Magnon squeezing in antiferromagnetic MnF2 and FeF2

Quantum squeezing of a collective spin-wave excitation or magnon using femtosecond optical pulses has been used to generate correlated pairs of spins with equal and opposite wave vectors in antiferromagnetic MnF2 and FeF2. In the squeezed state, the fluctuations of the magnetization of a crystallographic unit cell vary periodically in time and are reduced below that of the ground state quantum noise. The results obtained, including their temperature dependence for FeF2, are compared with earlier spontaneous Raman scattering measurements. The squeezing effect is observed to be at least one order of magnitude stronger in FeF2 than in MnF2.