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 2025 • 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.

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).
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.

About this Article -

This article was crafted using a human-AI hybrid and collaborative approach. AI assisted our team with initial drafting, research insights, identifying key questions, and image generation. Our human editors guided topic selection, defined the angle, structured the content, ensured factual accuracy and relevance, refined the tone, and conducted thorough editing to deliver helpful, high-quality information.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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