20 research outputs found
Non-linear effects in hopping conduction of single-crystal La_{2}CuO_{4 + \delta}
The unusual non-linear effects in hopping conduction of single-crystal
La_{2}CuO_{4 + \delta} with excess oxygen has been observed. The resistance is
measured as a function of applied voltage U (10^{-3} V - 25 V) in the
temperature range 5 K 0.1 V) the
conduction of sample investigated corresponds well to Mott's variable-range
hopping (VRH). An unusual conduction behavior is found, however, in low voltage
range (approximately below 0.1 V), where the influence of electric field and
(or) electron heating effect on VRH ought to be neglected. Here we have
observed strong increase in resistance at increasing U at T < 20 K, whereas at
T > 20 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 non-Ohmic behavior is attributable to inhomogeneity of
the sample, and 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 (with T_{c} about 20 K) in poor-conducting matrix. The
results obtained demonstrate that transport properties of cuprate oxides may be
determined in essential degree by structural or stoichimetric inhomogeneities.
This should be taken into account at evaluation of "quality" of
high-temperature superconductors on the basis of transport properties
measurements.Comment: 12 pages, REVTex, 11 Postscript figures, To be published in Fizika
Nizkikh Temperatur (published by AIP as Low Temperature Physics
Magnetoresistive study of antiferromagnetic--weak ferromagnetic transition in single-crystal LaCuO
The resistive measurements were made to study the magnetic field-induced
antiferromagnetic (AF) - weak ferromagnetic (WF) transition in LaCuO
single-crystal. The magnetic field (DC or pulsed) was applied normally to the
CuO 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 LaCuO
single-crystal in the parallel - to - CuO layers direction. The results
provide support for the 3-dimensional nature of the hopping conduction of this
layered oxide.Comment: 8 pages, 7 figures, RevTe
Resistive and magnetoresistive properties of CrO2 pressed powders with different types of inter-granular dielectric layers
Resistive, magnetoresistive and magnetic properties of four kinds of pressed
CrO2 powders, synthesized by hydrothermal method of chromic anhydride have been
investigated. The particles in powders constituted of rounded particles
(diameter 120 nm) or needle-shaped crystals with an average diameter of 22.9 nm
and average length of 302 nm. All of the particles had a surface dielectric
shell of varying thickness and different types (such as oxyhydroxide -CrOOH or
chromium oxide Cr2O3). For all the samples at low temperatures we found
non-metallic temperature dependence of resistivity and giant negative
magnetoresistance (MR). The maximum value of MR at low temperatures (T \approx
5 K) is \approx 37% in relatively small fields (0.5 T). At higher temperatures
there was a rapid decrease of MR (up to \approx 1% / T at T \approx 200 K). The
main objective of this work was studying the influence of properties and
thickness of the intergranular dielectric layers, as well as CrO2 particle
shape, on the magnitude of the tunneling resistance and MR of the pressed
powder. The new results obtained in this study include: (1) detection at low
temperatures in powders with needle-like particles a new type of MR hysteresis,
and nonmonotonic MR behaviour with increasing magnetic field (absolute value of
the MR at first grows rather rapidly with the field, and then begins
diminishing markedly, forming a maximum), and (2) detection of non-monotonic
temperature dependence, where - a field in which the resistance in a magnetic
field has a maximum, as well as finding discrepancies in values of and
coercivity fields, (3) detection of the anisotropy of MR, depending on the
relative orientation of the transport current and the magnetic field, (4) a new
method of synthesis, to regulate the thickness of dielectric coating.Comment: 19 pages, 8 figure
COERCIVE FORCE IN THE SYSTEM OF FERROMAGNETIC GRANULES FOR HALF METAL CrO2 WITH PERCOLATION CONDUCTIVITY
Magnetic and magnetoresistive properties of several samples of compacted powders of ferromagnetic half-metal CrO2, consisting of needle-shaped or spherical nanoparticles coated with thin dielectric shells, were investigated in wide temperature range. The temperature dependence of the coercive force Hc(T) is compared with the temperature dependence of the field of maximum of positive tunneling magnetoresistance Hp(T). The dependence of Hp(T) was nonmonotonic one. It is found that in the low-temperature range (4.2 ÷ 70 K) the ratio Hp ≈ Hc, expected for compacted ferromagnetic powders with particles of submicron sizes, does not fulfilled. It is assumed that the possible reason of the difference between Hp and Hc is the mismatch between the orientation of the global magnetization of the entire sample and the orientations of the magnetic moments in some part of granules that form the optimal conducting channels at low temperatures. Such a mismatch may be due to the multidomain granules are more prone to the formation of optimal conducting chains in the transport channels. That leads to a change in the mechanism of magnetization reversal in these channels and to violation of the ratio Hp ≈ Hc