The hyperfine field at 50 K is equal in magnitude to the field at 18 K because the overall width of the velocity splitting is identical in both spectra.
As the sample cools from 50 K to 18 K, the internal magnetic moments transition from a fluctuating state to a static alignment. At 50 K, the thermal energy causes these moments to relax. By 18 K, this motion is effectively suppressed, “freezing” the magnetic environment into a stable configuration.