Stefano Sanvito Physics Department, Trinity College, Dublin 2, Ireland TFDOM-3 Dublin, 11th July 2002

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

Use the spin of an electron as well as its electric charge (GMR) SemiconductorsMetals Non-magneticMagnetic Very long spin-diffusion length (0.1mm) Need for magnetic semiconductors: Mn + III-V or II-VI LOGICDATA STORAGE

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

In the mean-field approximation

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

We perform density functional theory (DFT) calculations in the local spin density approximation (LSDA). Several implementations. For large systems: SIESTA  Localized multiple-  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)

Ga As Mn Super-cell method

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

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

Landauer-Büttiker formalism Extract a TB Hamiltonian H and overlap matrix S from LSDA 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, (1999)

CPP CIP

Small Large Current in the planes I

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

Macroscopic Average of Total Potential

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

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

As antisites destroy the half metallic state

however …… I

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

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

Stefano Sanvito Physics Department, Trinity College, Dublin 2, Ireland TFDOM-3 Dublin, 11th July 2002

Mapping onto a pairwise interaction model Remarkably the fit works fine 50% compensation