Hall Imaging of the History Dependence of the Magnetocaloric Effect in Gd $_{5}$ Si $_{2.09}$ Ge

The isothermal entropy change of polycrystalline Gd ₅Si _2.09Ge _1.91 during the magneto-structural transition is measured exposing the sample to two different temperature/field histories. The phase fraction involved in the transition is observed by using the Hall probe imaging technique. Both experiments confirm that the entropy change during the first-order phase transition depends on the history of the sample. This is understood by the hysteretic nature of the magneto-structural transition. A Preisach-type model accounting for hysteresis in field and temperature is able to predict the phase fractions involved in the transition. The result can be used to explain differences between different measurement methods of the isothermal entropy change of the magnetocaloric effect.