ALBERT

Notes: The abundance of the wiskered sole Monochirus hispidus in the Sado Estuary showed a marked seasonal pattern, the highest densities occurred in spring and summer. The density of this species was mainly related, non-linearly, to depth, water transparency and percentage of gravel, fine sand and mud in the sediment. Monochirus hispidus fed mainly on crustaceans and polychaets. Significant differences were found between the proportions of prey items eaten and total length class; larger fish consumed more decapods and polychaets. The von Bertalanffy growth equation coefficients differed between sexes. The asymptotic length L∞ obtained for females was higher compared to males, while the growth coefficient (K) was higher for males. Individuals with mature gonads were found between March and September, but the main spawning period of M. hispidus in the Sado Estuary was from March to June.

Notes: In ferromagnetic systems with random anisotropy (RA), long-range magnetic order is destroyed and the correlation function of the magnetization exhibits an exponential form e−r/Rf, where the correlation length Rf corresponds to the size of Imry–Ma domains. Also, the law of approach to magnetic saturation follows a 1/(square root of)H law [ferromagnet with wandering axis (FWA) regime]. We have calculated the effect of magnetic correlations on the electrical resistivity and magnetoresistance of a RA ferromagnetic system at low temperatures. We find that in zero magnetic field RA introduces a positive contribution into the electrical resistivity, which increases with the anisotropy to exchange ratio, D/T. In the FWA regime the magnetoresistance has a logarithmic dependence on the magnetic field with a slope Δρ/(ρ ln H) inversely proportional to Rf. These features have been observed in an experimental study of the magnetoresistance at low temperatures (T≈4 K) of the amorphous series (DyxGd1−x)Ni in which the D/T ratio can be varied from near zero up to one from GdNi to DyNi. A quantitative analysis of the results enables the direct determination of the correlation length Rf and of the volume of structural correlations Ωc. We find that Rf varies from about 12 A(ring) in DyNi up to 300 A(ring) in (Dy0.1Gd0.9)Ni and Ωc is the same throughout the series (Ωc≈500 A(ring)3) in excellent agreement with the values determined from magnetic measurements on the approach to saturation. These results provide an independent confirmation of the exponential decay with distance and field dependence of magnetic correlations in random anisotropy systems.

Notes: The magnetic behavior of bulk La2/3Ca1/3MnO3 (TC=267 K) at low fields in the paramagnetic phase was studied. Near TC(T−TC〈30 K) we find a sequence of steplike features in the effective Curie constant C(T)=M(T−TC)/H at well defined temperatures and intermediate plateaus, accompanied by temperature hysteresis. On approaching TC, the C values at each plateau follow a geometrical progression, with the effective spin S doubled at each step. The first plateau gives S=5.5, corresponding to a cluster of three Mn3+ ions sharing one extra hole. This suggests that close to TC short range magnetic order at low fields develops hierarchically through a series of most stable cluster states. © 1998 American Institute of Physics.

Notes: We report measurements of the thermoelectric power (S) and electrical resistivity (ρ) in sputtered multilayers of M(10 A(ring))/Au25Cu75(t) (with M=Co or Ni) directly grown on Si (100), and in UHV—evaporated Pt 20 A(ring)/Ni(x) multilayers grown on Si(111). The multilayers have thicknesses t=21 A(ring), 23 A(ring), and x=19.5 A(ring), 39 A(ring), and the data spans the temperature range 10–300 K. All the samples exhibit an anomalous thermopower at high temperatures, with a huge maximum in S and a sharp peak in the temperature derivative dS/dT at a characteristic temperature T*, e.g., ∼190 K for Ni/Pt and ∼260 K for M/Au25Cu75 multilayers. In all cases the thermopower decreases abruptly at T*, from a giant positive value (S≈80 μV K−1 in Ni/Pt and S≈12–50 μV K−1 in M/Au25Cu75) to values close to zero below T*. The electrical resistivity also displays an anomaly over the same temperature range, characterized by a rapid resistivity enhancement Δρ∼1 μΩ cm (within ΔT ∼60 K) when T decreases below T*.This produces a sharp minimum in the derivative dρ/dT at T*. The shape of the S(T) anomaly is remarkably similar in all cases (mutatis mutandis) for ρ(T), suggesting a general underlying physical mechanism. We found that a suitable buffer (e.g., 100 A(ring) Pt in Ni/Pt, Fe50 A(ring) in Co/Au25Cu75 and 50 A(ring) in Ni/Au25Cu75) suppresses the anomalies, indicating a structural-related underlying effect. In this case, S values are small and negative (as occurs with Ni and Co), exhibiting the usual smooth variation over the whole temperature range. At this stage, it is not clear what is the precise physical mechanism responsible for the observed anomalies. Interfacial atomic mixing in the nonbuffered multilayers may cause individual magnetic atoms or clusters to have entirely nonmagnetic environments (Au, Cu, or Pt in our samples). This situation could give an anomalous transport contribution, through conduction electron scattering at virtual bound states (VBS) localized in the (3-D) magnetic atoms. A sharp peak occurs in the local density of states, N(E), at a characteristic energy EL. A giant thermopower may result when the Fermi level is close to EL (S∝dN/dE; very large).The VBS model successfully explains large S values observed in bulk noble metals diluted with 3-D magnetic impurities such as Co and Ni. However, the standard treatment does not lead to a sharp cutoff-temperature, below which S(T) gets negligible. Another contribution is the usual s-d electron transition mediated by phonon scattering. This leads to a thermopower proportional to the energy derivative of the splitted ferromagnetic 3D band density of states at EF, which can be very large when sub-band filling is almost complete. © 1996 American Institute of Physics.

Notes: Epitaxial thin films of La0.62±0.05Ca0.33±0.02MnO3−δ were grown by laser ablation on SrTiO3. On (100) substrates the films grow with the larger c axis perpendicular to the plane. The films deposited on (110) SrTiO3 grow with both the c (long) axis and a (or b, short) axis in the plane of the film. The electrical resistivity (ρ) and the magnetoresistance (Δρ/ρ) show crystalline anisotropy. The resistivity ratio between the a and c axis is constant (0.8) from 10 K up to 120 K and decreases to 0.77 between 125 and 225 K, shows a small peak anomaly at Tc (257 K), and is almost constant in the paramagnetic phase. This temperature dependence is associated with anisotropic local lattice distortions. The magnetoresistance anisotropy (Δρ/ρ(parallel)−Δρ/ρ⊥) with the applied field in the plane of the film, is small at low temperatures, peaks close to Tc, and is slightly larger for measurements along the a axis. The contributions of domain rotation and magnetocrystalline anisotropy to the anisotropic magnetoresistance associated with spin–orbit coupling are discussed. © 2000 American Institute of Physics.

Notes: We describe the competing magnetic, localization, and phonon effects on the transport properties of amorphous magnetic UxT1−x films, with T=Fe, Ni, Gd, Tb, and Yb. Amorphous UxFe1−x films change from collinear to random ferromagnetism as x increases, and the temperature dependence of the resistivity denotes the competing effects of spin-flip and non-spin-flip exchange scattering processes. The resistivity has a minimum at Tf rising sharply below this temperature. The sign of the magnetic resistivity and the magnetoresistance indicates 〈Si ⋅Sj 〉〉0, while the anisotropic magnetoresistance indicates a local exchange gap. Amorphous UxGd1−x and a-UxTb1−x are, respectively, spin glasses and random anisotropy dominated systems. The resistivity increases smoothly through Tf and has a slight upturn at low temperatures that we associate with weak localization. The magnetoresistance is negative in both systems and the anisotropic magnetoresistance is null, although the applied field induces anisotropic behavior in the Tb containing films (asperomagnets). All samples show quadratic and positive field dependence of magnetoresistance well inside the paramagnetic regime, and a linear regime below Tf. At low temperatures and in the a-UxGd1−x films, negative (H)1/2 and H2 regimes occur and are associated with weak localization processes dominated by the inelastic mean free path.

Notes: High-field magnetoresistance experiments performed on thick amorphous alloys offer a simple way to study three-dimensional (3D) weak localization of conduction electrons. After the precursor work of Fert et al. on nonmagnetic amorphous alloys, we found it interesting to study how these effects would disappear under substitutions of magnetic impurities (1% to 10% of Dy) in a nonmagnetic amorphous alloy (YNi). The experiments, performed between 1.5 to 50 K and in magnetic fields up to 20 T, showed (i) in YNi, characteristic features of the magnetoresistance due to weak localization under strong spin-orbit scattering and (ii) in DyxY1−xNi, a coexistence of weak localization effects with the classical contribution of spin alignment by the applied magnetic field, saturating at negative values. This last contribution dominates the behavior of Dy-richer samples whereas weak localization is clearly observed for x≤3%. In all the samples a dramatic increase of the initial magnetoresistance slope Δρ/ρH2 (where ρ=resistivity and H=applied field) is observed when magnetic impurity concentrations increase. We explain this increase of weak localization effects, in the framework of the available weak localization theory, by an enhancement of the Zeeman spin splitting due to interactions between localized (4f ) and delocalized (d, s) electronic states.

Notes: The electrical resistivity has been measured in ceramic La2/3Ca1/3MnO3 as a function of temperature and magnetic field, including the vicinity of the Curie temperature TC≅267 K and down to low fields. We found that the field dependence of the magnetoresistance Δρ/ρ reveals peculiar features close to TC, unknown in conventional ferromagnets. Particularly, a finite linear response in field, besides the usual quadratic term, is found above TC. We propose that this behavior is due to the coexistence of two different types of magnetic fluctuations, related to itinerant and localized charge carriers. © 1997 American Institute of Physics.

Notes: High-resolution electrical resistivity measurements (ρ,dρ/dT) were performed in three series of [Fe30ÅCrtÅ] multilayers in the temperature range 15–300 K, both at zero and under saturation magnetic field. The different series were prepared by MBE on MgO (100) substrates, by sputtering on MgO (100) substrates, and by sputtering on Si (100) substrates. In the temperature range 15 K(approximately-less-than)T〈50 K we always observe ρ=βT3 where β is a sample-dependent constant, indicating the dominance of phonon-assisted interband (s-d) electron scattering (ρ∝T3 when T(very-much-less-than)aitch-thetaDebye). For the samples grown on MgO we observe that β decreases with t(Cr) whereas for the samples grown on Si, the coefficient β increases with t. For T〉150 K the resistivity attains the classical dependence with ρ∝T also predicted by this s-d model. In spite of these differences our results show that ρ=βf(T) where f(T) is the same function of temperature for all the different samples studied. © 1997 American Institute of Physics.

Notes: Co48Ag52 films prepared by sputtering at 77 K were characterized by magnetoresistance (MR) and magnetization. MR measurements give evidence for an important positive anisotropic magnetoresistance (AMR) in the as-deposited samples that reaches ∼11% at 15 K and saturates at H(approximate)2 kOe. For higher applied fields the magnetoresistance is reversible and well described by a square Langevin function indicating a giant magnetoresistance behavior mostly due to superparamagnetic particles. The positive AMR becomes negligible after annealing at 350 °C for 10 min, probably due to stress relief. © 1997 American Institute of Physics.