Controlling antiferromagnetic magnon polarization by interfacial exchange interaction*

Antiferromagnetic (AFM) spintronics holds the great promise of ultrafast spin operation and immunity against magnetic field perturbations. However, manipulating the spin degree of freedom in AFM materials requires magnetic fields orders of magnitude stronger than in ferromagnets. Here we realize a highly efficient control of the magnon spins by the interfacial exchange interaction in an insulating type of thin film heterostructures of ferrimagnetic yttrium iron garnet (YIG) and AFM Cr₂O₃. At low temperatures, the exchange interaction lifts the degeneracy between the two AFM magnon modes, resulting in a net spin polarization from the more populated left-handed magnons even at zero applied magnetic field. This effect manifests as a sign change of the spin Seebeck effect (SSE) signal in YIG/Cr₂O₃(t)/Pt (t=0.7 nm and 12 nm) as temperature is varied. In addition, the SSE signal polarity flips when a magnetic field switches the YIG magnetization but not sufficiently strong to induce the spin-flop transition in Cr₂O₃, demonstrating the ability to control the magnon quantum states in AFM materials via interfacial exchange coupling. Our findings pave the way towards insulating spintronics where magnons function with two quantum spin states in analogy to electrons in metallic spintronics.