Graphene-modulated interfacial exchange coupling across organic molecular/ferromagnet spin interfaces

Yu Wang, Zheng Wang, and Xiaoguang Li

Phys. Rev. B 109, 014428 – Published 26 January 2024

Abstract

Magnetic molecules on ferromagnetic metallic substrates have been widely explored to exploit the potential for molecular magnetic storage and spintronics applications. Recent advances in these hybrid interfaces integrated with two-dimensional materials have been proposed as a flexible platform for realizing new spin-related effects. Herein, the impact of inserting graphene on the electronic and magnetic properties of a family of transition metal phthalocyanines (TMPcs, TM = Cr, Mn, Fe, Co, and Cu) deposited on ferromagnetic Ni(111) surfaces have been systematically rationalized by density functional theory analysis. Our calculations reveal that the magnetic exchange interaction across the molecule-substrate interfaces can be significantly mediated by the introduction of a graphene interlayer. Interestingly, these TMPcs exhibit ferromagnetic coupling with the Ni substrate. However, the strength of this coupling is reduced in the presence of a graphene decoupling layer, with the exception of CoPc. In the case of CoPc, the original ferromagnetic coupling with Ni(111) can be altered to antiferromagnetic when a graphene interlayer is introduced. By analyzing the different channels of communication involved in the spin interaction between the molecule and the magnetic substrate, we attribute these significant differences to the varied influences on the exchange interaction caused by the intermediary graphene layer. The presence of the inserted graphene layer may block the direct exchange interaction between the TMPc molecule and substrate, while the indirect superexchange interaction facilitated by the nitrogen atoms of the organic ligands is only reduced. Our study thus demonstrates that the inserted graphene can serve as an optimal intermediary layer for mediating the magnetic couplings across the molecule-substrate interfaces while allowing effective spin communication between them. These findings provide important insights into relevant experiments and offer a promising strategy to control the magnetic exchange interactions via utilizing graphene at metal-molecule interfaces.

Received 1 August 2023
Revised 28 November 2023
Accepted 10 January 2024

DOI:https://doi.org/10.1103/PhysRevB.109.014428

Physics Subject Headings (PhySH)

Exchange interaction Magnetic interactions Magnetic order Molecular magnetism Spintronics

Density functional theory

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Yu Wang , Zheng Wang , and Xiaoguang Li ^*

Institute for Advanced Study, Shenzhen University, Shenzhen 518060, China

^*xgli@szu.edu.cn

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Vol. 109, Iss. 1 — 1 January 2024

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Images

Figure 1
(a) The chemical structure of a TMPc molecule. (b) Typical adsorption sites for single TMPc molecule on Gr/Ni(111). The labels 1, 2, 3, and 4 represent four high-symmetry positions within graphene, i.e., TOP1, TOP2, HEX, and Bridge, corresponding to the TM ion of the TMPc molecule located above two inequivalent C atoms, the hexagon center and above the center of a C–C bond, respectively. The graphene layer has top-fcc stacking with respect to the Ni(111) surface. (c) Top and side views of TMPc/Gr/Ni(111) with the bridge adsorption conformation. The Ni(111) substrate is omitted for a clear comparison in the upper figure.
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Figure 2
The three panels (left to right) show the $d$ -projected DOS of Co and $s p$ -projected DOS of the N atoms in the (a) free CoPc, (b) the Bridge absorption configuration of CoPc/Ni(111), and (c) the Bridge absorption configuration of CoPc/Gr/Ni(111). We also show the out-of-plane $d$ -projected DOS ( $d_{z^{2}} + d_{π}$ ) of the Ni atom in topmost layer in (b) and (c), and the $s p$ -projected DOS on the two different (by symmetry) C atoms of graphene in (c). The red vertical dash lines indicate the Fermi energy $E_{F}$ that is set to zero. (d)–(f): top and side views of the system's spin density distributions. Blue (red) denotes positive (negative) spin density. The isosurface level for the spin density is $0.005 {Bohr}^{- 3}$ . The spin densities are analyzed by vaspkit [114] and rendered by vesta [115].
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Figure 3
The two panels show schematic diagrams of the interaction mechanism in TMPc/Ni(111) and TMPc/Gr/Ni(111) composites (TM=Co, Cu). (a) The half-filled out-of-plane $d_{z^{2}}$ orbital of the Co ion can directly interact with the out-of-plane orbitals of the Ni substrate, or indirectly with the out-of-plane orbitals of the Ni substrate via the $N − p$ orbital of the organic ligands. (b) The half-filled out-of-plane $d_{z^{2}}$ orbital of the Co ion indirectly interacts with the comparable Gr/Ni(111) substrate by the aid of the N atoms of the organic ligands. (c) The half-filled in-plane $d_{x^{2} - y^{2}}$ orbital of the Cu ion indirectly interacts with the out-of-plane orbitals of the Ni substrate via the $N − p$ orbitals of the organic ligands. (d) The half-filled in-plane $d_{x^{2} - y^{2}}$ orbital of the Cu ion participates in the indirect superexchange interaction with the comparable Gr/Ni(111) substrate by the aid of the $N − p$ orbitals of the organic ligands. Inset images in (a)–(d) show the involved magnetic $TM − d$ orbitals. Box in (b) and (d) denotes the comparable metallic substrate consisting of Gr/Ni(111).
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Figure 4
The three panels (left to right) show the spin-polarized PDOS of the molecule in the (a) free CuPc, (b) the bridge configuration of CuPc/Ni(111), and (c) the bridge configuration of CuPc/Gr/Ni(111). (a)–(f) adopt the same convention as in Fig. 2.
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