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The overlap of first and second order phase transitions and related magnetic entropy changes in Ni2+xMn1-xGa Heusler alloys

机译：Ni2 + xMn1-xGa Heusler合金中一阶和二阶相变的重叠以及相关的磁熵变化

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From application point of view, offstoichiometric Ni2MnGa is of interest and therefore many studies on offstoichiometric Ni2MnGa have been reported. Special focus has been paid on Ni rich Ni2MnGa, where substitution of Ni for Mn results in an increase in TM and decrease of TC, resulting in an overlap of the two transitions for certain Mn concentration [1]. As mentioned earlier, the martensitic transition is a first order transition where as the transition at TC from ferromagnetic order to a paramagnetic order is a second order transition. It is found that the overlap of TM and TC results in a first order magnetostructural transition from the paramagnetic cubic phase to ferromagnetic martensitic phase. Recent investigations show that this first order magnetostructural transition, due to the overlap of TM and TC, results in a fairly large magnetic entropy change, SM, in offstoichiometric Ni2MnGa. In this paper we present a study on the gradual overlap of the first order and second order phase transitions and the related magnetic entropy changes of polycrystalline Ni2+xMn1-xGa (x = 0.16, 0.17, 0.18, 0.19, 0.20). Ingots of approximately 5gm of polycrystalline Ni2+xMn1-xGa (x = 0.17, 0.18, 0.19, 0.20) were fabricated by conventional arc melting in an argon atmosphere using Ni, Mn, and Ga or 4N purity. The samples were annealed in vacuum for 144 hours at 800掳C, and slowly cooled down to room temperature.
机译：从应用的观点来看，化学计量的Ni 2 MnGa是令人关注的，因此已经报道了许多关于化学计量的Ni 2 MnGa的研究。特别关注富镍的Ni2MnGa，用Ni代替Mn会导致TM升高和TC降低，从而导致某些Mn浓度两个跃迁重叠[1]。如前所述，马氏体转变是一阶转变，其中在TC处从铁磁阶到顺磁阶的转变是二阶转变。发现TM和TC的重叠导致从顺磁立方相到铁磁马氏体相的一阶磁结构转变。最近的研究表明，由于TM和TC的重叠，这种一级磁结构转变导致了化学计量的Ni2MnGa中相当大的磁熵变化SM。在本文中，我们对多晶Ni2 + xMn1-xGa（x = 0.16，0.17，0.18，0.19，0.20）的一阶和二阶相变的逐渐重叠以及相关的磁熵变化进行了研究。通过使用Ni，Mn和Ga或4N纯度的氩气中的常规电弧熔化，制造了约5g的多晶Ni 2 + x Mn 1-x Ga（x ＝ 0.17、0.18、0.19、0.20）的铸锭。将样品在800°C的真空中退火144小时，然后缓慢冷却至室温。

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