Thermomagnetic writing on low-temperature phase MnBi films was investigated. By applying short laser pulses, the undesirable transformation to the quenched high-temperature phase is inhibited. The low-temperature phase proved to be stable when heated for magnetic switching with pulses shorter than 10 μs. The transient temperature during laser heating of a 20 μm diameter film spot was determined using a thin germanium film as a reference target. The temperature shift of the transmission spectrum (λ = 633 nm) of this semiconductor material serves for optical temperature indication. The threshold temperature for stable short-pulse thermomagnetic writing on MnBi was determined by this technique to be 550°C ± 25%. The decomposition temperature was determined to be 820°C ± 25%.