  The theory of special relativity is one of the most significant achievements
in modern physics, with several important predictions such as time dilation,
size contraction for a moving object and mass-energy equivalence. Recent
studies have demonstrated size contraction for an antiferromagnetic (AFM)
domain wall (DW). Here, we show the mass-energy equivalence by numerically
investigating the excitation of terahertz (THz) magnons from a moving AFM DW
under the magnetic anisotropy energy gradient. The energy of magnons comes from
the loss of DW mass, accompanied with a DW width broadening, overcoming the
Lorentz contraction effect. Our results pave the way to study relativistic
physics in AFM textures and to efficiently generate THz magnons by electric
means.
