Theory of tunnel magnetoresistance in magnetic tunnel junctions with hexagonal boron nitride barriers: mechanism and application to ferromagnetic alloy electrodes

TL;DR

This work addresses the problem of achieving high tunneling magnetoresistance (TMR) in magnetic tunnel junctions (MTJs) with hexagonal boron nitride barriers by uncovering a surface-state-assisted tunneling mechanism. Using first-principles transport calculations, the authors show that a Delta1-like down-spin surface state near the Gamma point enables strong spin-polarized transmission in hcp-Co1-xNi_x/h-BN/hcp-Co1-xNi_x(0001) junctions, and that Ni-doping shifts the Fermi level to concentrate this transmission, effectively realizing Brillouin-zone spin filtering. The interfacial distance between the barrier and electrodes critically modulates the surface-state contribution, with physisorption preserving high TMR while chemisorption suppresses it; the mechanism appears transferable to other 2D barriers. These findings provide a principled route for interface engineering in 2D-barrier MTJs and highlight the potential of surface-state-mediated tunneling in next-generation spintronic devices.

Abstract

Hexagonal boron nitride ($h$-BN), with its strong in-plane bonding and good lattice match to hcp and fcc metals, offers a promising alternative barrier material for magnetic tunnel junctions (MTJs). Here, we investigate spin-dependent transport in hcp-Co$_{1-x}$Ni$_{x}$$/$$h$-BN$/$hcp-Co$_{1-x}$Ni$_{x}$(0001) MTJs with physisorption-type interfaces using first-principles calculations. We find that a high TMR ratio arises from the resonant tunneling of the down-spin surface states of the hcp-Co$_{1-x}$Ni$_{x}$, having a $Δ_1$-like symmetry around the $Γ$ point. Ni doping tunes the Fermi level and enhances this effect by reducing the overlap between up-spin and down-spin conductance channels in momentum space under the parallel configuration, thereby suppressing antiparallel conductance and increasing the TMR ratio. This mechanism is analogous to Brillouin zone spin filtering and is sensitive to the interfacial distance but not specific to $h$-BN barriers; similar behavior may emerge in MTJs with other two-dimensional insulators or semiconductors. These findings provide insight into surface-state-assisted tunneling mechanisms and offer guidance for the interface engineering of next-generation spintronic devices.

Figures (11)

• Figure 1: Supercells of X$/$h-BN$/$X junctions, where X = (a) hcp-Co_1-xNi_x (top), (b) fcc-Co (middle), and (c) $L1_1$-CoNi (bottom). The $c_\mathrm{e}$ denotes the lattice constant of the ferromagnetic electrode $X$ along the $c$ axis. The $d_\mathrm{BN}$ represents the interlayer spacing of the h-BN monolayers, and $d$ is the interfacial distance between h-BN and the $X$ electrode.
• Figure 2: (a) Three-dimensional Brillouin zone of the hexagonal lattice. (b) Two-dimensional surface Brillouin zone projected onto the (0001) plane. (c) Complex band structure of bulk h-BN calculated using a unit cell containing two layers, each composed of one B and one N atom, along several out-of-plane directions: $\Gamma$–A ($\Delta$ line), $\Gamma^*$–A$^*$ ($\Delta^*_1$ line, with $\mathbf{k}_\parallel = (0, 0.1)$), K–H (P line), and L–M (U line). The horizontal axis represents $k^2 = (k_z - k_z^0)^2$faleev2015, where $k_z^0$ is the wave vector corresponding to the lowest conduction-band energy at a given in-plane momentum $\mathbf{k}_\parallel$; $k_z^0 = 0$ for $\mathbf{k}_\parallel = \Gamma, \Gamma^*$, and M, and $k_z^0 = 0.5$ for $\mathbf{k}_\parallel =$K.
• Figure 3: $\mathbf{k}_{\parallel}$-resolved conductances at the Fermi level: (a)–(c) up-spin $G_{\mathrm{P},\uparrow}$, (d)–(f) down-spin $G_{\mathrm{P},\downarrow}$ in the parallel configuration, and (g)–(i) up-spin $G_{\mathrm{AP},\uparrow}$ in the antiparallel configuration for X$/$h-BN$/$X junctions with X = hcp-Co (a, d, g), fcc-Co (b, e, h), and $L1_1$-CoNi (c, f, i). The unit of conductance is $e^2/h$.
• Figure 4: Spin-resolved band structures along (a)–(e) the $\Gamma$–A line and (f)–(j) the $\Gamma^*$–A$^*$ line with $\mathbf{k}_\parallel = (0, 0.05)$ for X = hcp-Co (a, f), fcc-Co (b, g), $L1_1$-CoNi (c, h), hcp-Co_0.8Ni_0.2 (d, i), hcp-Co_0.6Ni_0.4 (e, j). Majority- and minority-spin bands are shown in red and blue, respectively. For hcp-Co, bands with $\Delta_1$ and $\Delta^*_1$ symmetry are indicated. The unit of $k_z$, $\pi / c_\mathrm{e}$, is defined using the lattice constant of the ferromagnetic electrode along the $c$ axis, as illustrated in Fig. 1.
• Figure 5: Structure- and composition-dependent conductances and TMR ratio (green) for X$/$h-BN$/$X, where X = $L1_1$-CoNi, fcc-Co, and hcp-Co_1-xNi_x. Conductances for $G_{\mathrm{P},\uparrow}$, $G_{\mathrm{P},\downarrow}$, and $G_{\mathrm{AP}}$ are shown in red, blue, and purple, respectively.