Structure and magnetic properties of intermetallic compounds Gd5Si2-xGe2-xM2x

1 Introduction Nowadays, there is much interest in magnetic refrigeration materials, as they offer the prospect of an energy-efficient and environment friendly alternative for the traditional vapour cycle refrigeration technique. Magnetic refrigeration is…     

磁一级相变La(Fe,Si)13基化合物的研究进展

对NaZn13型La(Fe,Si)13化合物在磁性、相变特性及磁熵变规律和制备方法等方面的研究进展进行总结和论述.低Si含量的La(Fe,Si)13化合物具有良好的软磁特性,表现出特殊的磁相变特征:在居里温度处的热诱导一级磁性转变和高于居里温度时的磁场诱导巡游电子变磁性转变,并伴随着巨大的磁熵变和磁致伸缩.随Si含量增高,化合物的这种特殊磁相变特征逐渐减弱,呈现二级相变特征.总结了元素替代和添加对La(Fe,Si)13化合物性能的影响,一定的合金化可以改变居里温度并保持巨大的磁熵变,这对于近室温磁致冷有着重要意义.快速凝固技术的应用解决了化合物合成困难的问题,降低了材料的制造成本,使得NaZn13型La(Fe,Si)13化合物成为最有应用前景的近室温磁致冷工质之一.本文还探讨了这种化合物用做磁致伸缩材料的可能性.     

MAGNETIC PROPERTIES OF Nd_2(Fe_(1-x)M_x)_(14)B(M=C,Si)ALLOYS

The magnetic properties of Nd_2(Fe_(1-x)M_x)_(14)B(M=C and Si) compounds have been investi-gated.Substituting Fe by small amount of metalloid C and Si atoms,the compounds are intetragonal structure and have uniaxial anisotropy.The substitution of C for part of Fe de-creases the Curie temperature of the compounds and the intrinsic coercivity of bond samples atlow temperature.The replacement of Fe by Si makes the Curie temperature increase with amaximum at the vicinity of x=0.15.When the range of the content of Si is 0≤x≤0.10,thecoercivity distinctly increases at room temperature.from 62.7 kA/m at x=0 to138.7kA/m at x=0.06,where the residual magnetization has a maximum:in the mean timethe saturation magnetization decreases only by a small amount.     

Gd Mössbauer effect and magnetic properties of GdNi2Sb2, GdCu2Sb2 and GdAl2Ga2

We have investigated the magnetic properties and the ¹⁵⁵Gd Mössbauer spectra of the ThCr₂Si₂-type compounds GdNi₂Sb₂, GdCu₂Sb₂ and GdAl₂Ga₂. These three compounds were found to order antiferromagnetically, with T_N=6.5 K, 15.8 K, and 42.4 K respectively. The electric field gradient V₂₂ derived from the quadrupole splitting of the Mössbauer spectra gives rise to a sign change at the end of the T3d series in GdT₂Sb₂, as was observed previously also for the corresponding compounds with Si and Ge. This behaviour was explained in terms of decreasing hybridization between the Gd valence electron states and the d electron states of the T atoms.     

Magnetic phase transition in Ti-doped ErCo2 alloys

The homogeneity range of U₂Co_17−xSi_x system with the hexagonal Th₂Ni₁₇-type crystal structure extends from x = 1 to 3.4. The variation of the magnetic properties within the homogeneity range was studied on single crystals. All the compounds are ferromagnetic, M_s and T_C decrease monotonously with increasing Si content. The strongly modified magnetic anisotropy of U₂Co_17−xSi_x, as compared to isostructural Lu₂Co_17−xSi_x with the non-magnetic Lu, points to a magnetic state of U up to x = 3.0. The U contribution to K₁ decreases with increasing Si content and vanishes at x = 3.4 that can be treated as a transition from magnetic to non-magnetic state of U. Spin reorientation was observed with varying temperature in compounds with x ≤ 3 due to a competition of the U and Co sublattices anisotropies which occurs as two second-order phase transitions of the “plane–cone” and the “cone–axis” type.     

Overview of magnetoelastic coupling in(Mn,Fe)_2(P,Si)-type magnetocaloric materials

(MnFe)_2(P, Si)-type compounds are, to date, one of the best candidates for magnetic refrigeration and energy conversion applications due to the combination of giant magnetocaloric effect (MCE), tunable working temperature range and low material cost. The giant MCE in the (Mn,Fe)_2(P, Si)-type compounds originates from strong magnetoelastic coupling, where the lattice degrees of freedom and spin degrees of freedom are efficiently coupled. The tunability of the phase transition, in terms of the critical temperature and the character of the phase transition, is essentially attributed to the changes in the magnetoelastic coupling in the (Mn, Fe)_2(P, Si)-type compounds. In this review, not only the fundamentals of the magnetoelastic coupling but also the related practical aspects such as magnetocaloric performance, hysteresis issue and mechanical stability are discussed for the (Mn, Fe)_2(P, Si)-type compounds. Additionally, some future fundamental studies on the MCE as well as possible ways of solving the hysteresis and fracture issues are proposed.     

Mn1.2Fe0.8P0.48Six化合物的结构和磁热效应

对Mn1.2Fe0.8P0.48Si0.52和非化学计量比Mn1.2Fe0.8P0.48Si0.49化合物的物相与磁热效应(MCE)进行了研究。结果表明:两种化合物均为Fe2P型六角结构(空间群为P-62m),化合物中含有少量的(Fe,Mn)3Si第二相。当Si的含量x由0.52降到0.49时,化合物的居里温度由268K升到282K,而Si含量的变化对化合物的热滞没有明显的影响。Mn1.2Fe0.8P0.48Si0.52和Mn1.2Fe0.8P0.48Si0.49化合物在外磁场变化为0~1.5T下的最大磁熵变分别为11.7J/kg·K和9.0J/kg·K。低成本的原料、较大的磁熵变使得Mn1.2Fe0.8P0.48Six化合物成为一种理想的室温磁致冷材料。     

Structure and magnetocaloric effect in melt-spun La(Fe,Si)13 and MnFePGe compounds

The magnetocaloric properties of melt-spun La(Fe,Si) 13 and MnFePGe compounds were investigated. Very large value of magnetic entropy change |ΔS|=31 and 35.4J·(kg·K)-1 under 5 T were obtained at 201 K in LaFe11.8Si1.2 melt-spun ribbons and at around 317 K in Mn1.1Fe0.9P0.76Ge0.24 melt-spun ribbons, respectively. The large magnetocaloric effect results from a more homogenous element distribution related to the very high cooling rate during melt-spinning. The excellent MCE properties, the low materials cost and the accelerated aging regime make the melt-spun-type La(Fe,Si)13 and MnFePGe materials an excellent candidate for magnetic refrigerant applications.     

铜模吸铸法制备的Fe74Al4Sn2P10Si4B4C2非晶合金圆棒及圆管块体

利用铜模吸铸法制备了直径1．0mm,1．5mm和2．0mm的Fe74Al4Sn2P10Si4B4C2块体非晶合金圆棒,利用振动样品磁强计测试了圆棒的磁性能,其饱和磁感应强度较高,矫顽力较低。利用铜模吸铸法制备了Ф5mm×庐7mm×15mm的Fe74Al4Sn2P10Si4B4C2块体非晶合金圆管,并测试了其DSC曲线,块体非晶合金圆管与块体非晶合金圆棒的热性质一致。Fe74Al4Sn2P10Si4B4C2块体非晶合金显示出优良的铸造性能。     

Neutron diffraction studies of CaMn2Ge2 and BaMn2Ge2 compounds: first examples of antiferromagnetic Mn planes in ThCr2Si2-type structure compounds

The magnetic properties of ThCr₂Si₂-type structure CaMn₂Ge₂ and BaMn₂Ge₂ compounds have been investigated by neutron diffraction experiments. In the whole temperature range studied (2–270 K), both compounds are purely collinear antiferromagnets (not detected by bulk magnetometric measurements) characterized by a stacking of antiferromagnetic (001) Mn planes. This peculiar Mn-sublattice magnetic behaviour seems to be related to the valency of the large metal. At 2 K, the total Mn moments are about 2.7 μ_B and 3.6 μ_B for CaMn₂Ge₂ and BaMn₂Ge₂, respectively. The results are compared with those of closely related RMnSi and RMnGe compounds and the isotypic alkali-metal manganese pnictides. The magnetic properties of the ThCr₂Si₂-type structure RMn₂X₂ (XSi, Ge) compounds are discussed.     

Neutron diffraction studies of LaMn2Ge2Si2 and LaMn2Si2 compounds: evidence of dominant antiferromagnetic components within the Mn planes

The magnetic properties of ThCr₂Si₂-type structure LaMn₂Ge₂ and LaMn₂Si₂ compounds have been reinvestigated by neutron diffraction experiments. The ferromagnetic ordering previously proposed to take place on the manganese sublattice is revised. At high temperature, both compounds are purely collinear antiferromagnets (not detected by magnetic measurements), characterized by a stacking of antiferromagnetic (001) Mn planes. Below T_c=310 and 325 K for LaMn₂Ge₂ and LaMn₂Si₂, respectively, both compounds exhibit an easy-axis ferromagnetic behaviour. However, the occurrence of a dominant antiferromagnetic component within the (001) Mn planes yields a conical magnetic structure for the germanide (cone semi-angle =58° at 2 K) and a canted magnetic structure for the silicide (φ=49°). At 2 K, the total Mn moments are about 3.0 and 2.4 μ_B for LaMn₂Ge₂ and LaMn₂Si₂, respectively. The results are compared with those of closely related RMnSi and RMnGe compounds and the magnetic properties of the ThCr₂Si₂-type structure RMn₂X₂ (XSi, Ge) compounds are discussed.     

Thermoelectric performance of Mg2−xCaxSi compounds

Thermoelectric materials Mg_2−xCa_xSi (x = 0, 0.01, 0.03, 0.05, 0.07, 0.1) compounds have been prepared by vacuum melting followed by hot-pressing. Effects of the substitution of Ca for Mg on phase structures and the thermoelectric properties of the hot-pressed compounds were investigated. It was found that the alloying of Ca in Mg₂Si based compounds increases the electrical conductivity and decreases the Seebeck coefficient of the compounds, due to the electronegativity difference between Ca and Mg. The dimensionless figures of merit of Mg₂Si and Mg_1.99Ca_0.01Si reach, respectively, 0.41 and 0.34 at 660 K.     

纳米晶SmCo_(8.9)Si_(0.9)永磁合金相变特征及其磁性能变化研究

首次制备出具有高稳定性的Sm Co_(8.9)Si_(0.9)纳米晶合金,进而系统研究了亚稳相Sm Co_(8.9)Si_(0.9)的相变特征及相应的磁性能变化规律。发现添加元素Si可以有效提高过饱和固溶体亚稳相SmCo_(9.8)的稳定性,随着热处理温度的升高,SmCo_(8.9)Si_(0.9)纳米晶合金由SmCo_(9.8)(H)结构的单相转变为Sm_2Co_(17)(H)和Co(fcc)相,且伴随相变,矫顽力提高。其机理源于析出的细小Co相造成钉扎机制增强。进一步升高热处理温度,Sm_2Co_(17)(H)相转变为Sm_2Co_(17)(R)相,同时晶粒长大明显且晶粒尺寸分布不均匀,导致磁性能下降。     

Elastic properties and electronic structure of vanadium silicides-a density functional investigation

Vanadium and silicon form several binary compounds; the most well characterized structures have the compositions V:Si = 3:1, 5:3, 6:5, 1:2. Spin-polarized density functional band-structure calculations using the projector augmented wave method have been carried out for the stable binary compounds in the Si–V system. As many rare earth and early transition metals are refractory materials, the study focuses on the ground state structures and their stabilities by determining their elastic properties. The magnetic properties are also investigated because structurally related Si–Mn compounds exhibit magnetism. All the silicides investigated were found to be refractory, hard, metallic materials and no magnetism was found in the ground state structures.     

Changes in the phase structure and magnetic characteristics of Gd5Si2Ge2 when alloyed with Mn

Gd₅Si₂Ge₂ parent compounds were alloyed with Mn in order to understand the underlying relation between the structural phases and the magnetic behavior of the pseudo ternary compounds formed. The alloying mechanism in Gd₅Si₂Ge₂ causes simultaneous substitution of the nonmagnetic Si and Ge atoms from the (Si + Ge) sublattice in equal amounts. No subsequent heat treatment was made on alloyed compounds. X-ray powder diffraction, magnetization versus temperature and isothermal magnetization measurements were carried out. X- ray diffraction patterns were used to qualitatively determine the existence of different structural phases in the alloys. It was observed that the starting, as-melted alloy with z = 0 has Gd₅Si₄-type orthorhombic structure at room temperature with traces of 1:1 stoichiometry phase which transforms totally to a Gd₅Si₂Ge₂-type monoclinic phase when heat treated. Similarly, increase in the Mn content leads to an increase in the monoclinic phase content of the originally orthorhombic compounds. Curie temperatures were determined from M(T) measurements and the magnetocaloric characterization was made using M(H) measurements by plotting the magnetic entropy change values against temperature. No giant magnetocaloric effect was observed for non heat treated samples.     

快淬带粉末冶金法制备块状La(Fe, Co, Si)13磁制冷材料

利用快淬带粉末通过常压烧结技术制备了块状La(Fe,Co,Si)13磁制冷材料,并研究了温度和烧结助剂LaFeSi对烧结体显微组织与性能的影响。研究表明:随着烧结温度的提高,烧结体的密度增加,相成分发生明显的变化,具有立方NaZn13型La(Fe,Co,Si)13相大量生成。当向快淬带LaFe10.8Co0.7Si1.5C0.2粉末添加20%LaFeSi粉末经压样成形后在1200℃烧结3 h后,得到的烧结体基本是单一的La(Fe,Co,Si)13相,致密度达到90%左右。     

Magnetic properties of new compounds RuMn2Sn and RuMn2Si

New compounds RuMn₂Z (Z = Si, Sn) have been synthesized. X-ray diffraction measurements have confirmed that RuMn₂Z (Z = Si, Sn) crystallizes in a Heusler-like cubic structure. The lattice parameters of RuMn₂Si and RuMn₂Sn at room temperature are estimated to be 5.8260 Å and 6.2195 Å, respectively. Magnetization measurements have been carried out in fields up to 50 kOe for RuMn₂Z (Z = Si, Sn). Furthermore, the temperature dependence of initial permeability of RuMn₂Sn has been studied. RuMn₂Sn shows ferrimagnetic behavior. The spontaneous magnetic moment at 5 K and the Curie temperature of RuMn₂Sn are found to be of 1.68 μ_B/f.u. and 272.1 K, respectively. RuMn₂Si exhibits spin-glass-like behavior with a freezing temperature estimated to be about 50 K.     

热处理对水雾化Fe74Cr2Mo2Sn2P10C2Si4B4非晶磁粉芯性能的影响

采用水雾化Fe74Cr2Mo2Sn2P10C2Si4B4非晶磁粉制备出了高频特性较好的磁粉芯。研究了去应力退火和磁场退火对磁粉芯磁性能的影响。结果表明:非晶磁粉芯压制后的去应力退火处理能有效提高磁导率和品质因数。过高热处理温度使非晶粉末晶化,析出导电性较差的非铁磁相,恶化磁性能。最佳退火温度为400℃,最佳的磁性能为:在3500 kHz的频率下,μ=40.5,Q=225。磁场退火对Fe74Cr2Mo2Sn2P10C2Si4B4非晶磁粉芯磁导率影响较小。纵向磁场退火能增大非晶磁粉芯的损耗,横向磁场退火能降低非晶磁粉芯的损耗,磁粉芯总损耗变化主要来源于磁滞损耗。     

Nano-grain Al2O3 crystals grown by sputtering of aluminium on CoCrPt thin films for potential application in ultra-high-density recording media

Designing supraceramic assemblies based on Al₂O₃ has remained a challenge due to the problems associated with the suitable dispersion in neat compounds and ability to control the preferred orientation in a unique fashion. Herein, granular HCP-(CoCrPt)_100−X(Al₂O₃)_X (X represents the percent weight) thin films with Si(1 0 0) substrates have been fabricated using sputtering technique followed by annealing treatment. Structural and magnetic properties of thin film have been investigated for potential application in magnetic recording media. It was shown that coercivity increased from 0.5 to 2.5 kOe by increasing the nano-grain Al₂O₃ content in the CoCrPt magnetic layers. In CoCrPt-Al₂O₃ thin films coercivity of 2.5 kOe has been obtained with increasing the Al₂O₃ content from 3 to 13 wt.% in the annealed thin films. The structural properties of the samples were studied using X-ray diffraction (XRD) and transmission electron microscope (TEM) equipped with selected area electron diffraction (SAED). The magnetic properties of the samples were measured with a vibrating sample magnetometer (VSM). The VSM results showed that the HCP-CoCrPt-Al₂O₃ granular films are a promising candidate for ultra-high-density recording media because of its low Al₂O₃ content and simple manufacturing process.