All-electrically controlled spintronics in altermagnetic heterostructures

Abstract

The recent discovery of altermagnets, which exhibit spin splitting without net magnetization, opens new directions for spintronics beyond the limits of ferromagnets, antiferromagnets, and spin orbit coupled systems. We investigate spin selective quantum transport in heterostructures composed of a normal metal and a two dimensional d-wave altermagnet, and identify a universal mechanism for achieving perfect spin polarization. The mechanism is dictated by Fermi surface geometry: closed Fermi surfaces in weak altermagnets yield partial and oscillatory spin filtering, whereas open Fermi surfaces in strong altermagnets intrinsically enforce fully spin polarized conductance. Exploiting these distinct transport regimes, we propose all electrical spin filter and spin valve architectures, where resonant tunneling produces highly spin polarized conductance tunable by gate voltage and interface transparency. Altermagnets with open Fermi surfaces further support gate reversible perfect spin polarization that remains robust against interface scattering, disorder, and temperature. We also demonstrate an electrically controlled spin valve that reproduces the functionality of magnetic tunnel junctions without magnetic fields or relativistic mechanisms. d-wave altermagnets with open Fermi surfaces thus provide a promising platform for low dissipation, scalable, and magnetic field free spintronic devices with potential for integration into next generation quantum and CMOS compatible technologies.

Figures (6)

• Figure 1: Schematic of electrically controlled spintronic devices based on AM heterostructures: (a) spin filter and (b) spin valve. (a) In the spin filter, a gate applied to the AM region generates fully spin-polarized conductance by selectively blocking one spin channel. (b) The spin valve consists of two gated spin filters, exhibiting electrically tunable switching between high and low conductance states. This behavior is analogous to the parallel and antiparallel magnetization configurations in conventional ferromagnetic bilayer spin valves, but achieved here without magnetic fields and net magnetization.
• Figure 2: Dispersion of (a) weak and (b) strong AM along $k_x$ and $k_y$ directions with $k_y=0$ and $k_x=0$, respectively. The insets exhibited the anisotropic Fermi surface at $E/t=1$ for weak altermagnet and $E/t=\pm1$ for strong altermagnet as indicated by the horizontal dashed line. The spin-up (spin-down) subband is in blue (red). The altermagnetic strength is (a) $J/t=0.5$ and (b) $J/t=1.5$. Parameters: $t=1$ is the energy unit and $a=1$ is the lattice constant.
• Figure 3: (a) Schematic illustration of the spin-filter effect. (a-i) The spin-filter is composed of normal metals and a gated altermagnet by $U_g$. The junction is along the $x$-direction, keeping periodic boundary conditions along the $y$-direction. (a-ii) The normal-metal leads exhibit spin-degenerate parabolic dispersions (black lines), while the weak AM features spin-dependent parabolic bands with the same curvature direction, with blue and red lines denoting the spin-up and spin-down states, respectively. The dispersions are exhibited along $k_x$ direction with $k_y=0$. (a-iii): Isotropic Fermi surface of normal-metal leads and anisotropic closed and open spin-resolved Fermi surfaces for weak AMs. The Fermi surfaces are exhibited in $k_x$-$k_y$ plane. Blue (red) shaded regions indicate transverse mode ($k_y$) ranges forbidden for spin-up (spin-down) electrons due to the AM band structure. (a-iv) and (a-v) are the same as (a-ii) and (a-iii), respectively, but for strong AMs. (b) Transmission probability, spin-resolved conductance and spin polarization for a weak AM. (b-i) and (b-iii): Energy and incident-angle ($\theta_k$) resolved transmission probabilities $T_{\uparrow}$ and $T_{\downarrow}$. (b-ii) and (b-iv): Transmission for normal incidence ($\theta_k = 0$) at various tunneling barrier heights $V$. The horizontal dashed line denotes the resonant energy levels [Eq. (\ref{['eq_Ens']})] due to the confinement condition [Eq. \ref{['eq_resonant']}]. (b-v) and (b-vi): Energy-dependent spin-resolved conductance and resulting spin polarization. (c) Same as panel (b), but for a strong AM. Parameters: $U_L = 0$, $U_g = 1$, $V = 0$ in (i, iii, v), and junction length $d = 20a$. The $d_{x^2-y^2}$-wave AM with $\theta_J=0$ is considered.
• Figure 4: Gate-controlled spin-filter effect for (upper panel) weak and (lower panel) strong altermagnets, corresponding to Fig. \ref{['fig2']}. (a) and (c): Spin-resolved conductance as a function of gate voltage $U_g$ at fixed tunneling barrier $V = 5$. (b) and (d): Spin polarization as a function of $U_g$ for various values of $V$. The Fermi level is set to $E = 0.1t$, and all other parameters are identical to those in Fig. \ref{['fig2']}.
• Figure 5: (a) Schematic illustration of a double-gate-controlled spin valve based on a strong altermagnetic heterostructure. The configuration enables gate-tunable switching between parallel ($U_{\text{g}1} U_{\text{g}2} > 0$) and antiparallel ($U_{\text{g}1} U_{\text{g}2} < 0$) regimes. (b) Total conductance and (c) spin polarization as functions of the gate voltages $U_{\text{g}1}$ and $U_{\text{g}2}$. The length of the altermagnetic region is $d = 50a$, and the tunneling barrier is set to $V = 0$.