ISSN:
1089-7550
Source:
AIP Digital Archive
Topics:
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.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1063/1.362019
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