Magnon Correlation Enables Spin Injection, Dephasing, and Transport in Canted Antiferromagnets

TL;DR

The paper addresses how magnon spin and quantum coherence propagate in noncollinear antiferromagnets under thermal and electrical injection. It develops a quantum kinetic framework in which magnon spin is described by a matrix ${\cal S}_{\alpha}({\bf k})$, revealing that spin resides not only in magnon populations but crucially in inter-branch correlations, especially in canting-induced noncollinearity. The key contributions are the demonstration that spin transfer can occur via magnon correlations even when diagonal population terms carry no spin, the identification of intrinsic dephasing from free-induction decay and extrinsic dephasing from spin exchange with adjacent metals, and the prediction of gate-tunable, nonlocal magnon transport and Hanle-like effects. This framework provides a route to coherent magnon spin signals in canted AFMs and related noncollinear magnets, with potential applications in spintronics devices leveraging materials such as $\alpha$-Fe$_2$O$_3$, FePS$_3$, Cr$_2$O$_3$, and beyond, by enabling electrical control of spin coherence via interfaces and gates.

Abstract

Thermal and electrical injection and transport of magnon spins in magnetic insulators is conventionally understood by the non-equilibrium population of magnons. However, this view is challenged by several recent experiments in noncollinear antiferromagnets, which urge a thorough theoretical investigation at the fundamental level. We find that the magnon spin in antiferromagnets is described by a matrix, so even when the diagonal terms -- spins carried by population -- vanish, the off-diagonal correlations transmit magnon spins. Our quantum theory shows that a net spin-flip of electrons in adjacent conductors creates quantum coherence between magnon states, which transports magnon spins in canted antiferromagnets, even without a definite phase difference between magnon modes in the incoherent process. It reveals that the pumped magnon correlation is not conserved due to an intrinsic spin torque, which causes dephasing and strong spatial spin oscillations during transport; both are enhanced by magnetic fields. Spin transfer to proximity conductors can cause extrinsic dephasing, which suppresses spin oscillations and thereby gates spin transport.

Figures (6)

• Figure 1: (a) Description of magnon spin in AFMs in terms of a matrix. (b) A net spin-flip of an electron in normal metals creates the magnon correlation between two bands in AFMs. An optional configuration to pump magnon correlation is to use canted AFMs with Néel vector along $\hat{\bf z}$ driven by a spin accumulation $\parallel \hat{\bf z}$ created by a charge current $I$ along $\hat{\bf y}$ via the spin Hall effect.
• Figure 2: A general magnetic structure with ${\cal N}$ noncollinear spins in one unit cell. ${\pmb \eta}_{l=\{1,2\cdots,{\cal N}\}}$ represents their directions in the ground state.
• Figure 3: The structure of the spin matrix.
• Figure 4: (a) Two-sublattice model of AFM. The dashed square represents a unit cell with a lattice constant $a$. (b) and (c) Polarizations ${\bf m}$ by green arrows of the anti-phase mode "$1$" and in-phase mode "$2$" with the Néel vector ${\bf n}$ of mode "1" ("2") oscillating along the $\hat{\bf y}~(\hat{\bf x})$-axis, indicated by orange arrows. (d) Band structure of two modes. Inset: average frequency difference $|\delta\omega|=|\omega_1-\omega_2|$ over $|{\bf k}|\in[0,0.5]$ nm$^{-1}$.
• Figure 5: (a) Exchange asymmetry $\Delta$ and magnetic-field dependencies of injected magnon correlation density. (b) Distribution of the injected magnon correlation and population of mode "1" in the Brillouin zone. (c) Time evolution of magnon correlation ${\rm Re}\rho_{12,{\bf q}}$ (the blue curve) and the fitting via the exponential decay $\sim e^{-t/\tau_{\bf q}}$ (the red curve). (d) Calculated dephasing time $\tau_{\bf q}$ due to the magnon-electron interaction, showing excellent agreement between numerical and analytical calculations.