ELECTROCHEMICAL OXIDATION OF La2CuO4 SINGLE CRYSTALS

Bulk superconducting La₂CuO_4+δ single crystals are obtained by using electrochemical intercalation technique from the as-grown insulating samples. Oxidation is carried out by constant current I=10μA at temperature T=70℃ and room temperature, respectively. Structure and magnetic properties are studied by low-temperature X-ray diffraction and susceptibility measurements. A superconducting phase with T_c of 19K and δ-0.12 can be attributed to the formation of oxygen clusters. Room temperature oxidation is inhomogeneous: two superconducting phases with T_c1 of 24K and T_c2 of 8K and an antiferromagnetic phase are coexisting in the crystal. It is found that the appearance of T_c in this system has the "step" tendency.     

Metal--Insulator Transition in Ca-Doped Sr14-xCaxCu24O41 Systems Probed by Thermopower Measurements

High quality Sr14-xCaxCu24O41 single-crystals are successfully grown by floating-zone technique, and the trans- port properties are studied. The temperature dependence of resistivity along the c-axis direction is semiconductor- like for x ≤ 10 and it can be fitted by the thermal activation equation p = po exp（ △ /kBT） with kB being the Boltzmann constant and A the activation energy. A break in the slope of thermopower （S） versus the inverse temperature （1 IT） corresponding to the formation of charge-density waves （CD W） is first observed for x ≤ 6. The temperature dependence of thermopower becomes metallic for x ≥ 8 while the resistivity is still semiconductorlike. We propose that the insulation behaviour of the resistivity in the Ca doping range 8 ≤ x ≤ 11 could result from the localization of the charge carriers due to the disorder induced by Ca doping and a revised electronic phase diagram is derived based on our observations.     

Magnetic properties and magnetoresistance effect of Y1-xGdxMn6Sn6 (x=0－1) compounds

The magnetic properties and magnetoresistance effect of Y_1-xGd_xMn₆Sn₆ (x=0－1) compounds have been investigated by magnetization and resistivity measurements in the applied field range (0－5 T). Compounds with x=0.4－1 display ferrimagnetic behaviours in the whole magnetic ordering temperature range, while compounds with x=0－0.2 display a field-induced metamagnetic transition, and the threshold fields decrease with increasing Gd content. The compounds with x=0.1－0.2 undergo an antiferromagnetic to ferromagnetic transition with increasing temperature. The cell-parameter a and c and cell-volume V of compounds (x=0－1) increase with increasing Gd content. It was found that the saturation magnetization M_s of the compounds (x=0.4－1) decreases, while the ordering points of the compounds (x=0－1)increase with increasing Gd content. A large MR effect was observed in the compound with x=0.2, and the maximum absolute value of MR at 5 K under 3 T is close to 19.3%.     

Thermodynamic and kinetic properties of an icosahedral quasicrystalline phase in the Al-Pd-Tc system

The properties of a quasicrystalline phase in the Al-Pd-Tc system are studied for the first time. X-ray investigations demonstrate that the quasicrystalline phase in the Al₇₀Pd₂₁Tc₉ alloy has a face-centered icosahedral quasi-lattice with parameter a=6.514 ?. Annealing experiments have revealed that this icosahedral phase is thermodynamically stable. The heat capacity of an Al₇₀Pd₂₁Tc₉ sample is measured in the temperature range 3–30 K. The electrical resistivity and magnetic susceptibility are determined in the temperature range 2–300 K. The electrical resistivity is found to be high (600 μΩ cm at room temperature), which is typical of quasicrystals. The temperature coefficient of electrical resistivity is small and positive at temperatures above 50 K and negative at temperatures below 50 K. The magnetic susceptibility has a weakly paramagnetic character. The coefficient of linear contribution to heat capacity (γ=0.24 mJ/(g-atom K²)) and the Debye characteristic temperature (Θ=410 K) are determined. The origin of the specific features in the vibrational spectrum of the quasicrystals is discussed. __________ Translated from Fizika Tverdogo Tela, Vol. 42, No. 12, 2000, pp. 2113–2119. Original Russian Text Copyright ? 2000 by Mikheeva, Panova, Teplov, Khlopkin, Chernoplekov, Shikov.     

boldmath Characterization of the BaBiO3-doped BaTiO3 positive temperature coefficient of a resistivity ceramic using impedance spectroscopy with Tc=155℃

BaBiO₃-doped BaTiO₃ (BB-BT) ceramic, as a candidate for lead-free positive temperature coefficient of resistivity (PTCR) materials with a higher Curie temperature, has been synthesized in air by a conventional sintering technique. The temperature dependence of resistivity shows that the phase transition of the PTC thermistor ceramic occurs at the Curie temperature, T_c=155 ℃, which is higher than that of BaTiO₃ ( ≤ 130 ℃). Analysis of ac impedance data using complex impedance spectroscopy gives the alternate current (AC) resistance of the PTCR ceramic. By additional use of the complex electric modulus formalism to analyse the same data, the inhomogeneous nature of the ceramic may be unveiled. The impedance spectra reveal that the grain resistance of the BB-BT sample is slightly influenced by the increase of temperature, indicating that the increase in overall resistivity is entirely due to a grain-boundary effect. Based on the dependence of the extent to which the peaks of the imaginary part of electric modulus and impedance are matched on frequency, the conduction mechanism is also discussed for a BB-BT ceramic system.     

AGNETORESISTANCE, SPECIAL HEAT AND AC MAGNETIC SUSCEPTIBILITY IN La1-xCaxMnO3 SYSTEM

We have measured the resistivity, special heat and AC magnetic suceptibility versus temperature in La_1-xCa_xMnO₃(LCMO) bulk samples with x=0.3, 0.4, 0.5, 0.6. In the LCMO (x=0.3 and 0.4) samples a transition from dρ/dT0 was observed, accompanied by a peak in specific heat. In AC magnetic suceptibility measurement, a ferromagnetic and an antiferromagnetic transitions were observed in LCMO (x=0.5) sample.     

STRUCTURE, MAGNETIC PROPERTIES AND GIANT MAGNETORESISTANCE OF YMn6Sn6-xGax (x=0-0.6) COMPOUNDS

Structure, magnetic and transport properties of YMn₆Sn_6-xGa_x (0≤x≤0.6) compounds with a HfFe₆Ge₆-type structure were investigated. It was found that the Ga substitution leads to a contraction of the unit-cell volume. A transition from an antiferromagnetic to a ferromagnetic (or ferrimagnetic) state can be observed for samples (0.1≤x≤0.2) with increasing temperature. The antiferro-ferromagnetic transition for samples with x≤0.2 can also be induced by an external field. The required field is very low, and decreases with increasing Ga concentration. More Ga concentration (x≥0.3) leads to the samples being ferromagnetic in the whole temperature range below the Curie temperature. The Ga substitution weakens the interlayer magnetic coupling between the Mn spins. Corresponding to the metamagnetic transition, a magnetoresistance as large as 32% under a field of 5 T was observed at 5 K for the sample with x=0.2.     

ENHANCEMENT OF FERROMAGNETIC CLUSTER INDUCED BY MAGNETIC FIELD IN THE PHASE-SEPARATED La0.5Ca0.5MnO3

Magnetic and transport properties of La_0.5Ca_0.5MnO₃ have been investigated by measuring the magnetization and resistance in zero-field-cooled (ZFC) and field-cooled (FC) modes. Conspicuously irreversible behaviors of magnetization/resistance in the two different modes were observed below the charge ordering transition temperature (T_CO). The ZFC and FC magnetizations at 5K, as functions of the magnetic field, coincide for μ₀H≤1T. Afterwards, the ZFC magnetization tends to an approximate constant, but the FC one increases linearly with increasing field. There exists an excellent correspondence between magnetization and resistance below T_CO. All the results suggest that the ferromagnetic clusters embed in the charge-ordered matrix. The phenomenon of ferromagnetic clusters growing up easily in the FC procedure has been interpreted according to the model of thermally activated two-level system.     

STRUCTURE AND MAGNETIC PROPERTIES OF LaCo13-BASED ALLOYS

The structure and magnetic properties of two series of LaCois-based alloys, LaCo_13-xFe_x(0≤x≤6) and La_1-xR_x(Co, Fe)₁₃ (x=0.1,0.2; R=Y, Ge, Pr, Nd Sm, Gd, Tb, Dy, Ho, Er, and Yb), are studied by X-ray diffraction and magnetic mea- surements. In the above-mentioned composotions, all the samples display a structure of NaZn₁₃ type (space group: Fm3c). In LaCo_13-xFe_x system, as the iron content increases, its Curie temperature decreases, while thc magnetic moment per 3d atom increases. This may be explained by a rigid band model. In the other alloy system, the Curie temperature goes through a maxium at R=Gd, and this reflects the influ- ence of R - T interaction. It is also observed that saturation magnetization varies with R and gets lower when R is a heavy rare earth element. The contribution of R is close to that of a free-ion moment. La_0.9Pr_0.1Co₉Fe₄ has the largest value 29μ_B/f.u.     

An investigation of phase separation by magnetic force microscopy in La0.45Sr0.55MnO3-δ(δ≈0.01)

We have found phase separation in La_0.45Sr_0.55MnO_3-δ (LSMO) by means of electron spin resonance, magnetic force microscopy (MFM) and magnetic measurements. Ferromagnetic and antiferromagnetic phases can coexist at low temperatures, and ferromagnetic and paramagnetic phases coexist when the temperature lies between the Néel and Curie temperatures. The size and shape of the ferromagnetic phases (the minority phases) was first observed directly from MFM images. It is suggested that the phase separation in LSMO is not the charge segregation type, but an electroneutral type due perhaps to the nonuniform distribution of oxygen vacancies.     

La2-xBaxCuO4单晶反常的正常态输运性质研究

测量了La2-xBaxCuO4系列单晶样品的电阻率和热电势,我们发现,当＝0．125（x＝1／8）时,La2-xBaxCuO4的截流子是所有样品中局域化最强的,但数据分析结果显示,它又是弱域化行为,文中我们讨论了Tc的被压制的原因,得出这可能与低温下LTO到LTT的结构相变,空穴与自旋的静态条纹有序有关,热电势结果观察不到任何声子曳引的痕迹,表明在LBCO体系中电声子的相互作用很弱。     

On the interpretation of the phase diagram of La<Subscript>2−x</Subscript>M<Subscript>x</Subscript>CuO<Subscript>4</Subscript>

The difference between the phase diagrams of La_2?x(Ba,Sr)_xCuO₄ and Nd_2?xCe_xCuO₄ is discussed. It is proposed that the discrepancy of x-values corresponding to the transition from antiferromagnetic dielectric state to conducting one (respectively, x≈0.06 in La_2?x(Ba,Sr)_xCuO₄ and x≈0.125 in Nd_2?xCe_xCuO₄) results from non-homogeneous doping of La_2?x(Ba,Sr)_xCuO₄ over the range 0.06<x<0.125, when localized holes are added to each second CuO₂ layer. Therefore the actual phase diagram of the “holedoped” superconductor La_2?x(Ba,Sr)_xCuO₄ coincides with one for the “electron-doped” superconductor Nd_2?xCe_xCuO₄. It is shown that all features observed in La_2?x(Ba,Sr)_xCuO₄ around x=0.125 are clarified on this basis.     

Kinetic effects in manganites La1 − x Ag y MnO3 (y ≤ x)

We have measured the resistivity, magnetoresistance, and thermopower of ceramic manganite samples La_{1 ? x} Ag _y MnO₃ (y ≤ x) doped with silver as functions of temperature (4.2–350 K) and magnetic field (up to 26 kOe). A metal-insulator phase transition is observed in all investigated samples at temperatures close to room temperature. The behavior of the resistivity and thermopower in the high-temperature paramagnetic region is interpreted using the concept of small radius polaron; the activation energy decreases with increasing doping level. The resistivity in the low-temperature ferromagnetic region is approximated by the expression ρ_FM(T) = ρ₀ + AT ² + BT ^4.5 presuming the existence of electron-electron and electron-magnon interactions. A resistivity minimum and a strong magnetoresistive effect are observed at low temperatures. The latter effect is associated with scattering of charge carriers at grain boundaries, which are antiferromagnetically ordered relative to one another. The temperature dependence of thermopower in the magnetically ordered phase is described in the framework of a model taking into account the drag of charge carriers by magnons.     

The transport properties in Nd0.75Sr1.25CoO4 film: Evidence for polaronic transport

The experiments of electrical resistivity and thermopower on Nd_0.75Sr_1.25CoO₄ film in the temperature range 90 K<T<310 K were carried out. The great difference in the activation energies estimated from thermopower and resistivity, a characteristic of small polarons, is observed, providing strong evidence for polaron-dominated transport mechanism in this material. Furthermore, the activation energy at intermediate-temperature region is larger than that at low-temperature region in resistivity, but it is not observed in thermopower, indicating that the energy for the creation of the carriers is slightly lower at low-temperature region than that at intermediate-temperature region. At the same time, the abrupt drop in the thermopower and the abnormal peak in the differential curve of resistivity indicate that a phase transition between a paramagnetic state and a ferromagnetic state occurs at temperature about 218 K. The positive thermopower in the whole temperature range measured suggests that the carriers are holes in this system.     

Suppression of charge ordering and thermal hysteresis of electronic transport and magnetisation in La0.5Ca0.5Mn1−xNixO3

A series of polycrystalline La_0.5Ca_0.5Mn_1?xNi_xO₃ (x = 0.00, 0.025, 0.050, 0.075, 0.100 and 0.125) was synthesised using solid state reaction. Measurements in a cooling and warming cycle between 300 and 80 K were carried out to study the Ni-doping effects on the electrical resistivity, thermopower and magnetisation of single-phase La_0.5Ca_0.5Mn_1?xNi_xO₃. Partial substitution of Ni for Mn leads to the suppression of charge ordering state, the evidence of which is shown by the dramatic decrease in electrical resistivity and thermal hysteresis width in electrical resistivity, thermopower and magnetisation. However, the magnitude of both electrical resistivity and thermopower increases with increasing Ni content. This can be attributed to an increase in the Mn⁴⁺ concentration, which favours the antiferromagnetic state and leads to a gradual disappearance of ferromagnetic double exchange interaction. Besides, the metal–nonmetal transition temperature decreases with increasing Ni content until x = 0.075, which might arise from increased electron–phonon coupling due to less ordered spins at temperatures above ferromagnetic transition. For samples with x greater than 0.075, no metal–nonmetal transition is observed due to the suppression of double exchange mechanism.     

Heat treatment effects in amorphous metals: Zr70Cu30 and La70Cu30

We measured the electrical resistivity (4–300 K), superconducting critical temperature and thermal conductivity (0.5–7 K) of the amorphous metals Zr₇₀Cu₃₀ and La₇₀Cu₃₀. Heat treatments below crystallization temperature induced changes in these properties. In particular, in the first stage of the annealing of Zr₇₀Cu₃₀ there are systematic changes in the thermal conductivity and the critical temperature, while the electrical resistivity remains constant. We show that there is no simple correlation between the thermal conductivity processes in the low temperature and plateau regions. We also show that the thermal conductivity of as quenched La₇₀Cu₃₀ is typical of amorphous metals, contrary to information previously reported.     

Magnetic order in La2-x Bax CuO4 and La1.6-x Nd0.4 Srx CuO4 for x=0.125

We present μSR experiments on La_2-x Ba_x CuO₄ and La_1.6-x Nd_0.4 Sr_x CuO₄ for x=0.125. Both of these materials order magnetically with T_N\approx30\ K, while a superconducting sample of La_1.4 Nd_0.4 Sr_0.2 CuO₄ showed no evidence for static copper moments. Our results support the conclusion that the so‐called “1/8” anomaly in La_2-x Ba_x CuO₄ is a result of static (pinned) charge segregation. This revised version was published online in August 2006 with corrections to the Cover Date.     

La2CuO4+δ和La2-xSrxCuO4系统中的电子输运性质

 本文解释了La₂CuO_4+δ（0≤δ≤0.09）和La_2-xSr_xCuO₄（0≤x≤0.3）两种p型系统含铜稀土氧化物中的电阻和Seebeck系数与温度的依赖关系，在室温以上，一氧大气压下的La₂CuO_4+δ系统趋于失氧；在500 K以上，超导样品显示出失氧的一级相变，并且恢复到反铁磁相。在转变温度T₁≈300 K以下，对0<δ<0.05成份的样品，相分离成反铁磁相和超导相；而在T_cρ≈100 K的温度范围内，超导相进一步分离成富空穴和贫空穴畴。在0.04≤δ≤0.09范围内，T_c处的电阻陡降出现了台阶；我们认为，它反映了电子成对的起伏。在La_2-xSr_xCuO₄系统中，对于成分为01≈300 K以上，空穴的运动是弥散的，但是ΔH_m=0；而对于x≥0.22的样品，经历了从平滑到Fermi液态的转变。成份为0c1范围（其中空穴继续以弥散方式运动）是亚稳的，但是，在T_cρ≤150 K范围，出现了电荷起伏。当样品冷却通过T₁时，对于成份为0.15≤x≤0.2的样品，经历了由弥散到强质量增强巡游电子状态的转变；在T_c处，从均匀的修饰电子的正常态凝聚成超导的载流子对。在超导成份样品的正常态中，不寻常的电子-晶格相互作用，可以归结为在CuO₂面上从更离子性的到共价性的Cu:3d_x²-r²─O:P_σ键合的转变；通过这种转变，轨道杂化和Hubbard U参量随Cu─O键长和Cu原子上的外表局域氧化态都产生灵敏的变化。     

Thermopower and electrical resistivity of La0.875Sr0.125MnO3 single crystals under hydrostatic pressure

The thermopower α and the electrical resistivity ρ of La_1?x Sr_xMnO₃ single crystals with a strontium content x=0.125, which corresponds to the stoichiometric composition of the new charge-ordered (CO) phase, are measured in the temperature range 77–300 K at pressures up to 12 kbar. The dependence α(T) exhibits two maxima. The first low-temperature maximum is associated with the formation of the charge-ordered phase. The second high-temperature maximum is attributed to the O → O′ structural transition between the orthorhombic phases and the formation of ferromagnetic clusters. It is found that the phase transition observed at P>9.2 kbar is accompanied by a substantial shift of both maxima toward the low-temperature range.     

Elastic and kinetic properties of single-crystal La<Subscript>0.75</Subscript>Ba<Subscript>0.25</Subscript>MnO<Subscript>3</Subscript>

An experimental study of the temperature behavior of longitudinal sound velocity, internal friction, electrical resistivity, and thermopower of single-crystal La_0.75Ba_0.25MnO₃ is reported. A structural transition accompanied by a large jump (18%) in the sound velocity was found to occur at T _S ≈170 K. Within the interval 156–350 K, the temperature dependences of the sound velocity and internal friction reveal a temperature hysteresis. An internal-friction peak due to relaxation processes was detected. The metallic and semiconducting regions are separated by a transition domain about 80 K wide lying below the Curie temperature T _C =300 K.