Electric and thermoelectric properties of antiferromagnetic metals

Wu, Junyi

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https://hdl.handle.net/2142/132690 Copy

Description

Title

Electric and thermoelectric properties of antiferromagnetic metals

Author(s)

Wu, Junyi

Issue Date

2025-12-05

Director of Research (if dissertation) or Advisor (if thesis)

• Lorenz, Virginia O
• Cahill, David G

Doctoral Committee Chair(s)

Hoffman, Axel

Committee Member(s)

Rakheja, Shaloo

Department of Study

Physics

Discipline

Physics

Degree Granting Institution

University of Illinois Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• antiferromagnet
• thermoelectrics

Abstract

Metallic antiferromagnetic materials are promising candidates for next-generation spintronic technologies because of their robustness against external magnetic field, absence of stray fields and THz spin dynamics. However, reading the order parameter of antiferromagnets (AFs) has always been difficult because of the lack of net magnetization. In this thesis, we investigated collinear and non-collinear antiferromagnetic metals that share similar magnetic structures to materials predicted to exhibit electrical switching. we demonstrated experimentally that magneto-transport and thermoelectric measurements could serve as effective readout mechanisms of AFM order. In addition to the readout methods, we identified materials that provide large readout signals linked to their magnetic order. First, I used anisotropic magnetoresistance (AMR) to probe the Néel vector orientation in the collinear AF Fe$_2$As. AMR depends on the relative direction of magnetization with respect to the current, but is even in magnetization and is therefore present in AF. In single-crystal Fe$_2$As, we observed a large AMR response associated with Néel vector reorientation, reaching 1.3$\%$ at 5 K. This AMR is an order of magnitude larger than previously reported for comparable easy-plane AFs. The field dependence of the AMR signal confirms Fe$_2$As is in single-domain state with saturation fields less 1 T. The magnitude of the AMR shows a strong temperature dependence. Next, for noncollinear systems, I developed a second-harmonic voltage method for detection of the anomalous Nernst effect (ANE). The ANE is a transverse thermoelectric effect in which an electric voltage is generated perpendicular to both the magnetization and the applied temperature gradient. In some topologically nontrivial noncollinear AFs, a sizable ANE emerges even in the absence of net magnetization, providing a direct probe of the magnetic order parameter. Beyond its value as a readout mechanism, the ANE is of practical interest because materials with large ANE coefficients are promising for heat-flux sensing and thermoelectric energy harvesting. The second harmonic technique basically uses joule heating to generate an out-of-plane temperature gradient, and use lock-in detection with a balanced bridge circuit to separate ANE signal in the second harmonics from longitudinal magnetoresistance. The magnitude of the temperature gradient can be calculated from the dissipated power and the thermal conductivity of the sample. This technique is first validated in ferromagnetic film, such as Ni and Ni–Fe–Pt alloy films, where a temperature-dependent ANE signal is observed. We observed a moderate ANE coefficient for Ni$_3$Pt of 0.4 $\mu$V/K at 300 K and peaks near 0.6 $\mu$V/K at 200 K. We then shifted the ANE peak towards room temperature by tuning the composition of Ni$_{3-x}$Fe${_x}$Pt alloy. Finally, I applied this second-harmonic method to the chiral AF Weyl metal Mn$_3$Sn thin film, and correlated the ANE with its magnetic phase transitions. Non-collinear AF Mn$_3$Sn has a non-zero Berry curvature from time reversal symmetry breaking, and therefore have a large anomalous Hall effect (AHE) and large ANE at room temperature. A large zero-field ANE is observed in the inverse-triangular spin state and collapses upon entering the spiral phase, demonstrating that the ANE directly tracks the noncollinear AFM order rather than the small net magnetization. The ANE reaches 0.11 $\mu$V/K at 300 K for polycrystalline Mn$_3$Sn, exceeding the conventional scaling expected from magnetization-driven ANE. To sum up, this thesis uses AMR and ANE as probes of antiferromagnetic order. These results provide practical electrical and thermoelectric pathways for detecting and characterizing AF spin structures, advancing the development of AFM-based spintronic devices.

Graduation Semester

2025-12

Type of Resource

Thesis

Handle URL

https://hdl.handle.net/2142/132690

Copyright and License Information

Copyright 2025 Junyi Wu

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