Pathways to new magnetic semiconductors and half metals G.A.Sawatzky Department of Physics and Astronomy, University of British Columbia, Canada

collaborators I.Elfimov UBC S.Yunoki Trieste P.Steeneken Philips H. Tjeng Cologne A.Damascelli UBC K.Shen UBC D.Hawthorn UBC N.Ingle UBCb T.Hibma Groningen P.Abbamonte Illinois A.Rushdy Hamburg

Nanostructuring can dramatically alter physical properties Bad for conventional devices based on semiconductors Interfaces may dominate the properties May be good for otherwise boring materials Change a transparent non magnetic insulator into a half metallic ferromagnet

Some ( Nano) ways to dramatically change properties Electronic reconstruction of polar surfaces 2. Interface engineering 3. Controlled Defects and symmetry 4. Large Hund’s rule coupling of O,N ALL BASED ON SURFACES OR THIN FILMS and MULTILAYERS

Novel Nanoscale Phenomena in Transition-Metal Oxides 7 Ionic Oxide Polar Surfaces Stabilization of polar surfaces by epitaxy Correlated Electron System Surfaces Kinks and steps stabilized by epitaxy NiO (100) 1D Metallic steps Superconducting Copper oxides Applications: Novel SC; QuBits Sr O -1 +2 -2 +1 < 10 ML LaMnO3 eg t2g Mn3+ 3d 4 Strained 2D Layers Positive and negative pressure Applications: CMR; M-I Transition; Orbital Ordering Transparent insulator ½ metallic FM Applications: Spintronics; CMR Electronic Structure of Interfaces Metal-Insulator interface: gap suppression Applications: Molecular Electronics; Fuel Cells; Thermal Barrier Coatings Artificial Molecules Embedded into a Material Ca, Mg, Sr, Ni vacancies or O-N substitution in oxides New class of magnetic materials by ‘‘low-T’’ MBE growth Applications: Spintronics; Novel Magnets J O N

Introduction Rock-salt structure Band insulator Ca : [Ar](4s)2 -4 -2 2 4 6 8 10 12 L  X W K Energy (eV) Ca : [Ar](4s)2 O : [He](2s)2(2p)4

Correlated Electrons in a Solid U : dn dn  dn-1 dn+1 Cu (d9) O (p6) Δ : p6 dn  p5 dn+1 U = EITM – EATM - Epol If Δ < (W+w)/2  Self doped metal Δ = EIO – EATM - Epol + δEM EI ionization energy EA electron affinity energy EM Madelung energy EM is strongly reduced at surfaces Prop. to coordination no. ΔS<< ΔB J.Hubbard, Proc. Roy. Soc. London A 276, 238 (1963) ZSA, PRL 55, 418 (1985)

Neutral (110) surfaces of NiO Slab of 7 NiO layers LSDA+U: U=8eV J=0.9eV Band gap at the surface decreases from 3 eV to 1.2 eV Step edges could be 1D strongly correlated metals

POLAR SURFACES ELECTRONIC RECONSTRUCTION For review see Noguera J.Phys. Condens Matter 12 (2000) R367)

Polar (111) Surfaces of MgO Finite slab of charged planes NiO,MnO,EuO,CaO,SrO, MnS,EuS,----- 2+ 2- 2+ 2- Will reconstruct!! Unless we terminate it properly ΔV=58 Volt per double layer!

The surface atoms are electron or hole doped!!! Can also atomically reconstruct Or strongly charge an overlayer (2D gas) Demonstrated above is ELECTRONIC RECONSTRUCTION

LSDA Band Structure of CaO (111) Slab terminated with Ca and O -10 -5 5 10 Γ K M A L H Energy (eV) -10 -5 5 10 Γ K M A L H Ca 4s Spin Up Spin Down O 2p Bulk material is an insulator The O terminated surface is a half metallic ferromagnet NOTICE THE CROSSING OF THE VALENCE AND COND BANDS . IN VERY THIN LAYERS THEY WILL HYBRIDIZE

ELECTRONIC RECONSTRUCTION Transfer one electron from O layer to Sr layer Sr O -1 +2 -2 +1 < 10 ML

Defects in ionic insulators leading to Effective imbedded magnetic molecules Cation vacancies in simple Oxides I think these can only be made in MBE Ultra thin film growth Elfimov et al;Phys. Rev. Lett. 89, 216403 (2002)

Cation Vacancy Total Density of States Partial Density of States projected on Ca-vacancy site

Anion Vacancy Total Density of States Partial Density of States projected on O-vacancy site

Definition of hopping parameters Cluster model Definition of hopping parameters tpp=1/2(tpp-tpp) t’pp=1/2(tpp+tpp)

Exact diagonalization results Single-particle picture Three lowest states for two particles HOLES in anion orbitals and ELECTRONS in cation orbitals. ELECTRONS in cation orbitals and HOLES in anion orbitals. Solid symbols are for triplet state

Example of two particles in U= limit 1 2 Triplet Singlet “+” for singlet; “-” for triplet Energy level diagram for holes (t>0) -2t -t t 2t Triplet Singlet MOLECULAR HUND’S RULE HUGE STABILIZATION OF S=1

Point structural defects in crystals such as vacancies can indeed confine the compensating charges in molecular orbitals formed by atomic orbitals on the nearest neighbours. Under certain conditions “local” magnetic moments will be formed due to a kind of molecular Hund’s rule coupling with energetic determined by kinetic energy and symmetry considerations rather than exchange interactions.

Strange magnetic materials This could be the origin of the high Tc materials such as Co in TiO2, or ZnO, or in oxides of non magnetic materials like HfO2 Prelier et al Phys Cond. Matter 15,R1583 (2003) Venkatesan et al Nature 430, 630 ( 2004)

Anion substitution Replace O with N

N substitution for O in simple non magnetic Oxides Use N Hunds rule coupling Use impurity band resulting from N spanning the fermi energy This again seems only possible in MBE thin films

3 Recall O2 is magnetically ordered!! Hunds rule coupling of O 2p or N 2p is as large as Mn!!! All we need is: Holes in O 2p or N 2p Small band widths ( large lattice constant) Prevent dimerization and Nitroxide formation

DFT Study of Anion Substitution in SrO

25% Nitrogen

How to make N substituted Oxides with out nitroxide formation ? First work by Hibma”s group in Groningen on Fe Oxides MBE WITH NO2 Rather than O2 Low temperature (350C) use the high surface Diffusion RBS and ion channeling show substitutional N

XPS Valence Band Spectra of SrO1-xNx Films In agreement with the results of band structure calculations the N 2p peak is found to be about 2 eV lower in binding energy relative to the position of O 2p peak. Relative change in the intensities of these two peaks upon doping indicates that the growth process is indeed a process of substitution. This is also supported by RHEED and LEED data.

X-ray Absorption Spectra of SrO1-xNx Films Nitrogen K-edge Oxygen K-edge Nitrogen and Oxygen K-edges spectra both show pre-edge peak resulted from the presence of a hole in the Nitrogen 2p states. Note that neither Ta3N5 nor SrO has this particularity in their K-edge spectra.

N 1s core-level XPS spectra of SrO0.75N0.25 Low binding energy double peak structure is due to the interaction of core hole with charge-compensating hole in the Nitrogen 2p orbitals. Peak1 and peak2 are triplet and singlet states, respectively.

Manipulating Material Properties magnetic : (super) exchange, TC, TN electrical : (super) conductivity, TC, M-I-T optical : band gaps How about using Image Charge Screening ? Coulomb energy : Charge transfer energy : Band gap :

Molecular Orbital Structure is conserved ! Combined photoemission (solid lines) and inverse photoemission (dots with solid lines as guide to the eye) spectra of the C60 monolayer on Ag(111) (upper panel) and the surface layer of solid C60 (lower panel). Also included are the photoemission spectra (dashed lines) of the fully doped C60 (“K6C60”) monolayer on Ag(111) and the surface layer of solid K6C60. Band gap is reduced ! Molecular Orbital Structure is conserved !

EF + Bending Egap ~ 1eV Depends on Orientation! Orientation changes the gap at interface ! Orientation disorder is really bad !!

Molecules Si, Ge, GaAs Band width ~ 0.5 eV >10 eV Exciton B.E. ~20 meV Polarons ћ 0 ~ W ( ~ >1) — Electr. – Electr. UW U<<W Magnetism Yes (T-S~0.5eV) No Cond. Gap Egap  W Egap << W

The influence of external polarizable media For band width small compared to the response time of the polarizable medium we should treat the quasi particles as dressed electronic polarons The band gap = Ionization potential – electron affinity Will be strongly reduced at the interface. This can amount to a gap closing of more than 1.5 eV for a molecular system on a metal or on Si or GaAs which also are highly polarizable.

Summary Reduced dimensionality enforces correlations and can drastically change the properties of material, surface gap reduction Substitution of Oxygen for Nitrogen is very promising path to a new class of magnetic materials Under certain conditions local magnetic moments can be formed due to a kind of molecular Hund’s rule coupling with energetic determined by kinetic energy and symmetry considerations rather than exchange interactions. Strong charge transfers can result at interfaces of dissimilar materials and especially for crystallographic orientations involving polar surfaces. Organic molecular systems will exhibit strong band gap reduction at interfaces. Both electrons and holes will be attracted to that interface.