The anisotropy of Gd－Fe exchange interaction for Gd2Fe17 and Gd2Fe17H3

The magnetization curves along the crystal axes for Gd_2Fe_{17} and Gd_2Fe_{17}H_3 were analysed based on the single-ion model. If the Gd－Fe exchange interaction has been taken as isotropic as usual, the fitted values of magneto-crystalline anisotropy of the Fe sublattices in Gd_2Fe_{17} and Gd_2Fe_{17}H_3 would become unreasonably different from those of the corresponding Y or Lu compounds. It was shown that the large difference is caused by the neglect of the anisotropy of the Gd－Fe exchange interaction.     

Crystal field analysis of the magnetic properties of RFe11Ti and RFe11TiH （R=Sm, Tb, Ho）

The crystalline-electric-field parameters Anm for RFe11Ti and RFe11TiH （R=Sm, Tb, Ho） are evaluated by fitting calculations to the magnetization curves measured on the single crystals or on magnetically aligned powder samples at 4.2K and higher temperatures. Interstitial hydrogen atom in RFe11Ti has been found to have a significant effect on crystalline-electric-field parameters Anm. By using the parameters of exchange field 2μBHex estimated from inelastic neutron scattering experiments and the fitted Anm, the calculations can reproduce the experimental curves well.     

Theoretical study on the structure for R2Co17 (R=Y, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er) and R2Co17T (T=Be, C)

The structural stability of the intermetallic compounds R₂Co₁₇ and R₂Co₁₇T (T=Be, C) is tested by many means including random atom shifts, global deformations and high temperature disturbances under the control of the pair potentials. The structure type and crystal constants of R₂Co₁₇ and R₂Co₁₇Be are close to experimental results. The addition of Be and C in the interstice of R₂Co₁₇ causes a decrease of the cohesive energy, and Be and C only occupies the 9e interstitial site with the Th₂Zn₁₇-type structure or the 6h interstitial site with the Th₂Ni₁₇-type structure. All the above results indicate that the potentials are valid for studying the structural properties of these kinds of anisotropy materials.     

Magnetocaloric effect in R2Fe17 (R=Sm,Gd, Tb,Dy, Er)

A series of R₂Fe₁₇ (R=Sm, Gd, Tb, Dy, Er) have been synthesized. The magnetocaloric effect (MCE) of these compounds has been investigated by means of magnetic measurements in the vicinity of their Curie temperature. The Curie temperature of Er₂Fe₁₇ is 294 K. The maximum magnetic entropy change of Er₂Fe₁₇ under 5 T magnetic field is ∼3.68 J/kg K. In the R₂Fe₁₇ (R=Sm, Gd, Tb, Dy, Er) system, the maximum magnetic entropy change under 1.5 T magnetic field is 1.72, 0.89, 1.32, 1.59, 1.68 J/kg K corresponding to their Curie temperature (400, 472, 415, 364, 294 K), respectively.     

Magnetic structures of R3Cu4Si4 (R=Dy, Ho and Er)

Polycrystalline samples of R₃Cu₄Si₄ (R=Dy, Ho, Er) intermetallics were studied with neutron diffraction methods. All of them crystallize in the orthorhombic structure of Gd₃Cu₄Ge₄-type and order antiferromagnetically at low temperatures. Magnetic moments localized at the rare earth atoms, that occupy two non-equivalent 2d and 4e sublattices, order simultaneously in Dy₃Cu₄Si₄. The order is described by the propagation vector accompanied by , δ=0.025(2). In Ho₃Cu₄Si₄ two propagation vectors are needed to model the magnetic order. These are: for the 4e sublattice, which disorders as the first when the temperature increases, and for the 2d sublattice. A similar situation is observed for Er₃Cu₄Si₄ where the propagation vectors are: k=(0,1−δ,0), δ=0.097(2) for the 4e sublattice, which disorders as the first with increasing temperature, and , δ=0.0015(6) for the 2d sublattice.     

Magnetic characteristics of R6Mn23 hydrides (R = Gd,Ho or Er)

R₆Mn₂₃ systems, with R = Gd, Ho and Er, were hydrided to the composition R₆Mn₂₃H_x where x ap; 22. Magnetic properties of these systems and the parent intermetallics were studied over the temperature range 4 to 300 K and at applied field up to 21 kOe. Since Y₆Mn₂₃H₂₅ was established earlier to exhibit only Pauli paramagnetism, the magnetism of the R₆Mn₂₃ hydrides must originate with the rare earth sublattice. Gd₆Mn₂₃H₂₂ orders at ≈ 150 K, whereas ordering in Gd₆Mn₂₃ occurs at 468 K. The moment measured at 4 K indicates a non-collinear structure, perhaps generated by competition involving exchange between nearest and next nearest neighbors. The hydrides involving Ho and Er appear to remain paramagnetic to the lowest temperatures studied, perhaps because the reduced de Gennes factor exchange is insufficient to produce magnetic ordering. The possibility cannot be excluded, however, that they are antiferromagnetic.     

Influence of Si and Co substitutions on magnetoelastic properties of R2Fe17 (R=Y, Er and Tm) intermetallic compounds

The magnetostriction of the off-stoichiometric R₂Fe₁₇-type intermetallic compounds based on R₂Fe_14−xCo_xSi₂ (R=Y, Er, Tm and x=0, 4) was measured, using the strain gauge method in the temperature range 77-460 K under applied magnetic fields up to 1.5 T. All compounds show sign change and reduction in magnetostriction values compared to the R₂Fe₁₇ compounds by Si substitution. For Y₂Fe₁₄Si₂ and Er₂Fe₁₄Si₂, saturation behaviour is observed near magnetic ordering temperature (T_C), whereas for Tm₂Fe₁₄Si₂, saturation starts from T>143 K. Also, Co substitution has different effects on the magnetostriction of R₂Fe₁₄Si₂ compounds. In Er₂Fe₁₀Co₄Si₂ and Tm₂Fe₁₀Co₄Si₂, saturation occurs below the spin reorientation temperature (T_SR). In addition, in Er₂Fe₁₄Si₂, a sign change occurs in the anisotropic magnetostriction (Δλ) as well as the volume magnetostriction (ΔV/V) at their T_SR values. The volume magnetostrictions of the Tm-containing compounds show an anomaly around their T_SR. In R₂Fe₁₄Si₂ compounds, parastrictive behaviour is also observed in ΔV/V near their T_C values. In addition, the magnetostriction of the sublattices is investigated. Results show that in R₂Fe₁₄Si₂ compounds, the rare-earth sublattice contribution to magnetostriction is negative and comparable to the iron sublattice, whereas, in R₂Fe₁₀Co₄Si₂ compounds, the rare-earth sublattice contribution is positive and larger than Fe sublattice. These results are discussed based on the effect of Si and Co substitutions on the anisotropy field of these compounds. Influence of the spin reorientation transition on the magnetostriction of these compounds is discussed in terms of the anisotropic sublattice interactions.     

Magnetic susceptibility measurements for some of the intermetallic compounds R2Pt (R = Gd,Tb, Dy,Ho, Er)

The magnetic properties of some of the intermetallic compounds R₂Pt have been studied by means of susceptibility measurements. The compounds are probably ferrimagnetic or they possess some noncollinear structure. With the exception of Gd₂Pt two magnetic transitions with temperature are observed. At 4.2 K the magnetization of Tb₂Pt shows appreciable hysteresis. In the paramagnetic region the compounds confirm the free ion picture of the R³⁺ except for Gd₂Pt which is still in a nonparamagnetic state even at 300 K.     

Critical bond lengths and their role in spontaneous magnetostriction of R2Fe17CX (R=Y,Nd, Gd,Tb, Er)

High-energy high-flux synchrotron X-rays have been used to study the spontaneous magnetostriction of R₂Fe₁₇ (R=Y, Nd, Gd, Tb, Er) and their carbides in the temperature range 10–1100 K. Addition of interstitial carbon greatly increases both the Curie temperatures (T_C) and the spontaneous magnetostrain of the compounds, while reduces the anisotropy of the magnetostrain by expanding the distances between rare-earth and neighboring Fe sites. The increase of T_C with carbon is due to the increased spatial separation of the Fe hexagon layers. On the basal plane, the Fe hexagons are squeezed and the contribution of Fe sublattice to spontaneous magnetostriction is attenuated, while that of rare-earth sublattice is enhanced. The average bond magnetostrain around Fe sites are in linear relation with their hyperfine field intensities.     

Electrical transport and magnetic ordering in R 2Ti3Ge4 (R=Dy, Ho and Er) compounds

New R ₂Ti₃Ge₄ (R=Dy, Ho and Er) intermetallic compounds have been synthesized and characterized by X-ray diffraction and low temperature ac magnetic susceptibility, electrical resistivity and thermoelectric power measurements were carried out. The compounds crystallize in the parent, Sm₅Ge₄-type orthorhombic structure (space group Pnma) and lanthanide contraction is observed as one moves along the rare-earth series. The changeover from paramagnetic to antiferromagnetic phase happens at low temperatures and the ordering temperature scales with the de Gennes factor. The electrical resistivity is metallic with a negative curvature above 100 K. Thermopower displays a weak maximum at temperatures less than 50 K signifying the possible phonon and magnon drag effects.     

Molecular field theory analysis of R2Fe17C (R=PR,Nd, Gd,Tb, Dy,Ho, Er)

The temperature dependence of magnetization is analysed for R₂Fe₁₇C via the two-sublattice molecular field theory. The molecular field coefficients n_FF, n_RF and n_RR are obtained, by which T_C was calculated. Using the least-squares method, the fitted-form of H_R(T) varying with temperature for each compound is presented. The results are analysed. In addition, the parameters F=M_Fe²(0)n_FF/T_C was calculated for each R₂Fe₁₇C. By F, some phenomena different from the normal view were explained.     

Magnetic phase transition and the corresponding magnetostriction of intermetallic compounds RMnsGe2（R=Sm,Gd）

The temperature dependence of lattice constants a and c of intermetallic compounds RMn₂Ge₂ (R=Sm, Gd) is measured in the temperature range 10－800K by using the x-ray diffraction method. The magnetoelastic anomalies of lattice constants are found at the different kinds of spontaneous magnetic transitions. The transversal and longitudinal magnetostrictions of polycrystalline samples are measured in the pulse magnetic field up to 25T. In the external magnetic field there occurs a first-order field-induced antiferromagnetism－ferromagnetism transition in the Mn sublattice, which gives rise to a large magnetostriction. The magnitude of magnetostrictions is as large as 10^-3. The transversal and longitudinal magnetostrictions have the same sign and are almost equal. This indicates that the magnetostriction is isotropic and mainly caused by the interlayer Mn－Mn exchange interaction. The experimental results are explained in the framework of a two-sublattice ferrimagnet with the negative exchange interaction in one of the sublattices by taking into account the lattice constant dependence of interlayer Mn－Mn exchange interaction.     

Magnetic ordering in RCoSi2 (R = Tb, Dy, Ho) compounds

The crystal and magnetic structures of TbCoSi₂, DyCoSi₂ and HoCoSi₂ were studied by neutron diffraction using polycrystalline samples. At 4.2 K all three exhibit antiferromagnetic ordering: DyCoSi₂ and HoCoSi₂ of collinear. TbCoSi₂ of complex spiral types.     

Magnetic properties of off-stoichiometric R2Co3Zn14 (R=Y,Gd) single crystals

Single crystal X-ray diffraction data indicate that the R₂Co₃Zn₁₄ (R=Gd, Y) phase crystallizes non-stoichiometrically with a mixed occupancy of Co/Zn atoms on the 12-coordinated transition metal site and one of the three zinc sites. The crystals are rhombohedral with R-3m space group. Magnetization measurements provide no evidence of localized 3d electron moment in Y₂Co_2.3Zn_14.7 which is non-magnetic down to 1.8 K. Thermodynamic and transport measurements on two Gd₂Co_3+xZn_14−x crystals reveal that the extra cobalt influences temperature below which the samples enter into an antiferromagnetic state: T_N=31.5(3) K for Gd₂Co₃Zn₁₄ and 28(1) K for Gd₂Co_4.2Zn_12.8. A lower magnetic ordering temperature of T_mag=6.0(2) K is common in both Gd samples.     

金属间化合物RMn2Ge2(R=La,Pr,Nd,Sm,Gd,Tb和Y)中的自发磁相变 及相变时的磁弹性异常

在10—800K的温度范围内用X射线衍射方法测量了RMn₂Ge₂(R=La,Pr ,Nd,Sm,Gd,Tb和Y)的晶格常数与温度的变化关系.在各种类型的自发磁相变观察到晶格常数 的磁弹性异常现象.实验得出,自发磁相变时的磁弹性异常主要由Mn次晶格引起,并且Mn-Mn 交换相互作用能不仅与晶格常数a有关,而且与晶格常数c有关.用Kittle的交换反转模型讨 论了低温时的铁磁—反铁磁一阶相变. 关键词： 稀土金属间化合物 磁相变 磁弹性     

Enhancement of the recoilless fraction in R2Fe10Co4Si2 (R=Gd, Er, Y) offstoichiometric compounds after hydrogenation

Samples of R₂Fe₁₀Co₄Si₂ (R=Gd, Er and Y) solid solutions were prepared by arc melting and exposed to hydrogenation. Characterization of hydrogenation effects was done using Mössbauer spectroscopy. The values of the magnetic hyperfine fields corresponding to the four inequivalent lattice sites were found to increase after hydrogenation in all offstoichiometric compounds investigated. Slight changes in the relative site occupancies were observed. Transmission Mössbauer spectra were also recorded for absorbers consisting of the intermetallic and the stainless steel etalon. The recoilless fraction was derived from these spectra using the method developed by us. Our study is the first to evidence that the recoilless fraction is enhanced after hydrogenation.     

A crystal field investigation of the properties of RCuAs2 (R=Pr, Nd, Sm, Tb, Dy, Ho, Er and Yb)

A crystal field (CF) investigation of the magnetic properties and heat capacities of RCuAs₂ (R=Pr, Nd, Sm, Tb, Dy, Ho, Er and Yb) has been carried out using the observed average magnetic susceptibilities (1.8-300 K) of the title compounds. The CF parameters proposed for the systems show a systematic variation throughout the rare-earth series. Other physical properties dependent on the CF are also computed and compared with available experimental data. The experimental heat capacity data reported for a limited range of temperature agree well with computed heat capacity for all the compounds (except SmCuAs₂ and YbCuAs₂). CF J mixing was found to be appreciable for all the samples except YbCuAs₂.     

Luminescent properties of R2(MoO4)3:Eu (R=La, Y, Gd) phosphors prepared by sol-gel process

A series of red phosphors R_0.8Eu_1.2(MoO₄)₃ (R=La, Y, and Gd) have been synthesized by sol-gel method. The crystallization processes of the phosphor precursors were characterized by X-ray diffraction (XRD) and thermogravimetry-differential thermal analysis (TG-DTA), and the properties of these resulting phosphors have also been characterized by photoluminescence (PL) spectra and reflectance spectra. Field emission scanning electron microscopy (FE-SEM) was also used to characterize the shape and the size of the samples. The results of TG-DTA and XRD indicated that all of the R_0.8Eu_1.2(MoO₄)₃ (R=La, Y, and Gd) phosphors crystallized completely at 650 °C. Y_0.8Eu_1.2(MoO₄)₃ and Gd_0.8Eu_1.2(MoO₄)₃ have two structures, monoclinic and orthorhombic, while La_0.8Eu_1.2(MoO₄)₃ only adopts monoclinic structure. The luminescent properties of phosphors R_0.8Eu_1.2(MoO₄)₃ (R=La, Y, and Gd) are dependent on their structures to some extent. The orthorhombic Y_0.8Eu_1.2(MoO₄)₃ and Gd_0.8Eu_1.2(MoO₄)₃ phosphors show very similar luminescent properties, which differ from those of phosphors with monoclinic structure. For all of R_0.8Eu_1.2(MoO₄)₃ (R=La, Y, and Gd) phosphors, intense red emission is obtained by exciting at ∼394 and ∼465 nm which are owing to the sharp ⁷F₀→⁵L₆ and ⁷F₀→⁵D₂ lines of Eu³⁺. Two strongest lines at 394 and 465 nm in excitation spectra of these phosphors match well with the two popular emissions from near-UV and blue GaN-based LEDs, so they could be used as red components for white light-emitting diodes.     

Analyses of crystal field and exchange interaction of Dy3Ga5O12 under extreme conditions

This paper theoretically investigates the effects of crystal field and exchange interaction field on magnetic properties in dysprosium gallium garnet under extreme conditions (low temperatures and high magnetic fields) based on quantum theory. Here, five sets of crystal field parameters are discussed and compared. It demonstrates that, only considering the crystal field effect, the experiments can not be successfully explained. Thus, referring to the molecular field theory, an effective exchange field associated with the Dy--Dy exchange interaction is further taken into account. Under special consideration of crystal field and the exchange interaction field, it obtains an excellent agreement between the theoretical results and experiments, and further confirms that the exchange interaction field between rare-earth ions has great importance to magnetic properties in paramagnetic rare-earth gallium garnets.