Electrons spinning within a bismuth crystal structure.

The Elusive Spin: Unraveling the Mysteries of Spin-to-Charge Conversion

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Reese Marlowe in Tech & Innovation September 2026 • 5 min read.

"A Deep Dive into Bismuth Films and Their Unexpected Behavior in Spin Current Experiments"


In the ever-evolving realm of spintronics, researchers are constantly seeking new ways to harness the power of electron spin for technological advancements. One promising avenue involves the conversion of spin current—a flow of angular momentum without the movement of charge—into charge current, which could lead to more efficient and less energy-intensive devices. However, the path to achieving this conversion is riddled with challenges and unexpected twists.

A recent study published in Physical Review Letters has thrown a wrench into the works, challenging previous claims about the effectiveness of bismuth (Bi) and bismuth/silver (Bi/Ag) bilayers in facilitating spin-to-charge conversion. This article will discuss this research, dissecting its findings and exploring the implications for the future of spintronics.

The original study delved into the behavior of Bi films and Bi/Ag bilayers when subjected to thermal spin injection, a method where heat is used to generate a spin current. The researchers' goal was to observe and measure the conversion of this spin current into an electrical charge current, a phenomenon that could pave the way for novel electronic applications.

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Defining the Field

Spin-to-charge conversion refers to the ability of materials to bridge the use of electric charge for information processing with the use of electron spins, allowing spin currents to be converted into voltages. The efficiency of these conversions is characterized by parameters such as the spin Hall angle, defined as the ratio between the transverse spin and charge currents. The method based on conversion of the electronic spin state of NV centers to a charge-state distribution enables single-shot readout at room temperature, demonstrating a contrast similar to the population of the metastable singlet state.

Ultrafast Conversion Mechanisms

Ultrafast spin-to-charge current conversion refers to physical mechanisms whereby a rapidly varying spin current, driven by optical, electrical, or magnetization dynamics on sub-picosecond timescales, is converted into an electrical charge current. A spin-to-charge conversion method assisted by near-infrared light can suppress spin-flip errors by leveraging high spin-selectivity of cryogenic resonance excitation and flexibility of photoionization. However, extracting precise spin states after the initial excitation and calibration process typically requires multiple measurements, limiting single-shot fidelity.

Foundational Material Discoveries

Evidence for spin-to-charge conversion in GeTe(111) represents a fundamental milestone in the investigation of ferroelectric Rashba materials for spin-orbitronics, with the aim of bridging the gap between material properties and their deployment. Large multi-directional spin-to-charge conversion in low-symmetry semimetals at room temperature brings new flexibility to the design of spin logic devices, where any in-plane polarization of injected spins results into charge conversion. The spin-to-charge conversion in graphene on ferromagnetic substrates is explained by extrinsic spin-orbit interaction induced by proximity effect with the ferromagnetic layer.

Spin-to-Charge Conversion: A Closer Look

Electrons spinning within a bismuth crystal structure.

The fundamental principle at play here is spin-to-charge conversion, a process that transforms a flow of electron spin into an electrical current. This phenomenon is particularly attractive because spin currents, unlike conventional electrical currents, don't generate as much heat. This could lead to more energy-efficient electronic devices. The key to detecting spin-to-charge conversion lies in identifying the Inverse Spin Hall Effect (ISHE), where a spin current generates a charge current perpendicular to the spin direction. Heavy metals with strong spin-orbit coupling are typically used for this purpose.

Bismuth, the star of this study, is the heaviest non-radioactive element and seemed like a prime candidate for efficient spin-to-charge conversion due to its large spin-orbit coupling. Prior research had even suggested promising results. However, this new study injected a dose of skepticism into the field. The research team meticulously investigated Bi films and Bi/Ag bilayers, structures composed of thin layers of bismuth and silver, to observe how they behaved when subjected to a pure spin current. They used a technique called thermal spin injection, where a temperature gradient is applied to generate a spin current in an adjacent material, in this case, Yttrium Iron Garnet (YIG).

Here's what they found:
  • Minimal Conversion: Despite injecting a spin current into the Bi layer and Bi/Ag bilayer, there was surprisingly little evidence of spin-to-charge conversion.
  • Nernst Effect Dominance: Instead of the expected ISHE signal, the researchers primarily detected a Nernst signal originating from the Bi layer. The Nernst effect is a thermoelectric phenomenon where a magnetic field and a temperature gradient produce a voltage perpendicular to both.
  • Challenging Previous Claims: These results directly contradicted earlier studies that had reported significant spin-to-charge conversion in similar systems, particularly those attributing it to the Inverse Rashba-Edelstein Effect (IREE).
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Weyl Semimetals and Novel Materials

Spin-to-charge current conversion properties in the Weyl semimetal TaP have been demonstrated at room temperature using the inverse Rashba-Edelstein effect, with interfacial integration of this quantum material with the ferromagnetic metal Permalloy. Spin-to-charge conversion has also been studied in orthorhombic RhSi crystalline thin films, expanding the range of material systems investigated for these interconversion phenomena.

Temperature-Dependent Sign Changes

Spin-to-charge conversion induced by the Rashba-Edelstein effect was directly observed for the first time in samples with no magnetic layer, but measurements as a function of temperature exhibit a clear sign change. The spin-to-charge conversion signal in graphene/Pt lateral devices at room temperature has been reported as two orders of magnitude larger than previously reported metallic channels, though reproducibility across systems varies. Room temperature spin-to-charge conversion has been demonstrated in amorphous topological insulator materials, suggesting universality across structural phases.

Measuring Conversion Efficiency

Spin- and orbital-to-charge conversion phenomena in noncentrosymmetric materials with broken inversion symmetry can be treated by considering contributions from the Hall effect and the Rashba-Edelstein effect on an equal footing. The charge-to-spin conversion generates spin-orbit torque for non-volatile magnetic memory device applications, while spin-to-charge conversion is utilized in spin logic and neuromorphic operations. Room-temperature ferroelectric switching of spin-to-charge conversion in GeTe shows a magnitude comparable to platinum, but the charge current sign is controlled by ferroelectric polarization orientation.

To further investigate, the team performed a series of control experiments. They inserted a thin layer of magnesium oxide (MgO), an insulator, between the Bi layer and the YIG. This should have blocked any spin current from reaching the Bi layer. Surprisingly, the measured voltages remained unchanged, suggesting they were not related to spin-to-charge conversion at all. This reinforced the conclusion that the observed signals were primarily due to the Nernst effect within the bismuth itself. The team also varied the thickness of the Bi layers and examined different material interfaces, but the results consistently pointed to the absence of significant spin-to-charge conversion.

The Future of Spin Research

This study serves as a cautionary tale in the pursuit of efficient spin-to-charge conversion. While bismuth initially appeared to be a promising candidate, its thermoelectric properties, specifically the Nernst effect, can overshadow the desired spin-related phenomena. It also highlights the challenges of working at the nanoscale and the need for careful experimental design to isolate the effects being studied. As research progresses, scientists will likely explore alternative materials and heterostructures, focusing on those with strong spin-orbit coupling and minimal parasitic effects. Furthermore, a deeper understanding of the fundamental mechanisms governing spin-to-charge conversion is crucial for realizing the full potential of spintronics.

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Systematic Analysis Methods

A systematic and detailed analysis of efficient spin-to-charge conversion has been demonstrated in (110)-oriented RuO2 films, using amorphous CoFeB as the spin source. This approach allows for spin memory loss mitigation while maintaining high conversion efficiency, providing a framework for evaluating competing material systems.

Ultrafast Electronic Integration

Converting spin waves into digital signals for computing represents an ambitious spin-to-charge conversion that has long been a bottleneck in spintronics. Non-linear spin-to-charge conversion has been shown to be fully consistent with linear response measurements and is orders of magnitude faster. Voltage-controlled writing and spin-to-charge-conversion-based reading in spintronics devices establishes a foundation for advancing these technologies into future electronic devices.

Topological Materials and THz Emission

Central challenges in this field include the generation, manipulation, and detection of spin currents. The discovery of topological insulators has introduced a new class of materials with high spin-charge conversion efficiency due to strong spin-orbit coupling. Broad-band THz emission can be achieved through spin-to-charge conversion processes, opening applications in terahertz technology.

Tunable Material Properties

Non-volatile Fermi level tuning enables control of spin-charge interconversion in graphene-based systems through Rashba spin-orbit coupling. The impact of oxygen interdiffusion on spin-to-charge conversion at interfaces has been studied, with enhanced conversion at metal/oxide interfaces achieved by lowering temperature. These material engineering approaches are critical for developing practical spintronic devices.

About this Article -

Written with AI assistance from published research, and reviewed by the Mystum team. See our About page for more information.

Everything You Need To Know

1

What exactly is spin-to-charge conversion, and why is it considered important in the field of spintronics?

Spin-to-charge conversion is the process of transforming a spin current, which is a flow of electron spin, into an electrical charge current. This is desirable because spin currents ideally don't generate as much heat as conventional electrical currents, potentially leading to more energy-efficient electronic devices. The key phenomenon used to detect this conversion is often the Inverse Spin Hall Effect (ISHE).

2

Instead of observing spin-to-charge conversion, what did the researchers primarily detect, and what is the Nernst effect?

In the experiments, researchers primarily detected a Nernst signal originating from the bismuth layer instead of the Inverse Spin Hall Effect (ISHE) signal. The Nernst effect is a thermoelectric phenomenon where a magnetic field and a temperature gradient produce a voltage perpendicular to both. This Nernst signal overshadowed any potential signal from spin-to-charge conversion, leading to the conclusion that bismuth might not be as effective as initially thought.

3

What was the purpose of inserting a layer of magnesium oxide (MgO) in the experiment, and what did the results indicate?

The researchers inserted a thin layer of magnesium oxide (MgO) between the bismuth (Bi) layer and the Yttrium Iron Garnet (YIG). Magnesium oxide is an insulator, so it should block any spin current from reaching the bismuth layer. Surprisingly, the measured voltages remained unchanged, suggesting that the signals observed were not related to spin-to-charge conversion, further supporting the dominance of the Nernst effect.

4

What does this study reveal about the challenges and future directions in the search for efficient spin-to-charge conversion?

This study serves as a cautionary tale, highlighting the challenges of isolating desired spin-related phenomena from other material properties at the nanoscale. In the case of bismuth, its thermoelectric properties, specifically the Nernst effect, complicated the observation of spin-to-charge conversion. Future research will likely explore alternative materials and heterostructures with strong spin-orbit coupling while minimizing parasitic effects, as well as focusing on the deeper understanding of the fundamental mechanisms governing spin-to-charge conversion.

5

Besides the Nernst effect, what other spin-related phenomenon, like the Inverse Rashba-Edelstein Effect (IREE), is now being questioned in its effectiveness due to these findings?

While the study focused on bismuth (Bi) films and bismuth/silver (Bi/Ag) bilayers, it challenges previous claims of efficient spin-to-charge conversion attributed to the Inverse Rashba-Edelstein Effect (IREE). The Inverse Rashba-Edelstein Effect (IREE) suggests that an electrical current can generate a spin accumulation, or vice versa, at interfaces with specific symmetry properties. This study suggests that the observed phenomena in bismuth-based systems may have been misinterpreted, warranting further investigation into the roles of the Inverse Rashba-Edelstein Effect (IREE) and other effects in similar materials.

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