The use of Lorentz microscopy for the determination of magnetic reversal mechanism of exchange-biased Co30Fe70/NiMn bilayer

TL;DR

This study uses Lorentz microscopy combined with in-situ magnetizing experiments on a TEM to analyze the magnetic reversal mechanisms in an exchange-biased CoFe/NiMn bilayer, revealing two distinct reversal modes.

Contribution

It demonstrates the application of Lorentz microscopy with differential phase contrast on a conventional TEM to investigate magnetic reversal processes in exchange-biased bilayers.

Findings

Identification of two reversal modes: wall propagation and coherent rotation.

Mapping of magnetic induction at different fields using SIDPC.

Observation of exchange-bias field in the hysteresis loop.

Abstract

Lorentz transmission electron microscopy (LTEM) combined with in-situ magnetizing experiments is a powerful tool for the investigation of the magnetization of the reversal process at the micron scale. We have implemented this tool on a conventional transmission electron microscope (TEM) to study the exchange anisotropy of a polycrystalline Co35Fe65/NiMn bilayer. Semi-quantitative maps of the magnetic induction were obtained at different field values by the differential phase contrast (DPC) technique adapted for a TEM (SIDPC). The hysteresis loop of the bilayer has been calculated from the relative intensity of magnetic maps. The curve shows the appearance of an exchange-bias field reveals with two distinct reversal modes of the magnetization: the first path corresponds to a reversal by wall propagation when the applied field is parallel to the anisotropy direction whereas the second is a reversal by coherent rotation of magnetic moments when the field is applied antiparallel to unidirectional anisotropy direction.