Dynamic effects in the characterization of the magnetocaloric effect of LaFeSi-type alloys

In this work, the influence of the rate of change of the applied field to three magnetocaloric samples is studied: a bulk Gd sample used as a reference material, and fully hydrogenated La_0.7Ce_0.3Fe_11.7Si_1.3H_1.8 and La_0.7Ce_0.3(Fe_0.882Mn_0.018Si_0.1)₁₃H_1.8 alloys, denoted as ERASTEEL-NX and ERASTEEL-11688, respectively . Bulk samples ERASTEEL-NX and ERASTEEL-1688 were prepared from powder batches obtained by gas atomization process as described in [4] . The powder was then sintered at 1150°C for 5 hours in Ar (it has to be noticed that this is not the optimal temperature for maximizing the proportion of NaZn₁₃ type phase), and the obtained blocs were treated at 400°C for 1 hour in hydrogen . The final proportions of NaZn₁₃ type phase is respectively 80 .5% (instead of 92% for optimized heat treatment) and 85% (instead of 92% also for optimized heat treatment) in ERASTEEL-NX and ERASTEEL-1688 . ERASTEEL-NX, with 30% of Ce substituted to La and no substitution on the Fe site shows a very strong 1st order transition, with a wide hysteresis . ERASTEEL-1688 contains some Mn atoms in substitution to Fe . This induces a softening of the 1st order character of the magnetic transition in this sample . The adiabatic temperature change of the samples has been measured using a commercial setup consisting of a set of Halbach cylinders as a variable field source up to 1.78 T, and a thermocouple placed between two pieces of sample, with dimensions of 8 x 4 x 0 .75 mm, which registers the temperature difference between the sample and the sampleholder . The sample is kept in vacuum inside a bath cryostat . The temperature of the sampleholder is stabilized before performing each of the adiabatic measurements . The rate of change of the field ranged from 0 .25 T s^-1 to 3 T s^-1, which is equivalent t- frequencies of a magnetic refrigerator operating between 0 and 1 .78 T ranging from 0 .07 Hz to 0 .84 Hz .