Abstract
Metamagnetic shape memory alloys have aroused considerable attraction as potential magnetic refrigerants due to the large inverse magnetocaloric effect associated to the magnetic-field-induction of a reverse martensitic transformation (martensite to austenite). In some of these alloys, the austenite phase can be retained on cooling under high magnetic fields, being the retained phase metastable after field removing. Here, we report a giant direct magnetocaloric effect linked to the anomalous forward martensitic transformation (austenite to martensite) that the retained austenite undergoes on heating. Under moderate fields of 10 kOe, an estimated adiabatic temperature change of 9 K has been obtained, which is (in absolute value) almost twice that obtained in the conventional transformation under higher applied fields. The observation of a different sign on the temperature change associated to the same austenite to martensite transformation depending on whether it occurs on heating (retained) or on cooling is attributed to the predominance of the magnetic or the vibrational entropy terms, respectively.
Highlights
Magnetic shape memory alloys display unique properties associated to the occurrence of a thermoelastic martensitic transformation (MT) between magnetically ordered structural phases
As a consequence of the counterbalance between the vibrational and the magnetic contributions to the entropy change at the transformation, the forward MT can be inhibited on cooling by the application of a strong magnetic field[23,24,25,26,27,28,29,30,31,32], and the austenitic phase be stabilized at low temperatures, far below Tm
We show that the low temperature regime in which it occurs favors the predominance of the magnetic contribution to the total entropy change, and this leads to the observation of a direct magnetocaloric effect (ΔS < 0), contrary to the inverse effect (ΔS > 0) observed in the conventional forward MT
Summary
Let us describe the magnetization behavior of the Ni-Mn-In-Co alloy. The ZFC-FC magnetization measurement at 100 Oe (red dots) in Fig. 1 shows a jump at around 250 K linked to the standard martensitic transformation, as well as a temperature splitting around 50 K corresponding to a spin-glass like behavior already reported[29]. The entropy change linked to the Martensitic Transformation is similar at low and high temperatures but the temperature change at low temperature is higher due to the smaller value of the specific heat This giant magnetocaloric effect linked to the undercooled austenite phase shows the relevance of the entropy contributions and in particular the outstanding role of the magnetic entropy. In turn, the retention of ferromagnetic austenite and the low temperature regime in which it transforms to martensite favors the, otherwise impossible, predominance of the magnetic contribution to the total entropy change at the transformation, as long as the vibrational entropy of both phases approaches to zero This leads to the peculiar situation where the sign on the temperature change associated to the austenite to martensite transformation is different depending on whether it occurs on cooling (conventional) or on heating (retained), and highlights the outstanding role of magnetism on driving the structural transformation. From the application point of view, the magnetocaloric effect at low temperatures could compete with other materials like molecular magnetic compounds for magnetic refrigeration at cryogenic temperatures[38,39], while the occurrence of direct and inverse magnetocaloric effects associated to the same forward martensitic transformation could be useful in the design of refrigeration devices based on more complex thermodynamic processes
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