🔬 mesoscale physics

Electrothermal control of spin-reorientation transition in Co/Fe_3GaTe_2 heterostructures

This study demonstrates reversible electrothermal control of a spin-reorientation transition in Co/Fe₃GaTe₂ heterostructures, where Joule heating modulates the competition between the metallic Co overlayer and the van der Waals magnet to enable heat-assisted, device-level manipulation of magnetic anisotropy.

Original authors: Po-Wei Chen (Department of Physics, National Taiwan Normal University, Taipei 11677, Taiwan, Department of Physics, The University of Osaka, Toyonaka, Osaka, Japan, Institute of Physics, Academia Sini

Published 2026-07-20

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CC BY 4.0

Original authors: Po-Wei Chen (Department of Physics, National Taiwan Normal University, Taipei 11677, Taiwan, Department of Physics, The University of Osaka, Toyonaka, Osaka, Japan, Institute of Physics, Academia Sinica, Taipei, Taiwan), Ming-Hsien Hsu (Department of Physics, National Taiwan Normal University, Taipei 11677, Taiwan), Cheng-Ying Hsiao (Department of Physics, National Taiwan Normal University, Taipei 11677, Taiwan), Ming-Yang Ho (Department of Physics, National Taiwan Normal University, Taipei 11677, Taiwan), Masahiro Haze (Institute for Solid State Physics, The University of Tokyo, Kashiwa, Chiba, Japan), Yan-Ru Chu (Department of Physics, National Taiwan Normal University, Taipei 11677, Taiwan), Yu-Cheng Shao (National Synchrotron Radiation Research Center, Hsinchu, Taiwan), Po-Chun Chang (Department of Physics, Tamkang University, New Taipei City, Taiwan), Chen-Yu Ou (Department of Physics, National Taiwan Normal University, Taipei 11677, Taiwan), Ruei Chen (Department of Physics, National Taiwan Normal University, Taipei 11677, Taiwan), Ko-Fan Chen (Department of Physics, National Taiwan Normal University, Taipei 11677, Taiwan), Chung-Ting Ke (Institute of Physics, Academia Sinica, Taipei, Taiwan), Chao-Hung Du (Department of Physics, Tamkang University, New Taipei City, Taiwan), Yukio Hasegawa (Institute for Solid State Physics, The University of Tokyo, Kashiwa, Chiba, Japan), Wen-Chin Lin (Department of Physics, National Taiwan Normal University, Taipei 11677, Taiwan), . (Department of Physics, The University of Osaka, Toyonaka, Osaka, Japan)

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

Imagine the world of tiny magnets as a bustling city where information is stored in the direction a magnetic arrow points. In modern electronics, we want to flip these arrows quickly and efficiently to write data, but doing so usually requires massive amounts of electricity, which generates heat and wastes energy. Scientists are on a quest to find "smart" magnets that can be controlled with a gentle nudge rather than a heavy shove. This is where a concept called "magnetic anisotropy" comes in. Think of anisotropy as the magnet's favorite direction: some magnets love to point straight up (out-of-plane), while others prefer to lie flat (in-plane). Usually, a magnet is stubborn and sticks to its favorite direction no matter what. However, if you can find a way to make the magnet "soften up" or change its mind about which direction is best, you could switch its state with very little energy. This paper explores a clever way to do exactly that by playing a game of tug-of-war between two different magnetic materials.

The researchers in this study built a special sandwich using two magnetic ingredients: a layer of Cobalt (Co) and a flake of a material called Fe3GaTe2 (pronounced "Fay-Ga-Te-two," or FGaT for short). FGaT is a "van der Waals" magnet, which is a fancy way of saying it's made of atomically thin sheets that stick together like sticky notes, rather than being welded into a solid block. The scientists wanted to see what would happen if they let these two magnets fight over which direction the combined system should point. They discovered that by simply heating the sandwich—either with a hot plate or by running a tiny electric current through it—they could make the FGaT magnet get "tired" and lose its grip on the "up" direction. As the FGaT weakens, the Cobalt layer takes over and forces the whole system to flip and lie flat.

The team found that this switch happens at a specific "reorientation temperature" of about 311 Kelvin (roughly 38°C or 100°F), which is just a bit warmer than a typical room. When they heated the device, the magnetic loop (a graph showing how the magnet behaves) changed from a sharp, square shape (indicating a strong "up" preference) to a flat, sloping line (indicating a "flat" preference). Crucially, they proved this wasn't just the FGaT melting or losing its magnetism entirely; the Cobalt layer was actively pulling the system in a new direction because the FGaT had weakened. They even built a working device where they could control this switch using electricity. By sending a power input between 80 and 100 milliwatts, they could reversibly flip the magnet's favorite direction back and forth five times in a row without breaking anything.

To make sure this was truly a magnetic switch and not just a chemical change, they looked at the atoms with powerful microscopes and X-rays. They found that the atoms didn't rearrange or change their chemistry; the material was just getting softer magnetically. They also tested a version of the device without the Cobalt layer and found that the electricity could still lower the energy needed to flip the magnet's direction, acting like a "power-tunable" barrier. This suggests that by using a little bit of heat (Joule heating) generated by electricity, we can create a device that controls magnetic states efficiently. The paper suggests this could be a stepping stone for future "spintronic" devices that use heat-assisted writing to store data more efficiently, though the authors note that more work is needed to turn this into a fast, practical technology.

Technical Summary: Electrothermal Control of Spin-Reorientation Transition in Co/Fe3GaTe2 Heterostructures

Problem Statement
Electrical control of magnetic anisotropy in van der Waals (vdWs) magnets is a critical objective for developing reconfigurable two-dimensional spintronics. While electric fields can tune carrier density and interfacial properties, the specific dynamics of how a conventional metallic ferromagnet competes with a vdWs magnet across a direct interface remain largely unexplored. Specifically, the behavior of a Co/Fe3GaTe2 (FGaT) bilayer, where a standard ferromagnet (Co) interacts with a robust room-temperature vdWs ferromagnet (FGaT), requires investigation to understand if anisotropy competition can be driven by electrothermal effects to induce a spin-reorientation transition (SRT).

Methodology
The study employed a multi-modal characterization approach combining atomic-scale imaging, magnetic microscopy, and magneto-optical measurements:

Sample Preparation: High-quality Fe3GaTe2 single crystals were grown via chemical vapor transport. FGaT flakes were mechanically exfoliated onto substrates. Cobalt (Co) layers were deposited in situ using electron-beam evaporation within an ultra-high vacuum (UHV) environment to ensure oxide-free interfaces.
Atomic-Scale Characterization: Low-temperature Scanning Tunneling Microscopy (STM) was used to characterize the cleaved FGaT surface (identifying subsurface Fe-deficiency signatures) and the growth morphology of Co (tracking the evolution from Volmer-Weber clusters to quasi-continuous layers upon annealing).
Magnetic Imaging: Magnetic Force Microscopy (MFM) was utilized to visualize domain textures (stripe vs. bubble domains) under zero-field cooling (ZFC) and field cooling (FC) conditions. Polar Magneto-Optical Kerr Effect (MOKE) microscopy was employed for both temperature-dependent and power-dependent hysteresis loop measurements.
Device Fabrication and Testing: A multilayer device structure (AlOx/Pt/Pd/Co/FGaT/SiO2) was fabricated. Electrical bias was applied in a two-terminal configuration. Due to leakage through the top oxide layer, the electrical input was treated as an electrothermal drive (Joule heating) rather than a purely electrostatic gate. Control devices without the Co layer were also tested to isolate the response of the FGaT layer.
Structural Analysis: X-ray Absorption Spectroscopy (XAS) was performed to verify that thermal cycling across the Curie temperature ( $T_C$ ) did not induce irreversible changes in the electronic structure or local atomic arrangement of FGaT.

Key Results

Interface and Morphology: STM revealed that Co initially nucleates as isolated nanoscale clusters on the Te-terminated FGaT surface due to geometric mismatch and weak van der Waals interactions. Annealing at 180°C induced lateral spreading and coalescence, creating a quasi-continuous, lattice-guided Co coverage. STM also identified subsurface Fe-deficiency signatures consistent with Dzyaloshinskii-Moriya interaction (DMI) active disorder.
Temperature-Driven SRT: Temperature-dependent MOKE measurements on the Co/FGaT heterostructure demonstrated a reversible spin-reorientation transition. At low temperatures (297.5 K), the system exhibited a square out-of-plane (OOP) hysteresis loop dominated by FGaT's perpendicular magnetic anisotropy (PMA). As temperature increased, the OOP loop became less square, and the in-plane (IP) response became more square.
- The transition was centered at a reorientation temperature ( $T_R$ ) of approximately 311 K, well below the bulk FGaT $T_C$ (350–380 K).
- The nucleation fields ( $H_n$ ) crossed zero at $T_R$ , indicating a shift in the stability of the remanent OOP state.
- The transition was reversible over five thermal cycles, with no evidence of magnetic degradation.
Electrothermal Control: In the device geometry, Joule heating induced a nearly identical evolution of the hysteresis loops.
- Increasing input power (0 to ~254 mW) caused the OOP loop to collapse and the IP loop to become square, mirroring the temperature-driven transition.
- The critical power window for the transition (where $H_n$ crosses zero and remanence collapses) was identified between 80–100 mW.
- Repeated power cycling confirmed the reversibility of the electrothermal switching.
Mechanism Verification:
- Exchange Coupling: The observation of a single abrupt switching step in the OOP loops, rather than two separate steps, suggests that the Co layer and FGaT layer reverse collectively via interfacial exchange coupling.
- Anisotropy Competition: The transition occurs because Joule heating weakens the PMA of the FGaT layer, allowing the shape anisotropy of the Co overlayer (which favors IP magnetization) to dominate.
- No Structural Change: XAS data confirmed that thermal cycling across $T_C$ produced no detectable irreversible changes in the valence states or local coordination shells of Fe or Ga, ruling out structural or chemical transformation as the primary mechanism.
Domain Nucleation in Co-free Devices: In Co-free FGaT devices, Kerr microscopy showed that Joule heating lowers the domain-nucleation barrier. A power-thresholded, field-assisted magnetization reversal was demonstrated at a threshold near 15 mW, where a fixed assist field could switch the magnetization state once the power exceeded the threshold.

Significance and Claims
The paper claims to demonstrate that electrothermal anisotropy competition serves as a viable route for heat-assisted and device-level control of vdWs magnetism. Key contributions include:

Device-Level Extension: The work extends previous local electrothermal writing schemes (typically performed via conductive AFM) to a device-level geometry using Joule heating.
Reversible Control: It establishes that the dominant magnetic anisotropy in Co/FGaT heterostructures can be reversibly switched between OOP and IP states using electrical power, without requiring large write currents or relying solely on electric fields.
Mechanism Clarification: The study clarifies that the SRT is driven by the competition between the Co overlayer and the FGaT layer mediated by exchange coupling, rather than a loss of ferromagnetism in FGaT or purely electrostatic effects.
Potential for Textures: By linking the power-tunable nucleation barrier to the presence of DMI-active disorder (Fe deficiency), the authors suggest a pathway for the electrothermally assisted nucleation of bubble or skyrmion-like textures, offering a device-level counterpart to local writing methods.

The authors maintain a modest scope, noting that while the correspondence between power and temperature is qualitative (due to the lack of direct temperature measurement during electrical driving), the phenomenological match strongly supports an electrothermal origin. They identify future work, such as pulsed operation to quantify switching dynamics and element-resolved probes to resolve layer-by-layer reorientation, as necessary next steps.

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