Absence of the Ordinary and Extraordinary Hall effects scaling in granular ferromagnets at metal-insulator transition

TL;DR

This study investigates the scaling behavior of the Hall effects in granular Ni-SiO2 films near the metal-insulator transition, revealing the absence of expected scaling in certain regimes and confirming recent theoretical models.

Contribution

It provides experimental evidence that Hall effect scaling breaks down in the weakly insulating regime of granular metals, supporting models predicting a transition in conductivity behavior.

Findings

Hall effect scaling is absent in the weakly insulating regime.

Transition from logarithmic to exponential conductivity occurs when inter-granular conductance drops below quantum conductance.

Hall effect scaling loss is observed when inter-granular conductance exceeds quantum conductance.

Abstract

Universality of the extraordinary Hall effect scaling was tested in granular three-dimensional Ni-SiO2 films across the metal-insulator transition. Three types of magnetotransport behavior have been identified: metallic, weakly insulating and strongly insulating. Scaling between both the ordinary and extraordinary Hall effects and material resistivity is absent in the weakly insulating range characterized by logarithmic temperature dependence of conductivity. The results provide compelling experimental confirmation to recent models of granular metals predicting transition from logarithmic to exponential conductivity temperature dependence when inter-granular conductance drops below the quantum conductance value and loss of Hall effect scaling when inter-granular conductance is higher than the quantum one. The effect was found at high temperatures and reflects the granular structure of material rather than low temperature quantum corrections.