On the magnetovolume effects in YNi3

The thermal expansions of polycrystalline samples of ferromagnetic YNi₃ and paramagnetic YNi₂ have been determined from 4.2 K up to 65 K employing the three-terminal-capacitance method. At low temperatures the expansion of YNi₃ is dominated by a negative magnetic contribution which rapidly vanishes near the Curie temperature. The magnetovolume effects in YNi₃ are discussed within the Stoner-Edwards-Wohlfarth model of very weak itinerant ferromagnetism.     

Forced volume magnetostriction in (rare earth) ni2 cubic intermetallic compounds

The forced volume magnetostriction has been measured for the RENi₂ intermetallic compounds (RE = Ce, Pr, Sm, Gd, Tb and Dy). In GdNi₂ both spontaneous and forced magnetostriction originate from pure exchange and it is necessary to invoke short-range spin correlations to explain the peculiar temperature dependence. In PrNi₂, SmNi₂, TbNi₂ and DyNi₂ the magnetostriction is contributed to by strains from both crystalline electric field and exchange, the temperature dependence in TbNi₂ and SmNi₂ being also peculiar. Magnetoelastic coupling parameters for both types of contribution have been determined. At low temperatures a cooperative Jahn-Teller distortion has been identified in PrNi₂ by measuring thermal expansion. The volume magnetostriction of CeNi₂ appears to be anomalous, probably as a consequence of Ce being in an intermediate valence state.     

Thermal expansion of ZrFe2 and some rare-earth iron compounds

The thermal expansion coefficient of the compounds ZrFe₂, TiFe₂, YFe₂, ErFe₂, ErFe₃, Er₆Fe₂₃ and Er₂Fe₁₇ has been determined from room temperature up to 700°C. The anomalous thermal expansion of some of these compounds observed at temperatures below their Curie temperature (T_c) is shown to be related to a strong pressure dependence of T_c. (dT_c/dP is large and negative).     

Heat capacity measurements of itinerant electron magnetism in Y3Ni13−xCOxB2 system

The heat capacity of the Y₃Ni_13−xCo_xB₂ series has been measured from 300 mK to RT. The magnetic ordering phase transitions have been characterized as second-order type and the T_c's determined. The electronic contribution to the low-temperature heat capacity for x=0 yields an electronic constant γ=54 mJ mol K², which is higher than those of YNi₅ and YNi₄B, proving experimentally that its density of states at the Fermi surface is larger than in those other compounds. The substitution of Ni by Co increases γ linearly. Electronic band calculations could explain these features.     

Vortex phase diagram studies in the weakly pinned single crystals of YNi2B2C and LuNi2B2C

We present a study of magnetization measurements performed on the single crystals of YNi₂B₂C and LuNi₂B₂C. For both the compounds, we find flux jumps in magnetisation values in the respective field regions, where the structural transitions in the flux line lattice symmetry have been reported in these systems via the small angle neutron scattering experiments. The magnetisation hysteresis loops and the AC susceptibility measurements show pronounced peak effect as well as second magnetisation peak anomaly for both YNi₂B₂C and LuNi₂B₂C. Based on these results, a vortex phase diagram has been constructed for YNi₂B₂C forH∥c depicting different glassy phases of the vortex matter.     

Investigation of phase transformations in Cu1.8S-Cu2S thin films by kinematic electron diffraction

The phase transformations of the compounds Cu_1.8S-Cu₂S have been investigated. All the changes in the sample due to a thermal treatment up to 300°C were continuously photographed by the kinematic electron diffraction method. It was shown that, depending on experiment conditions, the low-temperature ordered phase of digenite Cu_Z-XS transforms completely or partly into the hexagonal modification of chalcocite Cu₂S, which becomes ordered in its low-temperature orthorhombic modification. At temperatures from room to 100°C the orthorhombic chalcocite coexists with the low-temperature digenite, in a range from 100°C to 300°C the high-temperature hexagonal chalcocite coexists with the low-temperature digenite and at 300°C the sample contains a mixture of the hexagonal chalcocite and the f.c.c.-phase of digenite. Djurleite and other phases with intermediate compositions are not observed in these experiments.     

Pressure dependence of magnetic ordering temperatures of rare earth - Sn3 compounds

Measurements of the hydrostatic pressure dependence of the Neél temperatures, T_N, are reported for PrSn₃, GdSn₃, GdSn₃ and CeIn₃. T_N is found to increase with applied pressure for PrSn₃ and NdSn₃, whereas T_N is pressure independent within experimental error for GdSn₃ and CeIn₃. Slightly Sn-deficient RESn₃ (RE=rare earth) compounds are found consistently to be weakly ferromagnetic. The physical properties of the RESn₃ compounds exhibit analogies with the corresponding properties of dilute superconducting ($\text{La}$RE)Sn₃ alloys. The high pressure data for PrSn₃ and CeIn₃ are qualitatively consistent with a “Kondo necklace” model for magnetically ordered RE compounds with unstable 4f shells.     

Thermal expansion of orthorhombic and tetragonal phases of the highT c superconductors RE1Ba2Cu3O y (RE=Y,Gd, Dy)

Measurements of the coefficient of linear thermal expansion α have been carried out for the orthorhombic and tetragonal phases of RE₁Ba₂Cu₃O _y (RE=Y, Gd, Dy) compounds using a high resolution capacitance dilatometer in the temperature range 77–300 K. All the superconducting samples exhibit a jump Δα at their respective transition temperatures,T _c. Evidences of, sample-to-sample variation in α values and dependence of Δα on the sample preparation conditions, have been obtained. The non-superconducting samples, in general, exhibit lower values of α possibly because of lowering of oxygen content.     

Heat capacity and thermal expansion of CdCr2Se4 and CdCr2S4

The thermodynamic properties of the spinel ferromagnetic compounds CdCr₂Se₄ and CdCr₂S₄ have been investigated by making heat capacity and thermal expansion measurements on single crystals. For both compounds, the ferromagnetic transition is marked by λ-type thermal anomalies, and the results provide a pressure dependence of the transition temperatures that is in agreement with direct measurements. Below the transition, CdCr₂S₄ shows an anomalous heat-capacity contribution and negative thermal expansion, which are in contrast to the conventional behavior found in CdCr₂Se₄.     

Zero-field splitting in NiSiF6−6H2O and its pressure, stress and temperature dependences: A supplementary study

The zero-field splitting in NiSiF₆?6H₂O was studied as a function, of hydrostatic pressure and uniaxial stress at temperatures above and below 220 K, at which the crystal exhibits a secondorder phase transition. The anisotropy of the thermal expansion coefficients parallel and perpendicular to the trigonal symmetry axis was measured in a wide range of temperatures. On the basis of the obtained data the spin-phonon interaction has been separated from the thermal expansion of the crystal. At high and low temperatures the two contributions have opposite signs and exhibit much higher values above 220 K than below this temperature. This is explained by the high anisotropy of the thermal expansion of the crystal characteristic of the high-temperature phase.     

(Pb0.97La0.02)(Zr0.65Sn0.35-xTi x)O3反铁电陶瓷的热膨胀性质

在-100—200℃温度范围内，测量了(Pb_0.97La_0.02)(Zr_{0.65< /sub>Sn0.35-xTix)O3(PZST，0.1≤x≤0.14)反铁电陶 瓷的热膨胀性质.实验结果表明，组分在0.1 ≤x≤0.12的试样室温下为反铁电(AFEt)四方相，热膨胀系数(α)在低温段发生 “弯曲” ，而变温x射线衍射谱(XRD)显示材料保持四方相结构；当Ti含量在0.125≤x≤0.14时，室温 下是铁电三方相(FER)，温度升高时FER→AFEt相变体 积收缩，AFEt→立方顺电(PE c)相变体积增大；变温XRD谱证明了材料相结构随温度的转变过程.用多元复杂 化合物存在 纳米线度组分非均匀的观点解释了热膨胀性质随Ti含量演化的物理机理，并得到了该系统的 温度-Ti(x)含量相图. 关键词： 热膨胀性质 铁电/反铁电相界 反铁电陶瓷 PZST}     

High-temperature magnetic properties of AnFe2 (An = Np,Pu, Am) Laves phases

Magnetization measurements have been performed on NpFe₂, PuFe₂, and AmFe₂ (cubic C15-type Laves phase) between 300 and 1100 K. The Curie temperatures were found to be 492 ± 2, 564 ± 2, and 613 ± 3 K, respectively. A correlation was found between the Curie temperature and the magnetic moment per iron atom determined in previous neutron-diffraction experiments. Critical exponents determined for NpFe₂ and PuFe₂ were found to be close to previously reported values for the chemically ordered metallic system Pd₃Fe. The overall magnetic behavior of the compounds studied implies the presence of local magnetic moments, in contrast to the itinerant magnetic behavior of UFe₂ noted in the previous paper.     

Investigation of phase transitions and thermal expansion of some complex tungsten-containing oxides

The phase transitions have been investigated and the thermal expansion coefficients have been determined in compounds with a structure of brannerite, defect pyrochlore, and tungsten bronze of the general formula M ^I A ^VWO_{6 − δ} (M ^I = □, H₃O, Li, Na, K, Rb, and Cs; A ^V = V, Nb, Sb, and Ta; δ = 0, 0.5) by high-temperature X-ray diffraction and differential thermal analysis. It has been shown that brannerites and tungsten bronze do not undergo phase transitions and the compounds are characterized by a significant anisotropy of the thermal expansion, whereas pyrochlores undergo phase transitions of three types, and thermal deformations in them are determined by the size of M ^I atoms.     

Thermal expansion anisotropy of β-TlInS2 crystals in the course of phase transitions

Basing on the thermal expansion studies performed for β-TlInS₂ crystals in the course of their phase transitions, we have found that the crystals aged for one year are composed from at least two polytypes, which undergo phase transitions at different temperatures. The detailed analysis of anisotropy of the thermal expansion demonstrates that all of the diagonal components of the thermal expansion tensor associated with the eigensystem have almost the same modules but differ by their signs. This leads to appearance of an elliptical conical surface of zero thermal expansion. The orientation of this surface does not depend on the temperature in the temperature interval under study. We have also found that the thermal expansion coefficients along the crystallographic a and b axes are close to zero.     

Magnetic and thermal properties of CuFeS<Subscript>2</Subscript> at low temperatures

The magnetic moment M, the magnetic susceptibility χ, and the thermal conductivity of chalcopyrite CuFeS₂, which is a zero-gap semiconductor with antiferromagnetic ordering, have been measured in the temperature range 10–310 K. It has been revealed that the quantities χ(T) and M(T) increase anomalously strongly at temperatures below ∼100 K. The temperature dependence M(T) is affected by the magnetic prehistory of the sample. An analysis has demonstrated that the magnetic anomalies are associated with the presence of a system of noninteracting magnetic clusters in the CuFeS₂ sample under investigation. The formation of the clusters is most likely caused by the disturbance of the ordered arrangement of Fe and Cu atoms in the metal sublattice of the chalcopyrite, which is also responsible for the phase inhomogeneity of the crystal lattice. The inhomogeneity brings about strong phonon scattering, and, as a result, the temperature dependence of the thermal conductivity coefficient exhibits a behavior characteristic of partially disordered crystals.     

Mn替代对Dy2AlFe16－xMnx化合物的结构和磁性的影响

通过X射线衍射及磁测量手段研究了Dy₂AlFe_16-xMn_x化 合物的结构和磁性.研究结果表明Dy₂AlFe_16-xMn_x化合 物具有六角相的Th₂Ni₁₇型结构.对x=1，2的样品采用X射线热膨胀 测定法在104—647K的温度范围内测量了其热膨胀性质，发现这些化合物在低温下存在热膨 胀反常现象，在居里点附近出现负膨胀性质.对自发磁致伸缩的研究结果表明Dy₂AlFe_16-xMn_x化合物中存在着较强的各向异性的自发磁致伸缩，随 着Mn含量的增加，其低温下的自发体磁致伸缩减弱.磁测量结果表明Mn的替代导致Dy_{2< /sub>AlFe16-xMnx化合物的居里温度及自发磁化强度急剧下降. 关键词： 2}AlFe_16-xMn_x化合物')" href="#">Dy₂AlFe_16-xMn_x化合物 反常热膨胀 自发磁致伸缩 自发磁化强度     

Study of solid-state reactions and order-disorder transitions in Pd/α-Fe(001) thin films

The formation of the hard-magnetic ordered L1₀-FePd phase in thin bilayer Pd/α-Fe(001) films has been experimentally studied. Solid-state reactions initiated by thermal heating in bilayer Pd/α-Fe(001) films with a thickness of 50–60 nm (the atomic ratio Pd: Fe ≈ 50: 50) separated from the substrate have been studied using the in situ electron diffraction methods. It has been shown that the solid-state reaction between the palladium and iron layers in Pd/α-Fe(001) starts at 400°C with the formation of the disordered Fe-Pd phase. At 480°C, the ordered L1₀-FePd phase is formed. The order-disorder phase transition has been studied. It has been established that the transition of the ordered L1₀-FePd phase to the disordered FePd phase starts at 725°C. At 740°C, only the disordered FePd phase is present over the whole volume of the film. The observed temperature of the order-disorder phase transition is shifted from the equilibrium value by 35°C to higher temperatures. This effect is assumingly associated with the higher concentration of palladium atoms at the boundaries of the Fe-Pd crystal grains owing to the grain-boundary adsorption.     

Entropy increase in the amorphous-to-crystalline phase transition in zirconium tungstate

It has been shown that zirconium tungstate (ZrW₂O₈) exhibits isotropic negative thermal expansion and undergoes pressure-induced amorphization above 1.5 GPa, at room temperature. Now, we have found that amorphous ZrW₂O₈ undergoes endothermic recrystallization, and thus have an overall entropy lower than that of the crystalline phase. This counterintuitive behavior can be rationalized owing to the same low-energy modes already shown to be responsible for the isotropic negative thermal expansion and the anomalously high heat capacity at low temperatures exhibited by ZrW₂O₈. Our findings point to an entire class of materials that should behave similarly to ZrW₂O₈ and constitute direct experimental evidence for an overall entropy increase in an amorphous-to-crystalline transition.     

Thermal expansion and the heat capacity of nanocrystalline and coarse-crystalline silver sulfide Ag2S

The thermal expansion and the heat capacity of coarse-crystalline and nanocrystalline silver sulfide Ag₂S were studied by dilatometry and differential scanning calorimentry for the first time in the temperature range 290–970 K. It is found that the thermal expansion coefficient and the heat capacity of nanocrystalline silver sulfide in this temperature range are higher than those in the case of the coarse-crystalline sulfide. It is revealed that the transformation of α-Ag₂S acanthite to β-Ag₂S argentite and β-Ag₂S argentite to γ-Ag₂S phase are the first-order phase transitions; the temperatures and the enthalpies of these transformations have been determined.

     

Low-temperature x-ray diffraction studies of the crystallographic parameters and thermal expansion of (CH3)2NH2Al(SO4)2 · 6H2O crystals

The a, b, c, and β crystallographic parameters of the (CH₃)₂NH₂Al(SO₄)₂ · 6H₂O crystal (DMAAS) have been measured by x-ray diffraction in the 90–300-K temperature range. The thermal expansion coefficients along the principal crystallographic axes α_a, α_b, and α_c have been determined. It was shown that, as the temperature is increased, the parameter α decreases and b increases, whereas c decreases for T<T _c (where T _c is the transition temperature) and increases for T>T _c, so that one observes a minimum in the c=f(T) curve in the region of the phase transition (PT) temperature T _c ～ 152 K. The thermal expansion coefficients α_a, α_b, and α_c vary in a complicated manner with increasing temperature, more specifically, α_a and α_c assume negative values at low temperatures, and the α_a=f(T), α_b=f(T), and α_c=f(T) curves exhibit anomalies at the PT point. The crystal has been found to be substantially anisotropic in thermal expansion.