Thermal expansion of single crystal Tb-50% Ho

Data for the temperature variation of the c-axis strain in annealed Tb-50% Ho is compared with previous measurements on Tb-50% Ho and Dy. Although there is agreement near 273 K, it is considered that internal strain is responsible for deviations between present and previous data for Tb-50% Ho in the temperature range T_C < T < T_N. The data from annealed Tb-50% Ho and Dy are significantly different.     

Effect of magnetic field on the TC and magnetic properties for the aligned MnBi compound

In the compound MnBi, a first-order transition from the paramagnetic to the ferromagnetic state can be triggered by an applied magnetic field and the Curie temperature increases nearly linearly with an increase in magnetic field by ∼2 K/T. Under a field of 10 T, T_C increases by 20 and 22 K during heating and cooling, respectively. Under certain conditions a reversible magnetic field or temperature induced transition between the paramagnetic and ferromagnetic states can occur. A magnetic and crystallographic H-T phase diagram for MnBi is given. Magnetic properties of MnBi compound aligned in a Bi matrix have been investigated. In the low temperature phase MnBi, a spin-reorientation takes place during which the magnetic moments rotate from being parallel to the c-axis towards the basal plane at ∼90 K. A measuring Dc magnetic field applied parallel to the c-axis of MnBi suppresses partly the spin-reorientation transition. Interestingly, the fabricated magnetic field increases the temperature of spin-reorientation transition T_s and the change in magnetization for MnBi. For the sample solidified under 0.5 T, the change in magnetization is ∼70% and T_s is ∼91 K.     

The influence of thermal history on the elastic and expansion properties of magnetically ordered Dy and 50% Tb-Ho

Measurements of thermal expansion and elastic properties of Dy and 50% Tb-Ho have demonstrated that the magnetic anomalies in these properties are dependent on the state of internal strain in the samples. Changes of more than 10 K are found in the Néel temperature, T_N, and variations of ≈ 30% observed in the c-axis thermal strains.     

Anomalous thermal expansion in the paramagnetic phase of dysprosium and terbium-50% holmium

The c-axis thermal expansion and expansion coefficients have been measured over the temperature range T_N < T < 273 K in single crystal dysprosium and terbium-50% holmium. The data show abrupt changes in the expansion coefficients at approximately 226 K in dysprosium and 250 K, 212 K and 196 K in terbium-50% holmium. Departures from lattice Grüneisen behaviour occur at temperatures well in excess of the Néel temperatures.     

The nature of anharmonicity and anomalous piezoelectric properties of ferroelectric SbSBr crystal

The contribution of soft mode at Sb atom's sites, to the temperature dependences of Sb atom's equilibrium position's difference Δz(T) has been studied theoretically, when SbSBr crystal is deformed along a(x), b(y) and c(z)-axis in paraelectric phase and is deformed along c(z)-axis in ferroelectric phase. The largest change of Δz₃₃(T) occurs in the ferroelectric phase near the phase transition temperature in the range from 16 K to 21 K. The temperature dependence of Sb atom's equilibrium position's displacements Δz₃₃ is very similar to the temperature dependence of experimental piezoelectric modulus, when SbSBr crystal is deformed in the direction of c(z)-axis in ferroelectric phase.     

Ultrasonic attenuation study of erbium in a magnetic field

The ultrasonic attenuation coefficient of longitudinal waves propagating along the c-axis of a single crystal of erbium was measured over a temperature range which covers the transition temperature from the paramagnetic to the sinusiodal phase T₆ = 83.8 K and the transition temperature from the sinusoidal to elliptical phase T_⊥ = 53.9 K. These measurements were performed as a function of temperature in the presence of constant magnetic fields and at constant temperatures as a function of applied magnetic fields. A magnetic field parallel to the c-axis shifted T₆ to lower temperatures and reduced the peak attenuation. A broad new maximum emerged on the high temperature side of the field-shifted T₆. Application of H perpendicular to the c-axis produced little change in either T₆ or T_⊥. A model introduced by Tachiki and Mackawa is used to discuss the results.     

Electrical conduction of a quasi-one dimensional Nb3S4 single crystal

With respect to single crystals of Nb₃S₄ the electrical resistivity from 2.8 K to 300 K and the magnetoresistance at 4.2 K were measured. The resistivity is represented as a sum of a temperature independent and an intrinsic temperature dependent component. The temperature dependence of the intrinsic resistivity subjects to T₃ form between 7 and 50 K above which it becomes weaker than T₃ approaching a T linear from. This behaviour is discussed in terms of the electron-electron Umklapp scattering. The ratio of the resistivities perpendicular and parallel to the c-axis takes about 15 between room temperature and 50 K. The transverse magnetoresistance is proportional to the magnetic field. The longitudinal magnetoresistance is too small to be measured.     

Anisotropy of the electron momentum distribution in Bi2Sr2CaCu2O8+δ superconductor studied by positron annihilation

The temperature dependent (30-300 K) Doppler broadening of the positron annihilated γ-radiation measurement has been investigated on single crystalline Bi₂Sr₂CaCu₂O_8+δ (Bi-2212) high T_c superconducting samples along two different crystallographic orientations. It has been observed that throughout the temperature range the electron momentum distribution has a larger value along the crystallographic c-axis than in the a-b plane. The temperature dependent Doppler broadened positron annihilation γ-radiation lineshape analysis shows a step like increase of S-parameter at the temperature region 92-116 K.     

c-axis penetration depth in Bi2Sr2CaCu2O8+δ single crystals measured by ac-susceptibility and cavity perturbation technique

The c-axis penetration depth Δλ_c in Bi₂Sr₂CaCu₂O_8+δ (BSCCO) single crystals as a function of temperature has been determined using two techniques, namely, measurements of the ac-susceptibility at a frequency of 100 kHz and the surface impedance at 9.4 GHz. Both techniques yield an almost linear function Δλ_c(T)∝T in the temperature range T<0.5T _c. Electrodynamic analysis of the impedance anisotropy has allowed us to estimate λ_c(0)≈50 µm in BSCCO crystals overdoped with oxygen (T _c≈84 K) and λ_c(0)≈150 µm at the optimal doping level (T _c≈90 K).     

Anisotropic thermopower of (TMTSF)2PF6: 1D–2D cross over and SDW ordering

The thermopower of (TMTSF)₂PF₆ has been measured along the a-axis as well as along the b-axis. Marked anisotropy is seen in the whole temperature region studied. Close to 100 K, an anomaly is attributed to cross over from dominating one-dimensionality to dominating two-dimensionality. Precursor effects seen in S_a near T_c are attributed to SDW fluctuations. Well below T_c, both thermopower components exhibit typical semiconducting properties.     

Gigantic uniaxial pressure effect in single crystals of iron-based superconductors

In order to elucidate the anisotropic pressure effect on superconductivity in an iron-based superconductor, magnetization measurements have been performed in Ba(Fe_0.92Co_0.08)₂As₂ single crystals under uniaxial pressures applied along the c-axis. Gigantic T_c suppression, dT_c/dP_//c = −15 K/GPa, was observed when the anisotropic deformation with the a-expansion and c-compression was induced by the c-pressure, which should be compared with dT_c/dP < +1 K/GPa in the isotropic pressure case. This suggests that the a-axis (c-axis) compression has a positive (negative) contribution to T_c.     

Thermal expansion behaviour and phase stability of AFe<Subscript>2</Subscript>As<Subscript>2</Subscript> (A = Ca,Sr and Eu) using powder diffraction technique

The thermal expansibilities and phase stabilities of AFe ₂As ₂ (A = Ca, Sr and Eu) have been investigated by powder diffraction techniques in the temperature range 5–600 K. We found the anisotropic thermal expansivities with temperature for all the compounds. The lattice parameter in the tetragonal phase ( A_T) of CaFe ₂As ₂ contracts with increasing temperature, whereas C_T expands. The rate of contraction in A_T is lower than the rate of expansion in C_T. Other compounds show normal thermal expansion behaviour along both a- and c-axes. In-plane expansion (i.e., along the a-axis) is found to be the smallest for EuFe ₂As ₂ and the highest for BaFe ₂As ₂. However, the rate of change of thermal expansivities along out-of-plane (i.e., along the c-axis) is higher as we go from Ba, Sr, Eu and Ca, respectively. Above 600 K, we notice the appearance /disappearance of certain reflections which suggest that tetragonal phase is not stable above this temperature for these compounds.     

Effect of annealing on the magnetic and superconducting properties of single-crystalline UCoGe

Single-crystals of the new ferromagnetic superconductor UCoGe have been grown. The quality of as-grown samples can be significantly improved by a heat-treatment procedure, which increases the residual resistance ratio (RRR) from ∼5 to ∼30. Magnetization M(T) and resistivity ρ(T) measurements show the annealed samples have a sharp ferromagnetic transition with a Curie temperature T_C is 2.8 K. The ordered moment of 0.06 μ_B is directed along the orthorhombic c-axis. Superconductivity is found below a resistive transition temperature T_s=0.65 K.     

Anisotropy of the lattice vibration of Eu and Cu in highTc superconductor Ba2EuCu3O7 analyzed by151Eu and57Fe Mössbauer effects

The anisotropy of superconductivity in highTc superconductors indicates that it is important to clarify the anisotropies of microscopic properties in these materials. In this work, we observed the anisotropy of the lattice vibration ofEu andCu inBa ₂ EuCu ₃ O ₇ by¹⁵¹ Eu and⁵⁷ Fe Mössbauer effect. Thec-axis aligned samples were prepared by healing the mixture of epoxy andBa ₂ EuCu ₃ O ₇ powder in a magnetic field. For⁵⁷ Fe Mössbauer measurement the sample doped with⁵⁷ Fe,Ba ₂ Eu(Cu _0.99 Fe _0.01)₃ O ₇, was used. The temperature dependence of Mössbauer spectrum was measured in two cases,c-axis ‖ γ-ray andc-axis ⊥ γ-ray. The Debye temperature ofEu alongc-axis (230 K) was lower than that inab-plane (265 K). The Debye temperature of⁵⁷ Fe substituted forCu(2) was lower (378 K) alongc-axis than inab-plane (417 K). The Debye temperature of⁵⁷ Fe substituted forCu(1) having no oxygen atom ina-axis was 435 K alongc-axis and 416 K inab-plane. These results are interpreted by the oxygen configuration around the Mössbauer atoms.     

Ultrasound studies of single crystal thulium in an applied magnetic field

The paper reports the first measurements of the single crystal elastic constants of the heavy rare earth metal thulium as a function of temperature and magnetic field. The constants were obtained from ultrasonic velocity measurements over a temperature range of 4.2–296 K and in applied magnetic fields of up to 5 T. The elastic constants; C₁₁, C₃₃, C₄₄ and C₆₆=(C₁₁–C₁₂)/2 were determined from the ultrasonic velocities. Anomalies in the elastic constants were observed at 58 K from the c-axis propagated shear wave measurements and at 55 K from the c-axis propagated longitudinal wave measurements. Significant softening of the elastic constants C₃₃ and C₄₄ was observed close to T_N. Application of a magnetic field (>2 T) along the c-axis direction induced further softening of the material. Electromagnetic acoustic transducers (EMATs) were also employed in addition to conventional piezoelectric quartz transducers. A marked increase in the EMATs acoustic coupling efficiency (generation and detection efficiency) occurred close to T_N.     

Channelingeffekte bei der Lichtausbeute von Anthrazen-, Naphthalin- undp-Terphenylkristallen bei Anregung mit α-Teilchen

The scintillation light yield of anthracene atT=300 °K andT=80 °K and of naphthalene andp-terphenyl atT=80 °K was investigated for impact directions in the (a, c)-plane. The light yield was measured for welldefined integration times. The relative depth of the channeling-minimum observed for impact directions parallel to thec-axis of anthracene is found to increase with increasing integration times. For the first time channeling-minima of the light yield were observed at impact directions parallel to thec- and the [102]-axis of naphthalene and thec- and [¯102]-axis ofp-terphenyl. The [¯102]-axis ofp-terphenyl shows a more pronounced effect than thec-axis.     

Faraday rotation and optical investigation of the metamagnetic phase transition in dysprosium phosphate

The metamagnetic phase transition in dysprosium phosphate has been studied by optical methods. A theoretical expression for the Faraday rotation in this substance is derived, and experimental results are reported for a long needle-shaped specimen. Observations of domains indicate that in DyPO₄ with magnetic field parallel to the crystallographic c-axis a mixed phase exists up to a tri-critical temperature T_t = 1.95 K.     

Specific features in the behavior of the central peak in Raman spectra of lithium tantalate

Low-frequency Raman spectra of a LiTaO₃ ferroelectric crystal are studied in the temperature range 300–1273 K. The central peak associated with the relaxation susceptibility of the crystal lattice is recorded throughout the temperature range covered, including the temperatures both substantially lower and considerably higher than the Curie point (T _c = 900 K). The critical slowing down of the relaxation response time predicted by the Ginzburg-Landau-Devonshire theory is observed in the temperature range from 0.9T _c to 1.1T _c . In contrast to the critical slowing down, the width of the central peak γ_R increases below 0.9T _c and decreases above 1.1T _c with increasing temperature.     

Magnetization anisotropy of the Tm- and Fe-subsystems in Tm2Fe17

Magnetic and neutron diffraction measurements were carried out in order to study the spontaneous and induced spin-reorientation (SR) transition of the “easy axis–easy plane” type in the poly and single-crystalline samples of the hexagonal Tm₂Fe₁₇. We have determined the temperature dependence of the lattice parameters and the angle between the c-axis and the magnetic moment of the Tm-subsystem. We also find that the SR transition is accompanied by a large (about 20%) magnetization change of the Tm subsystem. In order to induce such a SR transition with the external magnetic field, μ₀H_cr=5 T is necessary to be applied along the hard-magnetization direction (the a-axis) at 4.2 K. The H_cr value decreases with an increasing temperature. The magnetization measurements demonstrate that at 10 K the saturation magnetization along the easy-magnetization direction (the c-axis) is smaller than that along the hard-magnetization direction. Based on this observation, we believe that Fe-subsystem of Tm₂Fe₁₇ is likely to have magnetization anisotropy.     

Probing the phase transition of methane with Mössbauer effect in 57Fe impurities

Mossbauer studies were performed in ⁵⁷Fe impurities (0.3% at.) in solid methane, in order to investigate the properties of the order-disorder phase transition taking place at T_c=20.4K. From the temperature dependence of the relative values of the recoil-free-fraction - f - it was found that near T_c a change occurs in the slope of f(T²) without discontinuity in the value of f. A consistent model is proposed by which immediately below T_c both phases, I and II coexist with a gradual evolution of the ordered state (II) on the account of the high temperature plastic state (I). The Debye temperature of phase II (θ_II = 101±7K) increases with respect to that of phase I (θ_I = 72±3K), indicating a hardening in the phonon spectrum and precluding the onset of rotational modes of the disordered molecules.