Chemical ordering in Ni-Mn-Ga Heusler alloys

Heusler alloys, L2{dollar}sb1{dollar} ternary compounds with the formula A{dollar}sb2{dollar}BC, are often ferromagnetic in spite of the fact that their constituent elements may be paramagnetic, diamagnetic or antiferromagnetic. Heusler alloys have attracted attention for their potential use as actuators because several of the alloys, including Ni{dollar}sb{lcub}50{rcub}{dollar}Mn{dollar}sb{lcub}25{rcub}{dollar}Ga{dollar}sb{lcub}25{rcub}{dollar}, exhibit a {dollar}rm BCCto BCT{dollar} martensitic phase transition as well as ferromagnetic and chemical order-disorder transitions. The purpose of this investigation is to characterize the {dollar}rm A2to B2to L2sb1{dollar} order-disorder transition in a quasibinary Heusler alloy Ni{dollar}sb{lcub}50{rcub}{dollar}Mn{dollar}rmsb{lcub}x{rcub}{dollar}Ga{dollar}rm sb{lcub}50-x{rcub}{dollar} over a composition range {dollar}rm 15 le x le 35{dollar} by performing in situ neutron diffraction measurements on the alloy.; Neutron powder diffraction data were collected for a total of nine Ni{dollar}sb{lcub}50{rcub}{dollar}Mn{dollar}rm sb{lcub}x{rcub}{dollar}Ga{dollar}rm sb{lcub}50-x{rcub}{dollar} alloys in the composition range {dollar}rm 15 le x le 35{dollar}. For each of the alloys, 6-8 three-hour diffraction scans were performed at successively lower temperature steps ranging from 850{dollar}spcirc{dollar}C to 450{dollar}spcirc{dollar}C using a high temperature vacuum furnace. In addition, selected alloys were studied using differential thermal analysis (DTA) to perform heating and cooling scans over a temperature range of 100{dollar}spcirc{dollar}-1000{dollar}spcirc{dollar}. An experimentally observed temperature-composition (T-c) phase diagram is presented to describe the {dollar}rm A2to B2 to L2sb1{dollar} ordering phase boundaries relative to the alloy's melting transition. Also, observed long range order parameters {dollar}etasb1{dollar}(T) and {dollar}etasb2{dollar}(T), representative of the {dollar}rm A2to B2{dollar} and {dollar}rm B2to L2sb1{dollar} ordering stages respectively, are presented for each of the alloys.; Experimental results are compared against the predictions of a simple mean field model based on the Bragg-Williams-Gorski (BWG) approximation. In general, the observed long range order parameters {dollar}etasb1{dollar}(T) and {dollar}etasb2{dollar}(T) are in fair agreement with {dollar}etasb1{dollar}(T) and {dollar}etasb2{dollar}(T) as predicted by the BWG model. The start of {dollar}rm A2to B2{dollar} ordering is not observed in any Ni{dollar}sb{lcub}50{rcub}{dollar}Mn{dollar}rm sb{lcub}x{rcub}{dollar}Ga{dollar}sb{lcub}rm 50-x{rcub}{dollar} alloys, however variable degrees of {dollar}rm A2to B2{dollar} ordering are observed in Ni{dollar}sb{lcub}50{rcub}{dollar}Mn{dollar}rm sb{lcub}15{rcub}{dollar}Ga{dollar}sb{lcub}35{rcub}{dollar}, Ni{dollar}sb{lcub}50{rcub}{dollar}Mn{dollar}sb{lcub}20{rcub}{dollar}Ga{dollar}sb{lcub}30{rcub}{dollar}, Ni{dollar}sb{lcub}50{rcub}{dollar}Mn{dollar}sb{lcub}22{rcub}{dollar}Ga{dollar}sb{lcub}28{rcub}{dollar}, Ni{dollar}sb{lcub}50{rcub}{dollar}Mn{dollar}sb{lcub}29{rcub}{dollar}Ga{dollar}sb{lcub}21{rcub}{dollar} and Ni{dollar}sb{lcub}50{rcub}{dollar}Mn{dollar}sb{lcub}30{rcub}{dollar}Ga{dollar}sb{lcub}20{rcub}{dollar}. The {dollar}rm B2to L2sb1{dollar} transition in Ni{dollar}sb{lcub}50{rcub}{dollar}Mn{dollar}sb{lcub}25{rcub}{dollar}Ga{dollar}sb{lcub}25{rcub}{dollar} and other near-stoichiometric alloys appears as a weak 1st order phase transition, while compositions further from stoichiometry appear to undergo transitions more characteristic of the 2nd order.