1 K. Baberschke FU Berlin „Lectures on magnetism“ #2, Fudan Univ. Shanghai, Oct. 2005 1. Magnetic anisotropy energy = f(T) 2. Anisotropic magnetic moment ≠ f(T) Lecture 2: Magnetic Anisotropy Energy (MAE) ≈ 1μeV/atom is very small compared to ≈ 10 eV/atom total energy but all important Characteristic energies of metallic ferromagnets binding energy 1 - 10 eV/atom exchange energy 10 - 10 3 meV/atom cubic MAE (Ni) 0.2 μeV/atom uniaxial MAE (Co) 70 μeV/atom T=300 K B (G) 100 100 200 0 0 500 300 300 M (G) [111] [100] Ni [100] [111] 0.1 G ∆μ L ~ ~
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1K. Baberschke FU Berlin „Lectures on magnetism“ #2, Fudan Univ. Shanghai, Oct. 2005
1. Magnetic anisotropy energy = f(T)2. Anisotropic magnetic moment ≠ f(T)
Lecture 2: Magnetic Anisotropy Energy (MAE)
≈ 1µeV/atom is very small compared to≈ 10 eV/atom total energy but all important
Characteristic energies of metallic ferromagnets
binding energy 1 - 10 eV/atom
exchange energy 10 - 103 meV/atom
cubic MAE (Ni) 0.2 µeV/atom
uniaxial MAE (Co) 70 µeV/atom
T=300 K
B (G)
100
100 20000
500
300
300
M (G
)
[111]
[100]
Ni[100]
[111]
0.1 G∆µL ~~
2K. Baberschke FU Berlin „Lectures on magnetism“ #2, Fudan Univ. Shanghai, Oct. 2005
There are only 2 origins for MAE: 1) dipol-dipol interaction ∼ (µ1• r)(µ2• r) and2) spin-orbit coupling ? L S (intrinsic K or ∆Eband)
Structural changes by ≈ 0.05 Å increase MAE
by 2-3 orders of magnitude (~0.2→100µeV/atom)
O. Hjortstam, K. B. et al. PRB 55, 15026 (’97)
R. Wu et al. JMMM 170, 103 (’97)
3K. Baberschke FU Berlin „Lectures on magnetism“ #2, Fudan Univ. Shanghai, Oct. 2005
4K. Baberschke FU Berlin „Lectures on magnetism“ #2, Fudan Univ. Shanghai, Oct. 2005
13K. Baberschke FU Berlin „Lectures on magnetism“ #2, Fudan Univ. Shanghai, Oct. 2005
14K. Baberschke FU Berlin „Lectures on magnetism“ #2, Fudan Univ. Shanghai, Oct. 2005
O. Hjortstam, K. B. et al. PRB 55, 15026 (’97)
2b ab initio calculations
anisotropic µL ↔ MAE
ξLSMAE ∝ ∆µL4µB
15K. Baberschke FU Berlin „Lectures on magnetism“ #2, Fudan Univ. Shanghai, Oct. 2005
The surface and interface MAE are certainlylarge (L. Néel, 1954) but count only for onelayer each. The inner part (volume) of a nano-structure will overcome this, because theycount for n-2 layers.
C. Uiberacker et al.,PRL 82, 1289 (1999)
SP-KKR calculation for rigit fcc and relaxed fct structures
R. Hammerling et al.,PRB 68, 092406 (2003)
layer resolved ∆Eb=ΣKi at T=0
16K. Baberschke FU Berlin „Lectures on magnetism“ #2, Fudan Univ. Shanghai, Oct. 2005
(a)
(b)
(c)
(a)
(b)
(c)
0 1
-2
2
[110]
[001]
[100] ||
⊥
K 4 ||
/2 π M 2
K 2 /2 π M 2
1
-1
1 [001]
Ni(111)/rough W(110) Ni(001)/Cu(001)
||
⊥
4 π K
⊥ /2 M 2
/2 π K 2
M 2 a)
b)
M. Farle et al., PRB 55, 3708 (1997)
A. Berghaus, M. Farle, Yi Li, K. BaberschkeAbsolute determ. of the mag. anisotropy of ultrathinGd and Ni/W(110).Second Intern. Workshop on the Magnetic Properties of Low-Dimensional Systems.San Luis Potosi, Mexico, Proc. in Physics 50, 61 (1989)
Only with K4 ≠ 0a continues SRT is possible!
Do not use Keff = 2πM2 – Ki …because f(T) and f’(T) are different.Use the ratio Ki / 2πM2 ⇒ f(T) / f’(T)
17K. Baberschke FU Berlin „Lectures on magnetism“ #2, Fudan Univ. Shanghai, Oct. 2005
Oxygen surfactant assisted growth: a new procedure to prepare ferromagnetic ultrathin films
• oxygen acts as surfactant for Fe, Co and Ni films on Cu(100)
• change of magnetic anisotropy by surfactant
• induced magnetic moment of surfactant
18K. Baberschke FU Berlin „Lectures on magnetism“ #2, Fudan Univ. Shanghai, Oct. 2005
C. Sorg et al., Surf. Sci. 565, 197-205 (2004)M. Farle, Surf. Sci. Perspectives 575, 1-2 (2005)
Improved growth by oxygen surfactant
19K. Baberschke FU Berlin „Lectures on magnetism“ #2, Fudan Univ. Shanghai, Oct. 2005
evaporate 5.5 ML Ni
O(√2 × 2√2)R45°/Cu(100)missing row reconstruction
47 eV
0 2 40
2
4
nm
nm[0
10]
[100]
top layer
bottom layermiddle layer
oxygen
0 2 4nm
0
1.5
3
nm
U = 0.110 VI = 1.55 nA
[010
]
[100]
oxygentop layerbottom layer
0 2.5 5nm
0
2
4
nm
U = 0.110 VI = 0.65 nA
c(2 × 2)O/Ni/Cu(100)
R. Nünthel et al., Surf. Sci. 531, 53-67 (2003)
from AES ⇒ oxygen floats on top of Ni film
[010
]
[001]
20K. Baberschke FU Berlin „Lectures on magnetism“ #2, Fudan Univ. Shanghai, Oct. 2005
Local structure: Surface EXAFS Ni on O/Cu(100)
Rnn= (1.85±0.03) Å
h = 0.41 Å
Comparison to theory(T.S. Rahman):
Rnn= 1.87 Å
h = 0.44 Å
R. Nünthel et al., Surf. Sci. 531, 53-67 (2003)
oxygen K-edge SEXAFS
300 K hν
21K. Baberschke FU Berlin „Lectures on magnetism“ #2, Fudan Univ. Shanghai, Oct. 2005
NEXAFS ⇒ no bulklike NiO is formed
Ni L2,3-edge O K-edge
Electronic structure from X-ray absorption spectroscopy
2pz3d
2pxy4sp
22K. Baberschke FU Berlin „Lectures on magnetism“ #2, Fudan Univ. Shanghai, Oct. 2005
K filmV
0 0.05 0.10
10d(ML) 57
0.15 0.20-20
-15
-10
-5
0
5
10
15
20
25
Ni/Cu(001)
Ni vacuum
Ni + Cu capNi + O surfactant
t=0.75
2 Mπ 2
1/d (1/ML)
K (
µeV
/ato
m)
2
K bulkV
≈ 0
10.8
7.6
7.3
6.8
4.9
-107
-59
-81
-70
-17
Ni/vacuum
Ni/Cu
Ni/CO (van Dijken et al.)
(van Dijken et al.)
(surfactant)Ni/O
Ni/H2
Interface KS ( eV/atom)µ d C (ML)
ExperimentTheory
Jisang Hong et al., Phys. Rev. Lett. 92, 147202-1 (2004)
( ) θπ 22
2 cos2~ ⊥− KMFd
KKKK
SSV
21 ++=
23K. Baberschke FU Berlin „Lectures on magnetism“ #2, Fudan Univ. Shanghai, Oct. 2005Jisang Hong et al., Phys. Rev. Lett. 92, 147202-1 (2004)
E (eV)
Γ X
MAE along ΓX axisDensity of states
Results of ab initio calculations
clean Ni
O/Ni
• DOS shows that topmost Ni moment is basically unchanged
• O-induced surface state seen in the vicinity of X-point is responsible for change in MAE
• theory reveals induced moment in surfactant oxygen
DO
S (
stat
es/e
V⋅a
tom
⋅spi
n)
clean Ni film
surfactant grown Ni film
24K. Baberschke FU Berlin „Lectures on magnetism“ #2, Fudan Univ. Shanghai, Oct. 2005
530 540 550
-2
0
2
-0.02
0.00
0.02
XA
S (
arb.
uni
ts)
Photon Energy (eV)
Experiment Theory
XM
CD
(ar
b.
units)
Induced magnetism in oxygen: Ni on O/Cu(100)
theory: Ruqian Wu (UC Irvine):
induced moments in oxygen:
µS=0.053 µBµL=0.0021 µB
oxygen K-edge XMCD → orbital moment µL
BESSY: UE56/2-PGM2
15 ML Ni on O/Cu(100)300K
2pz3d
2pxy4sp
25K. Baberschke FU Berlin „Lectures on magnetism“ #2, Fudan Univ. Shanghai, Oct. 2005
Induced magnetism in oxygen: Co and Fe films Co on O/Cu(100)
540 5600
1
2 grazing normal
Nor
m. X
AS
(ar
b. u
nits
)
Photon Energy (eV)
Fe on O/Cu(100)
540 560 5800
1
grazing normal
norm
. XA
S (
arb.
uni
ts)
photon energy (eV)
530 540 550
-0.08
-0.04
0.00
0.04
norm
. XM
CD
(arb
. uni
ts)
photon energy (eV)
4 ML Co on O/Cu(100)300K
3 ML Fe on O/Cu(100)300K
hν
hνM
M
BESSY: UE56/2-PGM2
2pz3d
2pxy4sp
norm
. XA
S (
arb.
uni
ts)
norm
. XA
S (
arb.
uni
ts)
530 540 550
-0.08
-0.04
0.00
norm
. XM
CD
(ar
b. u
nits
)
photon energy (eV)
26K. Baberschke FU Berlin „Lectures on magnetism“ #2, Fudan Univ. Shanghai, Oct. 2005
Conclusion
• spin reorientation transition changes dramatically with surfactant→ surface anisotropy is strongly reduced in magnitude
• Fe, Co and Ni induce magnetic moment in surfactant