Magnetic domain structure of epitaxial Ni-Mn-Ga films

TL;DR

This study investigates the magnetic domain structures in epitaxial Ni-Mn-Ga films, revealing a fundamentally different pattern from bulk materials due to finite thickness effects, and models it using an adapted Kittel band domain approach.

Contribution

It provides the first detailed analysis of magnetic domain patterns in thin epitaxial Ni-Mn-Ga films and introduces an adapted Kittel model to explain these features.

Findings

Magnetic domains in thin films are band-like and perpendicular to twinning variants.

Finite film thickness influences the equilibrium twinning and domain periods.

The observed domain pattern is well described by an adapted Kittel band domain model.

Abstract

For the magnetic shape memory effect, knowledge about the interaction between martensitic and magnetic domain structure is essential. In the case of Ni-Mn-Ga bulk material and foils, a staircase-like magnetic domain structure with 90{\deg}- and 180{\deg}-domain walls is known for modulated martensite. In the present paper we show that the magnetic domain pattern of thin epitaxial films is fundamentally different. Here we analyze epitaxial Ni-Mn-Ga films by atomic and magnetic force microscopy to investigate the correlation between the twinned martensitic variants and the magnetic stripe domains. The observed band-like domains with partially perpendicular outof-plane magnetization run perpendicular to the microstructure domains defined by twinning variants. These features can be explained by the finite film thickness, resulting in an equilibrium twinning period much smaller than the domain period. This does not allow the formation of a staircase domain patter. Instead the energies of the magnetic and martensitic microstructures are minimized independently by aligning both patterns perpendicularly to each other. By analyzing a thickness series we can show that the observed magnetic domain pattern can be quantitatively described by an adapted band domain model of Kittel.