Journal of Physics D: Applied Physics
ACCEPTED MANUSCRIPT • OPEN ACCESS
Terahertz spectroscopy for all-optical spintronic characterization of the spin-Hall-effect metals Pt, W and Cu80Ir20 To cite this article before publication: Tom Seifert et al 2018 J. Phys. D: Appl. Phys. in press https://doi.org/10.1088/1361-6463/aad536
Manuscript version: Accepted Manuscript Accepted Manuscript is “the version of the article accepted for publication including all changes made as a result of the peer review process, and which may also include the addition to the article by IOP Publishing of a header, an article ID, a cover sheet and/or an ‘Accepted Manuscript’ watermark, but excluding any other editing, typesetting or other changes made by IOP Publishing and/or its licensors” This Accepted Manuscript is © 2018 IOP Publishing Ltd.
As the Version of Record of this article is going to be / has been published on a gold open access basis under a CC BY 3.0 licence, this Accepted Manuscript is available for reuse under a CC BY 3.0 licence immediately. Everyone is permitted to use all or part of the original content in this article, provided that they adhere to all the terms of the licence https://creativecommons.org/licences/by/3.0 Although reasonable endeavours have been taken to obtain all necessary permissions from third parties to include their copyrighted content within this article, their full citation and copyright line may not be present in this Accepted Manuscript version. Before using any content from this article, please refer to the Version of Record on IOPscience once published for full citation and copyright details, as permissions may be required. All third party content is fully copyright protected and is not published on a gold open access basis under a CC BY licence, unless that is specifically stated in the figure caption in the Version of Record. View the article online for updates and enhancements.
This content was downloaded from IP address 181.215.28.169 on 24/07/2018 at 02:03
Page 1 of 13
cri pt
Terahertz spectroscopy for all-optical spintronic characterization of the spin-Hall-effect metals Pt, W and Cu80Ir20 T. Seifert1,2,3, N.M. Tran4, O. Gueckstock1, S.M. Rouzegar1, L. Nadvornik1,2, S. Jaiswal5,6, G. Jakob5,7, V.V. Temnov4, M. Münzenberg8, M. Wolf1, M. Kläui5,7, T. Kampfrath1,2* 1. 2. 3. 4. 5. 6. 7. 8.
Email:
[email protected]
us
*
Department of Physical Chemistry, Fritz Haber Institute of the Max Planck Society, 14195 Berlin, Germany Department of Physics, Freie Universität Berlin, 14195 Berlin, Germany Department of Materials, Eidgenössische Technische Hochschule Zürich, 8093 Zürich, Switzerland Institut des Molécules et Matériaux du Mans, UMR CNRS 6283, Le Mans Université, 72085 Le Mans, France Institute of Physics, Johannes Gutenberg University, 55128 Mainz, Germany Singulus Technologies AG, 63796 Kahl am Main, Germany Graduate School of Excellence Materials Science in Mainz, 55128 Mainz, Germany Institute of Physics, Ernst Moritz Arndt University, 17489 Greifswald, Germany
(a)
NM
dM
FM
an
Abstract. Identifying materials with an efficient spin-to-charge conversion is crucial for future spintronic applications. In this respect, the spin Hall effect is a central mechanism as it allows for the interconversion of spin and charge currents. Spintronic material research aims at maximizing its efficiency, quantified by the spin Hall angle 𝛩 and the spin-current relaxation length 𝜆 . We develop an all-optical contact-free method with large sample throughput that allows us to extract 𝛩 and 𝜆 . Employing terahertz spectroscopy and an analytical model, magnetic metallic heterostructures involving Pt, W and Cu80Ir20 are characterized in terms of their optical and spintronic properties. The validity of our analytical model is confirmed by the good agreement with literature DC values. For the samples considered here, we find indications that the interface plays a minor role for the spin-current transmission. Our findings establish terahertz emission spectroscopy as a reliable tool complementing the spintronics workbench.
js
jc
ISHE
Femtosecond pump
M
THz pulse
(b)
pte
𝑬
𝑬
𝑬 Substrate Metal
ce
Figure 1. Schematic of the experiment. (a) Terahertz emission experiment. A femtosecond nearinfrared pump pulse excites electrons in both the ferromagnetic (FM, in-plane magnetization 𝑴) and non-magnetic (NM) metal layer. Due to the asymmetry of the heterostructure, a spin current 𝒋𝐬 is injected from the FM into the NM material where it is converted into an in-plane charge current 𝒋𝐜 by the inverse spin Hall effect (ISHE). The sub-picosecond charge-current burst leads to the emission of a terahertz (THz) pulse into the optical far-field. (b) Terahertz transmission experiment. A THz transient 𝐸 is incident onto either the bare substrate or onto the substrate coated by a thin metal film. By comparing the two transmitted waveforms 𝐸 and 𝐸 , the metal conductivity at THz
Ac
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
AUTHOR SUBMITTED MANUSCRIPT - JPhysD-117068.R1
AUTHOR SUBMITTED MANUSCRIPT - JPhysD-117068.R1
cri pt
frequencies is determined.
1.0
Co40Fe40B20 | Pt
+M
0.0 -1.0
-M
0.0
0.5
1.0
1.5
2.0
Time t (ps)
(b)
(c) 1.0
2.5
an Amplitude
0.8
0.5
0.6
Absorptance Reflectance Transmittance
0.4 0.2
0.0
0.0
dM
RMS of THz Signal
1.0
40
80
120
Pump power P (mW)
(d)
us
-5
THz signal (10 )
(a)
4
8
12
16
Pt thickness dNM (nm)
2
Re Drude Im Drude
Re Im
Pt
0
0.5
Co40Fe40B20
pte
6
Conductivity (10 S/m)
4
0.0
1.0
2.0
3.0
4.0
5.0
Frequency /2 (THz)
ce
Figure 2. Typical THz emission raw data and sample characterization. (a) THz emission signal measured from a C40F40B20(3 nm)|Pt(3 nm) bilayer for two opposite orientations of the sample magnetization (±𝑴). (b) Normalized pump-power dependence of the THz signal amplitude (RMS) for one orientation of the sample magnetization. (c) Pump-light absorptance, transmittance and reflectance as function of the Pt-layer thickness. (d) Frequency-dependent THz conductivities measured by THz transmission experiments (black and red dots) along with fits obtained by the Drude model (black and red solid lines).
Ac
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 2 of 13
1.0 Cu80Ir20 rel=2.3 nm
W rel=1.4 nm
cri pt
Pt rel=1.2 nm 0.5
0
0
5
10
15
20 0
5
10
15
20 0
5
NM Thickness (nm)
10
15
20
an
us
Figure 3. Thickness dependence of the THz-emission signal. THz-signal amplitude (RMS) as a function of the NM layer thickness divided by the thickness-dependent pump absorptance for Co40Fe40B20(3 nm)|Pt (markers in left panel), Co20Fe60B20(3 nm)|W (center panel), and Fe(3 nm)|Cu80Ir20 (right panel). Grey solid lines show fits based on Eqs. (1) and (2) with the relaxation length 𝜆 and a global amplitude as free parameters. We obtain 𝜆 = (1.2 ± 0.1) nm, (1.4 ± 0.5) nm and (2.3 ± 0.7) nm for Pt, W and Cu80Ir20, respectively. For each NM material, the experimental data is normalized by the maximum amplitude.
1.0
Co40Fe40B20(3)|Cu(d1)|Pt(3)|Cu(8-d1)
0 nm
0.5
dM
THz signal (normalized)
(a)
8 nm
0.0
-0.5
0.0
0.5
1.0
1.5
2.0
Time t (ps)
1.0
experiment ~e
pte
RMS of THz signal
(b)
-d1/rel
with rel=4.0 nm
0.5
0.0
0
2
4
6
8
Thickness of first copper layer d1 (nm)
ce
Figure 4. Role of the interface in THz emission studies. (a) THz waveforms emitted from Co40Fe40B20|Cu(𝑑 )|Pt|Cu(8 nm−𝑑 ) structures with fixed Co40Fe40B20 and Pt thicknesses of 3 nm and 𝑑 ranging from 0 to 8 nm. (b) THz signal amplitudes (RMS) vs 𝑑 together with a single exponential fit ∝ exp(−𝑑 ⁄𝜆 ) with 𝜆 = (4.0 ± 0.1) nm. The experimental data is normalized by the maximum signal amplitude.
Ac
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
AUTHOR SUBMITTED MANUSCRIPT - JPhysD-117068.R1
THz signal (norm.)
Page 3 of 13
AUTHOR SUBMITTED MANUSCRIPT - JPhysD-117068.R1
6
Co20Fe60B20 | Pt vs W
SH =-5.6%
Fe | Pt vs Cu80Ir20
SH =+3.2%
raw
4 2
raw
0 0.0
0.5
1.0
1.5
2.0
2.5
3.0
us
Time t (ps)
cri pt
-5
THz signal (10 )
8
an
Figure 5. Comparison of raw THz-emission data from Pt, W, and Cu80Ir20. THz waveforms emitted from FM|NM structures where NM is W (red) and Cu80Ir20 (orange) in comparison to a FM|Pt reference structure with the same layer thicknesses (black curves). The values of 𝛩 indicate the spin Hall angles of the respective NM materials estimated by calculating the ratios of the bare THzsignal amplitudes (RMS) and assuming a spin Hall angle of 12.0% for the Pt reference.
dM
Table 1. Overview of samples used for THz-emission measurements together with the corresponding reference samples. Numbers in parentheses indicate the film thickness in nm. In the last row, 𝑑 ranges from 0 to 8 nm. The substrate thickness is 500 µm. NM material
Sample structure
Reference sample structure
Pt
Glass||Co40Fe40B20(3)|Pt( 2-16)
-
Glass||Co20Fe60B20(3)|W( 2-20)
Glass||Co20Fe60B20(3)|Pt (3)
Cu80Ir20
Glass||Fe(3)|Cu80Ir20(216)|AlOx(3)
Glass||Fe(3)|Pt(4)
Cu
Glass||Co40Fe40B20(3)| Cu(𝑑 )|Pt(3)|Cu(8-𝑑 )
ce
pte
W
Ac
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 4 of 13
-
Page 5 of 13
cri pt
Table 2. Sample details for THz-transmission measurements. Drude-model fit parameters 𝝈𝐃𝐂 and 𝜞, conductivity literature values 𝝈𝐥𝐢𝐭 𝐃𝐂 , and substrate-thickness variation 𝜟𝒅𝐬𝐮𝐛 . Numbers in parentheses indicate the film thickness in nm. The substrate thickness is 500 µm.
Ref.
𝜞 (2π·1012 Hz)
𝜟𝒅𝐬𝐮𝐛 (µm)
[1]
81
-0.1
[4]
51
5.8
-
25
-7.3
2.0
[2]
>100
-5.9
4.0
[3]
>100
-2.2
[39]
64
-0.6
Sample
𝝈𝐃𝐂 (106 S/m)
6 𝝈𝐥𝐢𝐭 𝐃𝐂 (10 S/m)
Co40Fe40B20
Glass||Co40Fe40B20(3)|MgO(4)
0.6
0.7
Pt
Glass||Pt(10)
3.9
5.4
Co20Fe60B20
Glass||Co20Fe60B20(6)
0.6
-
W
Glass||W(10)
1.9
Fe
MgO||Fe(3) |AlOx(3)
2.7
Cu80Ir20
Glass||Cu80Ir20(4)|AlOx(3)
1.0
an
us
Material
dM
2.0
Table 3. Spin-Hall parameters. Spin-current relaxation length 𝜆 along with the literature value 𝜆 . The table also provides the ratio of the bare THz emission signals 𝛩 between a FM|NM heterostructure and the corresponding FM|Pt reference sample as well as the relative spin Hall angle 𝛩 extracted by our analysis correcting for sample specific parameters [see Eqs.(1) and (2)] along with the corresponding literature value 𝛩 .
𝝀𝐫𝐞𝐥 (nm)
𝝀𝐥𝐢𝐭 𝐫𝐞𝐥 (nm)
Ref.
-2 𝜣𝐫𝐚𝐰 𝐒𝐇 (10 )
𝜣𝐒𝐇 (10-2)
-2 𝜣𝐥𝐢𝐭 𝐒𝐇 (10 )
Ref.
Pt
1.2
1.1
[4]
-
-
12.0
[40]
1.4
