10-6 10-5 10-4 10-3 10-21015

1016

1017

1018

1019

0 300 600 9001013

1015

1017

G

SIMS profile: sample C

A

C

F

E

D

B

NG

d (cm

-3)

Flux ratio JGd/JGa

NG

d (cm

-3)

Depth (nm)

Gd corporation varies linearly with JGd/Jg Electrically highly resistive

Reactive (NH3)molecular-beam epitaxy on 6H-SiC(0001) Substrate with Ts = 810° C

Growth

-30 0 30

-0.6

-0.3

0.0

0.3

0.6

-2 0 2

-0.3

0.0

0.3

2 K 300 K

M (e

mu/

cm3 )

H (kOe)

M (e

mu/

cm3 )

H (kOe)

Clear hysteresis at 2 K and 300 KMagnetization saturates at high fields

⇓Ferromagnetism

Sample C (NGd ~ 6 × 1016 cm-3)

Magnetic Hysteresis

Colossal Magnetic Moments

1016 1017 1018 1019 1020

101

102

103

p e (µB)

NGd (cm-3)

300 K

1016 1017 1018 1019 1020101

102

103

p e (µB)

NGd (cm-3)

2 K

Average moment per Gd atom ⇒ as high as 4000 µB

Fit returns 2 K: pm = 1.1 × 10-3 µB, r = 33 nm

300 K: pm = 8.4 × 10-4 µB, r = 25 nm

Magnetic Results: Sample B

200 400 600 8000.00

0.02

0.04

0.06

M (e

mu/

cm3 )

H = 200 Oe

Temperature (K)

Tc

-2 0 2-0.4

0.0

0.4

-15 0 15

-0.4

-0.2

0.0

0.2

0.4

M (e

mu/

cm3 )

H (kOe)

T = 300 K

M (e

mu/

cm3 )

H (kOe)

300 K

760 K

Motivation

Search for ferromagnetic (FM) semiconductor with Tc > 300 K TM doped wide band-gap semiconductors seem to be good candidates

Previous results from (Ga,Mn)N Homogeneous layer exhibits spin-glass behavior

Ferromagnetism with Tc > 300 K is caused by precipitates (clusters)1

Electrically resistive

Previous results from (Ga,Gd)NFound to be FM with Tc > 300 K2

Advantages of GdMagnetic moment 8µB, larger than that of any TM atom

Only rare earth element with both 4ƒ and 5d orbitals partially filled

[1] S. Dhar et al., Appl. Phys. Lett., 82, 2077 (2003)[2] N. Teraguchi et al., Sol. State. Commun., 122, 651 (2002)

Empirical Model

Origin of colossal moment: Gd atoms polarize the matrix

pe = pGd + pm ν No/NGd; ν = 1-exp(-v NGd)

Expected ⇒ pe decreases as NGd is increased ⇐ Experimentally observed

Ferromagnetism: overlap of spheres → ferromagnetic coupling

Expected ⇒ Tc increases with NGd ⇐ Experimentally observed

Magneto-PhotoluminescenceNo magnetic field: PL spectra for all samples dominated by (Do,X) transition⇒ characteristic for GaN

+3/2>

+1/2>

-1/2>

-3/2>σ-σ+⇔

+3/2>

+1/2>

-1/2>

-3/2>

σ-

σ+⇔

B = 10 T in Faraday geometry (B || c)

3.42 3.44 3.46 3.48

Sample B (D0,X)

σ−

σ+

Inte

nsity

(arb

. uni

ts)

Photon Energy (eV)

Ref. Sample (D0,X)

σ−

σ+

Donor responsible for (Do,X): Oxygenconcentration ≈ 1018 cm-3 (SIMS)

Polarization in sample B has opposite sign as compared to the Ref. Sample

Average Gd to (Do,X) distance in B:≈ 12 nm ⇒ Gd has a long-range influence on the GaN matrix