  In this work, the rich ground-state structure of orthogonally-coupled
symmetric all-antiferromagnetic junctions with easy-plane anisotropy is
reported. Spin reorientation process rather than the traditional spin flop (SF)
occurs, resulting in a novel phase in which N\'{e}el vectors preserve the
mirror-reflection symmetry (termed as ``MRS phase"). The phase transitions
between SF and MRS phases can be either the first- or second-order. After
disturbed by external stimuli, magnons with different parities emerge. For
in-plane dc fields, no couplings between magnons occur. When dc fields become
oblique, coherent couplings between magnons with opposite parity emerge,
leading to anticrossings in resonance frequencies. However, self-hybridization
among magnons with the same parity never happens. More interestingly, spin
waves based on MRS phase are linearly polarized and their polarization
directions can be fine controlled.
