Strain-enhanced edge ferromagnetism and bipolar magnetic semiconducting behavior in Janus graphene nanoribbons

TL;DR

This work addresses graphene's lack of intrinsic magnetism by proposing Janus graphene nanoribbons with edge-defect arrays (m=2) that host intrinsic edge ferromagnetism. Using a hybrid approach that combines density functional theory (DFT) and determinant quantum Monte Carlo (DQMC) simulations, the authors map the magnetic and electronic properties across widths $W=2$–$6$ and examine uniaxial tensile strain as a control parameter. They find robust ferromagnetic ground states with bandgaps exceeding $200$ meV for all studied widths; under $25\%$ strain, the Curie temperature reaches $T_c \approx 222$ K, and beyond about $10\%$ strain the VBM/CBM spin characters invert, realizing a strain-tunable bipolar magnetic semiconductor. The results are stabilized by HSE06 validation and χ measurements, and mechanical analysis yields an elastic limit of ~$25\%$ with $Y_M \approx 145$ GPa, highlighting $W=5$ as a particularly promising candidate for room-temperature 1D spintronic devices.

Abstract

Using first-principles density functional theory and determinant quantum Monte Carlo methods, we show that Janus graphene nanoribbons with topological defect arrays ($m=2$) exhibit robust intrinsic ferromagnetism across widths $W=2-6$, with bandgaps exceeding 200 $meV$ and stable ferromagnetic ground states. Notably, uniaxial tensile strain significantly enhances their ferromagnetic properties: at 25\% strain, the Curie temperature increases to $222K$, a fivefold improvement over unstrained systems and the highest reported for graphene-based nanoribbons. Strain also induces a reversible transition to a bipolar magnetic semiconductor, with spin-flipped valence and conduction band edges beyond 10\% strain. This dual functionality, strain-enhanced ferromagnetism and strain-induced spin flip, stems from strain-modulated $p_{z}$ orbital hybridization and strong direct exchange interaction. Among these, $W=5$ Janus graphene nanoribbons emerge as potential candidates for room-temperature spintronic devices and strain-programmable quantum transport systems.

Figures (7)

• Figure 1: (a) Shows Janus graphene nanoribbons (JGNRs) with $m = 2$ and $W = 2$. (b) Shows JGNRs with $m = 2$ and $W = 5$. Here, m represents the number of defective benzene rings and W represents the width of the nanoribbon.
• Figure 2: (a) Total bandgap, (b) local magnetic moment at zigzag edge, (c) Energy difference (FM-NM), and (d) Energy difference (FM-AFM) as a function of JGNRs width. The inset figures in panels (c) and (d) are schematic diagrams of the FM and AFM states, respectively.
• Figure 3: (a) and (b) The spin-polarized energy bands and spin-polarized density of states of the JGNRs for $m = 2$ and $W = 5$, respectively, where the inset of (b) shows the spin charge density of the corresponding structure. (c)-(f) Spin-polarized energy bands of the JGNRs for $m = 2$, $W = 2,3,4,6$, respectively.
• Figure 4: (a) and (b) represent the stress-strain curves for ZGNRs (W=5) and JGNRs (W=5), respectively. (c) shows the strain dependence of the local magnetic moment at the zigzag edge. (d) illustrates the energy difference (FM-AFM) as a function of strain.
• Figure 5: (a)-(f) Magnetic moments and magnetic susceptibility after applying 0%, 5%, 10%, 15%, 20%, and 25% uniaxial tensile strains to JGNRs with $m = 2$ and $W = 5$, respectively.