Effect of \gamma-irradiation on superconducting transition temperature and resistive transition in polycrystalline YBa_{2}Cu_{3}O_(7-\delta)

A bulk polycrystalline sample of YBa_(2)Cu_(3)O_(7-\delta) (\delta \approx 0.1) has been irradiated by \gamma-rays with ^{60}Co source. Non-monotonic behavior of T_{c} with increasing irradiation dose \Phi (up to 220 MR) is observed: T_{c} decreases at low doses (\Phi < 50 MR) from initial value (\approx 93 K) by about 2 K and then rises, forming a minimum. At higher doses (\Phi > 120 MR) T_{c} goes down again. The temperature width of resistive transition increases rather sharply with dose below 75 MR and drops somewhat at higher dose. The results observed are discussed, taking into account the granular structure of sample studied and the influence of \gamma-rays on intergrain Josephson coupling.Comment: 2 pages, to appear in the proceeding of M2S-HTSC-VI conference, Housto

Magnetoresistive study of the antiferromagnetic-weak ferromagnetic transition in single-crystal La₂CuO₄+δ

Resistive measurements were made to study the magnetic field-induced antiferromagnetic (AF)—weak ferromagnetic (WF) transition in the La₂CuO₄ single crystal. The magnetic field (dc or pulsed) was applied normally to the CuO₂ layers. The transition manifested itself in a drastic decrease of the resistance in critical fields of 5–7 T. The study is the first to display the effect of the AF–WF transition on the conductivity of the La₂CuO₄ single crystal in the direction parallel to the CuO₂ layers. The results provide support for the three-dimensional nature of the hopping conduction of this layered oxide

Transport and magnetotransport properties of cold-pressed CrO2_2 powder, prepared by hydrothermal synthesis

Submicron powder of CrO$_2$ was prepared by hydrothermal synthesis method from chromium trioxide. Particles obtained were of rounded form with mean diameter about 120 nm. The powder (stabilized with thin surface layer of \beta-CrOOH) has been characterized by structural, X-ray and magnetic measurements. The powder (with Curie temperature about 385 K) was cold-pressed and its transport and magnetotransport properties have been measured in the temperature range 4--450 K in magnetic field up to 1.6 T. The samples studied is characterized by non-metallic temperature behavior of resistance and large negative magnetoresistance (MR) in low temperature range. At T=5 K the MR magnitude has been -17% at H=0.3 T and -20% at H=1.4 T. Its magnitude decreased fast with increase in temperature reducing to 0.3% and less for T>200 K. It is shown that this MR behavior is inherent for a system of magnetic grains with spin-dependent intergrain tunnelling. Some peculiarities of MR behavior in low-temperature range (below 40 K) can be associated with percolating character of tunnelling conductivity of this granular system under conditions of availability of only few conducting current paths through the sample.Comment: 21 pages (including 9 separate figures), submitted to J. of Alloys and Compound

Spin-dependent transport in compacted powders of chromic dioxide CrO₂ with anisotropy of nanoparticle shapes

Resistive and magnetoresistive properties of compacted powders consisting of nanosized particles of chromic dioxide CrO₂ with dielectric β-CrOOH shells are studied. Anisotropy of tunnel magnetoresistance which is responsible for by the magnetic field orientation relative to the specimen plane is observed. It is shown that the main contribution to the anisotropy of tunnel magnetoresistance is made by formation of the magnetic texture under powder pressing

Non-linear effects in hopping conduction of single-crystal La₂CuO₄₊δ

The unusual non-linear effects in hopping conduction of single-crystal La₂CuO₄₊δ with excess oxygen has been observed. The resistance is measured as a function of the applied voltage U (voltage controlled regime) in the temperature range 5 K≤T≤300 K and voltage range 10⁻³−25 V. At relatively high voltage (approximately at U>0.1 V) the conduction of sample investigated corresponds well to variable-range hopping (VRH). That is, in the range 0.1 V20 K the resistance decreases with increasing U. The magnetoresistance of the sample below 20 K has been positive at low voltage and negative at high voltage. The observed unusual non-Ohmic behavior at low voltage range is attributable to inhomogeneity of the sample, namely, to the enrichment of sample surface with oxygen during the course of the heat treatment of the sample in helium and air atmosphere before measurements. At low enough temperature (below ≈20 K) the surface layer with increased oxygen concentration is presumed to consist of disconnected superconducting regions in a poorly conducting (dielectric) matrix. This allows us to explain the observed unusual non-linear effects in the conduction of sample studied. The results obtained demonstrate that in some cases the measured transport properties of cuprate oxides cannot be attributed to the intrinsic bulk properties. Remove selecte