应力下MnGa合金体电子性质及磁性质的研究

Investigation on electronic states as well as magnetic properties of the MnGa alloy under stress

采用密度泛函理论方法系统研究分析了四方结构MnGa合金体在a轴1GPa应力下的结构、形成、电子性质和磁性质.结果表明,应力下MnGa合金a,b轴晶格参数增大,c轴晶格参数减小,三轴夹角有偏离90°的趋势,晶胞体积增大.Mn-Ga不成键,Mn-Mn之间的强键作用进一步增强,Ga-Ga之间的强键作用消失.Mn-Mn结合键长和Ga-Ga结合键长均减小,而Mn-Ga间距增大,排斥作用减弱.应力下MnGa合金形成焓由-4.85 eV减小到-5.4 eV,其更容易生成.应力下其能带整体向下移动,导带和浅能级价带分布较宽,有效质量较小;深能级价带分布较窄,有效质量较大.自旋向下的电子能带没有带隙,自旋向上的电子能带有0.26 eV的间接带隙.s电子和p电子主要形成导带和浅能级价带,自旋极化较弱,d电子主要形成深能级价带,定域性和自旋极化最强.两种占位的Ga具有近乎相同的电子形态,Ga的s电子和Ga的p电子产生较明显的自旋极化,形成弱的磁性,Ga的d电子主要贡献深能级处的态密度,基本不贡献磁性质.两种占位的Mn主要贡献费米能处的态密度,Mn的d电子自旋极化最强,在费米能级下方Mn1自旋向下的电子态密度远高于Mn2的;在费米能级上方Mn1自旋向下的电子态密度远低于Mn2的.应力下MnGa产生了较明显的磁相变.

The formation,geometry,electronic as well as magnetic properties of the tetragonal MnGa alloy under 1 GPa stress along a-axis are investigated by density functional theory analyzing method.The results show that the lattice constants a and b are increased,but the constant c is decreased,the angles tend to deviate from 90°and the volume is increased under stress.There is no bond between Mn and Ga,the strong bond between Mn and Mn is enhanced,whereas the strong bond between Ga and Ga disappears.Both the distance between Mn and Mn and that between Ga and Ga are decreased,but the distance between Mn and Ga is increased,and their repulsive effect is weakened.The formation enthalpy decreases from-4.85 eV to-5.4 eV,and so the alloy is easier to be formed.The bands are all lowered,the conduction band and the shallow valance band are wide with smaller effective mass,whereas the deep valance band is narrow with larger effective mass.There is no band gap for the spin down band and an indirect band gap of 0.26 eV for the spin up band.The s and p electrons form the conduction band and the high energy valance band,there is weak spin polarization.The d electrons mainly form the deep valance band,the localization and spin polarization effect are the strongest.All Ga atoms have almost the same electronic configurations.The d electrons of Ga mainly contribute the deep density of states,they do not contribute to the magnetic properties.The s and p electrons of Ga form obvious spin polarization,they form weak magnetism.The two kinds of Mn contribute mainly to the density of states near Fermi energy and they form distinctively strong spin polarization,especially for the d electrons of Mn.Under and above Fermi energy,the density of state value for the spin down electrons of Mn1 is respectively very higher and very lower than that of the Mn2.Magnetic transition in MnGa is induced under stress.