Giant Thermoelectric Power in Co/Al₂O₃ Ferromagnetic Nanocomposites Below Percolation Threshold

Abstract

In nanocomposite layers consisting of Co nanoparticles in Al2O3 matrix the thermoelectric power was studied in a wide range of Co content. These layers revealed the giant thermoelectric power in a magnetic field for Co content below percolation threshold. This unusual phenomenon has been explained by hopping electron transport through the media which consist of both, nonmagnetic and magnetic centers in the conditions of temperature gradient.

Electronic Transport and Magnetic Properties of Co/SiO2Magnetic Nanocomposites

“…This is manifested by the shape of M ZFC ( T ) and M FC ( T ) curves with the average blocking temperature T B of particles corresponding to the temperature of the maximum in M ZFC ( T ) curve as well as reversible room temperature M ( H ) curve with a Langevine‐like shape. These results are in good agreement with those ones obtained for a nanocomposite system 28.2 at.% Co in Al 2 O 3 matrix (Co/Al 2 O 3 nanocomposite) …”

“…This can increase the magnetothermoelectric power, in contrast to the case when the magnetic moments of the impurities are randomly oriented. This effect is analogous to the mechanism of suppression of the Kondo effect by a magnetic field and was observed by us in Со/Al 2 O 3 nanocomposites at Co concentrations below the percolation threshold …”

“…For more magnetically concentrated sample N5 (65.4 at.% Co), the superparamagnetic behavior was not revealed up to 300 K. The non‐zero coercivity observed at room temperature (82 Oe) and temperature dependences of ZFC–FC curves (obtained after cooling down from a no equilibrium thermal state) indicate that this sample is composed of much larger particles in agreement with where for Co/Al 2 O 3 nanocomposite film it was shown that higher amount of cobalt promotes growth of nanograins leading to larger Co particle dimensions. Additionally it may also point to the presence of significant inter‐particle magnetic interactions.…”

“…A consequence of this is the appearance of a negative magnetoresistance, the enhancement of magneto‐optical effects and possibility of using them as protective screens from electromagnetic radiation, etc. Previously, in the Co/Al 2 O 3 nanocomposites with Co concentrations below the percolation threshold we discovered giant positive thermoelectric power in a magnetic field at the temperature above 85 K . It exceeds several times the thermoelectric power at the same temperature in the absence of a magnetic field.…”