Professor Thorsten Hesjedal FInstP

Abstract:

The effect of strain on the local phase transition temperature of MnAs/ GaAs was discussed. It was found that the characteristics temperature T* at which the as-grown films transformed to the paramagnetic β-phase was locally shifted up towards the value of unstrained bulk MnAs. The analysis showed that the film areas exhibiting the higher T* were identified as the region in which the strain in the MnAs film was allowed to relax.

Abstract:

The magnetic out-of-plane component in MnAs/GaAs film was discussed. Its temperature dependence was substantially different from the dominating in-plane magnetization. The analysis showed that the out-of-plane component was due to small isolated magnetic 'grains' within the film.

Abstract:

Self organized etching techniques for fabricating a quasiregular array of MnAs nanoislands were discussed. The strain balance in the MnAs layer grown on GaAs substrates collapsed when the heterostructure was immersed in a wet-chemical etch solution and regular row of cracks and submicron-wide strips were carved from it. It was also shown that MnAs islands could serve as a nearly ideal etch mask to create GaAs columns by dry etching.

Abstract:

Thin epitaxial films of MnAs--promising candidates for the spin injection into semiconductors--are well known to undergo simultaneously a first-order structural and magnetic phase transition at 10-40 degrees C. The evolution of stress and magnetization of MnAs/GaAs(001), both measured quantitatively with our cantilever beam magnetometer at the coexistence region of alpha-MnAs and beta-MnAs, reveal an orthorhombically distorted unit cell of the ferromagnetic phase, which provides important clues on the origin of ferromagnetism in MnAs.

Abstract:

The epitaxially grown MnAs films on GaAs(001) were studied using variable-temperature magnetic force microscopy (VT-MFM). The MnAs film underwent a first order structural phase transition around a critical temperature of 40°C. It was found that, due to the strain involved, the ferromagnetic α-MnAs and paramagnetic β-MnAs phase coexisted as stripes along MnAs[0001].