Changes in magnetic domain structure during twin boundary motion in single crystal Ni-Mn-Ga exhibiting magnetic shape memory effect

V Kopecký,L Fekete,O Heczko,O Perevertov

doi:10.1063/1.4943363

Abstract

The complexity of Ni-Mn-Ga single crystal originates from the interplay between ferromagnetic domain structure and ferroelastic twinned microstructure. Magnetic domain structure in the vicinity of single twin boundary was studied using magneto-optical indicator film and magnetic force microscopy technique. The single twin boundary of Type I was formed mechanically and an initial magnetization state in both variants were restored by local application of magnetic field (≈40 kA/m). The differently oriented variants exhibited either stripe or labyrinth magnetic domain pattern in agreement with the uniaxial magnetocrystalline anisotropy of the martensite. The twin boundary was then moved by compressive or tensile stress. The passage of the boundary resulted in the formation of granular or rake domains, respectively. Additionally, the specific magnetic domains pattern projected by twin boundary gradually vanished during twin boundary motion.

Highlights

Up to 12% magnetic field induced strain[1] can be obtained in off-stoichiometric martensitic Ni-Mn-Ga Heusler alloys by magnetically induced reorientation (MIR), one of the magnetic shape memory effects.[2]
As the interaction of magnetic domains with twin boundary may be important for twin boundary motion, here we present the study of the magnetic domains in vicinity of single macroscopic twin boundary and how the magnetic domain structure changes after passage of this boundary induced by mechanical stress
In order to simplify complex ferroelastic domains arrangement as much as possible, we did the experiment on single twin boundary Type I which is parallel to {101} plane and forms mirror plane between two ferroelastic variants with nearly perpendicular orientation of c-axis.[5]

Summary

INTRODUCTION

Up to 12% magnetic field induced strain[1] can be obtained in off-stoichiometric martensitic Ni-Mn-Ga Heusler alloys by magnetically induced reorientation (MIR), one of the magnetic shape memory effects.[2] The reorientation is caused by magnetic-field induced motion of twin boundaries These boundaries are coherent interfaces between differently oriented ferroelastic domains called twin variants.[3] As the boundary separates the domains with different crystal and magnetic orientation, the twin boundary can be considered as magnetic domain wall. The field for Type II can be less than 8 kA/m for Ni-Mn-Ga single crystal.[5,10,11] The Ni-Mn-Ga martensite, which is usually modulated, exhibits MIR and possesses large uniaxial magnetocrystalline anisotropy along crystallographic c-axis This results in well-defined magnetic domain pattern in single variant state.[12,13,14]. As the interaction of magnetic domains with twin boundary may be important for twin boundary motion, here we present the study of the magnetic domains in vicinity of single macroscopic twin boundary and how the magnetic domain structure changes after passage of this boundary induced by mechanical stress

EXPERIMENT

RESULTS AND DISCUSSION

CONCLUSION