Dielectric properties of (KBr)1−x (KCN) x

The temperature dependence of the dielectric constant of (KBr)_1–x (KCN) _x was measured at frequencies of 87.5 Hz, 1, 10 and 100 kHz for concentrations ofx=0, 0.01, 0.04, 0.14, 0.25, and 0.50. Both the real and the imaginary part showed maxima at characteristic temperatures which depend on the measuring frequency and the concentration. The results are analogous to the behaviour of the magnetic susceptibility in spin glasses and support the existence of an orientational glass state.     

Random field effects in the three dimensional Ising magnet: Fe x Zn1−x F2

We have carried out a comprehensive neutron scattering study of random field effects in the diluted three dimensional Ising antiferromagnet Fe _x Zn_1–x F₂ withx=0.35 and 0.5. Emphasis is on the global trands from the small to the large random field regimes. It is found, as in previous experiments, that when the system is cooled in a field it evolves from the high temperature paramagnetic state to a low temperature domain wall state. The low temperature peaks are well-described by Lorentzian squared profiles although for thex=0.5 sample extinction made the measurements difficult. In both samples, the results show that in the field-cooled state the correlation length varies asH ^–v withv=2.2±0.1. In thex=0.35 sample this power law holds over a length scale varying from 2 to 1500 lattice constants. At low fields pretransitional behavior similar to that observed previously in Fe_0.6Zn_0.4F₂ is found. AtT _N (H=0) it is found that the correlation length also scales algebraically withH but withv=0.86±0.04. Pronounced history-dependent effects are observed below the phase boundary determined by the peak in the critical scattering. For example, on cooling in zero-field, raising the field and then warming, long range order survives up to the phase boundary; at this point it appears to convert abruptly into the finite correlation length field cooled state although elucidation of the explicit nature of this transition is complicated by rounding due to a concentration gradient. These results are discussed in the context of recent theories incorporating metastability effects as well as recent experiments.     

Electron pairing mechanism in incipient ferroelectric BaPb1−x Bi x O3

The lattice dynamical properties of BaPb_1–x Bi _x O₃ are described within the polarizability model for perovskite oxides, but extended to take into account the valence instability of Bi. In a quantitative investigation it is found that a mode softening to a ferroelectric state is excluded, but a transition to a paired electron state is possible. The transformation of the classical Hamiltonian to its quantum-mechanical equivalent shows the parameter regimes for which pairing of electrons and thus possibly superconductivity occur.     

Phase transition from spin glass to long-range magnetic order in semiconductor spinels CuCr1.5+0.5x Sb0.5−0.5x S4 (x=0.34 and 0.4)

Reentrant behavior to a spin glass state is discovered in the solid solution system CuCr_{1.5+0.5x Sb x0.5−0.5 x S 4} with x=0.34 and 0.4. The spin-glass transition temperature _{T f} determined from the kink in the temperature dependence of the initial susceptibility in an alternating magnetic field depends on the measurement frequency ω. It is shown that the frequency dependence _{T f} (ω) is a power-law function 1/ω=(1/ω₀)[T _f /( _{T f} − _T *)]^zv with _zv =7.7 for both compositions. For the composition with _x =0.34 a maximum is observed near _T * in the temperature dependence of the resistivity. These facts indicate that the transition from the spin glass to long-range magnetic order is a phase transition. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 4, 265–269 (25 August 1996)     

Study of the amorphous Fe100−x B x and (Fe70Co30)100−x B x powder prepared by borohydride reduction

The amorphous fine particles of Fe_100−x B _x (20.7≤x≤36.5) and (Fe₇₀Co₃₀)_100−x B _x (26.9≤x≤40.2) were prepared by borohydride reduction. The amorphous state of the samples was identified with both X-ray and electron diffraction measurements. With increasing boron content, the average hyperfine magnetic fields decrease. Comparing the hyperfine parameters of the glassy powder with those of the ribbons having the same compositions, the average hyperfine fields and the isomer shifts are different. The differences may be due to the amorphous polymorphism which is produced by the different preparation methods. Project is partly supported by the National Natural Science Foundation of China.     

Possible reconstruction of the ferromagnetic state under the influence of superconductivity in epitaxial V/Pd1−x Fe x bilayers

Epitaxial V/Pd_1−x Fe _x (001) bilayers with a V thickness of the order of 40 nm and with a Pd_1−x Fe _x thickness in the range from 0.8 nm to 4.4 nm were prepared by molecular beam epitaxy techniques. The Curie temperature of the Pd_1−x Fe _x layers varies between 90 and 250 K. For a bilayer with a Pd_1−x Fe _x thickness of 1.2 nm the ferromagnetic resonance measurements revealed a decrease of the effective magnetization 4πM _eff of the ferromagnetic layer below the superconducting transition temperature of V. As a possible explanation for this decrease we suggest a spatial modulation of the ferromagnetic order in the Pd_1−x Fe _x layer due to modifications of the indirect exchange interaction of magnetic ions via conduction electrons in the superconducting state. A comparison with a recent theoretical investigation supports this possibility.     

Transport phenomena of SmSe1−x As x

Results are presented on the measurements of unit cell parameter and electrical resistivity under pressure on SmSe_1−x As _x forx=0.1, 0.2, 0.3 and 0.4. The electrical resistivity values are found to be decreasing with increase of pressure and also with increase of arsenic concentration. The semiconductor to metallic transition is induced by chemical alloying of SmSe with SmAs similar to that observed under pressure. The electrical resistivity values are also calculated which are in good agreement with the experimental values. In this calculation, the carrier mobility is of negative sign and so the sample SmSe_1−x As _x is found to ben-type semiconductor.     

57Fe Mössbauer study of Eu1−x Sr x FeO3−y

The samples of Eu_1–x Sr _x FeO_3–y (x=0.0–1.0) were prepared by the solid state reaction method. Their X-ray diffraction patterns and⁵⁷Fe Mössbauer spectra at room temperature were measured. It is found that Sr ions incorporate in the lattice of EuFeO₃, the change of crystal structure is related to the dopant.⁵⁷Fe Mössbauer spectra consist of one magnetic, one doublet and one single paramagnetic components. The Fe ions in the cubic phase are in intermediate valence state between Fe(III) and Fe(IV) and may participate in electron hopping.     

Phases and phase transitions of (NaCl)1−x(NaCN)x

(NaCl)_1–x(NaCN)_x mixed crystals with CN-concentrationsx of 0.87, 0.76, 0.71 and 0.65 have been investigated by X-ray powder diffraction. Apart from the cubic room temperature phase, a rhombohedral and an orthorhombic phase have been identified. In addition a glass state has been observed which is characterized by a strong broadening of the cubic powder lines. The phase diagram shows a wide coexistence gap between the non-cubic phases and the glass state.     

Effect of substitution of Ce on superconducting properties of Bi1.7Pb0.3Sr2Ca2−x Ce x Cu3O10+δ system

We have investigated the superconducting properties of the Bi_1.7 Pb_0.3Sr₂Ca_2−xCe _x Cu₃O_10+δ system with x=0.00, 0.02, 0.04, 0.08 and 0.1 by X-ray diffraction and magnetic susceptibility. The substitution of Ce for Ca has been found to drastically change the superconducting properties of the system. X-ray diffraction studies on these compounds indicate decrease in the c-parameter with increased substitution of Ce at Ca site and volume fraction of high T _c (2 : 2 : 2 : 3) phase decreases and low T _c phase increases. The magnetic susceptibility of this compound shows that the diamagnetic on set superconducting transition temperature (onset) varies from 109 K to 51 K for x=0.00, 0.02, 0.04, 0.08 and 0.1. These results suggest the possible existence of Ce in a tetravalent state rather than a trivalent state in this system; that is, Ca²⁺ → Ce⁴⁺ replacement changes the hole carrier concentration. Hole filling is the cause of lowering T _c of the system.