Magnetic behaviour of two AlB2-related germanides: CePd0.63Ge1.37 and CeAu0.75Ge1.25

The title compounds were prepared from the elemental components by arc-melting and subsequent annealing at 1020 K and 1170 K. Both CePd_0.63Ge_1.37 and CeAu_0.75Ge_1.25 crystallize in the AlB₂ structure type with a statistical distribution of the transition metal and germanium atoms on the boron site. The structure of CeAu_0.75Ge_1.25 was refined from single crystal X-ray data: P6/mmm, a=433.5(1) pm, c=422.6(1) pm, V=0.0688(1) nm³, Z=1, wR2=0.0504 for 157 F² values and 7 variables. Magnetic susceptibility data for both compounds show a full cerium moment and ferromagnetic ordering at 6.0(5) K for CeAu_0.75Ge_1.25 and 3.0(5) K for CePd_0.63Ge_1.37. The crystal structure and properties of CeAu_0.75Ge_1.25 are compared with those of equiatomic CeAuGe which adopts the NdPtSb structure type, an ordered variant of AlB₂.     

Magnetic and electronic properties of RCoxGe2 (R = Pr, Nd) compounds

Magnetization, magnetic susceptibility, neutron diffraction and X-ray photoemission spectroscopy measurements were performed on polycrystalline samples: PrCo_0.85Ge₂ and NdCo_0.82Ge₂, crystallizing in an orthorhombic structure of the CeNiSi₂-type. The magnetometric data indicate that both compounds are antiferromagnetic at low temperatures, PrCo_0.85Ge₂ below 5 K and NdCo_0.82Ge₂ below 2.1 K. The neutron diffraction data indicate an antiferromagnetic structure in PrCo_0.85Ge₂ at 1.5 K and give no evidence of any magnetic ordering in NdCo_0.82Ge₂. The X-ray photoemission data indicate that the valence bands are formed predominantly by R 4f5d6s and Co 3d bands. The spin-orbit splitting values determined from the Pr and Nd, 3d_5/2 and 3d_3/2 XPS spectra are equal to 20.9 eV for the Pr compound and 23.0 eV for the Nd compound. The analysis of these spectra performed on the basis of the Gunnarsson–Schönhammer model revealed a small hybridization between 4f-electrons of the rare earths with the conduction band which implies rather good stability of the f shell in these compounds.     

Magnetic properties of CeMn2−xCoxGe2

The structure and magnetic properties of CeMn_2−xCo_xGe₂ (0.0≤x≤1.0) were studied by X-ray powder diffraction and magnetization measurements. All compounds crystallize in the ThCr₂Si₂-type structure with space group I4/mmm. Substitution of Co for Mn leads to a linear decrease in the lattice constants and the unit cell volume. Increasing substitution of Co for Mn shows a depression of ferromagnetic ordering.     

Magnetic ordering in ternary RMn2Ge2 compounds (R = Tb, Ho, Er, Tm, Lu) from neutron diffraction study

The compounds RMn₂Ge₂ (R = Tb, Ho, Er, Tm, Lu) have been investigated by neutron diffraction. TbMn₂Ge₂ is a collinear ferrimagnet with the Mn and Tb moment aligned along the c axis (μ_TB = 8.81(59) μ_B: μ_Mn = 2.21(44) μ_B). HoMn₂Ge₂ exhibits incommensurale ordering below 2.1 K characterized by two wavevectors at 1.3 K: q₁ = (0.1543(4), 0.1543(4), 0) and q₂ = (0.210(1), 0.007(1), 0). The Mn sublattice remains antiferromagnetic down to 1.3 K (μ_Mn = 2.38(6) μ_B). The Er moments order ferromagnetically below 5.5 K in ErMn₂Ge₂ (μ_Mn = 6.81(31) μ_B). The moments are perpendicular to the c axis. The Mn sublattice remains antiferromagnetic down to 1.8 K (μ_Mn = 2.34(18) μ_B). The magnetic structure of TmMn₂Ge₂ is characterized by the propagation vector (0.0.1/2). the Tm moments lying in the basal plane. The ordering of the Tm moments yields a canting of the Mn moments (τ = 21(3)°); μ_Tm = 6.63(18) μ_B; μ_Mn = 2.28(27) μ_B). The antiferromagnetic structure of LuMn₂Ge₂ has been determined (μ_Mn = 2.32(14) μ_B). The evolution of the magnetic properties of the heavy rare earth compounds RMn₂Ge₂ is discussed.     

Synthesis and properties of CaCd2Sb2 and EuCd2Sb2

High density polycrystalline CaCd₂Sb₂ and EuCd₂Sb₂ intermetallics are synthesized by Spark Plasma Sintering and their thermoelectric properties are investigated. X-ray diffraction measurements reveal both materials have a structure in space group, containing a small amount of CdSb as a second phase. Thermoelectric measurements indicate both are p-type conductive materials. The figure of merit value of CaCd₂Sb₂ is 0.04 at 600 K and that of EuCd₂Sb₂ is 0.60 at 617 K. Theoretical calculations show that CaCd₂Sb₂ is a degenerate semiconductor with a band gap of 0.63 eV, while EuCd₂Sb₂ is metallic with DOS of 13.02 electrons/eV. For deeper understanding of the better thermoelectric properties of EuCd₂Sb₂, its low temperature magnetic, transport and heat capacity properties are investigated. Its Nèel temperature is 7.22 K, convinced by heat capacity anomaly at 7.13 K. Hall effect convinced that it is a p-type conductive material. It has high Hall coefficient, high carrier concentration and high carrier mobility of +1.426 cm³/C, 4.38 × 10¹⁸/cm³ and 182.40 cm²/Vs, respectively. They are all in the magnitude of good thermoelectric materials. The Eu 4f level around Fermi energy and antiferromagnetic order may count for the better thermoelectric properties of EuCd₂Sb₂ than that of CaCd₂Sb₂.     

Magnetic and electrical properties of RCu5.1In6.9 compounds

Magnetic and electrical properties of the RCu_5.1In_6.9 compounds with R=Y, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho and Er were investigated in the temperature range 4.2–300 K. Additionally, for the compounds of Gd, Tb and Dy exhibiting magnetic anomalies at low temperatures, the magnetization versus magnetic field up to 14 T was measured at 4.2 K. Investigated compounds follow the Curie–Weiss law with relatively small values of the paramagnetic Curie temperature and the values of effective magnetic moment being in fair agreement with the values for the free ions. The electrical resistivity exhibits metallic character. The results are discussed in terms of the differences and similarities with other rare earth intermetallics.     

Magnetic properties of Pr1−xGdxMn2Ge2 compounds

The structure and magnetic properties of the Pr_1−xGd_xMn₂Ge₂ (0.0≤x≤1.0) compounds have been investigated by means of X-ray diffraction (XRD), differential scanning calorimetry (DSC) techniques and AC magnetic susceptibility measurements. All compounds crystallize in the ThCr₂Si₂-type structure with the space group I4/mmm. The lattice constants and the unit cell volume obey Vegard’s law. Samples in this alloy system exhibit a crossover from ferromagnetic ordering for PrMn₂Ge₂ to antiferromagnetic ordering for GdMn₂Ge₂ as a function of Gd concentration x. At low temperatures, the rare earth sublattice also orders and reconfigures the ordering in the Mn sublattice. The results are summarized in the x–T magnetic phase diagram.     

Induced crystallization and physical properties of Li2O–CaF2–P2O5:TiO2 glass system: Part II. Electrical, magnetic and optical properties

The results of various physical properties namely, dielectric properties (dielectric constant, loss tan δ, ac conductivity σ, over a wide range of frequency and temperature and dielectric breakdown strength in air medium at room temperature), optical absorption, electron spin resonance (ESR) at liquid nitrogen temperature and magnetic susceptibility at room temperature of Li₂O–CaF₂–P₂O₅:TiO₂ glass-ceramics have been reported. The optical absorption and magnetic susceptibility studies indicated that the titanium ions exist in Ti³⁺ state in addition to Ti⁴⁺ state in these samples. However, the reduction seems to be the lowest in the sample containing 0.6 mol% of TiO_2. The dielectric constant and loss variation with the concentration of TiO₂ have been explained on the basis of space charge polarization mechanism. The dielectric relaxation effects exhibited by these samples have been analyzed by a pseudo Cole–Cole plot method and the spreading of dielectric relaxation has been observed. The ac conductivity in the high temperature region seems to be related both with electronic and ionic movements. The low temperature (or the nearly temperature independent) part of conductivity could be explained on the basis of quantum mechanical tunneling model. The studies on dielectric breakdown strength indicated the highest insulating strength for the sample containing 0.6 mol% of TiO₂.     

Magnetic and anomalous transport properties in Fe2MnAl

This study investigated the magnetism and anomalous transport properties of the melt-spun ribbon Fe₂MnAl alloy. It is found that the alloy exhibits ferromagnetic order with the Curie temperature of ∼150 K, followed by another magnetic transition at T_R ∼ 51 K. It is suggested there is antiferromagnetism pinning effect on the ferromagnetic matrix at the temperature below T_R. A steep rise of the resistance below T_R has been observed, which can be understood in terms of antiferromagnetic scattering. An appearance of the resistance maximum near the Curie temperature and the negative temperature dependence of the resistance above Curie temperature are associated with the magnetic phase transition from ferromagnetism to paramagnetism. A negative magnetoresistance at low temperature arising from inhomogeneous magnetic scattering is reported.     

Influence of the TbFe2 crystallization on the magnetic and magnetostrictive properties of (Fe3Ga/TbFe2)n heterostructures

In this work we have investigated (Fe₃Ga/TbFe₂)_n multilayers grown by sputtering at room temperature. These multilayers exhibit a large coercivity associated to the crystalline TbFe₂ Laves phase. To reduce the coercivity it is necessary to control the crystallization of that material. In this work, we focus on the analysis of the properties of the TbFe₂ layers. In the as-grown heterostructures we have found evidence of nanoaggregates in the TbFe₂ layers. The Fe₃Ga thickness and the thermal treatments have an influence on the volume of these nanoprecipitates. In the annealed samples, when increasing the Fe₃Ga thickness we observe a decrease in the nanoaggregate volume and thus in the coercivity. The experimental results indicate that the crystallization of the TbFe₂ depends on the Tb diffusion promoted by the thermal treatment and on the stiffness factor (Y/α) of the Fe₃Ga layer. The magnetostrictive properties are also strongly influenced by the crystallization of the TbFe₂. We have achieved a maximum magnetostriction constant of nearly 550 ppm with a coercive field close to 400 Oe.     

MoSi_2含量对MoSi_2/Al_2O_3复合涂层介电与力学性能的影响(英文)

通过喷雾干燥法制备MoSi2包覆Al2O3的壳核结构混合粉,利用该混合粉以等离子喷涂技术制备MoSi2/Al2O3复合涂层材料。研究MoSi2/Al2O3质量比涂层材料的力学和介电性能的影响。结果表明:随着MoSi2含量从0增加到45%,复合材料的抗弯强度和断裂韧性分别从198MPa和3.05MPa·m1/2增加到324MPa和4.82MPa·m1/2,随后又降到310MPa和4.67MPa·m1/2。在8.2-12.4GHz微波频率波段内,随着MoSi2含量的增加,复合材料的介电损耗增加,而介电常数的实部却呈减小趋势。这主要是由于MoSi2颗粒熔化后的凝聚及导电网络结构的形成导致电导率的增加引起的。     

USb2单晶的生长及磁性和输运性质研究

采用Sb自助熔剂法成功生长高质量的USb₂单晶,并研究了磁化率、电阻、磁阻和比热等性质。研究表明,中等关联强度的USb₂中的5f电子具有巡游和局域双重特征。USb₂中的5f电子在260 K附近开始发生相干,203 K由顺磁态转变为反铁磁态,进行费米面的重构。在113 K以下局域的5f电子与传导电子发生第一次杂化使费米面附近电子结构发生变化。在54 K以下通过第二次杂化使得费米面附近形成了杂化能隙。在更低温度下晶体场效应对物理性质也产生了一定的影响。     

Magnetic phase diagrams of the TbRh2−xPdxPdxSi2 and TbRu2−xPdxSi2 systems

The a.c. susceptibility and high field magnetization on TbRh_2−xPd_xPd_xSi₂ and TbRu_2−xPd_xSi₂ compounds were investigated up to 140 kOe. The (T,x) magnetic phase diagrams were determined. For both systems, an increase in the Pd content causes a decrease in the Néel temperature and changes the magnetization curves.     

Structure, hyperfine interactions and magnetization studies of mechanically alloyed Fe50Ge50 and Fe62Ge38

X-ray diffraction, Mössbauer spectroscopy and magnetization measurements were used to study the structure and some magnetic properties of Fe₅₀Ge₅₀ and Fe₆₂Ge₃₈ prepared by mechanical alloying from the elemental powders. In both cases in the early stages of milling the intermediate paramagnetic FeGe₂ phase was formed. The mechanical alloying process of Fe₅₀Ge₅₀ resulted in the formation of the paramagnetic FeGe (B20) phase with an average crystallite size of about 15 nm. In the case of the Fe₆₂Ge₃₈, the ferromagnetic Fe₅Ge₃ (β) phase with a Curie temperature of about 430 K was obtained. The average crystallite size was about 9 nm. The average hyperfine magnetic field of about 16 T allowed it to determine that more than four germanium atoms exist in the nearest environment of the ⁵⁷Fe isotopes in the Fe₅Ge₃ phase.     

Magnetic properties of a new iron lead vanadate Pb2FeV3O11

Temperature dependence of dc/ac magnetization and electron paramagnetic resonance (EPR) spectra of Pb₂FeV₃O₁₁ iron lead vanadate has been investigated. The dc magnetic measurements have shown the presence of antiferromagnetic interactions with Curie-Weiss temperature, T_CW = −15.2 K, in the high temperatures range while the field cooled (FC) magnetization revealed a maximum at T_N = 2.5 K which coincides with a long range magnetic ordering. Temperature dependence of χ′ has shown a maximum at the same temperature. EPR spectrum of Pb₂FeV₃O₁₁ at room temperature is dominated by nearly symmetrical, very intense and broad resonance line centered at g_eff ∼ 2.0 that could be attributed to the correlated system of iron ions. The temperature dependence of magnetic resonance parameters (amplitude, g-factor, linewidth, integrated intensity) has been determined in the 4-300 K range and it suggests the existence of short range correlated spin system up to high temperatures. The temperature dependence of the amplitude of the resonance line has shown a pronounced maximum at 12.5 K that indicates on the existence of two subsystems of weakly and strongly coupled iron pairs. Comparison of dc magnetic susceptibility and EPR integrated intensity points to the presence of correlated spin agglomerates that play an important role in determination of the magnetic response of Pb₂FeV₃O₁₁.     

Magnetic properties of crystalline and amorphous GdFe2Hx alloys prepared by hydrogenation

Magnetic properties of crystalline c-GdFe₂H_4.3 and hydrogen-induced amorphous a-GdFe₂H_3.3 which were prepared by hydrogenation of crystalline CdFe₂ with the C15 structure were investigated by magnetization measurements and ⁵⁷Fe Mössbauer spectroscopy. The hydrogenation decreases the magnetic ordering temperature. The Curie temperatures are 360 K in c-GdFe₂H_4.3 and 469 K in GdFe₂H_3.3, which are noncollinear ferrimagnets with the compensation temperatures of 202 and 45 K, respectively. The Mössbauer spectra of c-GdFe₂H_4.3 at 297, 77.3 and 4.2 K consist of a sharp quadrupole doublet and a broadened Zeeman sextet, which indicates that a quarter of the Fe atoms do not carry an ordered magnetic moment even at 4.2 K. The spectra of a-GdFe₂H_3.3 show a broadened sextet with a distribution of hyperfine fields. The maximum of the distribution probabilities is located at 365 kOe at 4.2 K, a value which is larger than that of α-Fe.     

Magnetic multiresonance behavior of Fe@Al2O3 nanoembedments and microstructural evolution during mechanosynthesis

The embedding of metal nanoparticles into an insulating ceramic matrix can provide encapsulation and prevent their oxidation and agglomeration. A nanoembedment powder with the Fe nanoparticles embedded into Al₂O₃ matrix is prepared by high-energy ball milling. Starting from the highly exothermic reactant mixture of magnetite and aluminum, Fe nanoparticles were in situ formed in Al₂O₃ matrix by mechanochemical reaction. It is found that a post-reaction milling significantly narrows the size distribution of Fe nanoparticles. The mechanism for the two-stage milling process was proposed. The microwave permeability of nanoembedments exhibited a multiresonance behavior, which was the evidence for monodispersed Fe nanoparticles. The Fe@Al₂O₃ nanoembedments are potential candidates as microwave absorber and left-handed materials.     

Thermoelectric properties and electronic structure of p-type Mg2Si and Mg2Si0.6Ge0.4 compounds doped with Ga

Mg₂Si:Ga_x and Mg₂Si_0.6Ge_0.4:Ga_x (x = 0.4% and 0.8%) solid solutions have been synthesized by direct melting in tantalum crucibles and hot pressing. The effect of Ga doping on the thermoelectric properties has also been investigated by measurements of thermopower, electrical resistivity, Hall coefficient and thermal conductivity in temperature range from 300 to 850 K. All samples exhibit a p-type conductivity evidenced by positive sign of both thermopower and Hall coefficient in the investigated temperatures. The maximum value of the dimensionless figure of merit ZT was reached for the Mg₂Si_0.6Ge_0.4:Ga(0.8%) compound at 625 K (ZT ∼ 0.36). The p-type character of thermoelectric behaviours of Ga-doped Mg₂Si and Mg₂Si_0.6Ge_0.4 compounds well corroborates with the results of electronic structure calculations performed by the Korringa-Kohn-Rostoker method and the coherent potential approximation (KKR-CPA), since Ga diluted in Mg₂Si and Mg₂Si_0.6Ge_0.4 (on Si/Ge site) behaves as hole donor due to the Fermi level shifted to the valence band edge. The onset of large peak of DOS from Ga impurity at the valence band edge, well corroborates with high Seebeck coefficient measured in Ga-doped samples.