Rare-Earth Nitrides, Intrinsic Ferromagnetic Rare-Earth Nitrides, Intrinsic Ferromagnetic SemiconductorsSemiconductors

Presentation By: Franck NATALISchool of Chemical and Physical Sciences,

Victoria University of Wellington,New Zealand

Deng Xiaoping In 1992, "There is oil in the Middle East; there is rare earth in China…."

““Rare Earths” are NOT rareRare Earths” are NOT rare

OutlineOutline

• Motivation, why are they interesting?• Epitaxial film growth• GdN, an intrinsic ferromagnetic semiconductor!• SmN, ferromagnetism with no magnetic moment!• EuN, still a lot of mystery!!• Conclusion

Rare Earth-N CompoundsRare Earth-N Compounds

• 17 rare earth mononitrides (RE-N). • Rocksalt structure.

•Unstable in air: oxidation to RE2O3.

RE & group V/VI elements

N

P

As

Sb

Bi

Sc

Y

O

S

Se

Te

Po

La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

N

P

As

Sb

Bi

Sc

Y

O

S

Se

Te

Po

La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

= RE ion= RE ion

= Nitrogen= Nitrogen

Why Rare-earth Nitrides?Why Rare-earth Nitrides?

• Strongly correlated 4f electrons(That means their magnetic moments align)

• Large magnetic moments as the 4f shell fills

• Semiconductors or semimetals? • Experimental picture murky, N vacancies

and decomposition in air• Goal: passivated epitaxial films, magnetic, electron transport and

band structure investigations• Spintronics? Can RENs succeed where DMS fail?

71 72 Lu Hf

RE ion

HistoryHistory

• RE nitrides were studied in the 1960s• NaCl crystal structure well established• Trivalent REs; RE3+/N3-(but see EuN below)• Ferromagnetic structure known for most• Stoichiometry poor (N vacancies, O contamination)• Almost nothing was known about the electronic

band structure• Theoretical interest strong since ~ 2000

The films are grown by deposition of the RE in the presence of N2

-pure N2 and sometimes activated nitrogen -RE thermal evaporation -Characterisation by RHEED-Passivation AlN cap layers-ambient T for polycrystalline films-high-temperature for epitaxial growth, 2’’ substrate holder.-Base pressure : <10-8Torr-Press. Process : 10-5- 10-4Torr

To date: GdN, SmN, DyN, ErN, EuN, LuN, NdN

Thermionic system

UHV deposition systemUHV deposition system

Epitaxial growth on (100) surfacesEpitaxial growth on (100) surfaces

NEED a growth procedure layer to prevent RE and Si diffusion formation of rare-earth silicide compounds

First epitaxial growth GdN(100) on MgOAppl. Phys. Lett. 90, 061919 (2007)

b)a)

5 nm 60 nm

SmN on YSZ by PLD*

GdN a = 4.974 ÅEuN a = 4.8 ÅSmN a = 5.035 Å

* B. Ludbrook et al., J. Appl. Phys. 106, 063910(2009) F. Natali et al., Phys. Stat. Sol. (c) to be published (2011)

= RE ion= RE ion

= Nitrogen= Nitrogen

{111} plane

Rocksalt (111) surface hcp (0001) surface

22a

RE-Nitrides StructureRE-Nitrides Structure

AlN22aRE-N {111} on III-N {0001} Lattice Mismatch (%)

GaN InN Si(111)

GdN 3.518 13 10.3 -0.85 -8.38

EuN 3.522 13.2 11.3 0 -7.59

SmN 3.56 14.4 11.66 0.36 -7.26

GdN a = 4.974 ÅEuN a = 4.8 ÅSmN a = 5.035 Å

Rocksalt (NaCl) structure ReRocksalt (NaCl) structure Re3+3+NN3+3+

M.A. Scarpulla - MRS Fall Meeting 2010 Boston M.A. Scarpulla - MRS Fall Meeting 2010 Boston

} Strained

0 1 2 3 4 5 6 7 83.0

3.1

3.2

3.3

3.4

3.5

3.6

In

pla

ne la

ttice

par

amet

er (

Å)

GdN thickness (ML)

0 1 2 3 4 5 6 7 83.0

3.1

3.2

3.3

3.4

3.5

3.6

In

pla

ne la

ttice

par

amet

er (

Å)

GdN thickness (ML)

} Start to relax after 2-3MLs (onset of plastic relaxation)

(0001) AlN – 100-400nm

Si substrate

N2 + Gd

Growth of GdN layers on AlN/Si(111)Growth of GdN layers on AlN/Si(111)

Dislocation or other defect

Growth Conditions:Pure N2- Pg= 4 × 10-5 to 4 × 10-4 TorrVg = 60-80nm/h - Tg = 650-750ºCN2 flux = 102-103 Gd flux

F. Natali et al., J. Cryst. Growth 312, 3583 (2010)

Fully Relaxed (normal lattice constant = 3.52Å) }

0 1 2 3 4 5 6 7 83.0

3.1

3.2

3.3

3.4

3.5

3.6

In

pla

ne la

ttice

par

amet

er (

Å)

GdN thickness (ML)

25 30 35 40 45 50 55 60 65 70 75 80

10

100

1000

10000

100000

1000000

Inte

nsi

ty (

a. u

.)

2 ()A

lN(0

00

4)

Ga

N (

00

04

)AlN

(00

02

)

Ga

N (

00

02

)

Gd

N(2

22

)

Gd

N(1

11

)

Si(

11

1)

X-ray Diffraction : 2X-ray Diffraction : 2-scans -scans

Structural properties of REN layers Structural properties of REN layers

• GdN (111) epitaxial on AlN (0001)• GdN (111) || AlN (0001)

STM (300x300nm)STM (300x300nm)

GdN grains 20nm to 70 nmGdN grains 20nm to 70 nm

Rutherford Backscattering spectroscopy Rutherford Backscattering spectroscopy Measure thicknesses (surf. density) Stoichiometry: Gd:N ratio (only 1-2% accurate)

0 200 400 600 800 10000.0

2.0x103

4.0x103

6.0x103

8.0x103

1.0x104

Gd

N

Al buffer layer

Si

Random

Energy (MeV)

Bac

ksca

tterin

g Y

ield

(a.

u.)

Channel No

Al cap layer

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8

Structural properties of REN layers Structural properties of REN layers

F. Natali et al., J. Cryst. Growth 312, 3583 (2010)

0 200 400 600 800 10000.0

2.0x103

4.0x103

6.0x103

8.0x103

1.0x104

Gd

N

Al buffer layer

Si

Random

Energy (MeV)

Bac

ksca

tterin

g Y

ield

(a.

u.)

Channel No

Al cap layer

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8

Si substrate

(0001) AlN – 100nm

GdN – 20.5 nm

AlN cap – 35nm

Rutherford Backscattering spectroscopy Rutherford Backscattering spectroscopy Measure thicknesses (surf. density) Stoichiometry: Gd:N ratio (only 1-2% accurate)

Structural properties of REN layers Structural properties of REN layers

F. Natali et al., J. Cryst. Growth 312, 3583 (2010)

Epitaxial quality (channelling vs random conditions)

0 200 400 600 800 10000.0

2.0x103

4.0x103

6.0x103

8.0x103

1.0x104

Gd

N

Al buffer layer

Si

Random Channelling

Energy (MeV)

Bac

ksca

tterin

g Y

ield

(a.

u.)

Channel No

Al cap layer

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8

Rutherford Backscattering spectroscopy Rutherford Backscattering spectroscopy Measure thicknesses (surf. density) Stoichiometry: Gd:N ratio (only 1-2% accurate)

Strong yield reduction is a proof of the low lattice disorder in the film

Structural properties of REN layers Structural properties of REN layers

F. Natali et al., J. Cryst. Growth 312, 3583 (2010)

Si substrate

(0001) AlN – 100nm

GdN – 20.5 nm

AlN cap – 35nm

Begin with GdNBegin with GdN

• Gd3+ has a ½ filled 4f-level:

• Total spin angular momentum S = 7/2• Total orbital angular momentum L = 0• Magnetic state (M = B(L+2S)):atomic magnetic

moments tend to line up

ml = 3 2 1 0 -1 -2 -3

0 20 40 60 80 100

0

2

4

6

1/

M

( B

/Gd3+

)

Temperature (K)

0

2

4

Ferromagnetism below 70 K

Magnetization saturates at low magnetic field (<0.5T) Tc at ~70K Small coercive field

-3x104 -2x104 -1x104 0 1x104 2x104 3x104

-8

-6

-4

-2

0

2

4

6

8

-5.0x102 0.0 5.0x102

-6

-4

-2

0

2

4

6

M ( B

/ G

d3+

)

Field (Oe)

M ( B

/Gd3+

)Field (Oe)

Saturation magnetisation : 7 μB/Gd3+

coercive field of 125Oe

Magnetic properties of GdNMagnetic properties of GdN

These T-dependent resistivity data on polycrystalline and epitaxial films indicate clear semiconductor behaviour both above and below TC.

The films’ resistivity depends critically on the N2 pressure during growth, confirming that N vacancies are the major dopant in GdN.

Temperature dependent resistivityTemperature dependent resistivity

0 50 100 150 200 250 3000.05

0.10

0.15

0.20

0.25

0.30

0.35

T (K)

(m.

cm)

S. Granville et al., Phys. Rev. B 73, 235335 (2006)F. Natali et al., J. Cryst. Growth 312, 3583 (2010)

Polycrystalline film

Epitaxial film

Optical measurements, polycrystalline GdN filmOptical measurements, polycrystalline GdN film

The absorption in GdN develops a low-energy tail upon entering the ferromagnetic state.

PM band gap: 1.3 eV

FM band gap: 0.9 eV

Those disagreed with the best theoretical predictions, but give an opportunity to adjust a parameter Ud. (Introduced in the calculation as a fudge to treat the usual band-gap underestimate of LDA+U.) Trodahl et al., Phys Rev. B 76, 085211 (2007)

Red- majority spin

Blue dashed-minority spinThe direct gap is at X, here with 0.9 eV across the majority-spin gap. The spin-averaged gap is 1.3 eV, in agreement with the measured PM gap.

Note that:

1. GdN is an indirect-gap semiconductor, and

2. The holes () and electrons (X) in doped GdN have a common (majority) spin.

N 2p

Gd 5d

Trodahl et al., Phys Rev. B 76, 085211 (2007)

The adjusted GdN band structureThe adjusted GdN band structure

-8

-6

-4

-2

0

2

4

6

8

Ha

ll R

esi

sta

nce

()

Magnetic Field (T)

90K 150K 300K

-10 -8 -6 -4 -2 0 2 4 6 8 10

Bnqd

RHall1

Ordinary Hall effect

Magnetic state using Hall effectMagnetic state using Hall effectH

all R

esis

tan

ce (

oh

m)

Magnetic Field (Oe)

-8

-6

-4

-2

0

2

4

6

8

Hal

l Res

ista

nce

()

Magnetic Field (T)

90K 150K 300K

-10 -8 -6 -4 -2 0 2 4 6 8 10

Md

RB

nqdR SHall

1

-8

-6

-4

-2

0

2

4

6

8

-10 -8 -6 -4 -2 0 2 4 6 8 10

Ha

ll R

esi

sta

nce

()

Magnetic Field (Oe)

90K 70K 60K 5K

Anomalous Hall effect: RHall M

Bnqd

RHall1

Ordinary Hall effect

Magnetic state using Hall effectMagnetic state using Hall effect

Magnetic Field (Oe)

Hal

l Res

ista

nce

(o

hm

)

-8

-6

-4

-2

0

2

4

6

8

-10 -8 -6 -4 -2 0 2 4 6 8 10

Ha

ll R

esi

sta

nce

()

Magnetic Field (T)

5K 60K 70K 90K 150K 300K

Md

RB

nqdR SHall

1

Magnetic state using Hall effectMagnetic state using Hall effect

Magnetic Field (Oe)

Hal

l Res

ista

nce

(o

hm

)

-8

-6

-4

-2

0

2

4

6

8

-10 -8 -6 -4 -2 0 2 4 6 8 10

Ha

ll R

esi

sta

nce

()

Magnetic Field (T)

5K 60K 70K 90K 150K 300K

Md

RB

nqdR SHall

1

0 50 100 150 200 250 300

1021

2x1021

3x1021

4x1021

5x1021

Car

rier

Con

cent

ratio

n (c

m-3)

Temperature (k)

Carrier concentration

Carrier concentrations : 1021 cm-3

Magnetic state using Hall effectMagnetic state using Hall effect

The N vacancies are still of order 1%.

Hall effect

Magnetic Field (Oe)

Hal

l Res

ista

nce

(o

hm

)

• Experiment TC = 70 K

• Theory 20 K (Mitra, Lambrecht, PRB 2008)• So can carrier-mediated exchange enhance

TC? (Sharma, Nolting, PRB 2010)

What about Tc ?What about Tc ?

Control of N vacancies and carrier concentrationControl of N vacancies and carrier concentration

Polycrystalline films grown at ambient-T permit control of N vacancies and carrier concentration, and sure enough there is an enhancement (to above 70 K) in strongly conducting films. Up to 200K !!!

Plank et al., Appl. Phys. Lett. 98, 112503 (2011)

There is a relationship between the conductivity and TC, but we are a long way from establishing carrier-mediated exchange.

Relationship between the conductivity and TRelationship between the conductivity and TCC

Plank et al., Appl. Phys. Lett. 98, 112503 (2011)

Now SmN…Now SmN…

• Sm3+ has 5 4f electrons:

• Total spin angular momentum S = 5/2• Total orbital angular momentum L = 5• Magnetic moment: M = B(L+2S) = 0!!

ml = 3 2 1 0 -1 -2 -3

mtot = (LZ + 2 SZ) μB = 0.029 µB

P. Larson et al., Phys. Rev. B 75, 045114 (2007)

SmN electric and magnetic responsesSmN electric and magnetic responses

Semiconductor behaviour both above and below the anomaly at 27K. The 4f spins order ferromagnetically below 27 K, and the band structure shows the same character as GdN.

The very small ferromagnetic moment leads to weak coupling to an external field, and an enormous coercive field of more than 6 T, almost 1000 times harder than GdN. Saturation moment = 0.03 B/Sm ion (2K)

Meyer et al., Phys. Rev. B 78, 174406 (2008)

And EuNAnd EuN• Eu3+ has 6 4f electrons:

• Total spin angular momentum S = 3• Total orbital angular momentum L = 3• Hund’s rules: L opposes S • Total angular momentum J = 0!!

• EuN should be non-magnetic

ml = 3 2 1 0 -1 -2 -3

Magnetic moment not zero below 25 K, from SQUID data below (supported by extraordinary Hall effect)

EuN magnetic responseEuN magnetic response

EuN magnetic responseEuN magnetic response

1120 1130 1140 1150 1160 1170

XA

S (

arb.

u.)

Energy (eV)

680CEu2+

800C

Eu3+

Estimate ~percent Eu2+, varying with growth conditions. Likely origin N vacancies.

Eu M-edge X-ray Absorption

EuN is not ferromagnetic…

X-ray magnetic circular dichroism

EuN optical responseEuN optical response

Optical transmittance Band structure (QSGW calculation)

Eg~ 0.8-0.9eV

EuN indirect-gap semiconductorMinimum gap at Γ-X : 0.31 eV, Minimum direct gap at X is 0.94 eV.

J.H. Richter et al., submitted to Phys Rev. B

0 50 100 150 200 250 300

0.2

0.3

0.4

10.0

10.5

11.0

11.5

12.0

T=680C

( x1

0-3

cm

)

T (K)

T=800C

The semiconductor/metal question is not yet clear. The film can be either metallic or very heavily doped by nitrogen vacancies.

EuN electric responseEuN electric response

Temperature dependent resistivityTemperature dependent resistivity

ρ=ρ0+a*lnT+b*Tα

Resistivity upturn at low T causes by Kondo effect (scattering of electrons due to magnetic impurities).Tunable with N vacancies?

71 72 Lu Hf

semiconductors

Ferromagnetic

Lu is non-magnetic

no magnetic data yet for NdN

Other RE-N films (that we have so far grown)Other RE-N films (that we have so far grown)

Research focused on poorly studied and challenging materials:

Epitaxial thin films oriented (100) and (111)

So far all appear to be semiconductors

Most of them show ferromagnetism

With a high potentials for applications:

Magnetic sensors, IR detectors, MRAN, spintronic devices….

ConclusionConclusion

• VUW collaborators VUW collaborators B. Ruck, J. Trodahl, N. Plank, J. Richter, J. Galipaud, B. Le Do, M. Azeem, H. Warring, P. Murmu, J. Stephen.• MacDiarmid collaborators J. Kennedy (GNS), G. Williams (IRL), A. Hyndman (IRL), N. Gaston (IRL), S. Hendy (IRL). • International collaboratorsF. Semond-S. Vezian (CRHEA/CNRS, Valbonne), L. Hirsch (IMS, Bordeaux), S. Sorieul (CENBG/IN2P3, Gradignan), C. Meyer (Institut Neel/CNRS, Grenoble).

Funding from the MacDiarmid Institute, FRST (New Economy Research Fund), Marsden Fund….and Ben/Joe

AcknowledgementsAcknowledgements