Scaling of the Hall effects beyond the quantum resistance threshold in oxidized CoFeB

TL;DR

This study investigates how the ordinary and extraordinary Hall effects in oxidized CoFeB evolve across a wide resistivity range, revealing nonlinear scaling and polarity reversal beyond the quantum resistance threshold, differing from existing theories.

Contribution

It provides new experimental insights into Hall effects in amorphous ferromagnets across metal-insulator transition regimes, highlighting behaviors not predicted by current models.

Findings

Hall effect polarity reverses with increased resistivity

Both Hall effects scale quadratically beyond the quantum resistance threshold

Extraordinary Hall effect scales linearly with the ordinary Hall effect across all regimes

Abstract

The ordinary and the extraordinary Hall effects were studied in gradually oxidized amorphous CoFeB ferromagnets over six orders of resistivity from the metallic to the strongly insulating regime. Polarity of the extraordinary Hall effect reverses, and the amplitude of both the ordinary and the extraordinary Hall effects increases quadratically with resistivity when resistance exceeds the quantum resistance threshold. The absolute value of the extraordinary Hall effect scales linearly with the ordinary one in the entire range over eight orders of magnitude between the metallic and the insulating states. The behavior differs qualitatively and quantitatively from theoretically predicted and experimentally known in other materials.