Abstract
With the purpose to optimize the functional properties of Heusler alloys for their use in solid-state refrigeration, the characteristics of the martensitic and magnetic transitions undergone by Ni50Mn25−xGa25Cux (x = 3–11) alloys have been studied. The results reveal that, for a Cu content of x = 5.5–7.5, a magnetostructural transition between paramagnetic austenite and ferromagnetic martensite takes place. In such a case, magnetic field and stress act in the same sense, lowering the critical combined fields to induce the transformation; moreover, magnetocaloric and elastocaloric effects are both direct, suggesting the use of combined fields to improve the overall refrigeration capacity of the alloy. Within this range of compositions, the measured transformation entropy is increased owing to the magnetic contribution to entropy, showing a maximum at composition x = 6, in which the magnetization jump at the transformation is the largest of the set. At the same time, the temperature hysteresis of the transformation displays a minimum at x = 6, attributed to the optimal lattice compatibility between austenite and martensite. We show that, among this system, the optimal caloric performance is found for the x = 6 composition, which displays high isothermal entropy changes (−36 J·kg−1·K−1 under 5 T and −8.5 J·kg−1·K−1 under 50 MPa), suitable working temperature (300 K), and low thermal hysteresis (3 K).
Highlights
Heusler-type ferromagnetic shape memory alloys (FSMAs) undergo a structural, martensitic transformation (MT), where the high and low temperature phases do often present different magnetic orders [1,2]
Magnetocaloric effect (MCE) [7,8,9] is characterized by positive isothermal entropy change induced by the magnetic field (∆SM ) in the case of the ferromagnetic to paramagnetic transition, as undergone by common metamagnetic shape memory alloys (MMSMAs) [9,10]; on the contrary, paramagnetic to ferromagnetic magnetostructural transformation is accompanied by direct MCE, which yields negative ∆SM upon increasing the magnetic field
While for x = 9, a non-hysteretic magnetization increase is observed in the martensitic region— corresponding to the ferromagnetic transition in martensite at the corresponding Curie temperature, TC m —for x = 7, the magnetization increase occurs simultaneously with the MT and shows the temperature hysteresis that is characteristic of this first order transition
Summary
Heusler-type ferromagnetic shape memory alloys (FSMAs) undergo a structural, martensitic transformation (MT), where the high and low temperature phases do often present different magnetic orders [1,2]. Magnetocaloric effect (MCE) [7,8,9] is characterized by positive isothermal entropy change induced by the magnetic field (∆SM ) in the case of the ferromagnetic to paramagnetic (or weak magnetic) transition, as undergone by common MMSMAs (inverse MCE) [9,10]; on the contrary, paramagnetic to ferromagnetic magnetostructural transformation is accompanied by direct (or conventional) MCE, which yields negative ∆SM upon increasing the magnetic field. The compositions for which the above alloys show direct metamagnetic behavior are determined, and the transformation entropy change is correlated with the magnetic contribution. To assess the applicability of these alloys, the magnetocaloric and elastocaloric effects were evaluated, and the results are discussed in relation to the characteristics of the magnetostructural transformations
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