Delocalization and hybridization enhance the magnetocaloric effect in Ni2Mn0.75Cu0.25Ga Page: 3 of 20
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phase transition. In Ni2MnGa, these two transition temperatures are 225 and 384 K
respectively 12. Each of these phase transitions has an associated, weak magnetocaloric
It has been found that if the structural and magnetic transitions can be tuned to
occur at the same temperature, the entropy change (a measure of the magnetocaloric
effect) of the joint transition is significantly enhanced compared to that of the separated
transitions, leading to the GMC effect. For example, by increasing the Ni content,
entropy changes in the range -3.5 to -20.4 J/Kg K have been obtained in polycrystals,
while in single crystals the change observed has been as high as -86 J/kg K 6-8. The
transitions can also be tuned by atomic substitution at the Mn or at the Ga sites 12,13 In
particular, 25% Cu at the Mn site results in a AS of ~ -60 J/kg K in the temperature range
308 - 334 K 9,14. Ni2Mno 75Cuo.25Ga is thus promising as a magnetic refrigerant. But the
physics underlying the enhancement of GMC properties of this system has not been
Ferromagnetism in the Ni2MnGa family of compounds is localized, and is
primarily due to indirect RKKY interactions between neighboring Mn atoms, mediated
by conduction electrons 15,16 The minority spin electrons in the d band of the Mn are
almost excluded from the conduction band, and hence give rise to a localized magnetic
moment. Since the magnetic and structural properties are intricately linked by the lattice
parameter ratio c/a 17-20 as well as the density of 3d electrons at the Fermi level 20,21
replacement of Mn by Cu should cause site - and element - specific changes in the
electronic and magnetic properties. A detailed understanding of such changes is essential
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Roy, Sujoy; Blackburn, E.; Valvidares, S. M.; Fitzsimmons, M. R.; Vogel, Sven C.; Khan, M. et al. Delocalization and hybridization enhance the magnetocaloric effect in Ni2Mn0.75Cu0.25Ga, article, November 26, 2008; Berkeley, California. (digital.library.unt.edu/ark:/67531/metadc893936/m1/3/: accessed November 17, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.