fept nanomaterials for future magnetic data storage · coherent rotation-stoner wohlfarth model 0...
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Lake Ontario
Lake Erie
FePt Nanomaterials for FePt Nanomaterials for Future Magnetic Data Future Magnetic Data
Storage Storage
Hao ZengHao Zeng
Department of Physics, Department of Physics, University at BuffaloUniversity at Buffalo--SUNYSUNY
Summer School of Advanced Functional Materials 2006Shenyang, China, July 6, 2006
What’s Enabling Google Earth? 200 TB of Hard Disk Drives
92% of new information stored on magnetic media!
“Moore's Law” for Data Storage
?
Hard Disk Drive Technology
SNR ∝ 10log(N)
S S S SSSSN N N N NNN
Coherent Rotation-Stoner Wohlfarth Model
0 50 100 150 200-2.5
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
2.0
2.5
E
θ
ϕ =180°
EB
)cos(sin2 ϑϕϑ −−= VHMKVE s
Easy axis
H
θϕ
H
Ms
Setting EB =0,One get Hc = 2K/Ms
B.D. Cullity, Introduction to Magnetism and Magnetic Materials
Superparamagnetism
KV >> kTMs
Ms
Hc
MrFM
KV < 25kT
Ms
Ms
SP
1/τ = f0e -∆E/kT
Magnetic reversal is thermally activated,the probability of crossing over the energy barrier
∆E=kTln(f0 τ)
f0 –attempting frequency ~ 109 Hz, τ –measurement time ~ 100 s ∆E=25 kT
])25(1[2 2/1
VKTk
MKH
u
B
s
uc −=
u
BB
u
BB
KTkV
KTkV
60
,25
' =
= 100 s
10 year
Fighting SP Limit-Advanced Recording Schemes
AFC media
Ru
SNR∝Mrt∝(d1-d2)
Veff∝(d1+d2)
Perpendicular recording
Pack more data-same volume with smaller bit size
Stabilizes stored bits
High Ku Media Materials
D. Weller et al., IEEE Trans. Magn. vol 36, No 1, January 2000, p. 10-15
FePt –The Hardest Magnet
5 nm, 4.5 K
T. Shima et al., Appl. Phys. Lett. 85, 2571 (2004)
Thermally Assisted Recording
Lower the energy barrier for reversal by laser heating
PRB 72, 172410, 2005
Origin of Magnetocrystalline Anisotropy of FePt
Origin of anisotropy is spin-orbit coupling
Fe has relatively moderate spin-orbit interactions. MAE is very small in the bulk Fe.
Pt has very large spin-orbit coupling constant, but has no magnetic moment in pure form.
FePt gets the best of two worlds: Fe has large magnetic moment, and induces sizable magnetic moment on Pt (0.4µB). Pt provides spin-orbit interactions. Because of these the anisotropy is not single ion in origin and depends on the interaction between Fe and Pt.
Anisotropy Calculations
∑
∫=
∆+=
−=∆
∞−
−
ijij
E
easye
hardeanis
E
TtdETr
EEEF
]1[Im10
1
π
c
a
t∆ Is SO perturbation and T0 is a scattering T-matrix describing interactions
L10 FePt Thin Films for Magnetic Recording Media
R. F. C. Farrow, D. Weller, R. F. Marks, and M. F. Toney, JAP 79, 5967 (1996).
Texture achieved by MBE growth on MgO single crystal substrate
Achieving Texture Non-epitaxially
Fe55Pt45 annealed at 550 °C for 5 sec
• d⊥ ≈ 4. 5 nm
• d⊥ ≈ 6.3 nm
• d⊥ ≈ 6.7 nm
H. Zeng, M. L. Yan, N. Powers et al., Appl Phys Lett 80 (13), 2350-2352 (2002).
Three Stages in Texture Evolution
20 30 40 50 600
500
1000
1500
2000
2500
Inte
nsity
(a.u
.)
(111) (d)
2θ
0
200
400
600
800
1000
(002)(200)(001)
(111) (c)
0
100
200
300(002)(200)
(001)
(111)
(111)
(b)
0
50
100
150
200
250
(002)(200)
(001)(a)
t
Typical Hysteresis Loops
-15 -10 -5 0 5 10 15-0 .15
-0 .10
-0 .05
0 .00
0 .05
0 .10
0 .15
H (kO e)
-0 .10
-0 .05
0 .00
0 .05
0 .10
m (m
emu)
Top: 100 Å Fe55Pt45, 550 °C, 5 sec
Bottom: 100 Å Co55Pt45, 700 °C, 300 sec
//
//
⊥
⊥
FePt-based Nanocomposite Films
-1200
-800
-400
0
400
800
1200
(a)
H (kOe)H (kOe)
out of plane in plane
M (e
mu/
cc)
(b)
-40 -20 0 20 40
-600
-300
0
300
600 (c)
-40 -20 0 20 40
(d)
(a) FePt single layer; (b) (FePt 32Å/B2O38Å)5; (c) (FePt 32Å/B2O3 12Å)5; and (d) (FePt 32Å/B2O3 48Å)5. These films were annealed at 550°C for 30 minutes.
(a) TEM image of (FePt 32Å/B2O3 12Å)5 and (b) HR-TEM image of (FePt 32Å/B2O3 20Å)5 annealed at 550°C for 30 minutes
C.P. Luo, Ph.D. dissertation
Hysteresis loops as a function of B2O3 thickness
Concept of Patterned Media
• x
“Single-domain magnetic pillar array of 35 nm diameter and 65 Gbits/in2
density for ultrahigh density quantum magnetic storage”S.Y. Chou, M.S. Wei, PR Krauss, PB Fischer JAP 76, 6673 (1994)
single grain per bit (elimination of random √N noise)superparamagnetic limit applicable to a single bit, not to each grains within a multigrain bitdensity/cost limited by lithographic technology
Building Patterned Media Bottom-up — SOMA
4 nm FePt
S. Sun et al., Science 287, 1989 (2000).
FePt Nanoparticle Synthesis
FeCO
CO
CO
CO
OC
O
O
CH3
CH3
O
O
H3C
H3C
Pt
- COHeat,
FePt
COOH
NH2
reduction
FePt Nanocrystal Recording Tests(A. Moser, D. Weller)
0 5 1 0 1 5 2 0 2 5- 2 0- 1 8- 1 6- 1 4- 1 2- 1 0
- 8- 6- 4- 2024
MR
signa
l [m
V]
x [µ m ]
1040 fc/mm
2140 fc/mm
5000 fc/mm
500 fc/mm
Reading
4 nm ferromagneticFePt particle assembly
Writing
Phase Transformation & Aggregation
25nm
(a)
25nm1nm
(b)
600 °C
Fe
Pt
fctHigh Ku
fccLow Ku
Aggregation and Magnetic Properties
-1.0
-0.5
0.0
0.5
1.0
-1.0
-0.5
0.0
0.5
1.0
-60 -40 -20 0 20 40 60
-1.0
-0.5
0.0
0.5
1.0
(a)
(b)
(c)
H (kOe)
0 5 10 15 20 25 30 35 40 45 50
0.0
0.5
1.0癈 700 60 min FG癈 600 60 min FG癈 550 60 min FG
δM
H (kOe)
H. Zeng, et al, APL, 80, 2583(2002).
600 °C, moderate aggregation
interparticle interactions550 °C, little aggregation
700 °C, severe aggregation
L10 ordered FePt by RTA
Temperature for the onset of chemical ordering significantly loweredHowever, aggregation is not solved by reduction of ordering temp.
H. Zeng , Shouheng Sun, R. L. Sandstrom and C. B. Murray, JMMM 266, 227 (2003).
Discrete fct Ordered FePt Nanoparticle Arrays
-40000 -20000 0 20000 40000
-1.0
-0.5
0.0
0.5
1.0
m (a
rb. u
nit)
H (Oe)
8 nm, 560 °C 30 min N2
• highly ordered• nonexchange-coupled• thermally stable
H. Zeng et al, unpublished
Size and Surface Effects of FePt NPs
⎟⎠⎞
⎜⎝⎛ −∞=
dtMdM ss
61)()( (unpublished)
Both isotropic exchange (single ion) and anisotropic exchange (two-ion) stabilize ferromagnetic order!
(present theory does not explain the drastic decrease in Ms with decreasing size!)
Mossbauer Spectra of FePt NPs
F 298K652000656000660000664000668000 FePt - 298K - 4nm
645000650000655000660000665000 FePt - 78K - 4nm
162000164000166000168000170000 FePt - 4.2K - 4nm
-10 -5 0 5 10750000800000850000900000950000
1000000 Fe - 298K
Velocity(mm/s)
2000025000300003500040000 FePt - 298K - 8nm
740000
750000
760000
770000FePt, 78K, 8nm
Hhf (T) Qs (mm/s) Is (mm/s)4 nm 27.5 0.29 0.208 nm 27.5 0.26 0.20Fe 33
H. Zeng, unpublished results
Curie Temperature
v
c
cc
dd
TdTT
/1
0)()()(
−
⎟⎟⎠
⎞⎜⎜⎝
⎛=
∞−∞
No phase transition below 3 nm!
How to Make a Perfect Patterned Media
Magnetic easy axis
High orderingHigh densityAligned easy axisCorrect symmetry
Magnetic Nanodot Array in Self-Assembled Porous Template
Thickness Tunable Porous Templates
Co Nanodot Arrays
Magnetic Properties of Co Nanodots vs. Films
-2000-1500-1000 -500 0 500 1000 1500 2000-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
m (m
emu)
H (Oe)
10 K 300 K
-2000-1500-1000 -500 0 500 1000 1500 2000
-0.10
-0.05
0.00
0.05
0.10
m (m
emu)
H (Oe)
10 K 300 K
Dot Array Film
FePt Nanodots
001 Texture and Perpendicular Anisotropy Achieved
2 0 4 0 6 0
Inte
nsity
(a.u
.)
2 θ (d e g .)
5 5 0 , g la s s s u b s tra te
G 6
G 5
G 4
(001) (002)
-20000 0 20000
-0.00008
-0.00006
-0.00004
-0.00002
0.00000
0.00002
0.00004
0.00006
0.00008
m (e
mu)
H (Oe)
⊥
//
Future Work
Templates with smaller diameter and Higher Density
20 nm pitch would lead to 250 Gb/cm2 (or 1.6 Tb/in2)10 nm pitch – 6.4 Tb/in2
Dot array with the “correct” symmetry
Alternative technology-MTJ MRAM Architecture
Potential Advantages:Non-volatile high density memory (∼ DRAM)Short access time (∼ SRAM)Low power consumption
Reading a bitWriting bits“0” “1”
-+
+
+ + ++++
Courtesy of Arunava Gupta, U. Alabama
Domain Wall Memory
Write head read head
Iin Iout
Density comparable to HDDNo moving partsCompatible with Si technology
Acknowledgment
Chaeyun Kim, Bi-ching Shih-UBYucheng Sui, D.J. Sellmyer-UNLShouheng Sun-Brown U., Ping Liu, UTAGanping Ju-SeagateArunava Gupta-U Alabama
NSF and IRCAF for financial support
41
Research
Department of Physics: Taxonomy
2005-2006 2122
1190706
Distinguished professors Full professors
Associate professorsAssistant professorsGraduate students
Physics MajorsStaff positions
2006-20072131
13110756