Klizni valovi spinske gustoce: proučavanje strujnog šuma, ovisnosti o magnetskom polju i Hallovog otpora

We have studied the current-voltage characteristics of the (TMTSF)2PF6 in the spin density state (SDW), and in zero and finite external magnetic field. For the oscillating part of the nonlinear voltage response to the applied DC electric field, the fundamental frequency distribution (as a function of this field) and a nonlinear relation between the frequency and the SDW current reveal the growth of parallel conduction channels characterized by lower velocities and larger cross-sections. The number of fundamental frequencies, their amplitude and the level of low-frequency noise as well as the depinning behaviour provide a consistent indication of the sample inhomogeneities and associated local field variations, and might be well understood within the framework of the phase slippage model. The increase of the threshold electric field with the applied magnetic filed can be explained by the Bjeli-Maki theory, if the imperfect nesting is taken into account. Finally, the electric-field dependence of the Hall resistivity is consistent with the sliding mechanism of the SDW conduction.Proučavali smo karakteristike napon – struja materijala (TMTSF)2PF6 u stanju valova spinske gustoće (SDW). Za oscilatorni dio nelinearnog naponskog odziva na istosmjerno električno polje, osnovna frekventna raspodjela (kao funkcija tog polja) i nelinearan odnos frekvencije i SDW struje pokazuju rast usporednih kanala vođenja označenih manjim brzinama i većim udarnim presjecima. Broj osnovnih frekvencija, njihove amplitude i razina niskofrekventnog šuma, kao i otkočno ponašanje sustavno pokazuju na nehomogenosti uzorka i pridružene varijacije lokalnog polja, i mogu se shvatiti u okviru modela klizne faze. Rast praga električnog polja s magnetskim poljem može se protumačiti Bjeliš-Makijevom teorijom, ako se uzme u obzir ugniježdenje. Konačno, nalazimo da je ovisnost Hallovog otpora o električnom polju u skladu s kliznim mehanizmom SDW vođenja struje

Klizni valovi spinske gustoce: proučavanje strujnog šuma, ovisnosti o magnetskom polju i Hallovog otpora

We have studied the current-voltage characteristics of the (TMTSF)2PF6 in the spin density state (SDW), and in zero and finite external magnetic field. For the oscillating part of the nonlinear voltage response to the applied DC electric field, the fundamental frequency distribution (as a function of this field) and a nonlinear relation between the frequency and the SDW current reveal the growth of parallel conduction channels characterized by lower velocities and larger cross-sections. The number of fundamental frequencies, their amplitude and the level of low-frequency noise as well as the depinning behaviour provide a consistent indication of the sample inhomogeneities and associated local field variations, and might be well understood within the framework of the phase slippage model. The increase of the threshold electric field with the applied magnetic filed can be explained by the Bjeli-Maki theory, if the imperfect nesting is taken into account. Finally, the electric-field dependence of the Hall resistivity is consistent with the sliding mechanism of the SDW conduction.Proučavali smo karakteristike napon – struja materijala (TMTSF)2PF6 u stanju valova spinske gustoće (SDW). Za oscilatorni dio nelinearnog naponskog odziva na istosmjerno električno polje, osnovna frekventna raspodjela (kao funkcija tog polja) i nelinearan odnos frekvencije i SDW struje pokazuju rast usporednih kanala vođenja označenih manjim brzinama i većim udarnim presjecima. Broj osnovnih frekvencija, njihove amplitude i razina niskofrekventnog šuma, kao i otkočno ponašanje sustavno pokazuju na nehomogenosti uzorka i pridružene varijacije lokalnog polja, i mogu se shvatiti u okviru modela klizne faze. Rast praga električnog polja s magnetskim poljem može se protumačiti Bjeliš-Makijevom teorijom, ako se uzme u obzir ugniježdenje. Konačno, nalazimo da je ovisnost Hallovog otpora o električnom polju u skladu s kliznim mehanizmom SDW vođenja struje

Switching a spin-valve back and forth by current-induced domain wall motion

We have studied the current-induced displacement of a domain wall (DW) in the permalloy (Py) layer of a Co/Cu/Py spin valve structure at zero and very small applied field. The displacement is in opposite direction for opposite dc currents, and the current density required to move DW is only of the order of 10^6 A/cm^2. For H = 3 Oe, a back and forth DW motion between two stable positions is observed. We also discuss the effect of an applied field on the DW motion.Comment: 4 pages, 3 figure

Spin canting and ferromagnetism in a AuFe alloy: Mössbauer and magnetic measurements

We have made Mössbauer polarization and susceptibility measurements above technical saturation on Au-19 at.% Fe. Just below the magnetic ordering temperature the system behaves as a ferromagnet, but random local canting sets in at lower temperatures. A well-defined canting transition temperature cannot be established from the data, but canting begins above the temperature at which the low-field susceptibility shows a sharp falloff

Resistivity of spin-glasses

We analyze experimental data on the temperature-dependent part of the resistivity of AuFe, CuMn, AgMn, AuMn, and AuCr spin-glass alloys, using a model derived from the excitation approach of Walker and Walstedt. The model gives a satisfactory description of the experimental behavior for all the alloys. We find an energy scaling parameter for the excitation density of states that turns out to be approximately proportional to the spin-glass freezing temperature Tg. From the resistivity results we can estimate the local spin-relaxation time

Influence of doping on the Hall coefficient in Sr_{14-x}Ca_xCu_24O_41

We present Hall-effect measurements of two-leg ladder compounds Sr_{14-x}Ca_xCu_24O_41 (0 <= x <= 11.5) with the aim to determine the number of carriers participating in dc transport. Distribution of holes between the ladder and chain subsystems is one of the crucial questions important for understanding the physics of these compounds. Our Hall effect and resistivity measurements show typical semiconducting behavior for x < 11.5. However, for x=11.5, the results are completely different, and the Hall coefficient and resistivity behavior is qualitatively similar to that of high temperature copper-oxide superconductors. We have determined the effective number of carriers at room temperature and compared it to the number of holes in the ladders obtained by other experimental techniques. We propose that going from x=0 to x=11.5 less than 1 hole per formula unit is added to the ladders and is responsible for a pronounced change in resistivity with Ca doping.Comment: 8 pages, 7 figures, revised versio

The Hall effect in the organic conductor TTF-TCNQ: Choice of geometry for accurate measurements of highly anisotropic system

We have measured the Hall effect on recently synthesized single crystals of quasi-one-dimensional organic conductor TTF-TCNQ, a well known charge transfer complex that has two kinds of conductive stacks: the donor (TTF) and the acceptor (TCNQ) chains. The measurements were performed in the temperature interval 30 K < T < 300 K and for several different magnetic field and current directions through the crystal. By applying the equivalent isotropic sample (EIS) approach, we have demonstrated the importance of the choice of optimal geometry for accurate Hall effect measurements. Our results show, contrary to past belief, that the Hall coefficient does not depend on the geometry of measurements and that the Hall coefficient value is around zero in high temperature region (T > 150 K), implying that there is no dominance of either TTF or TCNQ chain. At lower temperatures, our measurements clearly prove that all three phase transitions of TTF-TCNQ could be identified from Hall effect measurements.Comment: Revised version; 7 pages, 5 figure

Transport, magnetic and superconducting properties of RuSr2RCu2O8 (R= Eu, Gd) doped with Sn

Ru{1-x}Sn{x}Sr2EuCu2O8 and Ru{1-x}Sn{x}Sr2GdCu2O8 have been comprehensively studied by microwave and dc resistivity and magnetoresistivity and by the dc Hall measurements. The magnetic ordering temperature T_m is considerably reduced with increasing Sn content. However, doping with Sn leads to only slight reduction of the superconducting critical temperature T_c accompanied with the increase of the upper critical field B_c2, indicating an increased disorder in the system and a reduced scattering length of the conducting holes in CuO2 layers. In spite of the increased scattering rate, the normal state resistivity and the Hall resistivity are reduced with respect to the pure compound, due to the increased number of itinerant holes in CuO2 layers, which represent the main conductivity channel. Most of the electrons in RuO2 layers are presumably localized, but the observed negative magnetoresistance and the extraordinary Hall effect lead to the conclusion that there exists a small number of itinerant electrons in RuO$_2$ layers that exhibit colossal magnetoresistance.Comment: 10 pages, 9 figure

The angular dependent magnetoresistance in alpha-(BEDT-TTF)_2KHg(SCN)_4

In spite of extensive experimental studies of the angular dependent magnetoresistance (ADMR) of the low temperature phase (LTP) of alpha-(BEDT-TTF)_2KHg(SCN)_4 about a decade ago, the nature of LTP remains elusive. Here we present a new study of ADMR of LTP in alpha-(ET)_2 salts assuming that LTP is unconventional charge density wave (UCDW). In the presence of magnetic field the quasiparticle spectrum in UCDW is quantized, which gives rise to striking ADMR in UCDW. The present model appears to account for many existing ADMR data of alpha-(BEDT-TTF)_2KHg(SCN)_4 remarkably well.Comment: 6 pages, 4 figure