物理工学コースからのおしらせ

物理工学ユニットでは、以下のように談話会を 対面形式で開催させて頂きます。

講演者：Dr Anna Kaleta
　　　　　Institute of Physics, Polish Academy of Sciences, Poland
　　　　　CIMAP, CNRS, France
演題：　Strain -mediated high TC of GaAs:MnAs granular system -
　　　　　microscopic studies of Mn clustering in WZ-(Ga,Mn)As by in-situ transmission electron microscopy
日時：　2023年11月20日（月）15：00 -
場所：　総合研究棟　W701室

概要：Wurtzite (WZ) (Ga,Mn)As, possessing similar symmetry to hexagonal α-MnAs, can be obtained when (Ga,Mn)As is grown as the shells of WZ-GaAs nanowire (NW) cores [1]. Experimental evidence has demonstrated that annealing WZ-(Ga,Mn)As leads to the formation of strained α-MnAs nanocrystals (NCs) embedded semi-coherently within the WZ-GaAs matrix, exhibiting ferromagnetism above 127 °C [2], unlike the bulk α-MnAs with a Curie temperature (TC) of approximately 40 °C.
The formation of α-MnAs NCs can be observed by employing in-situ transmission electron microscopy (TEM) with a Protochips system (Fig. 1 left) and suitable conditions to enhance the contrast of Mn clusters during scanning TEM (STEM). Sequential imaging during the annealing process allows for the visualization of MnAs precipitation in WZ-(Ga,Mn)As (forming the NW shell), as revealed in the initial insitu STEM results presented in Fig. 1 (right).
To accurately track Mn segregation, an algorithm based on the advanced image processing procedures was developed for automatic and reliable analysis of NWs images. It made it possible to identify, and measure the size and the distribution of the detected particles. Through this analysis, the various stages of MnAs precipitation were determined:

• nucleation of the coherent WZ nano-clusters (NCLs) with Mn atoms in a substitutional positions MnGa, significantly stressed in the WZ-GaAs matrix (starting at ~300 °C)
• NCLs phase transition from WZ-MnAs to α-MnAs (~350 °C)
• growth of semi-coherent α-MnAs NCs, partially strained in the WZ matrix (~400 °C)

During my presentation, I will provide insights into the potential thermal control of the granular WZ-GaAs:MnAs system to achieve the desired size and distribution of MnAs NCs within core-shell NWs. In this context, I will also highlight the role of the (Ga,Al)As inner shell and the importance of the WZ-GaAs crystal structure’s anisotropy in the process of Mn diffusion.



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世話人：Hannes Raebiger