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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: Experimental observation of the first-order nature of the spin reorientation transition in polycrystalline Er2Fe14B through high resolution resistivity, thermopower and 57Fe Mössbauer data is reported. Both a steplike decrease in resistivity and an increase in thermopower over a very narrow temperature interval around 323 K indicate a discontinuous spin reorientation transition. This is consistent with the change in sign of the quadrupolar shifts at all Fe sites at this temperature, derived from the Mössbauer spectra, which are indicative for the transition from planar to uniaxial anisotropy with increasing temperature. The symmetrical lineshape of the sharp singularity recorded in the temperature derivative of resistivity provides unambiguous evidence for the first-order nature of the spin reorientation. © 1997 American Institute of Physics.

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: Here we report a detailed study providing complete H-T phase diagrams for both NdRu2Si2 and TbRu2Si2 intermetallic compounds, based on high resolution SQUID magnetization measurements, in the temperature range 2–300 K and in applied magnetic fields up to 5 T, using c-axis magnetically oriented powders. For NdRu2Si2, our results are in good agreement with recent neutron diffraction work under applied magnetic fields, confirming the appearance of an intermediate (temperature and field) ferrimagnetic phase responsible for the two-step metamagnetic process observed in the temperature range 16–22.5 K. Our isofield magnetization curves provide unambiguous evidence on the decomposition of this ferrimagnetic phase into ferromagnetic and antiferromagnetic components. Furthermore, at low temperatures and low fields, we observe, in this compound, the presence of thermal irreversibility effects. For TbRu2Si2, we present an experimental account, on its H-T phase diagram, which shows striking differences with respect to that of NdRu2Si2. In particular, the ferrimagnetic phase, which sets in at 50 K, persists down to the lowest temperature. In addition, we observe reentrant ferrimagnetism in this compound (ferri-ferro-ferri) under applied fields in the range 28–31 kOe.

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: 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: Accurate measurements of the electrical resistivity (ρ,dρ/dT) and of the thermoelectric power (S,dS/dT) were performed in the ternary compounds U2Ni2In and Nd2Ni2Sn from 4 K to 250 K, and the results are compared with those previously obtained in U2Ni2Sn. The pair U2Ni2In/U2Ni2Sn enables us to study the influence of the p-electron character (and other band effects related to the elements In/Sn) on the transport properties, whereas the pair Nd2Ni2Sn/U2Ni2Sn provides direct information on the role played by the 4f/5f electrons. All these compounds order initially in the antiferromagnetic state, exhibiting a characteristic minimum in dρ/dT at TN. In the U2Ni2In compound a drastic reduction is observed in ρ(T) slightly below TN, suggesting the coexistence of the antiferromagnetic state with a coherent Kondo effect when T≤0.8 TN. In Nd2Ni2Sn we observe two distinct phase transitions below TN, of first-order character and likely associated with order-order magnetic transitions. The anomalous behavior of ρ(T) in the paramagnetic phase of the Nd2Ni2Sn compound can be satisfactorily described in terms of crystal field effects associated with the 4f-electron levels. © 1996 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.

Notes: Recently it was shown that the electrical resistivity behaves anomalously both at low temperatures and near the Curie point in rare-earth iron borides due to the subtle interplay between 4f and 3d transition elements. Here a systematic experimental study on the behavior of the thermoelectric power (S) of R2Fe14B compounds with heavy (R=Gd, Tb, Dy, Ho, Er) and light (Nd, Sm) rare-earth elements is presented. Data for La2Fe14B, in the temperature range 10 K〈T〈350 K, is also included for comparison. The results are consistent with the spin-mixing model for electron scattering, with an intrinsic thermopower contribution related to Fe and a spin dependent impurity scattering term related to the rare-earth magnetic ions. From the interplay of both terms we can get maxima in S (for R=Td, Dy, Ho, Er samples) or minima (Gd, Nd, Sm) at low temperatures. The data indicates the ultimate dominance of impurity scattering at low temperatures, producing a linear variation in the magnetic thermopower (S). In La2Fe14B, only the intrinsic effect is observed, and S(T) does not follow a linear temperature dependence.

Notes: Discontinuous multilayered Co80Fe20(t)/Al2O3(30 Å) thin films have been prepared by ion-beam sputtering. We report on structural, magnetic, and transport (for current in plane geometry) results obtained in this system. With growing nominal thickness t of the metal layers, which effectively characterizes the granular structure, a transition from tunnel to metallic conductance is observed, indicating the onset of infinite conducting paths at t〉18 Å. At t〈18 Å, that is within the range of tunnel regime, a different characteristic value t〉13 Å was detected from the magnetization data which display here a transition from superparamagnetic to ferromagnetic behavior. The measurements of tunnel magnetoresistance (MR) show that a sharp maximum of MR sensitivity to field takes place at this thickness, reaching ∼24%/kOe at room temperature. At least, MR itself as a function of t has a break at the same value. All these features suggest that some specific kind of percolation with respect to magnetic order occurs in our system when the disordered granular structure is still well separated, as confirmed by the data of high resolution transmission electron microscopy. Hence such magnetic percolation is clearly distinct from usual electrical percolation in these discontinuous layers. At the same time, the highest MR (∼6.5% at room temperature) in this series is attained with decreasing t only at t=10 Å. © 2001 American Institute of Physics.