GROWTH AND INVESTIGATION OF HALF-METALLIC Fe3O4

THIN FILMS

B. Vengalis, V. Lisauskas, A. Lisauskas, K.Šliužienė, V. Jasutis

Semiconductor Physics Institute, Vilnius Lithuania

M. A. Bari, J.J. Versluijis, J. M. D. Coey

Physics Department, Trinity College, Dublin 2, Ireland

GROWTH AND INVESTIGATION OF HALF-METALLIC Fe3O4

THIN FILMS

B. Vengalis, V. Lisauskas, A. Lisauskas, K.Šliužienė, V. Jasutis

Semiconductor Physics Institute, Vilnius Lithuania

M. A. Bari, J.J. Versluijis, J. M. D. Coey

Physics Department, Trinity College, Dublin 2, Ireland

MAGNETIC DEVICES BASED ON THIN FILM MULTILAYERS11-12 July 2002, Dublin, Ireland

Magnetite as promising material for magnetoelectronics

Fe3O4 thin films and related technological problems

Growth of Fe3O4 thin films by magnetron sputtering

Characterization of crystalline structure

Electrical and magnetic properties

Conclusions

Short outline of this report

Crystalline structure

Cubic inverse spinel structure Fd3m :O2- ions form frame of face centered cubic lattice, a = 0,8398 nm

A

B

a/2

O

Ionic model: [Fe3+]A [Fe3+Fe2+]B O2-

Fe3+ occupies 1/8 tetrahedral positions (A)Fe3+and the same amount of Fe2+ occupy 1/2 possible B positions

Magnetite: crystalline structure, attractive properties

Ferrimagnetic ordering

at T<TC 860 K (M = 4 B )Charge ordering at T<TV=120 K(Verwey transition)

B

A Fe3+

Fe3+

Fe2+

3d 4s

Electrical conductivity:

( 300 K) 10 mcm due to hopping of spin-polarized electrons between magnetically ordered Fe3+

ir Fe2+ states in B positions

Phase diagram of Fe-O

1600

1400

1200

1000

800

600

400 0.20 0.22 0.24 0.26 0.28 0.30

FeO+Fe3O4 Fe2O3 +Fe3O4

FeO

Fe3O4

-Fe+FeO

Fe2O3

Liquid oxide

Oxygen (wt %)

Fe3O4

-Fe+Fe3O4

Magnetite: phase diagram, technology problems

•High TC value compared to other HM oxides La2/3Sr1/3MnO3 , Sr2FeMoO6, CrO2

• Simple structure, one element•Low deposition temperature

Advantages :

Iron Fe (Cubic)Maghemite - Fe2O3 (Rhomohedr)Magnetite - Fe3O4 (Cubic)Wuestite FeO (Rhombohedral)

• Presence of isostructural phases in Ph. D.• Limited choice of lattice-matched substrate materials• There is a need in suitable isolating and conducting materials for heterostructures • Stability of Fe3O4 in various oxygen ambient needs to be studied• Stability of interfaces needs to be studied

Problems:

Technology Target P(O2),

Pa

Ts,

CSubstrate Film

qualityReferences

PLD -Fe2O3

-Fe2O3

-Fe2O3

-Fe2O3

-Fe2O3

-Fe2O3

Fe3O4

3x10-1

<1x10-1

<1x10-1

<1x10-1

<1x10-1

1x10-3

10-4

350

350

350

350

570

350

350

Si

MgO(100)

SrTiO3(100)

-Al2O3

SrTiO3

MgO(100)

MgO(100)

P

E (0.3%)

E ( 8 % )

E

E

E

E

JAP 83 (1998) 7049

PR(B)57(1998)7823

PR(B)64(2001)205413

DC-MS Fe 2x10-1 500 MgO(100)

MgAl2O4

E

E (4 % )

PR(B) 53(1996)9175

RF-MS Fe 5x10-1 20, 400 -SiO2 P J.A.P.75(1994) 431

RF-MS Fe3O4 10-2 250 MgO E PR(B)80(2002)823

DC-MS Fe 1.5x10-1 350-450 MgO(100) E This work

Preparation of Fe3O4 thin films by various authors

Target: Fe disk, 35 mm diam (h=0.5 mm)

Substrates: Cleaved MgO(100) (aMgO=0.42 nm ½ aFe3O4) Glass Temperature: Ts =300, 400, 450C

Gas ambient: Ar:O2 30:1, (p 5 Pa)

Film thickness: (d=50600 nm)

Preparation of Fe3O4 thin films in this work

0

20

40

60

80

30 40 50 60 70

x, mm

DR

, nm

/min

MgO

Glass

x

Fe

Deposition rate versus substrate to target distance at Idisch= 95 mA.

DC Magnetron sputtering.

Reflected High Energy Electron Diffraction (RHEED)

Microstucture of the grown Fe3O4 thin films

Regions of deposition rate resulting growth of single phase, epitaxial (E) and policrystalline (P) Fe3O4 at p(O2)0.15 Pa as found from XRD, RHEED and resistivity measurements

0 10 20 30 40 50DR, nm/min

Fe3O4 Fe3O4+ Fe

Fe2O3+ Fe3O4

450

400

350

T, C

E PP

Fe3O4 / Glass Fe3O4 / MgO Fe3O4 / MgO

0.2 0.6 1 1.4 1.8 2.2

2,1,)1()(222

2

ij

K

j ii

i

p

p

T=I / Is (h) - ln T / d

I0

I Is

d

, 104 cm-1

5.5

4.5

3.5

2.5

d, () = 0.16 0.27

0.42

Fe3O4 Fe3O4

MgO

Fe3O4 thin films on MgO and Glass. Optical absorption

E, eV

50 100 150 200 250 300

102

103

104

Fe3O

4 / MgO

R,

Temperature T, K

50 100 150 200 250 3000

2000

4000

6000

R,

(T) = exp(-Ea /kT)

(T) = A exp(B /T)1/4

T >TV

T <TV

Variable range hopping (Motts low)

Activation R(T) behaviour

Resistance versus temperature of Fe3O4 thin films

grown epitaxially on MgO(100) at 400C

50 100 150 200 250 300

102

103

104

105

106

R

,

Temperature T, K

100 150 200 250 300350102

103

104

105

106

R,

T, K

Ts=350C

250100 150 200

Ts=450C

Resistance versus temperature of Fe3O4/MgO thin films

Resistance anomaly at TVv was only seen for Fe3O4/MgO films grown at 350 and 400 C Activation energy of R(T) behavior at T>TV for epitaxial Fe3O4 films depends sensitively on

crystalline quality

50 100 150 200 250 300

103

104

105

106

107

R,

T, K

75 150 225300

103

104

105

106

107

R,

T, K

DR,nm/min 42

3427

27

34

1

4

1

4

DR,d

1/T1/T

Stability of Fe3O4 thin film during heating (dT/dt=7deg/min)

Fe3O4 thin film is stable during heating in vacuum up to 650 C.

• Nonreversible resistance change appears at 200 and 400C during heating in oxygen at P(O2)=105 Pa and 0,16 Pa, respectively

0 100 200 300 400 500 600 700

T, C

102

103

104

105

106

R,

10-4

10-4

0,15P(O2), Pa =105

Fe3O4/MgOd=0.35

1. Magnetite is realy an intersting material!

2. It likes vacuum and doesn’t like oxygen

3. High quality Fe3O4 thin films exhibiting resistance anomaly in the vicinity of Verway transition point were grown heteroepitaxially at 350 and 400 C on lattice-matched MgO(100) substrates by a reactive DC magnetron using metallic Fe target. You can try also.

3. We point out the Fe/O2 ratio (sputtering rate at a fixed oxygen pressure) of key importance for growth of single phase films.

Conclusions