🔬 mesoscale physics

Low magnon damping of a EuSe film down to millikelvin temperatures

This study demonstrates that epitaxial EuSe films exhibit exceptionally low magnon damping at millikelvin temperatures, with a Gilbert damping parameter upper bound of $1.1\times10^{-3}$ and a magnon lifetime of 2 ns, establishing them as promising candidates for hybrid quantum systems and quantum magnonics.

Original authors: Rostyslav O. Serha, Jan Hofer, Khrystyna O. Levchenko, Andreas Ney, Sergej O. Demokritov, Gunther Springholz, Andrii V. Chumak

Published 2026-10-16

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CC BY 4.0

Original authors: Rostyslav O. Serha, Jan Hofer, Khrystyna O. Levchenko, Andreas Ney, Sergej O. Demokritov, Gunther Springholz, Andrii V. Chumak

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

In the quest to build the next generation of quantum computers, scientists are searching for materials that can carry information without losing it to heat or noise. One promising candidate for this role is the magnon, a tiny ripple of magnetism that travels through a solid material like a wave on a pond. Unlike electrons, which are the standard carriers of electricity, magnons do not require a flow of charge, making them ideal for use in insulating materials where they can interact with superconducting circuits and light. However, for these ripples to be useful in a quantum machine, they must survive long enough to do their work. This requires materials where the ripples do not fade away quickly, a property known as low damping. While one material called yttrium iron garnet has long been the champion of this field, it has a significant flaw when cooled to the extreme temperatures needed for quantum experiments: it interacts with its surroundings in a way that causes it to lose energy. Researchers are therefore looking for a new class of magnetic semiconductors that can maintain their clarity even at temperatures just a fraction of a degree above absolute zero.

A team of physicists has now turned their attention to a specific material called europium selenide, a compound that behaves like a magnetic semiconductor. This material is unique because its magnetic properties come from electrons that are locked tightly to their atoms, rather than flowing freely like in a metal. This structure suggests that the material should naturally resist the internal friction that usually kills magnetic waves. To test this, the researchers grew a thin, high-quality film of europium selenide on a special crystal base that does not interfere with magnetism. They then placed this film inside a dilution refrigerator, a machine capable of cooling samples to temperatures as low as 30 millikelvin, which is colder than deep space. Inside this freezing environment, they used microwaves to excite the magnetic waves and carefully measured how long those waves lasted before fading away.

The results revealed a striking difference between this new material and the traditional favorites. In most magnetic materials, the waves lose energy faster as the frequency of the signal increases, a behavior that limits their usefulness. In contrast, the waves in the europium selenide film became sharper and more stable as the frequency and the applied magnetic field increased. When the researchers cooled the film to 30 millikelvin and subjected it to a strong magnetic field, they found that the waves survived for two billionths of a second. This is a remarkably long time for such a tiny ripple, corresponding to a level of energy loss that is among the lowest ever recorded for a thin film. The team calculated that the intrinsic friction within the material is so low that it is nearly as good as the best bulk crystals ever made, suggesting that the film itself is of exceptional quality.

Crucially, the researchers found that this low level of energy loss did not change as the temperature rose from 30 millikelvin up to 750 millikelvin. This stability is vital because it proves that the waves are not being scattered by random thermal jitters or by the magnetic substrate beneath them, a problem that plagues other materials. Instead, the remaining loss appears to come from small, fixed imperfections in the film, such as tiny variations in thickness or stray atoms, rather than from the fundamental nature of the material itself. The study also showed that the material can be switched between different magnetic states using very weak magnetic fields, allowing it to act as a magnet with no net magnetic field when turned off, or as a strong magnet when turned on. This ability to toggle between states without generating stray magnetic fields makes it an ideal partner for delicate superconducting components.

The findings suggest that europium selenide is a powerful new tool for the emerging field of quantum magnonics. By combining the ability to carry magnetic waves with very low energy loss, the capacity to operate at ultra-low temperatures, and the flexibility to change its magnetic state, this material offers a path forward for building hybrid quantum systems. While the current film still has some room for improvement compared to the purest crystals found in nature, the results demonstrate that the material's potential is real and accessible. The work opens the door to using these magnetic semiconductors not just as passive components, but as active, tunable parts of future quantum devices, bridging the gap between superconducting circuits and the magnetic world.

Technical Summary: Low Magnon Damping of an EuSe Film Down to Millikelvin Temperatures

Problem Statement
The rapid advancement of quantum magnonics has created a critical demand for magnetic materials that maintain low magnon losses at millikelvin temperatures while offering functionalities beyond established materials like yttrium iron garnet (YIG) and ferromagnetic metals. While insulating magnets are preferred to avoid conduction-electron losses, the current material base is narrow. YIG, which possesses the lowest known magnetic damping, is typically grown on gadolinium gallium garnet (GGG). However, GGG is paramagnetic; its large susceptibility at low temperatures generates inhomogeneous stray fields that increase magnon losses in YIG films by up to an order of magnitude below 1 K. Although diamagnetic substrates can eliminate this mechanism, the field lacks insulating magnets with properties superior to YIG that are compatible with such substrates. Europium chalcogenides (EuO, EuS, EuSe, EuTe) are promising candidates due to their magnetic semiconductor nature, high spin moments ( $7 \mu_B$ ), and negligible orbital momentum ( $L=0$ ), which theoretically implies weak spin-orbit coupling and intrinsic damping. However, the magnon damping of EuSe, particularly in film form and at temperatures relevant to quantum experiments (below 1 K), remains largely unexplored.

Methodology
The authors characterize a 240 nm thick epitaxial film of the metamagnet EuSe grown on a diamagnetic BaF $_2$ (111) substrate via molecular beam epitaxy. The study employs two primary characterization techniques across a temperature range from 10 K down to 30 mK:

Magnetometry: Vibrating sample magnetometry (VSM) was performed in a Physical Property Measurement System (PPMS) between 1.8 K and 10 K to map metamagnetic transitions and saturation magnetization.
Broadband Ferromagnetic Resonance (FMR): FMR spectroscopy was conducted using a coplanar waveguide geometry. Measurements were performed in the PPMS (15–70 GHz, 1.8–10 K) and in a dilution refrigerator with a superconducting vector magnet (up to 67.7 GHz, 30 mK–1.8 K). The complex transmission ( $S_{21}$ ) was recorded while sweeping the in-plane static magnetic field.

Data analysis involved fitting transmission traces with complex Lorentzians to extract the resonance field ( $B_{res}$ ) and the loaded full width at half maximum ( $\Delta B_L$ ). Intrinsic linewidths ( $\Delta B$ ) were derived by correcting for the radiative coupling contribution based on the depth of the transmission dip. The resonance data were analyzed using the Kittel equation for in-plane magnetized films to determine the effective magnetization ( $M_{eff}$ ) and gyromagnetic ratio ( $\gamma$ ).

Key Results

Magnetic Phases and Magnetization: The film exhibits the characteristic metamagnetic behavior of EuSe. Below the Néel temperature ( $T_N \approx 4.6$ K), the system transitions from an antiferromagnetic (AFM) state to a ferrimagnetic (FiM) state at low fields, and finally to a field-aligned ferromagnetic (FM) state at fields below 0.25 T. At 1.8 K and 2.5 T, the saturation magnetization reaches 997 kA/m, approximately 91% of the theoretical spin-only value. The effective magnetization ( $M_{eff}$ ) derived from FMR fits at millikelvin temperatures is 1108 kA/m, consistent with the full spin-only moment of Eu $^{2+}$ ions.
Linewidth Behavior: The intrinsic FMR linewidth ( $\Delta B$ ) exhibits behavior distinct from Gilbert-damped ferromagnets. Instead of increasing linearly with frequency, the linewidth of EuSe decreases as frequency and field increase. At 30 mK, the linewidth drops from 24 mT at 20 GHz to 5.45 mT at 67.7 GHz.
Temperature Dependence: Below 750 mK, the linewidth at high frequencies becomes independent of temperature. Above $T_N$ , the linewidth increases significantly at low frequencies but continues to decrease with increasing frequency, consistent with field-polarized paramagnons.
Damping Parameters: At 30 mK, 68 GHz, and 1.8 T, the linewidth reaches 5.5 mT. This value corresponds to a magnon lifetime ( $\tau$ ) of 2 ns. Assuming the entire linewidth is due to Gilbert damping yields an effective damping parameter ( $\alpha_{eff}$ ) of $1.1 \times 10^{-3}$ , which the authors identify as an upper bound. The actual Gilbert damping is likely lower, as the residual linewidth is attributed to extrinsic factors.

Significance and Claims
The paper establishes that epitaxial EuSe films on diamagnetic BaF $_2$ substrates exhibit low magnon damping down to 30 mK, comparable to bulk single crystals and significantly lower than YIG films on paramagnetic substrates at similar temperatures. The material offers a unique combination of properties:

Low Losses: A magnon lifetime of 2 ns and an upper bound for Gilbert damping of $1.1 \times 10^{-3}$ .
Zero-Field State: In the absence of an external field below $T_N$ , EuSe is antiferromagnetic with no net magnetic moment or stray field, making it compatible with superconducting elements.
Tunability: The magnetic order can be switched between antiferromagnetic, ferrimagnetic, and ferromagnetic phases using fields below 0.25 T.
High Saturation Magnetization: The saturation magnetization is more than five times that of YIG at low temperatures.

The authors conclude that EuSe and the broader family of europium chalcogenides are promising candidates for hybrid quantum systems and quantum magnonics. They note that the remaining broadening is extrinsic (likely due to impurities and lattice defects) and suggests that further material purification, similar to the history of YIG development, could reduce losses further. The study does not claim to have demonstrated a specific quantum device but rather identifies EuSe as a versatile platform for future quantum magnonics and spintronics applications.

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