Diluted Magnetic SemiconductorsDiluted Magnetic Semiconductors

Diluted Magnetic Semoconductor (DMS)- A ferromagnetic material that can be made by doping of i

mpurities, especially transition metal elements, into a semiconductor host.

- Conducting spin polarized carriers of DMS exhibit similar characters those of host semiconductors.

- Compatible with current semiconductor industry.

Local Magnetic Moment

Role of TM Impurities in DMS Material

Role of TM Impurities in DMS Material

TMTM

Splitting Valence Band

Role of TM Impurities in DMS Material

Role of TM Impurities in DMS Material

Spin Polarized Carrier

Finally Used for Spin Manipulation

Which Impurity is Possible for DMS?Which Impurity is Possible for DMS?

TMTM

Local Moments and Splitting Valence Bands Simultaneously

Success and Failure of Ga1-xMnxAsSuccess and Failure of Ga1-xMnxAs

• Mn substitutes Ga in zincblende structure– Structure is compatible

with GaAs 2DEG

• Tc is correlated with carrier density

• Ferromagnetic semiconductor with ordering temperature ~ 160 K

Ku et al., APL 82 2302 (2003)

Mn

As stateTotal DOS and Mn d state*10

Density of States of GaMnAsDensity of States of GaMnAs

Localized Moment due to MnDelocalized Carrier

due to p-d Exchange Interaction

Comparison of GaMnAs and GaMnNComparison of GaMnAs and GaMnN

• Total magnetic moment: 4 μB

• Mn local moment of Mn > 4 μB

d5+h configuration

• Fermi level under VBM

• Spin split of valence band

• Strong p-d hybridization

• Strong d character of holes

• -4.5 eV of exchange constant

• Hole mediated mechanism

GaAs:Mn

• Total magnetic moment: 4 μB

• Mn local moment of Mn < 4 μB

d4 configuration

• Fermi level on the narrow impurity band

• No spin split of valence band

• Not strong p-d hybridization.

• Very strong d character of holes

• Much higher value of exchange constant

• Double exchange mechanism

GaN:Mn

Transition Metal

1st NN Nitrogen 4th Nitrogen

2nd NN Nitrogen 3rd NN Nitrogen

Structure of 64-Atom GaNStructure of 64-Atom GaN

5th Nitrogen

Planewave Pseudopotential Method: VASP.4.6.21 XC functional: GGA(PW91) Cutoff energy of Planewave: 800 eV 4X4X4 k point mesh with MP Electronic Relaxation: Davidson followed by RMM-DIIS Structure Relaxation: Conjugate Gradient Force Convergence Criterion: 0.01 eV/A Gaussian Smearing with 0.1 eV for lm-DOS Treatment of Ga 3d state

Semicore treatment for GaN Core treatment for GaAs

MethodsMethods

More-than Half filled

Total and Local Magnetic MomentsTotal and Local Magnetic MomentsLess-Than Half filled

Magnetic moments of V, Cr, and Mn doped systems are concentrated on TM ion itself. On the contrary, magnetic moments of Fe, Co, Ni, and Cu are rather long-ranged.Localization of magnetic moments on the TM ion is not appropriate for successful DMS materials since delocalized spin polarized carrier is important for spin manipulation

Induced Magentic Moments of NitrogenInduced Magentic Moments of Nitrogen

For V, Cr, and Mn induced magnetic moments of N with the distance from the TM ion are much smaller than those of Fe, Co, Ni, and Cu. This also suggest that the V, Cr, Mn are not a candidate for DMS application.Materials having long-ranged interaction such as Fe, Co, Ni, and Cu passed the first test for DMS application.

GaFeN: Magnetic Insulator GaCoN: Half Metal

GaNiN: Magnetic Insulator GaCuN: Half Metal

Partial DOSs having More-Than Half Filled StatesPartial DOSs having More-Than Half Filled States

Up Spin Down SpinUp SpinUp Spin

t2g

eg

Filled Electron Unfilled Electron

Up Spin Down SpinUp SpinUp Spin

GaN:Mn(7)-half metal GaN:Co(9)-half metal

Up Spin Down SpinUp SpinUp Spin

GaN:Ni(10)-insulator

Up Spin Down SpinUp SpinUp Spin

GaN:Cu(11)-half metal

Electron Occupation in GaNElectron Occupation in GaNNo

Splitting of Valence p-

band

V, Cr, Mn

Fe, Co, Ni, Cu

Interaction Range of Transition Metals in GaN

Interaction Range of Transition Metals in GaN

From the viewpoint of valence band splitting and half metallicity, Co and Cu doped GaN are most probable candidates

Hamiltonian based on p-d HybridizationHamiltonian based on p-d Hybridizationp-d hybridization results in a spin dependent coupling between the holes and the Mn ions.

pdH Ns S

TM in GaN ΔEvalence (eV) Noβ (eV)Local

Moment(μB)

Fe 0.4203 -3.3624 4

Co 0.2902 -3.0955 3

Ni 0.3780 -6.0480 2

Cu 0.3961 -12.6752 1

GaAs:Mn 0.3231 -2.0678 5

Cu showed larger value of N0ß that Co system

Cu doped system might be higher Curie temperature

Formation Energy of GaN:TMFormation Energy of GaN:TM

GaN

Transition Metal

GaN:TM

Ga

Reactant Product

: ( ( ) ( )( : ) )tot bulk total bulkf E GaN TH G E GaNaN EM TET GaM M

Formation Energy of GaN:TMFormation Energy of GaN:TM

ElementTotal E

GaN:TMTotal E

Pure TMDelta E(B-C)

Difference in Formation E

(eV)

Fe -397.01039 -8.36880 -388.64159 0

Ni -392.81011 -5.47975 -387.33036 1.3122

Cu -390.18394 -3.75937 -386.42457 2.2170

Since the formation energy for Cu is larger than other transition elements, the control of doping level might be difficult.

SummarySummary

Co, CuCo, Cu

Cu doped GaN is predicted to be the most probable candidate for DMS application among 3d transition elements because the system induces long range splitting of valence band and higher p-d hybridization.