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

We investigate the temperature induced ferromagnetic to paramagnetic phase transition in Co substituted La(FexCoySi1−x−y)13 with x = 0.9 and low Co content of y = 0.015 (Tc≃200 K) by means of magneto-optical 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 10−4 m/s during the continuous propagation and 4×10−3 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.