Berlin 2012 – wissenschaftliches Programm
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MA: Fachverband Magnetismus
MA 41: Joint Session Magnetic Shape Memory Alloys I (jointly with DS, MM)
MA 41.7: Vortrag
Donnerstag, 29. März 2012, 11:45–12:00, H 0106
Failure of the Maxwell relation for the quantification of caloric effects in ferroic materials — Robert Niemann1,2, Oleg Heczko3, Ludwig Schultz1,2, and •Sebastian Fähler1,2 — 1IFW Dresden, P.O. Box 270116, 01171 Dresden, Germany — 2Department of Physics, Institute for Solid State Physics, Dresden University of Technology, 01062 Dresden, Germany — 3Institute of Physics, Academy of Science of the Czech Republic, Na Slovance 2, 182 02 Prague, Czech Republic
Giant caloric effects were reported in elasto-, electro- and magnetocaloric materials near phase transformations. Commonly, their entropy change is indirectly evaluated by a Maxwell relation. We report the fundamental failure of this approach. We analyze exemplarily the Ni-Mn-Ga magnetic shape memory alloy. An applied field results in magnetically induced reorientation of martensitic variants, which form during the phase transformation. This results in a spurious magnetocaloric effect, which only disappears when repeating the measurement a second time. This failure is universal as the vector character of the applied field is not considered in the common scalar evaluation of a Maxwell relation.