Electrical injection and transport of coherent magnon superposition states in noncollinear antiferromagnets

Noncollinear antiferromagnets (nAFMs) with a small net magnetic moment offer new opportunities for ultrafast spintronic devices, owing to unique physical properties. While in ferromagnets and collinear AFMs the spin current polarization is locked to the magnetization $\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{\mathbf{m}}$ and N\'eel vector $\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{\mathbf{n}}$ directions, we predict that magnon spin currents injected by metal contacts into nAFMs can be polarized with both $\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{\mathbf{n}}$ and $\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{\mathbf{m}}$ components when carried by a coherent superposition of magnon eigenstates. The spin injection efficiency is governed by an interface spin conductance tensor that depends on the noncollinear magnetic texture. While the $\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{\mathbf{m}}$ component diffuses freely into the nAFM, the $\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{\mathbf{n}}$ component oscillates as a function of distance from the injector and applied magnetic field, analogous to the Hanle effect of electron spins in metals. Our findings reveal the potential of nAFMs as platforms for the study of tensorial coherent spin transport.