Dynamics of the magneto-structural phase transition in La(Fe0:9Co0:015Si0:085)13 observed by magneto-optic imaging

We investigate the temperature induced ferromagnetic to paramagnetic phase transition in Co substituted La(FexCoySi1xy)13 with x =0.9 and low Co content of y =0.015 (Tc ≃200 K) by means of magneto-optic imaging with indicator film and by calorimetry at very low temperature rates. We were able to visualize the motion of the ferromagnetic(FM)/paramagnetic(PM) front which is forming reproducible patterns independently of the temperature rate. The average velocity of the FM/PM front was calculated to be 104 m/s during the continuous propagation and 4·103 m/s during an avalanche. The heat flux was measured at low temperature rates by a differential scanning calorimeter and shows a reproducible sequence of individual and separated avalanches which occurs independently of the rate. We interpret the observed effects as the result of the athermal character of the phase transition.