spintronics a. kellou and h. aourag metallic thin films revisited: fe, co, ni multilayers
TRANSCRIPT
SpintronicsA. Kellou and H. Aourag
Metallic Thin Films Revisited: Fe, Co, Ni Multilayers
SpintronicsMetallic Thin Films Revisited: Fe, Co, Ni Multilayers
Spintronics: To Control a Spin of Electrons, not a Charge
Magnetic Nanostructures for Spintronics Magnetic Multilayers Magnetic Wires Magnetic Quantum Dots
Applications of Magnetic Nanostructures Reading Heads, Magnetic Field Sensors, MRAM Field Effect Transistor, Spin-Valve Transistor Quantum Computer
Basic Structure
The prototype device that is already in use in industry as a
read headmemory-storage cell
is the
giant-magnetoresistive (GMR) sandwich structure
which consists of alternating
ferromagnetic and nonmagnetic metal layers.
Basic Structure
Depending on the relative orientation of the magnetizations in the magnetic layers,
the device resistance changes from
small (parallel magnetizations) to large (antiparallel magnetizations).
This change in resistance (also called magnetoresistance) is used to sense changes in magnetic fields
Basic Structure
Basic Structure
two different approaches: 1) existing GMR-based technology - developing new materials with larger spin polarization of electrons
- making improvements or variations in the existing device that allow for better spin filtering.
2) finding novel ways of both generation and utilization of spin-polarized currents.
Basic Structure
Problems:
existing metal-based devices do not amplify signals (although they are successful switches or valves), whereas semiconductor based spintronic devices could in principle provide amplification and serve, in general, as multi-functional devices.
spin polarizers and spin valves
Magnetic Random Access Memory (MRAM)
Low Resistance High ResistanceReversible
Issues in Magnetic Multilayers Fabrication of Ordered Nanostructures on a Surface
A detailed understanding of the various atomic processes
that occur during the formation of nanosized islands on surfaces
Surfaces are not simply a static media onto which the
deposited atoms and diffuse
Deposition and nucleation on a surface is important
29III. Applications: ii) binary alloys
FeCr, CoCr, and NiCr:Structural and magnetic properties
a B E MX MCr M
FeCr Theory nm fm af
5.324 5.381 5.377
216231232
-4647.3167-4647.3330-4647.3390
--1.68
--0.37
--2.01
CoCr Theory nm fm af
5.360 5.409 5.425
189224179
-4888.6493-4888.6847-4888.6576
-1.25-
-1.59-
-2.98-
NiCr Theory nm fm af
5.453 5.489 5.511
213239217
-5143.3630-5143.3951-5143.3953
--0.60
--2.39
--3.29
120 130 140 150 160 170 180
-4647,35
-4647,34
-4647,33
-4647,32
-4647,31
-4647,30
-4647,29
-4647,28
-4647,27
FeCr NM FM AF
En
erg
y (R
y)
Volume (a.u.3)130 140 150 160 170 180
-4888,68
-4888,67
-4888,66
-4888,65
-4888,64
-4888,63
-4888,62
-4888,61
CoCr NM FM AF
Ene
rgy
(R
y)
Volume (a.u.3)
120 130 140 150 160 170 180 190 200-5143,40
-5143,38
-5143,36
-5143,34
-5143,32
NiCr NM FM AF
Ene
rgy
(R
y)
Volume (a.u.3)
30III. Applications: iii) Ternary alloys
Semi-Heusler alloys
•Half-metallic materials possess 100% electron polarization at the Fermi energy.
•New class of magnetic materials displaying metallic character for one electron spin population and insulating character for the other.
•Technological interest as potential pure spin sources for use in spintronic devices, data storage applications, and magnetic sensors.
•Difficult to confirm experimentally the half-metallicity charcter (clean stoichiometric surfac).
To known if the intermettallic alloys based on a ferromagnet -Ti -Cr can lead to a half-metallicity behavior.
31III. Applications: iii) Ternary alloys
260 270 280 290 300 310
-7040,070
-7040,065
-7040,060
-7040,055
-7040,050
-7040,045
-7040,040
-7040,035
-7040,030
FeCoTi NM FM
En
erg
y (R
y)
Volume (a.u3)
250 260 270 280 290 300 310 320 330
-7294,795
-7294,790
-7294,785
-7294,780
-7294,775
-7294,770
-7294,765
-7294,760
-7294,755
-7294,750
FeNiTi NM FM
En
erg
y (R
y)
Volume (a.u3)
220 230 240 250 260 270 280 290
-7434,20
-7434,19
-7434,18
-7434,17
-7434,16
-7434,15
FeCoCr NM FM
En
erg
y (R
y)
Volume (a.u3)
210 220 230 240 250 260 270 280 290 300
-8374,08
-8374,06
-8374,04
-8374,02
-8374,00
-8373,98
-8373,96 FeCoNi
NM FM
En
erg
y (R
y)
Volume (a.u.3)
250 260 270 280 290 300 310 320 330 340 350
-6596,225
-6596,220
-6596,215
-6596,210
-6596,205
-6596,200
-6596,195
-6596,190 CoTiCr
NM FM
En
erg
y (R
y)
Volume (a.u.3)
250 260 270 280 290 300 310 320 330 340
-6354,88
-6354,87
-6354,86
-6354,85
-6354,84
FeTiCr NM FM
En
erg
y (R
y)
Volume (a.u.3)
210 220 230 240 250 260 270 280 290
-7688,91
-7688,90
-7688,89
-7688,88
-7688,87
-7688,86
-7688,85
FeNiCr NM FM
En
erg
y (R
y)
Volume (a.u.3)260 270 280 290 300 310 320 330 340 350
-6850,928
-6850,920
-6850,912
-6850,904
-6850,896
-6850,888
-6850,880
NiTiCr NM FM
En
erg
y (R
y)
Volume (a.u.3)
Semi-Heusler alloys
Ground states from total energy calculations
• FeCoTi, CoTiCr, NiTiCr, and FeCoNi are predicted ferromagnetic.
• FeNiTi, FeNiCr, FeTiCr, and FeCoCr and are predicted antiferromagnetic.
• FeCoCr and FeNiCr are nonmagnetic.
32III. Applications: iii) Ternary alloys
0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8
-120
-80
-40
0
40
80
120
160
Dn
Up
FeCoTi
DO
S (
stat
es/s
pin)
Energy (Ry)0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8
-200
-160
-120
-80
-40
0
40
80
120
160
Dn
Up
FeNiTi
DO
S (
stat
es/s
pin)
Energy (Ry)0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8
-200
-160
-120
-80
-40
0
40
80
120
160
200
Dn
Up
FeCoNi
DO
S (
stat
es/s
pin)
Energy (Ry)
0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8
-200
-160
-120
-80
-40
0
40
80
120
160
Dn
Up
FeCoCr
DO
S (
stat
es/s
pin)
Energy (Ry)
0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9
-200
-160
-120
-80
-40
0
40
80
120
160
Dn
Up
FeTiCr
DO
S (
sta
tes/
spin
)
Energy (Ry)0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8
-200
-160
-120
-80
-40
0
40
80
120
160
Dn
Up
FeNiCr
DO
S (s
tate
s/sp
in)
Energy (Ry)
0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9-200
-160
-120
-80
-40
0
40
80
120
160
Dn
Up
CoTiCr
DO
S (s
tate
s/sp
in)
Energy (Ry)
0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8
-200
-160
-120
-80
-40
0
40
80
120
160
200
Dn
Up
NiTiCr
DO
S (s
tate
s/sp
in)
Energy (Ry)
• All alloys are polarized except FeNiCr and CoTiCr.• FeCoTi, FeNiTi, and NiTiCr have a majority spin in a deep minimum right the Fermi level, leading to a pseudo-gap which is responsible for 100% electron polarization.
Semi-Heusler alloys
Total DOS
33III. Applications: iii) Ternary alloys
• Stoichiometric composition X2YZ
• Electronic structure can range from metallic to semi-metallic or semiconducting behavior.
• Half-metallic ferromagnetism, in which the bandstructure for majority electrons is metallic while the bandstructure for minority electrons is insulating.
• Anomalous peak in the yield stress and high temperature strength and excellent oxidation and corrosion resistance.
Heusler alloys
a (paramètre du reseau)
MX MAl MX’
Fe2AlTi nm 11.005fm 11.014 11.115a
-0.62
--0.01
--0.19
Co2AlTi nm 11.019fm 11.005
11.058b
-0.370.34b
--0.00
--0.05-0.05c
Ni2AlTi nm 11.136fm 11.136
10.926a
-+0.00
-+0.00
--0.00
Fe2AlCr nm 10.685fm 10.684
-0.08
-0.00
--1.04
Co2AlCr nm 10.758 fm 10.797 11.134b
-0.690.78b
--0.03
-1.611.60e
Ni2AlCr nm 10.841fm 10.960
-0.26
--0.03
-2.39
• All alloys are ferromagnetic, except Co2AlTi and Ni2AlTi (paramagnetic).
• Large magnetization in Cr alloys .
34III. Applications: iii) Ternary alloys
Heusler alloys
Fe Co Ni
10.6
10.7
10.8
10.9
11.0
11.1
11.2
11.3
Ti Cr
a (A
ngös
trom
)
Element
Fe Co Ni140
160
180
200
220
240
Ti Cr
a (
An
gö
stro
m)
Element
35III. Applications: iii) Ternary alloys
Heusler alloys
• Cr has induced a volume contraction although Z(Ti) < Z(Cr).
• This fact is due to changes in bonding.
• Cr has allso induced large bulk modulii except ofr Ni2AlCr (large magnitzation, hgh volume)
Lattice parameters and bulk modulii
-0.6 -0.4 -0.2 0.0 0.2 0.4
-120
-100
-80
-60
-40
-20
0
20
40
60
80
100
120
Fe2AlTi
DO
S (
Sta
tes/
Spi
n)
E-EF (Ry)
-0.6 -0.4 -0.2 0.0 0.2 0.4
-120
-100
-80
-60
-40
-20
0
20
40
60
80
100
120
Fe2AlCr
DO
S (
Sta
tes/
Spi
n)
E-EF (Ry)
-0.6 -0.4 -0.2 0.0 0.2 0.4
-120
-100
-80
-60
-40
-20
0
20
40
60
80
100
120
Co2AlTi
DO
S (
Sta
tes/
Spi
n)
E-EF (Ry)
-0.6 -0.4 -0.2 0.0 0.2
-120
-100
-80
-60
-40
-20
0
20
40
60
80
100
120
Co2AlCr
DO
S (S
tate
s/S
pin)
E-EF (Ry)
-0.6 -0.4 -0.2 0.0 0.2 0.4
-120
-100
-80
-60
-40
-20
0
20
40
60
80
100
120
Ni2AlTi
DO
S (
Sta
tes/
Spi
n)
E-EF (Ry)
-0.6 -0.4 -0.2 0.0 0.2
-120
-100
-80
-60
-40
-20
0
20
40
60
80
100
120
Ni2AlCr
DO
S (
Sta
tes/
Spi
n)
E-EF (Ry)
• Cr has induced Fermi displacement to the right (anti-bonding states) with a prounounced half-metallicity character in Fe2AlCr and to the left in Co2AlCr and Ni2AlCr.
36III. Applications: iii) Ternary alloys
Heusler alloys
Total DOS
i) Transition element family
ii) Binary systems
iii) Ternary systems
iv) Layered structures Clean V(001), Cr(001) and Fe (100) surfaces TM/5Cr(001) (TM = Ti, V, Cr, Mn, Fe, Co, Ni) Fe/Cr(001) systems
III. Applications
37
38III. Applications: iv) Layered structures
The unit cell in film calculations.
Z
Interesting properties (GMR, MAE, high local moments …) when ferromagnetic and antiferromagnetic transition elements are layered.
Determination of interlayer exchange coupling (IEC).
Effect of magnetism in surface, interface, and superlattices phenomena
Ferromagnetic substrates are well studied: Cu(001), Ag(001), Au(001),
Fe(001), Co(001) …but not antiferromagnetic Cr !!!
Vacuum
Vacuum
39III. Applications: iv) Layered structures
Surface magnetism in the (001) direction: nonmagnetic V, antiferromagnetic Cr, and ferromagnetic Fe. 5-layers of V(001), Fe(001) and Cr(001) in repeated slab structure.
Magnetism occurs in V and is enhanced in Cr and Fe (001) surfaces because of the lying bonds (coordination number).
Clean V(001), Cr(001), and Fe(001) surfaces
M1 M2 M3
V(001) -0.17 -0.08 0.67
Fe(001) 2.53 2.42 3.02
Cr(001) 1.21 -1.56 2.62
M3 (Surface)
M2 (Sub-surface)
M1 (Central)Z=0
M3
BULK
0.00
2.26
± 0.77
40III. Applications: iv) Layered structures
Several theoretical and experimental studies were devoted to the surface properties of the magnetic 3d transition metal grown on noble metal (Cu, Ag, and Au) and ferromagnetic (Fe, Co, and Ni) but not Cr(001).
Study of total and surface energies of Cr(001) films, magnetic, and electronic properties of 3d transition-metal (Ti, V, Cr, Mn, Fe, Co, Ni) monolayer on Cr(001), with two opposite spin orientations leading to ferromagnetic and antiferromagnetic configurations.
TM on 5-Cr(001) layers (TM = Ti, V, Cr, Mn, Fe, Co, Ni)
(a) 3-Cr(001) (b) 5-Cr(001) (c) TM/5-Cr(001)
Cr(3)
Cr(1)
TM
Cr(2)
41III. Applications: iv) Layered structures
Difference in total energy
TM on 5-Cr(001) layers (TM = Ti, V, Cr, Mn, Fe, Co, Ni)
Ti V Cr Mn Fe Co Ni-2
-1
0
1
2
14.1
14.4
14.7
15.0
Antiferromagnetic
Ferromagnetic
E=E
FM-E
AF
M (
mR
y/a
tom
)
Element
Cr (S)
Ti, V, Cr
ferromagnetic coupled
TM Fe, Co, and Ni antiferromagnetic
coupled
Nothing about Mn (ferrimagnetic coupled ???!)
42III. Applications: iv) Layered structures
TM on 5-Cr(001) layers (TM = Ti, V, Cr, Mn, Fe, Co, Ni)
Ti V Cr Mn Fe Co Ni
-3
-2
-1
0
1
2
3 FM AFM
MT
M ( B
)
Element
Ti V Cr Mn Fe Co Ni-3
-2
-1
0
1
2
3 FM AFM
M ( B
)
Element
Transition metal and total magnetic moment
TM’ s magnetic moment increases from Ti to Mn and decrease from Mn to Ni, in both ferromagnetic and antiferromagnetic configurations.
Mn deposition induces the highest value, followed by Fe, Co, and Ni.
Total magnetic moment has the same behavior as TM magnetic moment.
43III. Applications: iv) Layered structures
TM on 5-Cr(001) layers (TM = Ti, V, Cr, Mn, Fe, Co, Ni)
Spin Density Waves in Cr thin films
Cr4 Cr3 Cr2 Cr1 Cr2 Cr3 Cr4-3,0
-2,5
-2,0
-1,5
-1,0
-0,5
0,0
0,5
1,0
1,5
2,0
2,5
3,0
M (
)
Layer
The periodic nature the oscillations in 7-Cr(001) is strongly related to the itinerant linear Spin-Density Waves (observed in Cr multilayers, bulk Cr and its alloys.
Cr thin films need SDW to have antiferromagnetic ground state.
44III. Applications: iv) Layered structures
Several theoretical and experimental studies were devoted to the surface properties of the magnetic 3d transition metal grown on noble metal (Cu, Ag, and Au) and ferromagnetic (Fe, Co, and Ni) but not Cr(001).
Study of total and surface energies of Cr(001) films, magnetic, and electronic properties of 3d transition-metal (Ti, V, Cr, Mn, Fe, Co, Ni) monolayer on Cr(001), with two opposite spin orientations leading to ferromagnetic and antiferromagnetic configurations.
TM on 5-Cr(001) layers (TM = Ti, V, Cr, Mn, Fe, Co, Ni)
(a) 3-Cr(001) (b) 5-Cr(001) (c) TM/5-Cr(001)
Cr(3)
Cr(1)
TM
Cr(2)
45III. Applications: iv) Layered structures
Study of the diffusion, the surface alloy formation, and the magnetic properties in Fe/Cr(001) systems and magnetic properties of Fen/Crn(001) superlattices.
Fe/Cr multilayer exhibit interlayer exchange coupling (IEC), giant magneto-resistance (GMR), …etc.
Experimental results, obtained by similar techniques, often contradict each another and theoretical calculations also demonstrated a very complex behavior and solutions with close energies.
Fe/Cr(001) systems
Fig. 5.24 Upper half-slab of the unit cell in: (a) 4Cr(001), (b) 1Fe/3Cr(001), (c) 2Fe/2Cr(001), (d) Fe50Cr50/3Cr(001), and (e) 1Fe/Fe50Cr50/2Cr(001). The first layer (I) corresponds to central layers.
Cr
(c) 2Fe/2Cr(001)
I-3I-2I-1
I
(a) 4Cr(001) (b) 1Fe/3Cr(001)
Fe
(e) 1Fe/Fe50Cr50/2Cr(001)(d) Fe50Cr50/3Cr(001)
46III. Applications: iv) Layered structures
Fe/Cr(001) systems
Total energies and total and partial magnetic moments
Ef (Ry/atom) M1 (B) M2 (B) M3 (B) M4 (B) M (B)
Bulk Fe - +2.32 - - - +2.25
Bulk Cr - +0.77 -0.77 - - +0.00
2Cr(001) ++ +-
79.0779.09
-1.82+1.84
+2.84-2.82
--
--
+4.52-4.56
3Cr(001) +++ +-+
47.2247.24
+1.15+1.17
-1.54-1.53
+2.64+2.62
--
+2.94+3.94
1Fe/2Cr ++/+ +-/+ ++/- +-/-
43.6142.7843.0543.07
+0.28+0.85-0.65-0.47
+0.31-0.81+0.54+0.54
+2.54+2.51-2.52-2.60
----
+6.19+4.45-4.14-4.67
1Fe/3Cr +-+/+ +-+/-
29.4629.72
-1.07+0.83
+1.05-0.84
-0.94+0.80
+2.55-2.52
+4.39-4.56
2Fe/2Cr +-/++ +-/+- +-/-+ +-/--
24.6237.3537.8624.69
+0.62+0.67-0.35-0.18
-0.74-0.53+0.16+0.27
+2.03+1.68-1.51-1.98
+2.95-2.75+2.77-2.96
+9.06-2.91+2.81-9.05
)( bulkCrCr
bulkFeFeStructf EnEnEE
47III. Applications: iv) Layered structures
Fe/Cr(001) systems
Bilayer formation against the monolayer formation
)(2
1)001(4)001(2/2)001(3/1 CrCrFeCrFeMB EEEE
This energy is positive (+0.54 mRy/unit cell) in the ferromagnetic
state and negative (-8.10 mRy/atom) in the nonmagnetic state.
This means that magnetic moments allow BL formation (2Fe/2Cr(001)), whereas nonmagnetic state favors ML formation (1Fe/3Cr(001)).
This result contradicts the description which was discussed for Cr (ML) on Fe(001) substrate, where ML formation is preferred for the ferromagnetic configuration.
48III. Applications: iv) Layered structures
Fe/Cr(001) systems
Diffusion and surface alloy formation against phase separation
))1(( UNCCOValloySA ExxEEE Ef (Ry/atom) M (B)
1Fe/3Cr nm fm
39.3629.46
-+4.39
1Cr/1Fe/2Cr nm fm
51.4537.36
-+7.96
2Cr/1Fe/1Cr nm fm
51.9241.11
-+0.19
ESA
(mRy/atom)
M (B)
Fe50Cr50/3Cr(001)
nm fm
-1.30+3.80
-+5.39
1Fe/Fe50Cr50/2Cr(001)
nm fm
-0.11+2.51
-+14.38
Fe do not diffuse to Cr bulk layers.
No magnetism favors phase separation or clustering, whereas magnetism favors formation of Fe50Cr50/3Cr(001) followed by Fe/Fe50Cr50/3Cr(001) ordered surface alloys (confirmed in recent experimental study).
50III. Applications: iv) Layered structures
Fe/Cr(001) systems
Fen/Crn(001) superlattices
1 2 3 4 5
0,0
1,5
3,0
4,5
6,0
7,5
9,0
n
M (
)
M
-5
0
5
10
15
E
(mR
y/atom)
E
The formation energy is stabilized after n = 4.
The total magnetic moment is growing with the number of Fe and Cr layers.
Total energies favor the following spin alignments: +/+, ++/--, +++/+-+, ++++/-+-+, +++++/-+-+-.
V. Conclusion
We have given additional results to structural, electronic, and magnetic properties the selected transition materials (Ti, V, Cr, Mn, Fe, Co, and Ni) and their related systems; binary alloys, ternary alloys in Half-Heusler and Heusler structures, thin films and superlattices.
We have shown the importance of d-states in the ground state properties in these systems.
We have also studied the equilibrium parameters and the stability mechanism from the different formation energies and from the position of the Fermi level in the density of states.
The new form of the GGA approximation is adequate for transition metals and their related alloys.
The obtained structural properties are in good agreement with experimental data and more efficient than LDA ones.
51
In the binary systems XTi and XCr (X=Fe, Co, Ni), effects of magnetism is studied and related to the structural and electronic structures.
The martensitic transformation (MT) phenomena of NiTi have been studied and optimized lattice parameters for B19’ were given.
The different roles of d-states were highlighted and are totally responsible for unexpected and controversial behaviors.
52V Conclusion
Binary alloys
Structural parameters, formation energies, magnetic moments, and electronic properties of XYZ Half-heusler and X2AlX’ Heusler alloys (X=Fe, Co, Ni; X’=Ti, Cr) were presented.
The obtained results of lattice parameters and local magnetic moments agree very well with the experimental results.
Cr sites carry large magnetic moments and the moments at the X sites are usually small, when compared to Ti substitution.
All the densities of states are marked by a pseudogap left the Fermi level, except for Fe2AlTi where the pseudogap is right EF.
Among the selected materials, the Fe2AlCr and Co2AlCr alloys present a pronounced half-metallicity character.
53
Ternary alloys V Conclusion
The existence of itinerant linear Spin-Density Wave (SDW) is responsible for antiferromagnetic coupling between two adjacent Cr layers in Cr(001).
Mn overlayer induces the highest magnetic moments and relies between two opposite spin alignments in TM/Cr(001). Ferrimagnetic (FI) coupling can occur. Further investigations within the c(2x2) unit cell are necessary.
Ti, V, and Cr overlayers are antiferromagnetically coupled to the Cr sub-surface layer; Mn, Fe, Co and Ni are ferromagnetically coupled.
Fe layers are always antiferromagnetically coupled to Cr layers in Fe/Cr systems.
Fe atoms prefer to be deposited as an overlayer rather than being diffused in the Cr layers with formation of an ordered surface alloy.
Magnetism is responsible for the BL formation and ordered surface alloying in Fe/Cr (GMR, Colossal RM)
54
Layered structures V Conclusion
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