R - AGH

Nanoelectronics
Magnetoelectronics
Tomasz Stobiecki
AGH Katedra Elektroniki
Semestr letni 2009
HDD for 50 years and now
First Hard Disk Drive with 24" Diameter Disks Compared with Modern 2.5" HDD. The first HDD was
introduced in 1956 with 50 disks of 24" diameter holding a total of 4.4 Mbytes of data. The purchase price of
this HDD was $10,000,000 per Gbyte. For comparison in the foreground a modern HDD is shown holding 160
Gbyte of data on two 2.5" diameter disks at a purchase price of less than $1 per Gbyte.
Miniaturyzacja
Areal data storage density vs. time for
inductive and MR read heads
Disc drive
The slider carrying the magnetic
write/read head. The slider is
mounted on the end of head
gimbal assembly (HGA)
The magnetic disks (up to 10) in
diameter 1 – 5.25 inches. 5.400 –
15.000 RPM it is related to about
100 km/h
The air-bearing surface (ABS)
allowing the head to fly at a distance
above the medium about 10 nm
Schematic representation of a longitudinal recording
process
Magnetic force micrograph (MFM)of
recorded bit patterns. Track width is
350 nm recorded in
antiferromagnetic coupled layers
(AFC media)
Historia spintroniki
1986 – oscillatory interlayer exchange coupling (IEC) in Fe/Cr/Fe multilayers
P. Grünberg et al. Phys. Rev.Lett. 57 (1986), 2442
1988 – Giant Magnetoresistance (GMR) in Fe/Cr/Fe multilayers
M. N. Baibich,..., A.Fert,.. et.al. Phys. Rev.Lett. 61 (1988), 2472
1991 – Spin Valve (SV) in NiFe/Cu/NiFe/FeMn
B. Dieny, et al. Phys. Rev.B (1991)
1995 – Tunnel Magnetoresistance (TMR =15%) in CoFe/Al2O3/Co
J.S. Moodera, et al. Phys.Rev.Lett, 74 (1995)
2004 – Giant TMR at room temperature with MgO(100) barrier; TMR
=220% CoFe/MgO/CoFe
S.S.P. Parkin et al.- Nature vol.3 December (2004), 86
2006 – Hayakawa et al.- APL 89 (2006),23 2510; TMR =472%
CoFeB/MgO/CoFeB
Interlayer Exchange Coupling (IEC)
Structure of Fe film/ Cr wedge/ Fe whisker illustrating the Cr thickness dependence of Fe-Fe exchange. Above, SEMPA image of domain pattern generated from top Fe film. (J. Unguris et al., PRL 67(1991)140.)
Thickness dependnce of Cu spacer
For ferromagnetic 3d metals D↑(EF) ≠ D↓(EF) ⇒
ρ↑ ≠ ρ↓
GMR ⇒ due scattering into the empty quantum states above the Fermi level ⇒ ρ ∝D(EF)
Spinowo zależne przewodnictwo elektryczne
Analogia do równoległego połączenia dwóch rezystancji
M
I
R duże
M
I
R małe
Spin Polarization, Density of States (DOS)
Density of states 3d
Spin Polarization
Normal metal (Cu)
Ferromagnetic metal (Fe)
↑
P =
n −n
↑
n +n
↓
E
E
EF
↓
N
n
EF
n
n
DOS
Material Polarizations
Ni 33 %
Co 42 %
Fe 45 %
Ni80 Fe20 48 %
Majority Spin Minority Spin
Majority Spin
n
DOS
Minority Spin
n↑ ( EF ) > n↓ ( EF ) n ↑ ( E F ) = n ↓ ( E F )
ρ↑ = ρ↓
ρ↑ > ρ↓
Co84 Fe16 55 %
CoFeB 60%
GMR ⇒ due scattering into the empty quantum states
above the Fermi level ⇒ ρ ∝D(EF)
Zasada działania zaworu spinowego
(Spin-Valve) w głowicy twardego dysku
AFM: FeMn, NiO, NiMn, IrMn
USignal
FM: Co, Fe, NiFe, CoFe
NM: Cu, Ag, Au
AFM
Warstwa mocująca
(pin-layer)
10
I = const
warstwa swobodna
10 zamocowana 1(free-layer)
0
warstwa
(pinned-layer)
kierunek ruchu nośnika informacji
SV – charakterystyki magnetorezystancyjne
Zależność rezystancji od wzajemnego położenia wektorów namagnesowania:
R↑↑+ ΔR
FeMn/Ni80Fe20/Cu/Ni80Fe20
R = R↑↑ +
R↓↑ − R↑↑
2
[1− cos(θ −θ )]
1
2
R [a.u.]
Antysymetryczna charakterystyka
M(H) zakresie małych pól
Duża czułość magnetorezystancyjna
SR =
ΔR
R↑↑
HEB
-1,5
-1,0
∂
(
R0 − RS
RS
∂H
R ↑↑ ≈ 16 %
) ⋅ 100%
SR ≈8%/Oe
HF
-0,5
Field [a.u.]
0,0
0,5
M.Czapkiewicz – praca doktorska (1999)
Write/read head of HDD
GMR & TMR- as read head
GMR & TMR effect can be described as a change of resistance in respect to the
angles Θ between magnetizations M of adjacent ferromagnetic layers
R = R↑↑ +
R↓↑ − R↑↑
2
[1− cos(θ −θ )]
1
2
Disk layer structure
Thin film disks
Substrate – Al Mg (or glass) + electroplated Ni80P20
(Tc<Troom). NiP undercoat layer make disk hard and smooth.
Cr underlayer is used to control microstructure and magnetic
properties the main magnetic recording layer of CoPtCr
doped with B. The magnetic layer is covered by a carbon
overcoat layer and lubricant. The last two layers are
necessary for the tribological performance of the head-disk
interface and for the protection of the magnetic layer.
Microscopic properties
Coercivity Hc - control and modification:
• magnetocrystalline anisotropy (grain shape anisotropy),
•selection of alloying elements (Al, Cr, Pt, Ta, B,...)
•determination of influence:
•deposition conditions and parameters: substrate temperature, bias
voltage, sputtering power (deposition rate), sputtering gas pressure
(Ar)
•microstructure: film stresses, grain size, texture (grain orientation),
grain boundaries, crystal defects.
If the grain structure is noticably voided, leading to reduced magnetic intractions and
lower transition noise.
Thermal stability
For high density recording the grains are small in comaprison to the bit cell. In a simplified model,
assuming isolated grains, the thermally induced switching of magnetization has to overcome an
energy barier. The switching probability f is given by an Arrhenius equation:
⎛ ΔW ⎞
f = f 0 exp ⎜ −
⎟
kT ⎠
⎝
where ΔW = KuV
(6)
ΔW is energy barier, Ku is the uniaxial anisotropy constant, V is grain volume. If the grains
become very small, the magnetization switch very easily which leads to superparamagnetic
efect.
Estimation of minimum grain size (example):
Ku=2×105 J/m3. Bit stored 10 years at room temperature
(f<3.33×10-9Hz at T=300 K), than diameter of spherical grain is 9 nm.
Granular media vs. patterned media
Antiferromagnetic – coupled (AFC) media
A precise control of the Ru thickness allows to establish an anti-parallel
(antiferromagnetic coupling) between two ferromagnetic layers. Decreasing the Mrδ
in AFC recording media leads to shrap transition, small grains and good S/N.
Mrδ (eff)= Mrδ (top) - Mrδ (bottom)
Storage density of AFC media >25Gbit/in2.
(6)
Longitudal recording vs. perpendicular
Perpendicular Recording
Schematic of the perpendicular recording scheme. The soft underlayer in the
medium acts as an efficient write field flux path and effectively becomes part
of the write head. The transmission electron micrograph (top right) shows a
cross-section of a prototype perpendicular recording head used in a recent
laboratory demo of 150 Gbit/in2 area recording density.
SV-MTJ Based Read Heads
SV-MTJ as a read sensor for high density (> 100Gb/in2) must fulfill requirements
- Resistance area product (RxA) < 6 Ω-μm2
- High TMR at low RxA
2006 – New world record of TMR
472% Anelva & Advanced Industrial Science and Technology
(AIST), Japan
128 Mbit ⇒ 370 mV
Tunneling in FM/I/FM junction
Material Polarizations
FM I (PI)
Ni 33 %
Co 42 %
Fe 45 %
FM II (PII)
E
E
E
EF
N
n
eV
n
EE
F F
N
nn
Barrier
Ni80 Fe20 48 %
n
N
nII↑ − nII↓
PII = ↑
nII + nII↓
n
Co84 Fe16 55 %
CoFeB 60%
DOS
Majority Spin Minority Spin
n I↑ − n I↓
↑ ↑
↓ ↓
PI = ↑
I
∝
n
n
+
n
I II
I n II
n I + n I↓ M ↑↑
↑
I
I↑
↓
I
TMR =
I M ↑↑ − I M ↑↓
2 PI PII
=
I M ↑↓
1 − PI PII
TMR =
I↓
I↑
↓
II
I M ↑↓ ∝ n n + n n
↑
II
DOS
DOS
Majority Spin Minority
Minority Spin
Spin
R↑↓ − R↑↑
R↑↑
I↓
Our results
Motivation
•How to optimize the multilayers structure of MTJ in
order to obtain desirable tunnelling and magnetic
parametrs?
Structure analysis
•Texture
•Interface roughness
•Correlations between microstructure exchange
coupling and tunnelling parameters of IrMn based
MTJs.
Conclusions
MTJ systems for electrical measurements
100×100 μm
10 mm
TIMARIS: Tool status
Tool #1 – process optimization on ∅200 mm wafers
since mid of March 03
Tool #2 – The Worlds 1st ∅300 mm MRAM System is
Ready for Process in August 03
Clean room
Multi (10) Target
Module
Oxidation /
Pre-clean Module
Transport Module
Sputtering System Tohoku
LL : wafer-in
Plasma
Oxidation
LL :
Bridge
Metal
depo.
Reactive
sputter :
surface smooth
Sputtering system Uni Bielefeld
Magnetic Random Access Memory (M-RAM)
current conductors
non-magnetic spacer
ferromagnet
antiferromagnet
0
1
≈ 150 nm
Ra - high
Rp - low
M-RAM fabrication compatible to CMOS technology
SV-MTJ Based Spin Logic Gates
(+, − ) IA
RM TJ3
NAND
RM TJ 4
MTJ 1
SV-MTJs
Programing Inputs
RM TJ2
IS
MTJ 2
NOR
MTJ 1
MTJ 2
2 VOUT
IS
VO UT
Logic Output
Logic Output
RM TJ1
VOUT= IS(RMTJ3 + RMTJ3 – RMTJ1 – RMTJ2)
Logic Inputs
(+, − ) IB
„1"
0
„0"
-2 VOUT
SV- MTJ as spin logic gates must fulfill
requirements
-
Thermal stability
Magnetic stability
Centered minor loop
Single domain like switching behaviour
Reproducibility of R and TMR
Siemens & Univ. Bielefeld: R. Richter et al. J. Magn.Magn. Mat. 240 (2002) 127–129
(0,0)
(0,1) (1,0) (1,1) (0,0) (0,1) (1,0) (1,1)
Logic Inputs MTJ 3, MTJ 4
Infineon and IBM Present World´s First 16 Mbit MRAM - Innovative
Chip Design Results in Highest Density Reported to Date
The increasing number of mobile applications such as smartphones and
notebooks with additional multimedia features results in the need for more
advanced memory chips.
MRAM is a promising candidate for universal memory in high
performance and mobile computing as it is faster and consumes less
power than existing technologies.
A new class of device based on the quantum of electron
spin, rather than on charge, may yield the next generation of microelectronics.
Pamięć Parkina
DW – przemieszczane impulsami prądu.
Porównanie ruchu dwóch DW pod wpływem impulsu pola i prądu
Flux of information
Information
Information
Information
transmission
Processing
storage
Outside
word
Input
Information
Output
DRAM, MRAM
Magnetic(HDD)
Optical (CD, DVD)
MAGNETOELECTRONICS
SPIN ENGINEERING
SPINTRONICS
Schedule
•Lecture 1 - Fundamentals of magnetism
•Lecture 2 - Spin depend electron transport: AMR, GMR
•Lecture 3 - HDD
•Lecture 4 - Spin depend electron transport: TMR
•Lecture 5 - MRAM
•Lecture 6 - Biosensor, Magnetic wireless actuator for medical
applications
•Lecture 7 – Millipede
Lecture 1
Fundamentals of Magnetism
Definitions of magnetic fields
(
r
r r
Induction: B = μ0 H + M
External magnetic field:
)
→
H
average magnetic moment of
M magnetic material
→
Magnetization
χ
Susceptibility
→
tensor representing anisotropic material
→
M =χH
→
→
B = μ 0 H (χ + 1) = μ H
where:
μ = μ0 (1 + χ )
permability of the material
Maxwell’s equations
r r
r
∇ o B = divB = 0
r r
r r
∇ × H = rotH = j
r r
∫ H o dl = i
[oe]
H=
l
r
r r
r
∂B
∇ × E = rotE = −
∂t
r r
∂ r r
∂φ
o
E
d
l
=
−
B
d
s
=
−
=U
o
∫
∫
∂t S
∂t
i
2πr
[A/m]
[oe]
H=
iN
l
[A/m]
Demagnetization field
poles density, magnetic „charge” density
r
r
⎛ B − μ0 M
∇o ⎜⎜
⎝ μ0
→
→ →
⎞
⎟ = − ∇o M = ρ m
⎟
⎠
r
r
Hd = −NM
Demagnetization field
when magnetic materials becomes magnetized by application of
external magnetic field, it reacts by generating an opposing field.
To compute the demagnetization field, the magnetization at all points must
be known.
r r
⎛ dMx dMy dMz ⎞
⎟⎟
+
+
ρm = −∇ o M = −⎜⎜
dy
dz ⎠
⎝ dx
[emu/cm4]
The magnetic field caused by magnetic poles can be obtained
from:
4 πρ dV
dH =
r2
The fields points radially out from the positive or
north poles of long line. The s is the pole strength
per unit length [emu/cm2]
H = 0.2s / r
[oe= emu/cm3]
Demagnetization tensor N
For ellipsoids, the demagnetization tensor is the same at all the points within the
given body. The demagnetizing tensors for three cases are shown below:
xx
yx
xy
yy
xz
yz
zx
zy
zz
0 0 0
0 0 0
0 0 4π
4π / 3
0
0
0
4π / 3
0
0
0
4π / 3
2π
0
0
2π
0
0
0
0
0
The flat plate has no demagnetization within its x-y plane but shows a 4π
demagnetizing factor on magnetization components out of plane. A sphere shows
a 4/3 π factor in all directions. A long cylinder has no demagnetization along its
axis, but shows 2π in the x and y directions of its cross sections.
H total = H S − H D
HS - the solenoid field
(4π)
Exchange coupling
The saturation of magnetization MS for body-centered cubic Fe crystal can
be calculated if lattice constant a=2.86 Å and two iron atoms per unit cell.
2.2 μ B
3
M S (T = 0) =
=
1700
emu
/
cm
(2.86 × 10 −8 )3
2
Electron Spin
The magnetic moment of spining electron is called the Bohr magneton
eh
μB =
= 0.93 × 10 − 20 emu
4πm
3d shells of Fe are unfilled and have uncompensated electron spin magnetic
moments
when Fe atoms condense to form a solid-state metallic crystal, the electronic
distribution (density of states), changes. Whereas the isolated atom has 3d:
5+, 1-; 4s:1+, 1-, in the solid state the distribution becomes 3d: 4.8+, 2.6-; 4s:
0.3+,0.3-. Uncompensated spin magnetic moment of Fe is 2.2 μB .
Electron spin
Orbital momentum
r r r
L=r×p
Magnetic moment of electron
L = rmv = r 2 mω
μL = i ⋅ S =
2π
ω=
T
r
L
μL
μL
r
r
i
L
r
p
=
μL =
e
2m
e 2
πr
T
eπr 2ω
μL =
2π
L=
eh
l (l + 1)
4πm
h
2π
l (l + 1)
Electron spin
Spin polarization of ferrmagnets
Energy
Energy
Energia
Magnetization
EF
d
d
d
Density of states
Spin
s
s
s
Spin Polarization, Density of States (DOS)
Density of states 3d
Spin Polarization
Normal metal (Cu)
Ferromagnetic metal (Fe)
↑
P =
n −n
↑
n +n
↓
E
E
EF
↓
N
n
EF
n
n
DOS
Material Polarizations
Ni 33 %
Co 42 %
Fe 45 %
Ni80 Fe20 48 %
Majority Spin Minority Spin
Majority Spin
n
DOS
Minority Spin
n↑ ( EF ) > n↓ ( EF ) n ↑ ( E F ) = n ↓ ( E F )
ρ↑ = ρ↓
ρ↑ > ρ↓
Co84 Fe16 55 %
CoFeB 60%
GMR ⇒ due scattering into the empty quantum states
above the Fermi level ⇒ ρ ∝D(EF)
„A new class of device based on the
quantum of elctron spin, rather than on
charge, may yield the next generation of
microelectronics.”