Magnetism control with enhanced hard magnetic temperature in heterostructures based on the van der Waals magnet ${\mathrm{Fe}}_{5}{\mathrm{GeTe}}_{2}$

Magnetism control with enhanced hard magnetic temperature in heterostructures based on the van der Waals magnet ${Fe}_{5} {GeTe}_{2}$

Chunsheng Wang, Kejia Zhu, Yupeng Ma, Longyu Lu, Tao Hu, Liang Li, Xucai Kan, Yimin Xiong, Yingying Zhang, Guopeng Wang, Zhengcao Li, and Mingliang Tian

Phys. Rev. B 108, 094408 – Published 5 September 2023

Abstract

The discovery of two-dimensional magnets with intrinsic ferromagnetic or antiferromagnetic orders has opened up promising prospects for exploring magnetism at the desired thickness, paving the way for novel spintronic applications. In practice, the manipulation of the magnetic state in van der Waals ferromagnets is highly demanding but remains a significant challenge. In this work, we demonstrate that the magnetic states of the ${Fe}_{5} {GeTe}_{2}$ (FGT)-based heterostructures can be substantially tuned through a natural oxidation process. During this process, the exchange bias effect is observed in the naturally oxidized FGT flakes, confirming the existence of exchange coupling between the ferromagnetic FGT layer and antiferromagnetic oxide layer. Through the exchange coupling, the formed oxide layer affects the magnetic ordering of the Fe1 sites and magnetic anisotropy, which is evidenced by the increment of hard magnetic temperature. Theoretical calculations shed light on the crucial role of the oxide layer in the magnetic state transitions. These results provide valuable insights into the understanding and manipulation of the emergent magnetic states, leading to significant advances in the realization of practical two-dimensional spintronics.

Received 13 April 2023
Revised 1 August 2023
Accepted 14 August 2023

DOI:https://doi.org/10.1103/PhysRevB.108.094408

Physics Subject Headings (PhySH)

Hall effect Magnetic coupling Magnetic interactions Magnetic phase transitions

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Chunsheng Wang^1,*, Kejia Zhu ^1,*, Yupeng Ma^1,*, Longyu Lu², Tao Hu², Liang Li³, Xucai Kan², Yimin Xiong^1,4, Yingying Zhang^5,†, Guopeng Wang^1,‡, Zhengcao Li⁵, and Mingliang Tian^1,§

¹School of Physics and Optoelectronics Engineering, Anhui University, Hefei 230601, China
²School of Materials Science and Engineering, Anhui University, Hefei 230601, China
³Key Laboratory of Materials Physics, Anhui Key Laboratory of Nanomaterials and Nanotechnology, Institute of Solid State Physics, Hefei Institutes of Physical Science, Chinese Academy of Sciences, Hefei 230031, China
⁴Hefei National Laboratory, Hefei 230028, China
⁵State Key Laboratory for New Ceramics and Fine Processing, Key Laboratory of Advanced Materials of Ministry of Education, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China

^*These authors contributed equally to this work.
^†Corresponding author: 20239@ahu.edu.cn
^‡Corresponding author: zyy2023@tsinghua.edu.cn
^§Corresponding author: mltian@ahu.edu.cn

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Vol. 108, Iss. 9 — 1 September 2023

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Figure 1
(a) Schematic illustration of the FGT oxidation process. (b) Cross-sectional HAADF-STEM image and the corresponding EDS elemental mapping of oxidized FGT (FGT-O) flakes with a gold passivation layer. From the STEM image, the thickness of the oxide layer (the region between the FGT and gold layer) is about 4 nm. (c) Normalized in-plane magnetization as a function of magnetic field ( $H$ //ab) for the FGT and oxidized FGT-O crystals at $T = 2 K$ and $T = 150 K$ (inset).
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Figure 2
EB effect in oxidized FGT flakes. (a) Anomalous Hall resistance ( $R_{xy}$ ) as a function of an external magnetic field for 14 nm-O15 flake under positive (PFC, $H_{cool} = 9 T$ ) and negative cooling field (NFC, $H_{cool} = - 9 T$ ). (b) EB field ( $H_{E}$ ) as a function of temperature for 14 nm-O15 and 14 nm-O30 flakes. (c) Cooling field $H_{cool}$ (PFC) dependence of $| H_{E} |$ for 14 nm-O15 and 14 nm-O30 flakes measured at $T = 2 K$ . (d) Thickness-dependent $| H_{E} |$ for FGT annealed with 30 min under PFC. The green square region represents the presence of EB in the oxidized FGT with the thickness ≤ 14 nm, and the red triangle region represents the absence of EB (No EB) in thicker oxidized FGT flakes.
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Figure 3
Anomalous Hall effect measurements performed on FGT and oxidized FGT flakes. $R_{xy}$ as a function of external magnetic field for the (a) 14 nm and (b) 14 nm-O15 flakes at various temperatures. (c) Remnant anomalous Hall resistance $R_{xy}^{r}$ as a function of temperature obtained from FGT and oxidized FGT flakes. $R_{xy}^{r}$ is normalized by its values at $T = 2 K$ . The black arrows highlight magnetic transitions temperature at $T_{m 1}$ and $T_{m 2}$ . (d) $T_{R}, T_{m 1}$ , and $T_{m 2}$ . The blue dash line indicates the transition temperature $T_{1}$ obtained from the $M − T$ curves of FGT bulk crystal.
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Figure 4
First-principles calculation of FGT slab models. (a) Structural configurations of FGT, FGT-V1, FGT-O1, and ${Fe}_{2} O_{3}$ /FGT slab models. FGT represents 3 uc FGT slab model. FGT-V1 and FGT-O1 represent 3 uc FGT slab models with the top tellurium atoms gone and substituted by oxygen atoms, respectively. ${Fe}_{2} O_{3}$ /FGT is constructed using 1 uc ${Fe}_{2} O_{3}$ and 3 uc FGT. (b) Calculated magnetic moment for FGT and oxidized FGT. The inset is the spin-resolved density of states (DOS) of FGT around Fermi energy. (c) Calculated magnetic anisotropy energy (MAE) and bond distance of Fe1 and Fe3 ( $D_{Fe 1 - Fe 3}$ ) for FGT and oxidized FGT. The $D_{Fe 1 - Fe 3}$ of FGT is set to 0 Å as a reference. The inset shows the diagram of $D_{Fe 1 - Fe 3}$ . (d) The schematic diagram of oxidation gradient along the $c$ axis of the entire flake. The oxide layer is divided into two sections: section 1 with strong oxidation and section 2 with low oxidation.
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Physical Review B

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Magnetism control with enhanced hard magnetic temperature in heterostructures based on the van der Waals magnet ${Fe}_{5} {GeTe}_{2}$

Chunsheng Wang, Kejia Zhu, Yupeng Ma, Longyu Lu, Tao Hu, Liang Li, Xucai Kan, Yimin Xiong, Yingying Zhang, Guopeng Wang, Zhengcao Li, and Mingliang Tian

Phys. Rev. B 108, 094408 – Published 5 September 2023

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Physical Review B

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Magnetism control with enhanced hard magnetic temperature in heterostructures based on the van der Waals magnet Fe5GeTe2

Chunsheng Wang, Kejia Zhu, Yupeng Ma, Longyu Lu, Tao Hu, Liang Li, Xucai Kan, Yimin Xiong, Yingying Zhang, Guopeng Wang, Zhengcao Li, and Mingliang Tian

Phys. Rev. B 108, 094408 – Published 5 September 2023

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Magnetism control with enhanced hard magnetic temperature in heterostructures based on the van der Waals magnet ${Fe}_{5} {GeTe}_{2}$