Magneto-optical Kerr effect of a Ni_{2.00}Mn_{1.16}Ga_{0.84} single crystal across austenite and intermartensite transitions

We carried out magneto-optical Kerr effect (MOKE) and magnetization measurements on a single crystal of Ni2.00Mn1.16Ga0.84 in a longitudinal geometry. While there had been reports of polar geometry measurements so far, we show that - against earlier predictions - surface magnetic states of the martensite and the austenite can be also probed efficiently via longitudinal MOKE. A single variant magnetic state, which occurs at room temperature, is characterized by ferromagnetic hysteresis loops similar to standard ferromagnets. Temperature dependencies of Kerr rotation were found to be linearly proportional to magnetization for martensitic phases. After passing through an inter martensitic structural transition at around 260 K in zero magnetic field, the coercive fields are more than doubled in comparison with the room temperature values. On the other hand, at higher temperatures above 338 K where an austenite structure is formed, the MOKE signals are dominated by quadratic contributions and the magnitude of Kerr rotation drops due to changes in the electronic and magnetic domains structure. After the sample is cooled across the austenite-martensite transition again, we observed a multiple beam reflected by the sample due to different types of structural domains present on its surface.