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MM: Fachverband Metall- und Materialphysik

MM 48: Joint Session Magnetic Shape Memory Alloys I (jointly with DS, MA)

MM 48.7: 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 Niemann^1,2, Oleg Heczko³, Ludwig Schultz^1,2, and •Sebastian Fähler^1,2 — 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.