Effect of temperature and compositional changes on the phonon properties of Ni-Mn-Ga shape memory alloys
Effect of temperature and compositional changes on the phonon properties of Ni-Mn-Ga shape memory alloys
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DOI：
10.1103/physrevb.86.144305
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发表时间：
2012-10-11
影响因子：
3.7
通讯作者：
Acet, Mehmet
中科院分区：
文献类型：
--
作者：
Ener, Semih;Neuhaus, Juergen;Acet, Mehmet
本文报道了铁磁形状记忆合金体系Ni-Mn-Ga在其化学计量Ni2MnGa和非化学计量Ni49Mn32Ga19成分中的振动特性。给出了不同温度下的弹性和非弹性中子散射测量，重点是奥氏体相，并与第一性原理计算进行了比较。除了TA(2)[xi xi 0]声学分支和光声子分支的显著例外，两种成分的全声子色散的总体行为是相似的。与Ni2MnGa相比，Ni49Mn32Ga19的光学声子在整个倒易空间中的色散较小，这是由于过量的Mn原子占据了规则的Ga位。在接近马氏体转变时，两种样品在奥氏体相中观察到TA(2)[xi xi 0]声子分支的明显软化。其在倒易空间的位置揭示了马氏体相变机制。奥氏体L2(1)结构通过在[1 $(1)/ bar $0]方向上的变换（110）面转变为四方调制马氏体结构，类似于在d(1)和d(2)过渡金属的马氏体转变中观察到的情况。虽然化学计量样品中TA(2)[xi xi 0]声子软化的温度依赖性与磁性和结构转变完全吻合，但非化学计量样品的情况并非如此。在这里，磁有序与振动性质之间的关系仍然是一个悬而未决的问题。
We report on the vibrational properties of the ferromagnetic shape memory alloy system Ni-Mn-Ga in its stoichiometric Ni2MnGa and off-stoichiometric Ni49Mn32Ga19 compositions. Elastic and inelastic neutron scattering measurements at different temperatures are presented with a focus on the austenite phase and compared to first-principles calculations. The overall behavior of the full phonon dispersion is similar for both compositions with remarkable exceptions for the TA(2)[xi xi 0] acoustic branch and optical phonon branches. Less dispersion is found in the optical phonons for Ni49Mn32Ga19 in the whole reciprocal space when compared to Ni2MnGa and is explained by the occupation of regular Ga sites by excess Mn atoms. A pronounced softening in the TA(2)[xi xi 0] phonon branch within the austenite phase is observed in both samples when approaching the martensitic transition. Its location in reciprocal space reveals the martensitic transition mechanism. The austenite L2(1) structure transforms to the tetragonal modulated martensite structure by shuffling (110) planes in the [1 $(1) over bar $0] direction, similarly to what has been observed at the martensitic transitions of the d(1) and d(2) transition metals. Whereas the temperature dependence of the softening of the TA(2)[xi xi 0] phonons in the stoichiometric sample coincides perfectly with the magnetic and structural transitions, this is not the case for the off-stoichiometric sample. Here the relation between the magnetic ordering and the vibrational properties is still an open question.