnetization in the CrSb films. Small magnetic signals were found in the Sb₂Te₃ buffer layer, likely due to diffusion of chromium during growth. However, these signals did not extend into the CrSb and did not affect its symmetry or electronic structure.
The core question was whether the spin splitting seen in bulk CrSb would still appear in the ultrathin regime. To answer this, the team performed angle-resolved photoemission spectroscopy, or ARPES. This method maps the energy of electrons as a function of their momentum and can reveal details of the band structure. The measurements were done both at low temperatures using synchrotron radiation and at room temperature using a helium lamp source.
In the 10 nanometer films, the ARPES spectra clearly showed bands that were split depending on the direction of electron momentum. The spin splitting reached values up to 700 millielectronvolts. These results match earlier data from bulk CrSb crystals and confirm that the defining feature of altermagnetism is preserved at the nanoscale. The band structure depended on momentum direction, as predicted for g-wave altermagnets. Along high-symmetry directions, splitting was minimal or absent. In other directions, clear splitting was observed.
Measurements at room temperature gave similar results. The band splitting was still visible and showed the same directional dependence. This suggests that the altermagnetic properties of CrSb remain stable not only under size reduction but also under thermal conditions relevant for devices. No anomalous Hall effect was detected at room temperature, which aligns with symmetry-based predictions. At lower temperatures, some weak magnetic features were observed, but these were linked to the buffer layer and not to the CrSb itself.
To understand how thin the material could become before the band structure changed, the team performed density functional theory calculations. These simulations showed that deviations from the bulk-like behavior begin to appear only below about 2.2 nanometers. Experimental data from 5 nanometer films suggest that spin splitting may persist even at this thickness, although disorder begins to blur the spectral features.
Together, these findings set a lower bound for maintaining the altermagnetic band structure in CrSb films. The study shows that the material can be thinned to at least 10 nanometers without losing its essential electronic properties. This is a critical threshold for real applications, where materials must be compatible with layered structures and scalable manufacturing techniques.
CrSb, in this configuration, offers a promising route for incorporating spin-split electronic states into future devices. Its ability to combine antiferromagnetic stability with large spin splitting, without relying on strong spin-orbit coupling, provides a platform for exploring new types of spin transport and spin-based logic. The results also point to a broader strategy for identifying and stabilizing altermagnetic materials in thin film form. By refining growth methods and exploring symmetry control at interfaces, future work may unlock further possibilities in this emerging class of quantum materials.
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