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Peer-reviewedCC BY 4.0Physical Review Research

Direct observation of thermal spin depolarization in a type IB half-metal NiCo2O4 by spin-resolved photoemission

  • Hirokazu Fujiwara1
  • Tomoki Higashikawa2
  • Enda Xiao3
  • Terumasa Tadano3
  • Lingling Xie4
  • Daisuke Kan4
  • Yuichi Shimakawa4
  • Kaishu Kawaguchi1
  • Ryo Mori1,5
  • Ayumi Harasawa1
  • Takeshi Kondo1
  • Takayoshi Yokoya2
5 affiliations
  1. 1The University of Tokyo
  2. 2Okayama University
  3. 3National Institute for Materials Science
  4. 4Kyoto University
  5. 5University of Maryland, College Park
Open at sourceDOICite

Abstract

Half-metals (HMs), characterized by 100% spin polarization at the Fermi level ( E F ), are ideal candidates for future spintronic memory devices. However, their spin polarization rapidly decreases with temperature, limiting their practical implementation. While this degradation has been theoretically attributed to nonquasiparticle (NQP) states—many-body electronic states arising from electron-magnon interactions that reduce spin polarization near E F —experimental verification remains limited. Here, we report the temperature evolution of the electronic states in epitaxial Ni Co 2 O 4 (NCO) thin films, a predicted type I B HM where the main spectral weight of NQP states lies below E F . Using high-resolution bulk-sensitive laser-based spin-resolved photoemission spectroscopy, we reveal that at 20 K, a metallic Fermi edge is observed in the minority-spin states but is absent in the majority-spin states, providing spectroscopic evidence for the half-metallicity of NCO. As temperature increases, majority-spin spectral weight emerges at E F , and spin polarization near E F decreases—consistent with the emergence of NQP states. The onset of this behavior occurs around 100 K. Our results suggest that NCO provides a promising platform for investigating NQP-related many-body effects in type I B HMs, guiding the design of spintronic materials with robust high-temperature spin polarization.

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Cite

BibTeXRIS
BibTeX
@article{fujiwara2026direct,
  title = {{Direct observation of thermal spin depolarization in a type $\mathrmI_\mathrmB$ half-metal $\mathrmNi\mathrmCo_2\mathrmO_4$ by spin-resolved photoemission}},
  author = {Hirokazu Fujiwara and Tomoki Higashikawa and Enda Xiao and Terumasa Tadano and Lingling Xie and Daisuke Kan and Yuichi Shimakawa and Kaishu Kawaguchi and Ryo Mori and Ayumi Harasawa and Takeshi Kondo and Takayoshi Yokoya},
  journal = {Physical Review Research},
  year = {2026},
  month = oct,
  doi = {10.1103/9hsj-vmwn},
  url = {http://link.aps.org/doi/10.1103/9hsj-vmwn},
  note = {CC BY 4.0},
}
RIS
TY  - JOUR
TI  - Direct observation of thermal spin depolarization in a type I_B half-metal NiCo_2O_4 by spin-resolved photoemission
AU  - Hirokazu Fujiwara
AU  - Tomoki Higashikawa
AU  - Enda Xiao
AU  - Terumasa Tadano
AU  - Lingling Xie
AU  - Daisuke Kan
AU  - Yuichi Shimakawa
AU  - Kaishu Kawaguchi
AU  - Ryo Mori
AU  - Ayumi Harasawa
AU  - Takeshi Kondo
AU  - Takayoshi Yokoya
PY  - 2026
DA  - 2026/10/29
JO  - Physical Review Research
DO  - 10.1103/9hsj-vmwn
UR  - http://link.aps.org/doi/10.1103/9hsj-vmwn
AB  - Half-metals (HMs), characterized by 100% spin polarization at the Fermi level ( E F ), are ideal candidates for future spintronic memory devices. However, their spin polarization rapidly decreases with temperature, limiting their practical implementation. While this degradation has been theoretically attributed to nonquasiparticle (NQP) states—many-body electronic states arising from electron-magnon interactions that reduce spin polarization near E F —experimental verification remains limited. Here, we report the temperature evolution of the electronic states in epitaxial Ni Co 2 O 4 (NCO) thin films, a predicted type I B HM where the main spectral weight of NQP states lies below E F . Using high-resolution bulk-sensitive laser-based spin-resolved photoemission spectroscopy, we reveal that at 20 K, a metallic Fermi edge is observed in the minority-spin states but is absent in the majority-spin states, providing spectroscopic evidence for the half-metallicity of NCO. As temperature increases, majority-spin spectral weight emerges at E F , and spin polarization near E F decreases—consistent with the emergence of NQP states. The onset of this behavior occurs around 100 K. Our results suggest that NCO provides a promising platform for investigating NQP-related many-body effects in type I B HMs, guiding the design of spintronic materials with robust high-temperature spin polarization.
SN  - 2643-1564
ER  -