Control of Mooij correlations at the nanoscale in the disordered metallic Ta-nanoisland FeNi multilayers.
2020
Localisation phenomena in highly disordered metals close to the extreme conditions determined by the Mott-Ioffe-Regel (MIR) limit when the electron mean free path is approximately equal to the interatomic distance is a challenging problem. Here, to shed light on these localisation phenomena, we studied the dc transport and optical conductivity properties of nanoscaled multilayered films composed of disordered metallic Ta and magnetic FeNi nanoisland layers, where ferromagnetic FeNi nanoislands have giant magnetic moments of 10^3-10^5 Bohr magnetons (\mu_B). In these multilayered structures, FeNi nanoisland giant magnetic moments are interacting due to the indirect exchange forces acting via the Ta electron subsystem. We discovered that the localisation phenomena in the disordered Ta layer can be associated with electronic inhomogeneities leading to the appearance of the low-energy excitations strongly superimposed in the optical conductivity with the Drude contribution of free charge carriers. From the consistent results of the dc transport and optical studies we found that with an increase in the FeNi layer thickness across the percolation threshold evolution from the superferromagnetic to ferromagnetic behaviour within the FeNi layer leads to the delocalisation of Ta electrons from the associated electronic inhomogeneities. On the contrary, we discovered that when the FeNi layer is discontinuous and represented by randomly distributed superparamagnetic FeNi nanoislands, the Ta layer normalized resistivity falls down below the MIR limit by about 60%. The discovered effect leading to the resistivity fall below the MIR limit can be associated with non-ergodicity and purely quantum localisation phenomena, which need to be challenged further.
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