Co_{40}Fe_{50}Ni_{10} and Co_{50}Fe_{45}Ni_{5} ternary alloys were prepared by mechanical alloying method. To check the stability of their structure thermal treatment was applied subsequently. As X-ray diffraction studies proved the final products of milling were the solid solutions with bcc lattice and the average grain sizes ranged of tens of nanometers. After heating of the Co_{50}Fe_{45}Ni_{5} alloy up to 993 K the mixture of two solid solutions with bcc and fcc lattices was formed. In other cases thermal treatment did not change the type of the crystalline lattice. Mössbauer spectroscopy revealed hyperfine magnetic field distributions which reflected the different possible atomic surroundings of ^{57}Fe isotopes. Results of the macroscopic magnetic measurements proved that both investigated alloys had relatively good soft magnetic properties.
The ferromagnetic Curie temperatures T_{C} derived from a temperature derivative of AC susceptibility are equal to 106 K and 161 K for the nanocrystalline and polycrystalline manganites, respectively. The magnetic susceptibility and electron spin resonance confirm that the Griffiths-like phase exists above the Curie temperature in paramagnetic matrix of the nanocrystalline manganite. An analysis of electron spin resonance spectra allows to detect the upper temperature limit for an existence of Griffiths-like phase at temperature T_{GI}=290 K, which is somewhat higher than the T_{G} of the magnetic susceptibility.
Mechanical alloying was used to prepare Co_{40}Fe_{60}, Co_{60}Fe_{35}Ni_{5}, Co_{40}Fe_{45}Ni_{15}, and Co_{40}Fe_{35}Ni_{25} alloys from the elemental powders. As X-ray diffraction studies proved the final products of milling were the solid solutions with bcc or fcc lattice and the average grain size between 20 and 50 nm. After heating of the alloys up to 993 K, the mixtures of two solid solutions with bcc and fcc lattices were formed in the case of Co-Fe-Ni alloys. Thermal treatment did not influence the type of the lattice of Co_{40}Fe_{60} alloy. The Mössbauer spectroscopy revealed hyperfine magnetic field distribution ranged from 33 to 38 T for Co_{40}Fe_{60} alloy and from 30 to 37 T for Co-Fe-Ni alloys. In the case of two-phase alloys, distributions were decomposed into two simple Gaussian functions using the numerical fitting. Magnetic measurements allowed to determine the effective magnetic moments and the Curie temperatures of the obtained alloys.
Structure and transport properties have been studied for a series of La_{0.75-x}RE_xCa_{0.25}MnO_3 manganites with heavy rare earth ions of Gd, Dy, Ho substituting La with x=0, 0.10, 0.25, 0.50, and 0.75. Polycrystalline samples were prepared by the carbonate precipitation route. The oxygen content was determined by the iodometric titration. The X-ray investigations carried out by the powder method show that the unit cell volume gradually decreases and orthorhombic distortion of the lattice increases with rising RE content. Below the room temperature the electrical resistivity is of the semiconducting type for all the samples studied. Electrical resistivity vs. temperature dependences were analyzed within different models: simple thermal activation, Mott's variable range hopping, adiabatic, nonadiabatic, and bipolaron. The Curie temperatures of Gd, Dy, and Ho substituted manganites determined from magnetization measurements show that at 280 K all the samples are in the paramagnetic phase. The increasing RE fraction reduces magnetization at 4 K as compared to La_{0.75}Ca_{0.25}MnO_3.
We calculated the differential conductance G as the function of the bias voltage V across the tunnel junction between a normal metal or a conventional superconductor and an inhomogeneous superconductor with charge density waves. Spatial averaging over random domains with varying superconducting and normal state properties was carried out. For high-T_c oxides, irregularly distorted charge density wave patterns with spatially scattered values of various parameters were earlier shown to manifest themselves in a great body of experimental data. The results of calculation were applied to explain the well-known dip-hump structure in the G(V) dependence for Bi_2Sr_2CaCu_2O_{8+δ} and other cuprates.
The voltage, V, dependences of the differential tunnel conductance G(V) were calculated for two kinds of junctions involving normal and superconducting charge-density-wave metals (CDWMs) in the external magnetic field H. The first is a non-symmetrical junction with a CDWM electrode being biased with respect to a ferromagnet. Calculations show that the paramagnetic splitting occurs between spin-up and spin-down components of G(V), similar to what is observed when a superconducting electrode is used instead of the CDWM one. The second setup is symmetric (CDWM-I-CDWM), where I denotes an insulator. If at least one of the CDWM electrodes is normal and H≢0, G(V) will also be spin-split.
Differential conductance G as a function of the bias voltage V was measured for break-junctions of superconducting Bi_2Sr_2CaCu_2O_{8+δ} and YBa_2Cu_3O_{7-δ}. The dependences G(V) for both materials clearly demonstrate the so-called dip-hump structures outside the gap region. A theory, which suggests the charge-density-wave origin of the dip-hump structures and explains its specific form by intrinsic inhomogeneity of cuprate materials, was developed. The well-known pseudogap features in the tunnel spectra of high-T_c oxides found both below and above the superconducting critical temperature are also described by the theory, which testifies that both the pseudogap and the dip-hump structures have the same origin. Competing theories and various G(V) peculiarities found for a number of superconducting oxides are briefly discussed.
Results of X-ray absorption fine structure measurements in manganites (La_{1-x}Ho_{x})_{2/3}Ca_{1/3}MnO_{3} with 0.15 < x < 0.50 are presented. When LaMnO_{3} is doped with a divalent element such as Ca^{2+}, substituting for La^{3+}, holes are induced in the filled Mn d orbitals. This leads to a strong ferromagnetic coupling between Mn sites. Ca ions in La_{1-x}Ca_{x}MnO_{3} introduce a distortion of the crystal lattice and mixed valence Mn ions (Mn^{3+} and Mn^{4+}). On the other hand, in manganites (La_{1-x}Ho_{x})_{2/3}Ca_{1/3}MnO_{3} the substitution of La for Ho causes a lattice distortion and induces a disorder, which reduces a magnetic interaction. The ferromagnetic transition temperature and conductivity decrease very quickly with increasing x. The magnetic and transport properties of compounds depend on the local atomic structure around Mn ions. The information on the bond lengths and Debye-Waller factor are obtained from the extended X-ray absorption fine structure (EXAFS) data analysis. The charge state of Mn is determined from the position of the absorption edge in X-ray absorption near edge structure (XANES) data. XAFS results are in good agreement with magnetic characteristics of the studied materials.
JavaScript is turned off in your web browser. Turn it on to take full advantage of this site, then refresh the page.