Transcript Magnetite Epitaxial and Polycrystalline Thin Films for

Magnetic behavior of Fe3O4
epitaxial and polycrystalline films
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:
•Fe3O4 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- Fe3O4 bilayers
•Projects: spin valves, MTJ’s, spin-LEDs
•Conclusion
Acknowledgements:
CNRS, Council of Midi Pyrénées, M. Guyot
1
Fe3O4 - 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
Fe3+ (5 µB).
 B site is octaedral with 8 Fe3+ (5
µB) and 8 Fe2+ (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
µB (480 emu/cm3).
• Structural, magnetic and electrical
transport phase change at Verwey
temperature (122 K in the bulk)
2
B sites
A sites
a=8.396 Å
Spin polarization in magnetite
• Band structure calculations
reveal minority spins are only
present at Fermi level above
Verwey transition: half metallic
ferromagnetic behavior
3
H. Feil, Sol. St. Comm 69, 245 (1989)
Experimental study by spin-resolved photoelectron
spectroscopy
-80% spin polarization
Dedkov et al, PRB 65 (2002)
4
Epitaxial magnetite films grown
on MgO (001)
5
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
• Fe2O3 facing targets
• reduction of Fe2O3 to Fe3O4 (more stable
phase)
• MgO (001) substrates
• In situ RHEED
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Epitaxial growth on MgO (001)
Azimuth: 100
IN SITU RHEED:
• confirmation of epitaxial
relationship “cube on
cube”.
• unit cell of Fe3O4 films is
almost twice the one of
MgO.
• sharper patterns with
increasing thickness.
Azimuth: 110
MgO
190 Å Fe3O4
1875 Å Fe3O4
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Structural
properties:
HR TEM
• Evidence of cube-on-cube
pseudomorphic epitaxy growth of Fe3O4
on MgO.
• Flat interfaces, no intermixing detected.
• Slight plastic relaxation between MgO
and Fe3O4 compensated by misfit
dislocations (misfit < 0.3%).
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Evidence for single phase Fe3O4 by
Mössbauer spectroscopy
Velocity ( mm / s )
-10
+10
1.04
A b so rp tio n ( % )
CEMS shows that:
• only Fe3+ 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)
9
0
1.00
-1
-1
co mpo nent
IS (mm.s )
2 (mm.s )
B hf (T )
abundance(% )
A -site
0.30  0.01
0.004  0.004
49.1  0.2
36
B -site
0.63  0.01
0.038  0.004
45.6  0.2
64
Kerr spectroscopy: other evidence for
magnetite
0.1 0
0.0 5
• 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
Fe2+ to Fe3+ on B sites).
• Experimental spectra very similar to the ones
obtained from bulk magnetite, within uncertainty due
to substrate reflectivity and film thickness.
0.0 0
-0.05
-0.10
-0.15
-0.20
exp eriment
-0.25
fit
0.1 0
0.0 5
0.0 0
-0.05
-0.10
-0.15
exp eriment
-0.20
fit
-0.25
1.0
1.5
2.0
2.5
3.0
Pho ton E nergy (eV)
1
3.5
4.0
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,
• MSat @ 1 Tesla lower than the one of bulk
(480 emu/cm3).
 results indicative of the presence of
magnetic frustration likely due to
ANTIPHASE BOUNDARIES
1
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 Fe3O4, 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 JBB = 3 K and JAA = -11 K,
• dominant interaction: JAB= -22 K (antiferromagnetic)
 at the APBs, the JAB interactions may loose their dominant
character and AF coupling is possible between the two domains
in antiphase.
1
Growth on MgO (001) of Fe3O4 with APBs
[110]
[110]
O
Mg
Fe
(oct. site)
1
(001) plane
Of MgO
substrate
TEM: evidence for APB’s
(antiphase boundaries)
Plane view
1
Cross section
Images collected from 220 diffraction
spot
HR TEM: a rare close up on atomic displacements at the
APB
Cross section HRTEM micrograph and its
simulation (T=20nm, Df=-80nm) of an APB
in a Fe3O4 [001] thin film.
1
Influence of APBs on the zero field
magnetic configuration by MFM
APB top morphology
APB-induced ripple
[010]
dust
TEM plane view, same scale
[100]
Atomic (a) and magnetic (b) force microscopy images of a 750 Å thick Fe 3O4 film.
J.F. Bobo et al, Euro. Phys. J B24 (2001) 43
1
MFM measurements on other epitaxial films of
magnetite
[100]
[010]
Thickness = 1870 Å
10*10 µm scale
1
Thickness = 95 Å
2*2 µm scale
Correlation magnetic rippleAPBs
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.
•Deeper study of the magnetic interactions generated by APBs
necessary.
•Full treatment by micromagnetic approach.
1
Polycrystalline magnetite films
and nickel oxide - magnetite
bilayers
1
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
Fe3O4 single layers.
• avoid intermixing NiO/Fe3O4
which occurs at higher deposition
temperatures.
2
EXPERIMENTAL PROCEDURES:
• deposition at room temperature
 Ar+ 10% O2 for NiO to get
good stoichoimetry
 pure Ar for Fe2O3 in order to
reduce to Fe3O4 stable phase
 in plane applied magnetic field
(1200 Oe) for magnetic ordering.
• electron microscopy, x-ray diffraction
• room temperature VSM, SQUID
Electron
diffraction
confirms the
presence of
magnetite
single phase
2
X-ray diffraction reveals magnetite single phase and
smooth surfaces
(311) - Irel=999
JCPDS # 86-1352 file
(222) - Irel=73
(333) - Irel=255
(220) - Irel=291
(400) - Irel=203
(422) - Irel=81
Fe3O4 - 8000 Å film
(331) - Irel=5
2 (°)
Réflectivité
Fit parameters:
thickness: Fe3O4 938 Å
Surface roughness = 10 Å
2
Angle d'incidence,  (°)
expériment
fit
Fe3O4 - 1000 Å film
Electron microscopy evidences flat
structures with columnar growth
Grain size ~ 200-300 Å
2
Columnar structure is coherent from NiO
to Fe3O4, indicative of grain epitaxy
2
HR TEM of the
NiO-Fe3O4
interface:
- no intermixing
- local continuity
of the atomic rows
2
Structural properties
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
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/cm3
• slight easy axis induced by the
deposition with 1200 Oe in plane,
unexpected presence of some
exchange bias
HE ~ 60 Oe
LOW SQUARENESS CAUSED
BY POLYCRYSTALLINITY.
H (kOe)
2
Fe3O4 - NiO bilayers: investigation of exchange coupling at
room temperature with 1000 Å NiO underlayer
no NiO underlayer
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 Fe3O4 films.
2
Magnetic ripple investigated by AFM/MFM
Fe3O4 1000 Å single layer
Surface morphology indicative of
a low roughness
Magnetic ripple with a constant
periodicity ~ 0.2 - 0.4 µm.
NiO1000 Å - Fe3O4 1000 Å bilayer
2
Close up
Surface roughness
3
Magnetic ripple
Conclusions concerning
magnetite films
•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. MgAl2O4).
•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.
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Potential applications of magnetite films
•Benefit of the magnetite large spin polarization
•Avoid the “low squareness” behavior
2. GMR
structures with
Fe3O4 magnetic
layers
1. Inserted
electrode-barrier
polarizing layer
3. Spin injector
layer in spin LEDs
3
Potential magnetoresistive applications
of such structures
Fe3O4
Fe3O4
insulating tunnel barrier (e.g. MgO)
metal spacer (Pt, Cu...)
Fe3O4
Fe3O4
CPP MTJ with fully spin polarized
electrodes
(Sénéor, Freitas, Suzuki)
CIP GMR structure with bad
conductor as ferromagnetic layers
(B. Diény)
epitaxial Fe3O4 film
Spinel MgAl2O4 substrate
(bicrystal or laser grooved)
3
APB area
CIP GMR structures - first trials
•smooth structures
•No magnetic decoupling
•No GMR so far at 300 K :-(
3
Spin LEDs
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
Samples structure:
Standard LED structure with semi transparent Co window for light detection.
Doping for Schottky barrier from 1017 to 2.1018 cm-3
Emitted light (500 µm window)
H
Au(1000Å)
-
+
Co(150Å)
AlGaAs:Si(700Å)
Pt(20Å)
AlGaAs(400Å)
AlGaAs(500Å)
AlGaAs:Be(5000Å)
GaAs:Zn substrate
3
GaAs(100Å) quantum well
Electroluminescence experiment
Out of plane applied field = 0.8 T
Co film magnetization not totally perpendicular (MzMsat/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
Electroluminescence results
s+
s-
Stationary mode =>
integration over time
of several radiative,
non radiative and spin
relaxation processes
3
Stationnary measurements
E
P 
s
+
s
+
-s
-
+s
-
P0

1+
 life
 spin
gap
hh
3
Further studies
• 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
General conclusion
• 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 Fe3O4 and NiO-Fe3O4 bilayers.
• Perspectives
– GMR, TMR or spin-LED structures based on magnetite,
– try to reduce APBs in epitaxial films (choice of different substrates
/ deposition conditions).
4