In an article recently published in the journal Nature Communications, researchers investigated the feasibility of electrical noise spectroscopy as a highly sensitive tool for studying and detecting magnons in a quantum Hall ferromagnet.
Study: Sensitive Magnon Detection via Electrical Noise Spectroscopy. Image Credit: Irina Chukantseva/Shutterstock.com
Background
Collective spin-wave excitations/magnons in magnetic materials are promising quasi-particles for advanced spintronics devices, like information transfer platforms. Thus, developing new techniques is crucial to detect these charge-neutral quasi-particles.
Although different experimental tools like terahertz spectroscopy have been utilized for magnon detection in bulk magnetic materials, detecting them in device geometries, essential for information processing-related applications, has remained difficult. Recently, researchers have successfully turned magnons into electrical signals in a quantum Hall ferromagnet in graphene.
When subjected to a perpendicular magnetic field, graphene displays unique quantum Hall phases related to its unusual Landau levels (LL) sequence, manifesting both valley and spin degrees of freedom. Although the charge excitations in most of the quantum Hall ferromagnet insulators have an exchange energy-determined gap, the magnons/spin waves have a Zeeman energy-determined gap and are the system's lowest energy excitations.
However, magnons do not significantly impact electrical transport as they do not carry electrical charge, which increases the challenges of detecting them. Specifically, the detection of charge-neutral excitations depends on the conversion of magnons into electrical signals in the form of excess holes and electrons in quantum Hall ferromagnets.
However, detecting an electrical signal becomes challenging when excess holes and electrons are equal. Thus, it is crucial for sensitive magnon detection to identify an alternative method that does not depend on the difference between excess hole and electron signals.
The Proposed Approach
In this study, researchers addressed the existing limitation by measuring the magnon-generated electrical noise. They utilized the symmetry-broken quantum Hall ferromagnet of the zeroth Landau level (ZLL) in graphene to launch magnons.
Researchers investigated whether electrical noise spectroscopy of magnons could be an effective method for achieving the required sensitivity for magnon detection. Encapsulated devices containing a heterostructure of hexagonal boron nitride, single-layer graphene, and graphite layers were fabricated using the dry transfer pick-up approach.
They established that the device hosts robust symmetry-broken quantum Hall phases and evaluated magnon transport when the bulk filling was maintained at ν = 1/three-quarters filling. Additionally, an edge current was injected through an ohmic contact to generate magnons.
A floating ohmic contact's non-local electrochemical potential was measured when the injected current flowed only in the downstream direction. The floating contact was located upstream from the source contact. All measurements were performed in a cryo-free dilution refrigerator with a 20 mK base temperature.
The standard lock-in technique was employed to measure electrical conductance, while an LCR resonant circuit at a 740 kHz resonance frequency was used to measure noise. The signal was amplified using a homemade preamplifier at 4 K and a room temperature amplifier, and eventually measured by a spectrum analyzer.
Moreover, a theoretical model was formulated in which the noise was produced by equilibration between propagating magnons and edge channels. The model also allowed researchers to pinpoint the ballistic magnon transport regime in the device.
Specifically, researchers modeled the magnon absorption and generation regions as line junctions of length L to compute the non-local resistance, tunneling current, and noise generated in the magnon absorption regions. These line junctions were modeled as extended segments containing two co-propagating edge channels where electrons tunnel along a series of tunnel junctions.
Importance of the Work
This study highlights the sensitivity of electrical noise spectroscopy in detecting magnons in a quantum Hall ferromagnet. No non-local signal was detectable when the injection contact's bias voltage corresponded to an energy lower than the Zeeman energy. However, a finite non-local signal was observed for negative bias voltages when the bias energy exceeded the Zeeman energy.
Interestingly, the non-local signal remained zero for positive bias voltages, indicating that magnons were not produced in this bias regime. Although no noise was detected below the Zeeman energy, the noise increased for both positive and negative bias voltages when the bias energy exceeded the Zeeman energy.
Noise contributions generated due to magnon absorption at various corners in the devices were additive, even when the average hole and electron currents were mutually canceled. This finding underscores the high sensitivity of electrical noise spectroscopy as a magnon detection tool.
Moreover, the theoretically calculated noise efficiently matched the experimental data, indicating that the detected noise resulted from an increase in the system's effective temperature due to equilibration between magnons and edge channels.
In summary, this study demonstrates the effectiveness of electrical noise spectroscopy as a highly sensitive method for detecting magnons in a quantum Hall ferromagnet.
Journal Reference
Kumar, R. et al. (2024). Electrical noise spectroscopy of magnons in a quantum Hall ferromagnet. Nature Communications, 15(1), 1-9. https://doi.org/10.1038/s41467-024-49446-z,https://www.nature.com/articles/s41467-024-49446-z
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Article Revisions
- Jun 20 2024 - Title changed from "Sensitive Magnon Detection via Electrical Noise Spectroscopy" to "Quantum Hall Ferromagnet Magnon Detection Via Noise Spectroscopy".