Kerr microscopy study of magnetic domains and their dynamics in bulk Ni-Mn-Ga austenite

TL;DR

This study used advanced Kerr microscopy to observe and analyze magnetic domain structures and their evolution in bulk Ni-Mn-Ga austenite, revealing surface effects and domain orientation peculiarities.

Contribution

It demonstrates the capability of Kerr microscopy to visualize magnetic domains in bulk Ni-Mn-Ga austenite and uncovers domain behavior influenced by surface treatments and crystallographic orientation.

Findings

Surface coercivity is much higher than bulk coercivity.

Electropolishing reduces coercivity and widens magnetic domains.

Domains are observed only along one surface in-plane cubic axis.

Abstract

The observation of magnetic domains on austenite Ni-Mn-Ga bulk samples has been a big challenge for many years. Using advanced Kerr microscopy, with automatic compensation of the sample motion and monochromatic LED light we were able to observe magnetic domains and follow their evolution with magnetic field on the _100_ faces of an austenite bulk single crystal. After mechanical polishing variable fine stress-induced domains patterns were observed at different locations. The surface coercivity visualized by the Kerr loop was two orders higher than the bulk coercivity from magnetometry measurement. After additional electropolishing, wide 180 domains were observed with a width of about 50 micrometers and the Kerr loop coercivity decreased to the level determined from the magnetometry. Surprisingly, the magnetic domains were observed only along one of two _100_ cubic axes lying in the surface plane.