四元Heusler合金Co_2[Mn,Fe]Ge_(1-x)Ga_x的晶体结构、半金属稳定性和磁性

基于密度泛函理论的第一性原理研究了四元Heusler合金Co2MnGe1-xGax和Co2FeGe1-xGax的晶体结构、半金属稳定性和磁性,发现随着掺入Ga比例x的改变,其晶格常数遵循Vigard原理。对于Co2MnGa1-xGex系列的合金,其原胞磁矩很好地符合SP(Slater-Pauling)值和实验值;而对于Co2MnGe1-xGax系列合金,其原胞磁矩虽然与实验值一致,但却随x的减小而逐渐偏离SP值。态密度显示,Co2MnGe1-xGax系列合金在x=0～0.75的掺杂比例内都有0.4～0.6eV的自旋向下带带隙宽度,而Co2FeGe1-xGax虽然在费米面附近也存在较高的自旋极化,但几乎不存在显著的自旋向下带带隙。此外,四元合金Co2MnGe0.75Ga0.25的费米面居于最小自旋带隙的中部,将具有较好的半金属稳定性。     

四元Heusler合金Co2[Mn,Fe]Gel-xGax的晶体结构、半金属稳定性和磁性

基于密度泛函理论的第一性原理研究了四元Heusler合金Co2MnGe1-xGar和Co2FeGe1-xGax的晶体结构、半金属稳定性和磁性,发现随着掺入Ga比例x的改变,其晶格常数遵循Vigard原理.对于Co2 MnGal-x Gex系列的合金,其原胞磁矩很好地符合SP(Slater-Pauling)值和实验值;而对于Co2 MnGe1-xGax系列合金,其原胞磁矩虽然与实验值一致,但却随x的减小而逐渐偏离SP值.态密度显示,Co2 MnGe1-xGax系列合金在x=0～0.75的掺杂比例内都有0.4～0.6eV的自旋向下带带隙宽度,而Co2 FeGe1-xGax虽然在费米面附近也存在较高的自旋极化,但几乎不存在显著的自旋向下带带隙.此外,四元合金Co2 MnGe0.75 Ga0.25的费米面居于最小自旋带隙的中部,将具有较好的半金属稳定性.     

双钙钛矿Sr_2Fe_(1-x)Cr_xMoO_6电子结构与磁学性质的第一性原理研究

利用第一性原理系统计算了Sr2Fe1-x CrxMoO6体系中不同Cr含量(x=0,1/3,0.5和1)所对应Sr2Fe1-xCrxMoO6晶胞结构的电子结构和磁学性质。计算结果表明,体系的晶格常数、总磁矩及Fe离子磁矩由于Cr掺杂而降低,晶格发生畸变,并发现当Cr掺杂含量增加至0.5时自旋向上费米面附近态密度明显增加,超胞呈金属性,但Sr2Fe1-xCrxMoO6(x=0,1/3,0.5和1)仍保持半金属性。得出的计算结果与现有的实验和理论值吻合较好,该结果为其材料的进一步实际应用提供了理论依据。     

添加Co对AB3.5型储氢合金结构及性能影响的理论与实验研究

基于密度泛函理论,采用总能量平面波赝势方法,设计了La0.75Mg0.25Ni3.5-xCox(x=0、0.25、0.5、0.75)系列合金,并研究了其晶体及电子结构.计算结果显示随着Co含量的增加,La原子上的电荷转移先增大后保持不变,在Co含量为0.5时达到最大;费米能级处的态密度值先增加后稍减小,Co含量在0.5时达到最大.利用悬浮感应熔炼法制备了该系列合金La0.75Mg0.25Ni3.5-xCox(x=0、0.3、0.5、0.7).对熔炼所得合金进行了结构和性能表征,XRD结果显示,随着Co含量的增加,合金的主相均为Ce2Ni7型AB3.5相;电化学测试显示,x=0.5时,合金的放电容量和循环性能均较好,为398.5 mAh/g,容量保持率S250为62%;PCT结果表明,在室温条件下合金的吸氢平台在0.04~0.09 MPa之间,当x=0.5时,吸氢平台压最低,为0.04 MPa,同时吸氢量最大,为1.587wt%.综合分析计算和实验的结果,AB3.5合金性能随Co添加量的变化趋势符合第一性原理计算的预测.     

四元Heusler合金CoFeTiSb的空位和反位缺陷效应

采用密度泛函理论研究了空位和反位缺陷效应对四元Heusler合金CoFeTiSb电子结构的影响。结果表明,Vd_(Co)、Vd_(Fe)空位缺陷和Ti_(Sb)反位缺陷具有负的形成能,能保持85%以上的自旋极化率,在CoFeTiSb合金工业制备过程中自发形成几率较大;Vd_(Ti)、Vd_(Sb)空位缺陷和Fe_(Co)反位缺陷虽然费米面处自旋极化率为100%,仍保持半金属性,但正的形成能表明在制备过程中它们出现的几率较小。此外,Sb_(Co)、Sb_(Fe)、Sb_(Ti)、Co_(Ti)、Fe_(Ti)反位缺陷由于具有较大的形成能和半金属性破坏,自旋极化率不同程度降低。     

新型稀磁半导体(La_(0.75)Ba_(0.25))(Ag_(0.75)Mn_(0.25))SO电子结构与光学性质的第一性原理计算

采用基于密度泛函理论的第一性原理赝势平面波方法,对新型稀磁半导体(La_(0.75)Ba_(0.25))(Ag_(0.75)Mn_(0.25))SO的电子能带结构、自旋极化态密度和光学性质进行了计算与分析。计算结果表明La_(0.75)Ba_(0.25)Ag_(0.75)Mn_(0.25)SO为直接带隙半导体,能隙约为1.2 eV,掺杂后体系带隙明显减少;费米面附近态密度发生劈裂,出现了明显的自旋有序排列,呈明显的铁磁性,磁性的产生是通过Mn3d和相邻S3p原子之间的p-d杂化作用实现的;La_(0.75)Ba_(0.25)Ag_(0.75)-Mn_(0.25)SO反射主峰与吸收曲线峰值都在紫外波段,而且集中在100~300 nm的范围。但在近红外和可见光区,此材料反射和吸收都较低,可用于可见光高透明的领域。     

Co2MnSi1-xZx(Z=B,P, As) Heusler合金的磁性和电子结构的第一性原理计算

通过第一性原理计算研究了Co₂MnSi_1-xZ_x(Z=B, P, As) Heusler合金的能带结构和自旋极化，同时也证明了具有不同价电子的其他元素Z对Si的取代可以控制费米能级在带隙中的移动。与此同时，随着同种Z元素浓度的变化，晶格参数具有接近线性变化的特征，并且这些合金的能带结构具有相似的特点。不同合金的磁矩随价电子数呈线性变化趋势，符合Slater-Pauling行为。由计算结果可知，当x=0.125左右，Co₂MnSi_1-xP_x和Co₂MnSi_1-xAs_x合金具有较好的电子性质和较高的自旋极化。     

Ta添加对(Fe_(0.46)Co_(0.36)Ni_(0.04)Zr_(0.14))_(100-x)Ta_x非晶合金复合材料磁性能和力学性能的影响

利用铜模吸铸法制备出直径为3 mm的(Fe0.46Co0.36Ni0.04Zr0.14)100-xTax(x=0,4,8,12)非晶合金复合材料柱状试样,通过X射线衍射(XRD)、选区电子衍射(SAED)、振动样品磁强计(VSM)、微机控制电子式万能力学试验机和扫描电镜(SEM)研究了不同Ta含量对合金的磁性能和压缩力学性能的影响。结果表明,随着Ta含量的增加,合金中不断有晶体相Co2Zr、Co2Ta0.5Zr0.5析出,合金的矫顽力Hc呈现出减小的趋势,而合金的饱和磁化强度Ms、抗压强度σbc和弹性应变εe则呈现出先增加后减小的趋势。当x=4时,合金的Ms、σbc和εe均达到最大值,分别为137.70A·m2/kg、1 814 MPa和4.583%。说明选取合适的Ta含量能够明显改善(Fe0.46Co0.36Ni0.04Zr0.14)100-xTax非晶合金复合材料的磁性能和力学性能。     

单层WxMo1-xS2合金电子和光学性质的第一性原理研究

采用基于密度泛函理论(DFT)的第一性原理方法对WxMo1-xS2(x=0,0.25,0.5,0.75,1)单层合金的电子结构和光学性质进行了较为系统的研究.计算结果表明:合金的带隙都为直接带隙,随着W含量增加可由1.802 eV增加至1.940 eV,但并不是线性增加.电荷差分密度图计算结果显示,随着W含量的增加,Mo原子失电子数逐渐增加,W原子得电子数逐渐增加.合金的光学性质可随着W含量的变化调谐.随着W含量的增加,WxMo1-xS2合金的静态介电常数逐渐减小,虚部吸收阈值逐渐增大,吸收边向高能区移动.与本征Mo16S32相比,WxMo1-xS2合金在紫外光区域(6～8.5 eV)表现出更强的紫外吸收能力,而W12Mo4S32和W16S32合金在可见光区域(～3 eV)有着更高的吸收系数,这在理论上表明此类单层合金在可见光及近紫外光区域可应用于光电信号的探测.     

Al替代Ni对La_2MgNi_(7.5-x)Co_(1.5)Al_x贮氢合金相结构及电化学性能的影响

采用X射线衍射、电子探针和电化学测试研究了La2MgNi7.5-xCo1.5Alx(x=0.0~0.5)合金的相结构和电化学性能。XRD结果和EPMA观察表明,Al替代改变了合金的物相组成和物相的丰度。LaNi3相消失,αLa2Ni7相丰度的变化表现为先增加(x从0.0~0.1)后减少(x=0.3和0.5),LaNi5相随合金中Al含量x的增加而明显增加。Al替代Ni降低了合金的最大电化学放电容量和活化性能,但明显改善了La2MgNi7.5Co1.5合金的电化学循环稳定性。     

A First Principle Study of Co2CrZ (Z=In, Sn, Sb) Based on FP-LAPW Method

The structural and electronic properties of Heusler alloys Co₂CrZ (Z=In, Sn, Sb) have been studied using the generalized gradient approximation (GGA) and local spin density approximation (LSDA), respectively. Among the systems under investigation, Co₂CrSb gives 100 % spin polarization at E _F . Co₂CrSb is the most stable Half-Metallic Ferromagnets (HMFs) with an energy gap 0.25 eV at the Fermi level in the spin-down channel. We have also found that the increase in the total magnetic moment as Z goes from In to Sb. The calculated density of states (DOS) and band structures shows the half-metallic ferromagnets (HMF) character of Co₂CrSb.     

Electronic,Elastic, and Magnetic Properties of the Full-Heusler with the 4d Transition Metal Element,Co<Subscript>2</Subscript>YSi,Co<Subscript>2</Subscript>ZrSi,and Co<Subscript>2</Subscript>Y<Subscript>0.5</Subscript>Zr<Subscript>0.5</Subscript>Si: a First-Principle Study

The full-potential all-electron linearized augmented plane wave plus local orbitals (FP-LAPW + lo) method, as implemented in the suite of software WIEN2K, has been used to systematically investigate the structural, electronic, elastic, and magnetic properties of the half-metallic ferromagnetic Heusler compounds with 4d transition elements Co₂YSi, Co₂ZrSi, and alloy Co2Y_0.5Zr_0.5Si presented. The theoretical formalism used is the generalized gradient approximation to density functional theory (GGA) with the Perdew–Burke–Ernzerhof (PBE) exchange-correlation functional. The calculated lattice parameters and magnetic moments agree well with the available theoretical results. The spin-polarized density of states (DOS) of Co₂YSi and Co₂ZrSi indicate a half-metallic behavior with vanishing electronic density of states for minority spin at the Fermi level, which yields a perfect spin polarization while for Co₂Y_0.5Zr_0.5Si shows a nearly half-metallic behavior with small spin-down electronic density of states at the Fermi level.     

First-Principles Study of Structural,Electronic, Magnetic and Half-Metallic Properties of the Heusler Alloys Ti<Subscript>2</Subscript>ZAl (Z = Co,Fe, Mn)

Using the first-principles calculations based on density functional theory within the generalized gradient approximation (GGA), we investigate the structural, electronic and magnetic properties of the Ti₂ZAl (Z = Co, Fe, Mn) alloys with the CuHg₂Ti-type structure. The optimized equilibrium lattice constants were found to be 6.08 Å for Ti₂CoAl, 6.07 Å for Ti₂FeAl and 6.16 Å for Ti₂MnAl. The Ti₂ZAl (Z = Co, Fe, Mn) alloys are found to be half-metallic ferromagnets. The total magnetic moment of Ti₂ZAl (Z = Co, Fe, Mn) is 2, 1 and 0 µ _B, respectively, which is in agreement with the Slater–Pauling rule M _tot=Z _tot- 18. The Ti₂ZAl (Z = Co, Fe, Mn) have a band gap of 0.64745, 0.57795 and 0.39327 eV, respectively.     

Effect of substituting sp-element on half metallic ferromagnetism in NiCrSi Heusler alloy

The investigation of the electronic and magnetic properties of semi Heusler alloy, NiCrSi and alloys obtained by substituting Si with Ge and P (i.e. NiCrGe and NiCrP) has been presented. The present work aims to search the new candidates showing half metallic (HM) ferromagnetism using all-electron full potential linearized augmented plane wave (FPLAPW) method within generalized gradient approximation (GGA) formalism for exchange and correlation (XC) effects. We predict that these alloys are true half metallic ferromagnets (HMFs), showing an indirect band gap in minority density of states (DOS). This gap is largest in case of NiCrSi (0.91 eV). The substitution of sp-element in NiCrSi results in the minority band gap reduction. The main contribution in total magnetic moment comes from Cr-3d states. Moreover, it has an integer value in present alloys as expected in accordance with the Slater-Pauling (SP) rule.     

Theoretical Investigations of Properties of New Half-Heusler Compounds NiFeZ (Z = Si,Ge)

The half-Heusler alloys NiFeZ (Z = Si, Ge) are investigated theoretically in the general framework of Density Functional Theory (DFT). We have calculated structural, mechanical, elastic, electronic, magnetic, and optical properties using WIEN2k simulation package. To incorporate the electronic exchange-correlation energy we have used the GGA+U approximation. Both the compounds are found structurally stable, however, elastic properties show that NiFeSi is ductile while NiFeGe is brittle. Electronically, the compounds are found to be half-metals with small band gaps in the spin-down channels whereas spin-up channels are conducting. The total magnetic moments for both the compounds are ≈ 2μ _B. Optical properties including dielectric function, refractive index, extinction coefficient, reflectivity, conductivity, and absorption coefficient are also calculated and analyzed.     

First-Principle Study of Half-Metallic Ferrimagnet Behavior in Titanium-Based Heusler Alloys Ti<Subscript>2</Subscript>FeZ (Z = Al,Ga, and In)

Using density functional theory with the full-potential linearized augmented plane-wave method (FP-LAPW), we have study the structural, electronic, and magnetic properties of Ti₂FeZ (Z = Al, Ga, and In) alloys with Hg₂CuTi-type structure. The magnetic stabilities reveal that all our compounds exhibit ferrimagnetic (FiM) behaviors. The electronic structure report the existence of a gap energy equal to 0.56, 0.60, and 0.64 eV for Ti₂FeAl, Ti₂FeGa, and Ti₂FeIn, respectively, in the spin-down state and divulge metallic intersections at the Fermi level for the spin-up state. These results indicate that our compounds have a half-metallic (HM) nature. In addition to this, the total magnetic moments are in agreement with the obtained one by the Slater-Pauling rule (M _tot = Z _tot ? 18), which indicates the 100% spin polarization for these compounds.     

Co对Nd-Fe-Al大块非晶合金的晶化行为和磁性能的影响

采用示差扫描量热法(DSC)，X射线衍射(XRD)和振动样品磁强计(VSM)研究了Co对Nd—Fe-Al大块非晶合金的非晶形成能力，晶化行为和磁性能的影响。结果表明：加入Co元素后可以显著提高Nd—Fe-Al大块非晶合金的非晶形成能力以及提高合金的居里温度。Nd60Fe30-xAl10Cox(z=0、5、10)大块非晶合金在室温有较高的内禀矫顽力，具有硬磁性。内禀矫顽力随着Co含量的增加变化不大．但是饱和磁化强度和剩磁则随着Co含量的增加有所下降。Nd60Fe3-xAl10Cox(x=0、5、10)大块非晶合金具有的硬磁性能来自于非晶相。合金少量晶化后，磁性能变化不大。完全晶化后合金的硬磁性迅速消失。     

Fe–Co magnetic nanoclusters by density functional theory calculations

ABSTRACT

We present density functional theory results referring to the structural, electronic and magnetic properties of 13, 55, 147 and 309 Fe–Co (magnetic–magnetic) icosahedral nanoclusters (ICO) comparing with our previous results on Fe–Cu (magnetic–nonmagnetic). It came out that the Fe atoms always favour the edge surface sites exhibiting higher average magnetic moment (MM) for Fe and FeCo ICOs than FeCu while the local Fe MM is greater for FeCu₁₂ and Fe₆Cu₄₉ ones. This is due to the strong hybridisation of Fe 3d–Co 3d states, while in Fe–Cu the Fe spin down states are restricted close to fermi without been affected by the corresponding Cu states. These results could be used for the design of environmental sustainable smart magnetic alloys.

This is part of a thematic issue on Nanoscale Materials Characterisation and Modeling by Advances Microscopy Methods - EUROMAT.