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Stefano Sanvito

TFDOM-3 Dublin, 11th July 2002. Diluted magnetic semiconductors for spintronic applications. Stefano Sanvito. Physics Department, Trinity College, Dublin 2, Ireland. Digital ferromagnetic heterostructures. Electronic Structure Ballistic Transport

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Stefano Sanvito

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  1. TFDOM-3 Dublin, 11th July 2002 Diluted magnetic semiconductors for spintronic applications Stefano Sanvito Physics Department, Trinity College, Dublin 2, Ireland

  2. Digital ferromagnetic heterostructures • Electronic Structure • Ballistic Transport • Effect of As antisites Outline • Spintronics • Diluted magnetic semiconductors • Conclusions Funded by NSF/ONR/ACS/EI

  3. Spintronics Use the spin of an electron as well as its electric charge (GMR) LOGIC DATA STORAGE Semiconductors Metals Non-magnetic Magnetic Need for magnetic semiconductors: Mn + III-V or II-VI Very long spin-diffusion length (0.1mm)

  4. LT-MBE Growth Formation of MnAs Ga 300 As Substrate T (C) Metallic Mn 200 Insulating Roughening Polycrystalline 100 0.02 0.04 0.06 x Hole-mediated ferromagnetism x (Ga,Mn)As Mn = local spin (5/2) + 1 hole H.Ohno, JMMM 200, 110 (1999)

  5. In the mean-field approximation Zener Model

  6. Digital Ferromagnetic Heterostructures R.Kawakami et al. APL 77, 2379 (1999) • No dependence of on Mn concentration • Small dependence of on doping the GaAs • For some samples the transport changes from hole to electron dominated

  7. Density Functional Theory We perform density functional theory (DFT) calculations in the local spin density approximation (LSDA). Several implementations. For large systems: SIESTA • Localized multiple-z Pseudo-atomic orbitals • Optimized Pseudopotential • Ceperley-Alder Exchange correlation • Large k-point sampling • Super-cells with up to 100 atoms D. Sánchez-Portal, P. Ordejón, E. Artacho, and J.M. Soler, Int. J. Quant. Chem. 65, 453 (1997)

  8. Ga As Mn In practice .... Super-cell method

  9. First questions Are the carriers spin-polarized? What is the real dimensionality of the system ? Why is ferromagnetism insensitive to what you do in the GaAs layers?

  10. Half-Metal !! The DOS shows a gap for the minority band at the Fermi level Large dispersion parallel to MnAs plane Small dispersion perpendicular to MnAs plane

  11. Write H and S in tridiagonal form Calculate the transport with Green’s function technique, generalized to non-orthogonal basis set and singular coupling matrices S.S. et al. PRB 59, 11936 (1999) Ballistic Transport Landauer-Büttiker formalism Extract a TB Hamiltonian H and overlap matrix S from LSDA

  12. Transport: CIP vs CPP CPP CIP

  13. Large Small CIP I Current in the planes

  14. Large Small CPP I The current is due to hopping between Mn planes

  15. Macroscopic Average of Total Potential

  16. Answers Are the carriers spin-polarized? • Digital magnetic heterostructures are half metallic What is the real dimensionality of the system ? • CIP current in plane • CPP current (small) = hopping between plane Why is ferromagnetism insensitive to what you do in the GaAs layers? • Total potential suggests spin-selective confinement

  17. x A further question ... In real samples there is a large hole compensation. This is due to As antisites (As atoms at the Ga sites) Is this picture valid when As antisites are present?

  18. As antisites destroy the half metallic state

  19. I however ……

  20. A further question ... Is this picture valid when As antisites are present? • As antisites destroy the half metallic state of digital ferromagnetic heterostructures • If the As antisites are far enough from the MnAs planes, charge and spin separation: Majority spin holes in the MnAs plane Minority spin electrons in the GaAs spacer

  21. Conclusion • DFT is a powerful tool to study structural, electronic, magnetic and transport properties of diluted magnetic semiconductors • Digital Ferromagnetic Heterostructures are 2D half-metal (if no As antisites are present) • As antisites in DFH destroy the half metallic state, but different spins are spatially separated

  22. TFDOM-3 Dublin, 11th July 2002 Diluted magnetic semiconductors for spintronic applications Stefano Sanvito Physics Department, Trinity College, Dublin 2, Ireland

  23. 50% compensation FM Coupling Mapping onto a pairwise interaction model Remarkably the fit works fine

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