Magnetism and transport properties of super-structure of diluted magnetic semiconductors

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The possibility of new types of diluted magnetic semiconductors has been found through first-principies calculation using the Green's function method and the coherent potential approximation. The computer code that calculates the transport properties of super structures have been also developed. By use of this method the tunneling magneto resistance of realistic systems were calculated and compared with the experimental observations1) The electronic structure of II-IV-V2 and I-III-VI2 chalcopyrite type diluted magnetic semiconductors was calculated and the ferromagnetic transition temperature was estimated. The results show that (Cd,Mn)GeP_2 and (Zn,Mn)GeP_2 free from any defects never shows ferromagnetism. The ground state of these systems is spin-glass like magnetic state. However the calculation shows that the same system but with various types of defects becomes ferromagnetic. This implies that the experimentally observed ferromagnetism is due to lattice defects or Mn atoms occupyi … More ng Ge sites instead of Cd or Mn sites. It was also found that I-III-VI2 chalcopyrite, (Cu,TM)AlS_2, where TM is V or Cr showed rather high ferromagnetic transition temperature.) The theory of transport properties of the super structure of Ni/Cu/insulator/Ni type was developed. The tunneling current flow across these samples through ideal leads is calculated and its dependency on the thickness of the Cu layer is investigated. It was found that the tunneling magneto resistance varies oscillatory as the thickness of the Cu layer was increased when the barrier height due to the insulator layer was big enough.. This seems to explain the experimentally observed quantum oscillation of the similar structure using Co instead of Ni. The calculation, however, failed to reproduce the decay of the amplitude of the oscillation. This failure is due the neglect of scattering process caused in the Cu layer, implying more sophisticated vestment such as the coherent potential approximation might be necessary for quantitative discussion.3) The computer code for screened KKR method, which enables us to calculate the transport properties of structures of large scale, has been developed The test run has been completed and we are ready to use the code for applications. Less