Magnetic properties of Fe-Ag multilayers with varying layer

Magnetic properties of Fe-Ag multilayers with varying layer

thickness and bilayer number

L.F. Kiss, J. Balogh, L. Bujdosó, D. Kaptás

Institute for Solid State Physics and Optics Wigner Research Centre for Physics

Hungarian Academy of Sciences, Budapest, Hungary

Outline

•Samples: Fe-Ag multilayers of the sequence

Si / buffer + (t

Fe + t

Ag)

+ cover

•Preparation

•Measurements: how does the blocking temperature (T

) depend on t

, t

and n?

•Consequences: general concepts determining the

magnetic structure of Fe-Ag multilayers

Sample preparation

Vacuum evaporation in 10^-7 Pa with 0.01 nm/s for ⁵⁷Fe Ag with electron guns

57Fe from Joule-heated W crucible target 1 (Ag)

target 2 (B, Nb)

substrate: untreated Si(111) wafer

Experimental details

•SQUID-magnetometer (Quantum Design MPMS-5S)

•Measuring range: 1.8 K  T  400 K and 0  H  5 T

•ZFC protocol: cooling in zero field and measuring during heating at a small field (typically 10 Oe)

•FC protocol: cooling at the measuring field and measuring during heating at the same field

•2 pieces of ca. 6 x 6 mm² Si wafer on one another (each with the multilayer on the surface)

Magnetic properties of small particles Blocking temperature

H

10-20 Å V, M_s, K m = M_sV

E = KV energy barrier H = 0

-dM/dt = f₀M e^-KV/kT  M /



M_r = M_i e^-t/,

where 1/





KV_p/kT = 25 V = const T_B = KV/25k

Structure of the multilayers

Si Ag Fe cover

Si Ag Fe cover

Si Ag Fe cover

SPM SPM + FM FM

1 2 4 5 6 7 10 14 15 28 826

0 1

5.7 324

8 75 75

10 118

13 75 75 75

20 92 59 30

26 10,75,162 10 10,20 10 10

30 92

40 92 59

50 1-20 1 1,2,10 1,10

52-55 35,75 75

70 2-20

80 92

104 32

t_Ag (A)

t_Fe (A)

SPM SPM + FM FM

bilayer numbers

1 2 4 5 6 7 10 14 15 28 826

0 1

5.7 324

8 75 75

10 118

13 75 75 75

20 92 59 30

26 10,75,162 10 10,20 10 10

30 92

40 92 59

50 1-20 1 1,2,10 1,10

52-55 35,75 75

70 2-20

80 92

104 32

t_Ag (A)

t_Fe (A)

SPM SPM + FM FM

bilayer numbers

Dependence on Fe thickness

0 50 100 150 200 250 300

-20 0 20 40 60

10 Oe FC

ZFC

Si/50 A Ag + t A Fe + 50 A Ag

M (emu/g)

T (K)

4 A 5 A 7 A 10 A

SPM SPM + FM 1 layer

Dependence on Fe thickness

0 50 100 150 200 250 300

0 50 100 150 200

30x 10 Oe

10x

30x

M (emu/g)

T (K)

2 A 4 A 5 A 7 A

Si/26 A + (t_Fe ⁵⁷Fe + 26 A Ag)₁₀

SPM SPM + FM

0 T

0.3 T

-5 0 5

(4 A Fe + 26 A Ag)₁₀

velocity (mm/s)

Intensity (a.u.)

0.5 T

(7 A Fe + 26 A Ag)₁₀

0 T

Intensity (a.u.)

0.1 T

0.3 T

-5 0 5

velocity (mm/s)

0.5 T

10 layers

RT

H_hf increases with external field showing small-particle behaviour

Dependence on Fe thickness

Below t_Fe ~ 2-7 Å, SPM;

Above t_Fe ~ 2-7 Å, SPM + FM BUT it depends on t_Ag and n

0 50 100 150 200 250 300

-200 -100 0 100 200

20x

Si/50 A Ag + (t_Fe Fe + 50 A Ag)₁₀

FC

ZFC

10 Oe

M (emu/g)

T (K)

4 A 7 A 10 A

SPM 10 layers

SPM + FM

1 2 4 5 6 7 10 14 15 28 826

0 1

5.7 324

8 75 75

10 118

13 75 75 75

20 92 59 30

26 10,75,162 10 10,20 10 10

30 92

40 92 59

50 1-20 1 1,2,10 1,10

52-55 35,75 75

70 2-20

80 92

104 32

t_Ag (A)

t_Fe (A)

SPM SPM + FM FM

bilayer numbers

Dependence on Ag thickness

0 50 100 150 200 250 300

-1 0 1 2 3 4

10 Oe

(1 A Fe + t_Ag Ag)₇₅

M (emu)/g

T (K)

8 A 13 A

0 50 100 150 200 250 300

0 10 20 30

30x 10x

4.5x

10 Oe

(2 A Fe + t_Ag Ag)₇₅

M (emu/g)

T (K)

8 A 13 A, 4.5x 26 A, 10x 54 A, 30x

SPM SPM SPM + FM

Dependence on Ag-layer roughness

0 50 100 150 200 250 300

-100 -50 0 50 100

12x 500 A

50 A

10 Oe

t_Ag Ag + 4 A Fe + 70 A Ag + 70 A Nb

M (emu/g)

T (K)

0 200 400 600

0 50 100

t_Ag (A)

TB (K)

T_B increases with t_Ag above t_Ag ~ 50 Å

Around t_Ag ~ 50 Å, T_B shows minimum, indicating pin-hole effect

J. Balogh, L. Bujdosó, D. Kaptás, I. Dézsi and A. Nakanishi, Phys.

Rev. B 85 (2012) 195429

1 2 4 5 6 7 10 14 15 28 826

0 1

5.7 324

8 75 75

10 118

13 75 75 75

20 92 59 30

26 10,75,162 10 10,20 10 10

30 92

40 92 59

50 1-20 1 1,2,10 1,10

52-55 35,75 75

70 2-20

80 92

104 32

t_Ag (A)

t_Fe (A)

SPM SPM + FM FM

bilayer numbers

Dependence on layer number

Why does T_B depend on the layer number?

First idea: interactions between the Fe clusters

Contradiction: only slight

difference in T_B between multi- layers with t_Ag = 50 and 70 Å

Second idea: the higher the layer position, the larger the cluster size because of the more and more irregular shape of the layers

0 5 10 15 20 25

0 50 100 150

solid: t_Ag = 50 A open: t_Ag = 70 A

50A Ag + (4A ⁵⁷Fe + t_Ag Ag)_n + cover

T B (K)

n

Dependence on layer position (n = 1)

0 50 100 150 200 250 300

-10 0 10 20 30

10 Oe top

bottom

Si/70 A Ag + 4 A Fe + (70 A Ag + 4 A Nb)₁₉

M (emu/g)

T (K)

bottom top

T_B is significantly higher for the top layer position

Si Ag Fe cover

Nb

19x

Si Ag Fe cover

Nb

19x

Dependence on buffer and cover layers

T_B depends mainly on the buffer layer

0 50 100 150 200 250 300

-200 -100 0 100 200

50 A buffer + (7 A Fe + 50 A Ag)₁₀ + 100 A cover

10 Oe

M ( emu/g)

T (K)

buffer: Ag, cover: Ag buffer: Ag, cover: W buffer: W, cover: W

Field dependence of magnetization

Description

M(H, T) = M₀ L(mH/k_BT) +



n cluster density m cluster moment L Langevin function



D_cluster ~ 2 nm

0 10000 20000 30000 40000 50000

0 50 100 150 200

250 Si/50 A Ag + 5 A ⁵⁷Fe + 50 A Ag + 100 A Ag

M (emu/g)

H (Oe)

5 K 50 K 100 K 150 K 200 K 250 K 300 K

Generally: D_cluster ~ 1.4 – 2 nm

1 2 4 5 6 7 10 14 15 28 826

0 1

5.7 324

8 75 75

10 118

13 75 75 75

20 92 59 30

26 10,75,162 10 10,20 10 10

30 92

40 92 59

50 1-20 1 1,2,10 1,10

52-55 35,75 75

70 2-20

80 92

104 32

t_Ag (A)

t_Fe (A)

SPM SPM + FM FM

bilayer numbers

Magnetic map of Fe-Ag multilayers

Magnetic map of Fe-Ag multilayers

Video

Modeling the ZFC and FC curves

𝑍𝐹𝐶

𝑠2

𝐵 𝐵

𝑚 0

𝐵 𝑇

0

∞

𝑇

𝐵 𝐵

𝐹𝐶

𝑠2

𝑚

0 𝐵 𝐵

∞

𝑇

𝐵

𝐵 𝐵

𝑇

0

Stoner-Wohlfarth model: noninteracting, randomly oriented, single-domain particles











Modeling the ZFC and FC curves

0 20 40 60 80 100 120 140 160

0 5 10 15 20 25

/(M s2 /3K)

T (K)

20 K, wide 80 K, wide 20 K, narrow

0 50 100

0.0 0.1 0.2 f (T_B)

T (K)

Summary

•Fe-Ag multilayers with different Fe (t_Fe) and Ag (t_Ag) thicknesses and bilayer numbers (n)

•Three organizing principles determining the magnetic structure of Fe-Ag multilayers

• -Critical Fe thickness t_Fe^crit ~ 2-7 Å below which SPM behaviour is observed depends strongly on t_Ag and n

• -Pin holes and interaction between Fe layers below t_Ag ~ 50 Å

• -Increasing T_B with increasing n is related to the increasing surface roughness due to columnar growth

•Typical cluster size D_cluster ~ 1.4 - 2 nm

L.F. Kiss, J. Balogh, D. Kaptás and L. Bujdosó, Phys. Rev. B 98 (2018) 144423

Thank you for your attention!