Abstract
The effect of Zn doping on Ni-Mn-Ga magnetic shape memory alloy was studied by the first-principles calculations using exact muffin-tin orbital method in combination with the coherent-potential approximation and projector augmented-wave method. Trends in martensitic transformation temperature TM and Curie temperature TC were predicted from calculated energy differences between austenite and nonmodulated martensite, ΔEA−NM, and energy differences between paramagnetic and ferromagnetic state, ΔEPM−FM. Doping upon the Ga-sublattice results in stabilization of martensitic phase which indicates the increase in TM. TC is affected only weakly or slightly decreases, because ΔEPM−FM of martensite does not change significantly with doping. The substitution of Mn atoms by Zn causes the decrease in both TM and TC. Comparing to Cu-doped Ni-Mn-Ga alloys, we predict that doping with Zn results in smaller decrease in TC but also in smaller increase in TM. Moreover, Cu doping upon the Ga-sublattice strongly decreases the magnetic anisotropy energy of martensite, whereas such strong effect was not observed for Zn doping. Based on the calculations of Zn-doped Ni-Mn-Ga alloys we suggest that simultaneous doping with Zn and an element increasing TC can result in significant increase in both transformation temperatures without strong decrease of magnetic anisotropy.
Highlights
Numerous scientific investigations have been done in order to study Ni-Mn-Ga Heusler alloys mainly because they exhibit magnetic shape memory (MSM) behavior
Based on the calculations of Zn-doped Ni-Mn-Ga alloys we suggest that simultaneous doping with Zn and an element increasing TC can result in significant increase in both transformation temperatures without strong decrease of magnetic anisotropy
Zn substitution instead of Ga has an opposite effect, as shown in recent ab initio calculations of Zn doping on Ga sublattice with 6.25 and 12.5 at. % [57]
Summary
Numerous scientific investigations have been done in order to study Ni-Mn-Ga Heusler alloys mainly because they exhibit magnetic shape memory (MSM) behavior. The macroscopic deformation of such materials in an external magnetic field, so called Magnetic Field-Induced Strain (MFIS), is caused by the motion of highly mobile twin boundaries in magnetically ordered martensite [1, 2]. Ni-Mn-Ga alloy is known for its 6 % MFIS in martensite with five-layered modulation (10M) [3, 4]. Practical usability of Ni-Mn-Ga is restricted by its low transformation temperature from cubic austenite with L21 structure to martensite which occurs at TM = 202 K in stoichiometric Ni2MnGa alloy [9]. For practical applications in small actuators [10] or energy harvesters [11], all relevant transformations should be at least several tens of kelvins above the room temperature. For more demanding applications e.g. for use in internal combustion engines, all transformation temperatures should be above 413K [12]
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