Thermopower and resistivity in ferromagnetic thin films near room temperature

We present measurements of thermopower (Seebeck coefficient) and electrical resistivity of a wide selection of polycrystalline ferromagnetic films with thicknesses ranging from 60–167 nm. For comparison, a copper film of similar thickness was measured with the same techniques. Both the thermal and electrical measurements, made as a function of temperature from 77–325 K, are made using a micromachined thermal isolation platform consisting of a suspended, patterned silicon-nitride membrane. We observe a strong correlation between the resistivity of the films and the thermopower. Films with higher resistivity and residual resistivity ratios, indicating a higher concentration of static defects such as impurities or grain boundaries, with rare exception show thermopower of the same sign, but with absolute magnitude reduced from the thermopower of the corresponding bulk material. In addition, iron films exhibit the pronounced low-temperature peak in thermopower associated with magnon drag, with a magnitude similar to that seen in bulk iron alloys. These results provide important groundwork for ongoing studies of related thermoelectric effects in nanomagnetic systems, such as the spin Seebeck effect.

Figure 1

Top panel: Suspended thermal isolation platform with a schematic view of the circuit for measuring thermovoltage. Bottom panel: Thermovoltage ($▵V$) vs temperature change across the bridge ($▵T$) at 299 K for Fe, Cu, Co, Ni, and Ni-Fe films. Inset: Closeup of the island structure with heater and thermometer wires and the lead for measuring thermopower and resistivity.

Figure 2

Measured $\rho$ and $\alpha$ of Ni (Filled circles: 83-nm evaporated film, Open circles: 50-nm sputtered film) and Ni-Fe alloy films (Open triangles: 75-nm films, Closed triangles: 60-nm film) compared to bulk literature values for Ni (Ref. 11) (solid lines).

Figure 3

Measured $\rho$ and $\alpha$ of Co (Asterix: 167-nm thick film, x-filled circle: 75-nm thick film) and Cu films compared to bulk thermopower values for Co (Ref. 14) and Cu (Ref. 15).

Figure 4

Fe $\alpha$ films (circles) compared to bulk data (lines 1–5) from Blatt et al. (Ref. 11) Line 1: bulk Fe; 2: $0.6\%$ Ni; 3: $1.45\%$ Ni; 4: $1\%$ Pt; and 5: $2\%$ Pt