1
3
4
Jean-François Bobo LPMC UMR CNRS 5830
Coworkers:
• U. Lüders, S. Couderc, D.Basso, D.Hrabovsky, R. Mamy, A.R.Fert LPMC Toulouse
•
E.Snoeck, C. Gatel, T. Blon CEMES Toulouse
•
S.Visnovsky, J. Hamrle, Charles University, Prague, Tcheck Republic
• J.Teillet, Groupe de Physique des Matériaux, UMR CNRS 6634 Rouen, France
Outline:
• Fe
3
O
4 material properties
• Epitaxial growth of single phase magnetite on MgO (001), influence of antiphase boundaries
•
Polycrystalline magnetite films deposited on float glass, exchange bias, NiO- Fe
3
O
4
• Projects: spin valves, MTJ’s, spin-LEDs bilayers
• Conclusion
Acknowledgements:
CNRS, Council of Midi Pyrénées, M. Guyot

2
Fe
3
O
4
- an inverse spinel
• Spinel structure, 8 FU per unit cell
(a = 8.397 Å).
• Two sites for Fe ions:

A type is tetraedral site with 8
Fe 3+ (5 µB).

B site is octaedral with 8 Fe 3+ (5
µB) and 8 Fe 2+ (4 µB).

Antiferromagnetic interaction between iron ions located on A and
B sites.
• Relatively high Curie temperature
(860 K).
• Saturation moment per FU is close to 4
µ
B
(480 emu/cm 3 ).
• Structural, magnetic and electrical transport phase change at Verwey temperature (122 K in the bulk)
B sites
A sites a=8.396 Å

3
Spin polarization in magnetite
• Band structure calculations reveal minority spins are only present at Fermi level above
Verwey transition: half metallic ferromagnetic behavior
H. Feil, Sol. St. Comm 69, 245 (1989)

4
Experimental study by spin-resolved photoelectron spectroscopy
-80% spin polarization
Dedkov et al, PRB 65 (2002)

5

6
Thin films growth by facing target sputtering
GROWTH CONDITIONS:
• Plassys MPU 600-S UHV chamber
• deposition temperature from 400 to
500°C
• rf plasma with pure argon
• Fe
2
O
3 facing targets
• reduction of Fe
2
O
3 to Fe
3
O
4 phase)
• MgO (001) substrates
• In situ RHEED
(more stable

7
Epitaxial growth on MgO (001)
Azimuth: 100 Azimuth: 110
IN SITU RHEED:
• confirmation of epitaxial relationship “cube on cube”.
• unit cell of Fe
3
O
4 films is almost twice the one of
MgO.
• sharper patterns with increasing thickness.
MgO
190 Å Fe
3
O
4
1875 Å Fe
3
O
4

8
Structural properties:
HR TEM
• Evidence of cube-on-cube pseudomorphic epitaxy growth of Fe
3
O
4 on MgO.
• Flat interfaces, no intermixing detected.
• Slight plastic relaxation between MgO and Fe
3
O
4 compensated by misfit dislocations (misfit < 0.3%).

9
Evidence for single phase Fe
3
Mössbauer spectroscopy
O
4 by
Velocity ( mm / s )
0
1.04
-10 +10
CEMS shows that:
• only Fe 3+ is present on A site (

• the ratio of abundances on A and B sites agrees with magnetite structure.
• orientation of moments corresponds to random (indicative of magnetic disorder)
1.00
component IS(mm.s
-1
) 2

(mm.s
-1
) B hf
(T) abundance(%)
A-site
B-site
0.30

0.01
0.63

0.01
0.004

0.004
0.038

0.004
49.1

0.2
45.6

0.2
36
64

Kerr spectroscopy: other evidence for magnetite
1
0
• Polar configuration (H=13.3 kOe)
• Both Kerr rotation and ellipticity collected
• Tabulated dielectric tensor elements by Fontjin et al
(PRB56 (1997) 5432) for fitting
• Spectral dependence mainly related to intervalence charge transitions (IVCT, i.e. electron transfer from
Fe 2+ to Fe 3+ on B sites).
• Experimental spectra very similar to the ones obtained from bulk magnetite, within uncertainty due to substrate reflectivity and film thickness.
0.10
0.05
0.00
-0.05
-0.10
-0.15
-0.20
-0.25
0.10
0.05
0.00
-0.05
-0.10
-0.15
-0.20
-0.25
e xp erime nt
fit
e xp erime nt
fit
1.0
1.5
2.0
2.5
3.0
Pho ton Energy (e V)
3.5
4.0

1
1
Bulk magnetization measurements
• SQUID 300 K,
• in plane magnetization easy axis,
• coercive field close to the one of the bulk
(300 Oe), except for the thinnest film.
• magnetic saturation not reached, even at 5
Teslas,
• M
Sat
@ 1 Tesla lower than the one of bulk
(480 emu/cm 3 ).
 results indicative of the presence of magnetic frustration likely due to
ANTIPHASE BOUNDARIES

1
2
Antiphase boundaries in magnetite
Initial work by D.T. Marguliès (PRL 79 (1997) 5162,
Further study by Hibma et al (JAP 85 (1999) 5291).
• during the growth of Fe
3
O
4
, many different nucleation sites are present on the MgO surface,
• coalescence of these sites when their size increases,
• continuity of the oxygen lattice but faults of the cation lattice,
• the cations lattice faults are combinations of 1/4 a <110>, they are located at the so called antiphase boundaries (APBs).
• magnetic exchange interactions modifieds @ APBs:
• minor interactions J
BB
= 3 K and J
AA
• dominant interaction: J
AB
= -11 K,
= -22 K (antiferromagnetic)
 at the APBs, the J
AB interactions may loose their dominant character and AF coupling is possible between the two domains in antiphase.

1
3
[110]
Growth on MgO (001) of Fe
3
O
4
[110] with APBs
O
Mg
Fe
(oct. site)
(001) plane
Of MgO substrate

1
4
TEM: evidence for APB’s
(antiphase boundaries)
Plane view
Cross section
Images collected from 220 diffraction spot

1
5
HR TEM: a rare close up on atomic displacements at the
APB
Cross section HRTEM micrograph and its simulation (T=20nm,
D f=-80nm) of an APB in a Fe
3
O
4
[001] thin film.

Influence of APBs on the zero field magnetic configuration by MFM
APB top morphology
APB-induced ripple
[010]
1
6 dust TEM plane view, same scale
[100]
Atomic (a) and magnetic (b) force microscopy images of a 750 Å thick Fe
3
O
4 film.
J.F. Bobo et al, Euro. Phys. J B24 (2001) 43

MFM measurements on other epitaxial films of magnetite
[100]
1
7
Thickness = 1870 Å
10*10 µm scale
[010]
Thickness = 95 Å
2*2 µm scale

Magnetic ripple has a larger length scale than APBs network
 strong suggestion that only some of them contribute to magnetic frustration or that APBs lead to larger scale ripple through local frustrations.
Magnetic ripple oriented along <110> directions
 Fe cations displacements at APBs are aligned along <110> and induce AF coupling between domains.
1
8
•Deeper study of the magnetic interactions generated by APBs necessary.
•Full treatment by micromagnetic approach.

1
9

2
0
Why and how polycrystalline growth?
OBJECTIVES:
• study of the influence of exchange bias in all oxide system
• bricks for large spin polarization devices like:
 magnetic tunnel junctions

GMR spin valves with current confined in the spacer layer
 spin injection layer for spin LEDs in the case of
Fe
3
O
4 single layers.
• avoid intermixing NiO/Fe
3
O
4 which occurs at higher deposition temperatures.
EXPERIMENTAL PROCEDURES:
• deposition at room temperature

Ar+ 10% O
2 for NiO to get good stoichoimetry
 pure Ar for Fe
2
O
3 in order to reduce to Fe
3
O
4 stable phase
 in plane applied magnetic field
(1200 Oe) for magnetic ordering.
• electron microscopy, x-ray diffraction
• room temperature VSM, SQUID

2
1
Electron diffraction confirms the presence of magnetite single phase

2
2
X-ray diffraction reveals magnetite single phase and smooth surfaces
(311) - Irel=999
JCPDS # 86-1352 file
(220) - Irel=291
(222) - Irel=73
(400) - Irel=203
(331) - Irel=5
(333) - Irel=255
(422) - Irel=81
2

( °)
Fe
3
O
4
8000 Å film
Fit parameters: thickness: Fe
3
O
4
938 Å
Surface roughness = 10 Å expériment fit
Fe
3
O
4
1000 Å film
Angle d'incidence,

( °)

2
3
Electron microscopy evidences flat structures with columnar growth
Grain size ~ 200-300 Å

2
4
Columnar structure is coherent from NiO to Fe
3
O
4
, indicative of grain epitaxy

2
5
HR TEM of the
NiO-Fe
3
O
4 interface:
- no intermixing
- local continuity of the atomic rows

2
6
Flat and smooth films compatible with spin electronics structures.
Polycrystalline films, but with good crystalline coherence along the growth direction and some inplane coherence.
Questions:
 Do these films behave like the epitaxial ones that contain APBs?
 Does the applied field during deposition induce in-plane anisotropy?
 Does exchange coupling with NiO induce exchange bias?

2
7
Magnetite single layers: magnetic behavior
BEHAVIOR SIMILAR TO THE
ONE OF EPITAXIAL FILMS
BUT WITH LESS
SQUARENESS:
• coercive field close to 300 Oe
• low squareness
• not saturated at 0.2 tesla
• saturation magnetization lower than 480 emu/cm 3
• slight easy axis induced by the deposition with 1200 Oe in plane, unexpected presence of some exchange bias H
E
~ 60 Oe
LOW SQUARENESS CAUSED
BY POLYCRYSTALLINITY.
H (kOe)

Fe
3
O
4
- NiO bilayers: investigation of exchange coupling at room temperature with 1000 Å NiO underlayer no NiO underlayer
2
8
Coercive field of magnetite layer increases with thickness from 200 Oe to
450 Oe.
Exchange bias field is close to 60 Oe (100 Oe max) and exists even for single Fe
3
O
4 films.

2
9
Magnetic ripple investigated by AFM/MFM
Fe
3
O
4 1000 Å single layer
Surface morphology indicative of a low roughness
Magnetic ripple with a constant periodicity ~ 0.2 - 0.4 µm.
NiO
1000 Å
- Fe
3
O
4 1000 Å bilayer

3
0
Surface roughness
Magnetic ripple

3
1
•Relatively easy to obtain either epitaxial or polycrystalline.
•Hard to get defect-free epitaxial samples on MgO => need to try other substrates like spinels (e.g. MgAl
2
O
4
).
•Difficult to obtain good squareness hysteresis loops, mainly due to AF intergrain or interdomain couplings.
•Magnetic ripple observed for all samples, magnetized, demagnetized, with or without applied field up to 70 Oe, with or without NiO underlayer.

3
2
Potential applications of magnetite films
•Benefit of the magnetite large spin polarization
•Avoid the “low squareness” behavior
1. Inserted electrode-barrier polarizing layer
3. Spin injector layer in spin LEDs
2. GMR structures with
Fe
3
O layers
4 magnetic

Potential magnetoresistive applications of such structures
Fe
3
O
4 metal spacer (Pt, Cu...)
Fe
3
O
4
CIP GMR structure with bad conductor as ferromagnetic layers
(B. Diény)
Fe
3
O
4 insulating tunnel barrier (e.g. MgO)
Fe
3
O
4
CPP MTJ with fully spin polarized electrodes
(Sénéor, Freitas, Suzuki)
3
3 epitaxial Fe
3
O
4 film
Spinel MgAl
2
O
4 substrate
(bicrystal or laser grooved)
APB area

3
4
•smooth structures
•No magnetic decoupling
•No GMR so far at 300 K :-(

3
5
Collaboration with:
•Quantum optoelectronics group - LPMC
X. Marie, Th. Amand, E. Vanelle, M. Senes, B. Liu
•Photonics group - LAAS
Ch. Fontaine, A. Arnoult
Objective:
•Injection of spin-polarized current from a ferromagnet to a semiconductor
•Probing the spin injection efficiency via electroluminescence
Structures chosen:
- GaAlAs-GaAs-GaAlAs quantum wells prepared by MBE
- sputtered ferromagnetic injection electrodes
- injection barrier : Schottky barrier
H.J. Zhu et al, Phys. Rev. Lett . 87 (2001) 016601.
A.T. Hanbiki et al, Appl. Phys. Lett. 80 , 1240 (2002).

3
6
Standard LED structure with semi transparent Co window for light detection.
Doping for Schottky barrier from 10 17 to 2.10
18 cm -3
H Emitted light (500 µm window)
-
+
Au
(1000Å)
Co
(150Å)
AlGaAs:Si
(700Å)
Pt
(20Å)
AlGaAs
(400Å)
AlGaAs
(500Å)
GaAs
(100Å) quantum well
AlGaAs:Be
(5000Å)
GaAs:Zn substrate

3
7
Out of plane applied field = 0.8 T

Co film magnetization not totally perpendicular (M z

M sat
/2)
EL pulses <10 ms, <10 V
Detection of s + and s circularly polarized photos emitted by electron-hole recombinations in the QW in a spectrometer
(stray scope).
T=12-15 K

3
8
s + s -
Stationary mode => integration over time of several radiative, non radiative and spin relaxation processes

3
9
E
P
 s s +
+ s
+ s
-
-

1
+
P
0

 life spin gap hh

4
0
• Increase the spin polarization with magnetite injection electrode
• Perform time-resolved EL to overcome the carrier and spin relaxation processes in the
QW’s:

4
1
• Epitaxial magnetite films
– pseudomorphic growth on MgO, but presence of APBs,
– APBs induce a magnetic ripple at submicron scale.
• Polycrystalline magnetite films
– single phase magnetite,
– smooth polycrystalline layers,
– evidence for exchange bias in Fe
3
O
4 and NiO-Fe
3
O
4 bilayers.
• Perspectives
– GMR, TMR or spin-LED structures based on magnetite,
– try to reduce APBs in epitaxial films (choice of different substrates
/ deposition conditions).