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Altermagnetic (AM) materials have recently attracted significant interest due to their non-relativistic momentum-dependent spin splitting of their electronic band structure which may be useful for antiferromagnetic (AFM) spintronics. So far, however, most research studies have been focused on conducting properties of AM metals and semiconductors, while functional properties of AM insulators have remained largely unexplored. Here, we propose employing AM insulators (AMIs) as efficient spin-filter materials. By analyzing the complex band structure of rutile-type altermagnets MF2 (M = Fe, Co, Ni), we demonstrate that the evanescent states in these AMIs exhibit spin- and momentum-dependent decay rates resulting in momentum-dependent spin polarization of the tunneling current. Using a model of spin-filter tunneling across a spin-dependent potential barrier, we estimate the TMR effect in spin-filter magnetic tunnel junctions (SF-MTJs) that include two magnetically decoupled MF2 (001) barrier layers. We predict a sizable spin-filter TMR ratio of about 150-170% in SF-MTJs based on AMIs CoF2 and NiF2 if the Fermi energy is tuned to be close to the valence band maximum. Our results demonstrate that AMIs provide a viable alternative to conventional spin-filter materials, potentially advancing the development of next-generation AFM spintronic devices.
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Spin-Filtered Tunneling Device Using a Topological Insulator
There has been much interest in the study of topological insulators (TI) recently. Due to their unique electronic structure, these new materials have been an active area of research to discover new quantum phenomena and their application in new technologies. Unlike the electronic structure observed in traditional semiconductors, the strong spin-orbit coupling induces a band inversion in the electronic structure of TIs. One of the side effects of this band inversion is creating metallic-like surface states at the material's surface that are protected by time invariance and whose spin angular momentum is locked to the direction of the momentum of the electron. These surface states are essentially resistant to scattering events that otherwise affect other materials. Leveraging the characteristic scattering resistance, the spin-momentum locking of the surface states, and the Dirac cone structure, a spin-resonant tunneling diode using topological insulators has been investigated to implement a negative differential resistance device. Utilizing the spin texture of the surface states, an additional spin-filter can help to suppress the valley current in a negative differential resistance device. In the spin-resonant tunneling diode, the tunneling process would also benefit from having protection from conventional scattering processes due to defects and thickness or line edge roughness. This research is focused on the manufacturing of a spin-filtered tunnel diode. Using molecular beam epitaxy to grow a three-layer heterostructure, with two layers of bismuth selenide as the topological insulator separated by a thin layer of tungsten diselenide as a tunnel barrier. The alignment of the Fermi levels of the topological insulator layers and the thickness of the tunnel barrier were investigated using X-ray Photoelectron Spectroscopy. The fabrication and initial electrical measurements of the spin-filtered tunnel diode were also investigated.
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- Award ID(s):
- 1917025
- PAR ID:
- 10281809
- Date Published:
- Journal Name:
- Masters Thesis of Engineering
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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