Spintronics

Spintronics
Summer school SFB 491
Claudia Felser

Content
• Concept
• Materials
• Methods
• Vision

Spintronics
P. Ball, Nature 404 (2000) 918,
G. A. Prinz, Science 282 (1998) 1660

Rational Design
Synthesis
computational
design
structure
nano particles
single crystals
thin films
ceramics
spectroscopy

Spin-electronics
Spin + Charge = Spintronics
C. Felser Angew. Chem. Int. Ed. 46 (2007) 668

Giant Magnetoresistance: GMR
P. Grünberg, PRB39, 4828 (1989);
A. Fert, PRL61, 2472 (1988)

Colossal Magnetoresistance: CMR
La 0.6 Ca 0.3 MnO 3

Colossal Magnetoresistance: CMR
Semiconductor to Metal Transition
in GdI 2
MR 0 = 60 % bei RT
Problem: high field effect,
small at room temperature
C. Felser et al. JSSC 19 147 (1999).

Half metallic Ferromagnet
Normal Metal P = 0 Halfmetallic ferromagnet P = 1

Tunnel Magnetoresistance: TMR
MRAM = Magnetic RAM

Tunnel Magnetoresistance: TMR
A
⇒ TMR = R − P
R P
R P
= 2P LP R
1 − P L
P R
Julliere formula

Spin injection into semiconductor
Ferromagnetic Semiconductor
with T C above room
temperature is still a challenge

Content
• Concept
• Materials
• Methods
• Vision

Ferromagnetic Metals

How to Design Halfmetallic Ferromagnets
• Precondition: a gap in the overall electronic structure:
• Covalent compounds – related to semiconductors Si
• Ionic compounds –oxides, halides, sulfides
Felser et al. Angew. Chem. Int. Ed. 46 (2007) 668

Half metallic Ferromagnets

Definition Half metals (Coey)
Type Density of states Conductivity ↑ Electrons at
E F
↓ Electrons at
E F
I A
half-metal metallic itinerant none
I B
half-metal metallic none itinerant
II A
half-metal nonmetallic localized none
II B
half-metal nonmetallic none localized
III A
metal metallic itinerant localized
III B
metal metallic localized itinerant
IV A
semimetal metallic itinerant localized
IV B
semimetal metallic localized itinerant
V A
semiconductor semiconducting few, itinerant none
V A
semiconductor semiconducting none few, itinerant

Structure Al
M auf 0 0 0
z.B. Al

Structure Si
()
M X auf 0 ¼¼¼ 0 0
M auf z.B. ¼ZnS
¼ ¼
z.B. Si

Half Heusler Compound CoTiSb
M auf 0 0 0
M auf ½ ½ ½
M auf ¼ ¼ ¼
z.B. LiAlSi oder NiMnSb
Kandpal et al. J. Phys. D 39 (2006) 776

Design of a Semiconductor
Valence electrons
Si 2
2 x 4 = 8
Ga
As
3 + 5 = 8
ZnS → C1b
9 + 4 + 5 = 18
Co
Ti
Sb

Half-Heusler Compounds
Charge density
CoTiSb
Charge density
CoMnSb
(e)
Spin density
CoMnSb
18 = 9+4+5
Semiconductor
21 = 9+7+5 3μ B
Half metallic Ferromagnet

Heusler Compound Co 2 CrAl
TM auf 0 0 0
M auf ½ ½ ½
TM auf ¼¼¼
TM auf ¾¾¾
Half metallic ferromagnet

Oauf 0 0 ½
Heusler- Double Perovskite

Design of half metallic ferromagnet
Valence electrons
Si 2
2 x 4 = 8
Cr
As
6 + 5 = 8 +3
3 μ B
ZnS → C1b
10 + 7 + 5 = 18 + 4
Ni
Mn
Sb
3 μ B

CrAs with ZnS structure

NiMnSb – Half Heusler

Ionic: CrO 2 – Rutil structure

Rational Design of New Materials
XYZ
Halb-Heusler
C1 b
X 2 YZ
Heusler L2 1

Heusler and the Periodic Table

Heusler Compounds: ~500

Slater-Pauling Rule for Heusler
Magic valence electron number 24
Valence electrons =
24 + saturation magnetisation
Co 2 CrAl
2*9 + 6 + 3 = 27
Saturation magnetisation :3μ B
Total spin moment: M t
(μ Β
)
7
6
5
4
3
2
1
0
−1
−2
Co 2
CrAl
Fe 2
MnSi
Ru 2
MnSi
Ru 2
MnGe
Ru 2
MnSn
M t
=Z t
−24
Mn 2
VAl
Co 2
MnSi
Co 2
MnGe
Co 2
MnSn
Co 2
VAl
Fe 2
MnAl
Fe 2
CrAl
Co 2
TiAl
Fe 2
VAl
Mn 2
VGe
Co 2
MnAs
Co
Co
2
MnSb
Rh 2
FeSi
2
MnIn
Rh Co 2
FeAl
2
MnTl
Co 2
TiSn
Co 2
MnAl
Co 2
MnGa
Rh 2
MnAl
Rh 2
MnGa
Ru 2
MnSb
Ni 2
MnAl
Rh 2
MnGe
Rh 2
MnSn
Rh 2
MnPb
−3
20 21 22 23 24 25 26 27 28 29 30 31 32
Number of valence electrons: Z t
Galanakis et al., PRB 66, 012406 (2002)

Heusler Compounds: ~500
Co 2 CrAl: 27e -
Co 2 FeAl: 29e -

Result
Large magneto resistance at room temperature and low fields
• High T C
• Doping
Co 2 Cr 0.6 Fe 0.4 Al +
15% Al 2 O 3
H
Block, et al. J. Solid State Chem. 176, 646 (2003)

Rational Design
Half metallicity
High density of states at E F
Large MR at room temperature
Intermag 2002: Co 2 Cr 0.4 Fe 0.6 Al
• First TMR device (Inomata et al.) 19% at RT
• TMR-device with MgO (Marukame et al. APL 90 (2007) 012508)
109% TMR at RT ⇒ 88 % spin polarisation at 4K
• Point contact 80% MR (Coey et al.)
• Patent (Felser, Block, DE 101 08 760, H01 L43/08 )
• Block, Felser, et al. J. Solid State Chem. 176, 646 (2003)

TMR-device with CCFA
IrMn
Co 90 Fe 10
Ru
Co 50 Fe 50
MgO
Co 2 Cr 0.6 Fe 0.4 Al
5 nm
(b)
a CCFA
= 0.5737 nm
a CMG
= 0.5743 nm
a CMS
= 0.5654 nm
a MgO
= 0.4212 nm
Marukame et al. APL 90 2007 012508

Heusler Compounds: ~500
Co 2 FeAl: 29e -
Co 2 FeSi: 30e -

Further Candidates
Heusler compounds:
Co 2 YZ
Halfmetallic
ferromagnets
High Curietemperatures
Curie Temperature T c
[K]
1400
1200
1000
800
600
400
200
0
Ni
Co
0.0 0.5 1.0 1.5 2.0 2.5
Fe
pure elements
Co 2
YZ
Fit Co 2
YZ
Co 2
FeSi this work
Magnetic Moment per Atom m [μ B
]
Expected Curie temperature for Co 2 FeSi : > 1000K
Fecher, J. Appl. Phys. 99 (2006) 08J106

Co 2 FeSi: Determination of Saturation Magnetization
Magnetic Moment per unit cell m [μ B
]
6
4
2
0
-2
-4
-6
5K
300K
775K
1.0%
0.5%
0.0%
-0.5%
-1.0%
-2.0k 0.0 2.0k
-3 -2 -1 0 1 2 3
Magnetic Field H [10 6 A/m]
SQUID magnetometry
Magnetic moment in saturation:
5.97μ B
±0.1μ B
at 5K
Extrapolation to 0K :Slater-
Pauling rule: 6 μ B
Co 2 FeSi is a halfmetallic ferromagnet
Wurmehl, et al ., Phys. Rev. B 72 (2005) 184434

Co 2 FeSi: Determination of the Curie-Temperature
Lake Shore
Specific Magnetization σ [Am 2 kg -1 ]
60
50
40
30
20
10
σ(T)
T C
= 1100 ± 20 K
μ 0
H = 0.1T
m = 47 μg
1/χ(T)
Θ = 1150 ± 50 K
T C
0
0
700 800 900 1000 1100 1200 1300
Temperature T [K]
800
600
400
200
Inverse Susceptibility 1/χ
Curie temperature of Co 2 FeSi is (1100 ± 20) K
Wurmehl, et al ., APL 88 (2006) 032502.

Curie Temperatures
Curie Temperature T c
[K]
1400
1200
1000
800
600
400
200
0
Ni
Calculated T C
[K]
Co
1200
0.0 0.5 1.0 1.5 2.0 2.5
Fe
pure elements
Co1000
2
YZ
Fit Co 2
YZ
Co 2
FeSi 800 this work
Magnetic Moment per Atom
600
m [μ B
]
400
200
T calc
= T meas
Co 2
VGa
Co 2
CrGa
Co 2
TiAl
Co 2
VSn
Co 2
MnAl
Co 2
FeSi
Co 2
MnSi
Co 2
MnSn
0
0 200 400 600 800 1000 1200
Measured T C
[K]
Kübler et al., submitted PRB

Tuning the Fermi Energy Co 2 FeAl 1-x Si x
Spin resolved density of states ρ(E) [eV -1 ]
10
5
0
5
10
10
5
0
5
10
10
5
0
5
10
10
5
0
5
10
10
5
0
5
10
Majority
Minority
-10 -5 0
(a)
(b)
(c)
(d)
(e)
Energy E − ε F
[eV]
0.8
0.6
0.4
0.2
0.0
-0.2
-0.4
-0.6
-0.8
(a)
LSDA - GGA
CBM
VBM
(b)
LDA +U
0.00 0.25 0.50 0.75 1.00 0.00 0.25 0.50 0.75 1.00
Si concentration x
Energy E − ε F
[eV]
G. H. Fecher, C. Felser J. Phys. D 40 (2007) 1582

200
Higher TMR using MgO Barrier
MgO(100)/Cr(40)/CFAS(30)/MgO(2)/CFAS(5)/CoFe(3)/IrMn(10)/Ta(15)
unit : nm
TMR = 174%
RT
150
Annealing at 500 °C
TMR (%)
100
50
0
-1000 -500 0 500 1000
H (Oe)
K. Inomata APL 89 (2006) 252508

Co 2 MnSi: TMR and spin polarization
Very high TMR ratios
P lower
=0.89 P upper
=0.81
Half metals
Strong temperature
dependence
Sakuraba et al. APL 89 (2006) 052508

TMR at Room Temperature
Co 2
MnSi/Al 2
O 3
/Co 2
MnSi,
580 %@2K
Sakuraba et al. APL 89 (2006) 052508

The Goal for Spinlogic 1000%
TMR at 4 K with La 0.6 Sr 0.3 MnO 3
A. Fert et al. APL
82 (2003) 233.

Half Heusler Compounds: ~250
Mn 2 MnGa:24e -

Mn 3 Ga
Mn 2 MnGa
Two magnetic sublattice
•24 Valence electrons – 0 μ B
•Mn at tetrahedral site – 4 μ B
⇒Compensated ferrimagnet

Balke et al. APL 90 (2007) 152504
Spin torque transfer application
Mn 2 MnGa
•Compensated ferrimagnet: 1μ B
•Theoretical Spinpolarisation: 88%
•Energy product: 52.5 kJ m -3 at 5K
•Curie temperature: 730 K
•Tetragonal

Spintronics
Summer school SFB 491
Claudia Felser

Content
• Concept
• Materials
• Methods
• Vision

Half metallic Ferromagnets

Covalent: MgAgAs and InAs
The half-Heusler (MgAgAs) crystal structure can be thought of as a
stuffed zinc blende, with ionic Mg 2+ stuffing a more covalent
(AgAs) 2- lattice. The (AgAs) 2- lattice is isostructural and
isoelectronic with InAs.

Semiconducting Half Heusler Compounds
Magic numbers: 8 and 18 valence-electrons
8
6
4
2
0
8
LiMgN
LiMgP
LiMgAs
LiMgBi
CePtSb
RENiSb
REAuSn
AgCdSb
…
DOS (states eV −1 cell −1 )
0
18
12
6
0
18
12
6
0
TiCoSb
VCoSn
NbCoSn
VFeSb
TiCoSb
YNiSb
Kandpal et al. J. Phys. D 39 (2006) 776

8-Electron Half-Heusler Compounds
Si LiAlSi LiMgN
(a) (b) (c)
Van Vlechten
(c) (d) (c)
Kandpal et al. J. Phys. D 39 (2006) 776

Design of a Semiconductor
Valence electrons
Si 2
2 x 4 = 8
Ga
As
3 + 5 = 8
ZnS → C1b
9 + 4 + 5 = 18
Co
Ti
Sb

Half Heusler Compounds: ~250
CoTiSb:18e -

Design of a magnetic Semiconductor ?
Valence electrons
Si 2
2 x 4 = 8
Ga
As
3 + 5 = 8
ZnS → C1b
9 + 7 + 5 = 18 +3
Co
Mn
Sb

XCoSb
Design of a magnetic Semiconductor ?
1. Obey Slater-Pauling rules
2. Most of them half-metals
3. Large exchange splitting

Design of a Magnetic Semiconductor
Valence electrons
Si
2 x 4 = 8
Ga
Mn
As
3 + 5 = 8 + x
Zinc-Blende → C1b
9 + 4 + 5 = 18 ± x
Co
Ti
Sb
Fe

Spin injection into semiconductor
Ferromagnetic Semiconductor
with T C above room
temperature is still a challenge

Semiconducting CoTiSb

For Spin Injection
XMCD-Investigation on Fe
DOS [eV -1 ]
DOS [eV -1 ]
6
4
2
0
-2
-4
-6
-8
6
4
2
0
-2
-4
-6
CoTiSb
CoTi 0.9
Fe 0.1
Sb
-8
-12 -10 -8 -6 -4 -2 0 2 4
Energy E [eV]
Kroth et al. APL 89 202509 (2006 )

Half Heusler Compounds: ~250
REPdSb:18e -
+ 4f n

Stuffed Wurtzites: ZnS
hexagonal semiconducting and half-metallic analogues
half-Heusler (MgAgAs)
Stuffed wurtzite (LiGaGe)

Structure
MgAgAs – ABC
LiGaGe – ABA
Test

Half-Heusler – LiGaGe-Type
Semiconducting Half-Heusler:
GdNiSb - MnNiSb
GdNiBi
GdPtSb
GdPtBi
LiGaGe-Type :
GdAgSn
GdPdSb
GdAuSn
GdAgPb
Gd 3+ - Mn 3+

Content
• Concept
• Materials
• Methods
• Devices
• Vision

Synthesis
computational
design
structure
nano particles
single crystals
thin films
ceramics
spectroscopy

Methods: Single crystals
Single crystal disc
-cut
- polished
Arc-melter
with Single Crystal Puller
Co 2 FeSi single crystal

Methods: Sample Preparation
Cutting
Polishing
Sample as-cast
Etching
CEMS
Target production
Relative intensity I / I 0
1.02
1.01
1.00
-8 -6 -4 -2 0 2 4 6 8
Doppler velocity v [mms -1 ]

Structural Characterisation
_
Im3m Pm3m Fm3m
Structural characterisation
by XRD not sufficient.

Heusler and the Periodic Table

Resonant XRD
1.6
_
1
(400)
(422)
(220)
0.1
(111)
(200)
(311)
(222)
(331)
(420)
0.01
Co 2
FeGa
hν = 7050 eV
Relative intensity I / Imax
1.4
1.2
1.0
(100)
(110)
Co 2
FeZ
Z =
Al (B2)
hν= 7050eV
Co 2 FeGa
0.8
0.6
0.001
1
0.1
0.01
30° 40° 50° 60° 70° 80° 90° 100°
Scattering angle 2θ
Co 2
FeGe
hν = 7050 eV
Relative intensity I / Imax
(111)
(200)
(311)
(222)
(331)
(420)
(400)
(422)
(220)
0.4
0.2
0.0
(111)
(200)
(220)
Si (L2 1
)
Ga (?)
Ge (?)
12° 15° 18° 21° 24° 27° 30° 33° 36°
Scattering angle 2θ
Co 2 FeGe
30° 40° 50° 60° 70° 80° 90° 100°
Scattering angle 2θ
Relative intensity I / Imax

EXAFS Investigation
Fe spectra
Short Fe-Co distance
Co spectra
Short Co-Fe distance
Short Co-Si distance
Co 2 FeSi crystallises in L2 1 structure
B. Balke et al. APL, accepted (2007)

Wurmehl et al. submitted APL
55
Mn-NMR: Co 2 Mn 0.5 Fe 0.5 Si
n=0 n=1 n=12
n=6
…
…
Yellow: Mn
Black: Fe
Random distribution of Mn/Fe
in third coordination shell
of 55 Mn. Each satellite is
attributed to certain numbers
of Fe atoms.
x= 0.114
x= 0.899

Mößbauer Characterisation Co 2 Cr 0.6 Fe 0.4 Al
Quelle Probe Detektor
~ 0.1 μ m
~ 1 μm
γ
γ
e -
Res. Transmission, %
100.00
99.50
99.00
98.50
98.00
-10 -5 0 5 10
4
a
Detektor
X, γ
Transmissionsgeometrie
3
2
T = 300 K
b
Rückstreugeometrie
a) Mößbauer spectroscopy in transmission
b) CEMS 57 Fe Mößbauer spectroscopy sputter
targetsCo 2 Cr 0.6 Fe 0.4 Al at RT
c) CEMS at 93 K
Res. Emission, %
0 1
1 2 3
-10 -5
T = 93 K
0
5
c
10
0
-10
-5
0
5
v, mms -1
10

High Energy Photoemission
Hard X-ray photoemission: bulk sensitive !
Electron mean free path (Angstrom)
100
10
" Universal " curve :
SXPES
Region for standard photoemission
Measured
Extrapolated
HAXPES
100
10
1
1 10 100 1000 10000
Electron kinetic energy (eV)
1

New High Energy Spectrometer in Mainz
SPECS PHOIBOS 225 HV (delivered June 2006)
Energy up to 15keV
2D detector
Angular resolved PES
Mott detector
Spin resolved PES

High Energy Photoemission

Tuning the Fermi Energy
Kandpal et al., Phys. Rev. B 73 (2006) 094422

High Energy Photoemission: HAXPES
True bulk sensitivity (≈ 40 Heusler cells)
Co 2
MnSi Co 2
Mn 0.5
Fe 0.5
Si Co 2
FeSi
15
10
hν = 8 keV
5
Intensity I [kcts]
0
-10 -5 0
4
3
2
-10 -5 0 -10 -5 0
2
3
2
1
High resolution
at ε F
(≈100 meV)
1
1
-1.5 -1.0 -0.5 0.0
-1.5 -1.0 -0.5 0.0
Energy E − ε F
[eV]
-1.5 -1.0 -0.5 0.0
Balke et al. PRB 74, 104405 (2006)
Fecher et al. J. Phys. D 40 1576 (2007)

HAXPES: Thin films
Electron mean free path λ [Å]
70
60
50
40
30
20
10
Co 2
MnSi
Experiment
Calculated
0
2000 2500 3000 3500 4000 4500 5000 5500 6000
Electron energy E kin
[eV]
1nm Pt
1 - 10 nm
Co 2
MnSi
First Results from
BESSY KMC-1
Electron Mean Free Path
Ta

HAXPES: CoTi 1-x Fe x Sb
Normalised intensity I / I max
1.0
0.8
0.6
0.4
0.2
hv = 2.5keV
CoTi 0.9
Fe 0.1
Sb
CoTiSb
2
CoTi 0.9
Fe 0.1
Sb
0.0
-14 -12 -10 -8 -6 -4 -2 0 2
Energy E − ε F
[eV]
DOS [eV -1 ]
0
-4 -2 0 2
Energy E [eV]
Fecher et al. JESRP accepted (2007)

1905
1983
2001
Functions of Heusler Compounds
• Magnetic material: Cu 2 MnAl
• Halfmetallic ferromagnet: NiMnSb
• Magneto-optical: PtMnSb
• Magneto-mechanic: Ni 2 MnGa
• Superconductor: Pd 2 YSn
Semiconductor: CoTiSb
• Heavy fermion:
Fe 2 VAl
• Li-conductor:
LiMnSb
• Magneto-electronic: Co 2 FeSi
• Thermo-electric: TiNiSn
• Magneto-caloric: CoMnSb:Nb

thermoelectrica
composite
4MBit MRAM
(Motorola 2003)
material
power
generation
cooling
Ni 2
ZrGa
Co 2
ZrGa
Ni 2
ZrGa
solar cells
multi-ferroics
magneto electronics

Heusler Compounds: ~500
Ni 2 ZrGa:27e -

New Superconductors
For superconductor-ferromagnet devices:
Ni 2 ZrGa – Co 2 ZrGa
Keitzer et al. Nature 439 (2006) 825
Winterlik et al. In preparation

Multiferroic Materials: Co 2 XY and BaTiO 3
Ferromagnetic pillars in
ferroelectric matrix
Multiferroic tunnel junctions
M
M
P
P
M
a few nm
Fully polarized
BaTiO 3
Co 2 MnSi
Zheng et al., Science 303, 661 (2004)
Picozzi, private communication

Hybrid materials
Spin injection into organic semiconductor
Magnetic Nanoparticle
Hybrid materials
•Heusler
•Organic or biological materials
Co 2
FeSi
OSC
GMR in π-conjugated organic
semiconductor barrier
N
ΔR/R of 40% at 11K
Xiong et al: Nature 427, 821 (2004)

X 2 YZ
XYXZ