Giant and Tunneling Magnetoresistance in Unconventional Collinear Antiferromagnets with Nonrelativistic Spin-Momentum Coupling

Giant and tunneling magnetoresistance are physical phenomena used for reading information in commercial spintronic devices. The effects rely on a conserved spin current passing between a reference and a sensing ferromagnetic electrode in a multilayer structure. Recently, we have proposed that these fundamental spintronic effects can be realized in unconventional collinear antiferromagnets with nonrelativistic alternating spin-momentum coupling. Here, we elaborate on the proposal by presenting archetype model mechanisms for the giant and tunneling magnetoresistance effects in multilayers composed of these unconventional collinear antiferromagnets. The models are based, respectively, on anisotropic and valley-dependent forms of the alternating spin-momentum coupling. Using first-principles calculations, we link these model mechanisms to real materials and predict an approximately 100% scale for the effects. We point out that, besides the giant or tunneling magnetoresistance detection, the alternating spin-momentum coupling can allow for magnetic excitation by the spin-transfer torque.

Popular Summary

In spintronics devices, substituting ferromagnets for antiferromagnets has demonstrated potential for orders-of-magnitude-faster information writing, insensitivity to perturbing magnetic fields, and absence of stray fields—opening new avenues for neuromorphic or dissipationless nanoelectronic components. However, finding viable counterparts for electrical readout remains an unsolved problem. Here, we predict these sought-after effects in realistic materials with unconventional antiferromagnetism.

The exponential growth of ferromagnetic hard-drive capacities was allowed by electric readout by giant-magnetoresistance (GMR) and tunneling-magnetoresistance (TMR) effects. The effects refer to resistance changes, controlled by mutual orientations of the magnetization of reference and sensing electrodes in a multilayer structure. Microscopically, the GMR and TMR effects rely on spin-dependent band-structure phenomena—transport and tunneling, respectively—that are absent in conventional antiferromagnets.

However, our unconventional antiferromagnets exhibit band structures with alternating spin polarization in momentum space that we show can exhibit GMR and TMR effects. We show that the unconventional nonrelativistic magnetization distribution in real space generates strongly anisotropic (elliptic rather than circular) spin-polarized bands and well-separated opposite spin channels in momentum space, which can enhance GMR and TMR effects. The strength of these effects can be 2 orders of magnitude larger than state-of-the-art readout effects in antiferromagnets based on relativistic spin-momentum couplings.

Protecting the spin is a central problem in spintronics’ quest to complement charge-based microelectronics, where charge is protected by the huge particle-antiparticle energy separation. Thus, our demonstration of spin protection in our unconventional antiferromagnets using a large energy gap (about 1 eV) between the spin-up and spin-down states opens up unprecedented opportunities in spin-physics research.

Figure 1

Archetype GMR and TMR model mechanisms in unconventional antiferromagnets. Red and blue correspond to up and down spins, respectively, and a gray arrow marks the applied current direction. (a) The GMR stack with a metallic spacer in a current-in-plane geometry. As an example, we show the antiparallel configuration of the Néel vectors in the two electrodes ${\mathrm{AF}}_1$ and ${\mathrm{AF}}_2$. Interfaces are oriented along one of the main axes of the elliptic spin-dependent bands. (b) Energy band cuts highlighting the anisotropic alternating spin-momentum coupling around the $\mathbf{\Gamma }$ point in the Brillouin zone, resulting in anisotropic spin-dependent conductivities. (c) The tight-binding model band dispersion. The dashed rectangle highlights the region with the anisotropic spin-momentum coupling around the $\mathbf{\Gamma }$ point. (d) The TMR stack with an insulating barrier. As an example, we show the parallel configuration of the Néel vectors in the two electrodes. (e) Energy band cuts highlighting the valley-dependent alternating spin-momentum coupling around the ${\mathbf{M}}_1$ and ${\mathbf{M}}_2$ points in the Brillouin zone, resulting in valley and spin-dependent densities of states. (f) The tight-binding model band dispersion. The dashed rectangles highlight the valleys with opposite spin splitting, marked by the magenta arrows.

Figure 2

Model transport calculations in a quasi-two-dimensional tunnel junction. (a) Schematics of the geometry of the leads representing the unconventional antiferromagnets, the scattering region separating the leads (spacer), and selected parameters used in the calculations. (b) Spin-up (red) and spin-down (blue) energy bands in the lead as a function of $k_x$ plotted for the discrete set of transverse wave-vector parameters corresponding to $N_y=20$ lattice points in the direction parallel to the lead-spacer interface. Hamiltonian parameters in the lead correspond to Fig. 1 and the first case described in the text. (c) Conductances for parallel (P) and antiparallel (AP) configurations of the Néel vectors in the leads and the corresponding relative difference of the P and AP conductances. (d) Top view of the energy bands with the alternating spin-momentum coupling in the lead in the first Brillouin zone. Hamiltonian parameters in the lead correspond to Fig. 1 and the second case described in the text. Magenta circles mark the alternating spin-polarized valleys. (e),(f) The same as (b),(c) for Hamiltonian parameters in the lead corresponding to Fig. 1 and the second case described in the text. In (b),(e), the magenta arrows mark the energy ranges with decoupled spin-up and spin-down channels due to the spin-polarized valleys in the band structure.

Figure 3

First-principles calculations of GMR in ${\mathrm{RuO}}_2$ [32]. (a) Crystal structure of ${\mathrm{RuO}}_2$. Red and blue mark opposite magnetic moments on the first and second Ru sublattices. The presence of an intersublattice transposing symmetry containing a crystal rotation operation (${\mathcal{C}}_{4z}$) and the absence of translation ($t$) and inversion ($P$) transposing symmetries are also highlighted. (b) Fermi surface plots for spin-up (left) and spin-down (right) states. The color coding corresponds to the group velocities, highlighting the strong anisotropy of the two spin-channel conductivities. (c) Spin-projected nonrelativistic energy bands of ${\mathrm{RuO}}_2$ with marked anisotropic spin-momentum coupling around $\mathbf{R}$ and $\mathbf{Z}$ points. (d) Longitudinal spin-up and spin-down conductivities. (e) GMR ratio. For comparison, also shown is the transverse spin current relative to the longitudinal charge current (SCR) calculated in Ref. [37].

Figure 4

First-principles calculations of TMR in ${\mathrm{RuO}}_2$ [32] and ${\mathrm{Mn}}_5{\mathrm{Si}}_3$ [31]. (a)–(c) are for ${\mathrm{RuO}}_2$ and (d)–(f) for ${\mathrm{Mn}}_5{\mathrm{Si}}_3$. (a) Wave-vector- and sublattice- (blue and red, respectively) resolved energy bands. (b) Sublattice- (blue and red) and spin- (left and right) resolved density of states as a function of energy. (c) Energy-dependent TMR and spin polarization parameter (see the text). (d)–(f) The same as (a)–(c) for ${\mathrm{Mn}}_5{\mathrm{Si}}_3$.