Mapping of magnetic domains by MFM in Ni₂MnGa

Abstract

Influence of structural transition in the evolution of the magnetic domains in the ferromagnetic shape memory alloy system Ni_2+xMn_1-xGa is reported here using Magnetic Force Microscopy (MFM) studies. Studies reported are with two samples with their martensite transition temperature T_M less than and greater than the Curie temperature T_c. Present results show an evolution of MFM across the Tc with a clear twin domains and sub domain structures inside the twins. The higher spatial resolution of MFM (~50nm) as compared to optical microscope (400nm) is useful in probing the domain walls. Force derivative of the MFM signal that may be used as an order parameter seems to scale the onset of magnetic order in the system. One can clearly see the vanishing of the MFM patterns for T>T_c. Results are discussed in the light of models available for tip-sample interactions that track the local magnetization.