Magnetization Precession Induced Spin Pumping in
Ta /Py, Ta /Fe3O4 /Py Multilayer Nanostructures
Nilamani Behera*, Ankit Kumar, Sujeet Chaudhary, Dinesh K. Pandya
Thin Film Laboratory, Indian Institute of Technology Delhi, Hauz Khas, New Delhi-110016, India,
Corresponding author’s e-mail:nila.behera@gmail.com, Tel.:+91-11-2659 6521; Fax: +91-11-2658 1114
*
Introduction
In bilayer paramagnet (PM) / ferromagnet (FM)
system, spin pumping mechanism is primarily due to
transfer of spin angular momentum from the precessing
local spins of FM layer to the conduction electrons of
PM layer [1]. The interfacial spin mixing conductance
relates the efficiency to create the spin current at
PM/FM interfacial sites and governs the spin pumping
mechanism. Recent studies are focused on interface
tailoring of PM/FM system using MgO and TiN
interfacial layers in order to enhance the spin pumping
efficiency. It is observed that the spin mixing
conductance is suppressed in the presence of MgO or
TiN at the interface of NM/FM bilayer [2-3]. In order to
enhance the spin pumping efficiency a new approach of
the role of interface tailoring on spin is employed by
inserting 1-3 nm Fe3O4 layer at interface, thus forming a
trilayer structure PM/Fe3O4/FM.
Experimental Details
Ta/Py/Ta, Ta/Fe3O4/Ta and Ta/Py/Fe3O4/Ta
multilayer samples of different thickness were
fabricated on Si substrate using pulsed DC magnetron
sputtering employing 99.99% pure metal targets and a
sputtering system of 2×10–7 Torr base pressure. The
bottom layer of β-Ta is kept in all samples and sample
stacks are capped with Ta layer. The resonance field Hr
and field swept line width ΔH are measured at constant
frequencies by using broadband (2-12GHz) Lock-In
Amplifier (SR-830) ferromagnetic resonance (LIAFMR) technique with help of a 8719ES vector network
analyzer (VNA) in an in-plane configuration. The
resonance signals are recorded at different frequencies
in the range 3GH to 10GHz.
Result and Discussion
Figure 1(a) shows the FMR spectra of Ta(6nm)/Py
(12nm)/Ta(2nm), Ta(6nm)/Fe3O4(2nm)/Py(12nm)/Ta (2
nm), and Py(12nm)/Ta(6nm) systems at 10 GHz
frequency. The FMR data are fitted with symmetric and
anti-symmetric Lorentzian functions. The observed line
shape parameters are fitted with ∆𝐻 = ∆𝐻0 + 4𝜋𝛼 ⁄√3𝛾
(shown in Fig. 1(b)) where ΔH0 is the zero frequency
intercept due to extrinsic inhomogeneous contribution to
line width, ƒ is the resonant frequency, α is the Gilbert
damping constant, and γ is the gyromagnetic ratio. The
estimated values of Gilbert damping constant are found
to decrease in the case of Ta (6nm)/Py(12nm) samples
from 0.0068 to 0.0061 as compared to Py, whereas they
increase to 0.0077 in case of Ta(6nm)/Fe 3O4(2nm)/
Py(12nm) samples. The interfacial spin mixing
conductance of these two samples is found to be |𝑔↑↓ | =
2.26×1018 m–2 and 6.37×1018 m–2, respectively. In
former case, the spin angular momentum transfer takes
place from Py (FM) layer to Ta (PM) layer in contrast to
the decrease in line-width due to spin accumulation over
the surface of PM layer. This spin accumulation
opposes spin pumping through back flow of spin
angular momentum into FM [4]. But in later case, the
presence of interfacial Fe3O4 (2nm) possibly locks the
back flow of spin angular momentum from Ta layer to
Py layer, which leads to the enhancement in Gilbert
damping constant and spin mixing conductance.
1.0
H(TFPT)
= 3.5 mT
H(PT) = 3.23 mT
PT= 0.0.0068
3.0 TFPT=
0.5
0.0077
2.5
0.0
-0.5
3.5
H (mT)
The spin back flow nature of Ta layer through
interface of Ta/Py to Py layer leads to decrease in Gilbert
damping constant (𝜶) and spin mixing conductance (𝒈↑↓ ).
But the presence of Fe3O4 (2nm) at the interface of Ta/Py
i.e. Ta (6 nm)/Fe3O4 (2nm)/Py (12nm) trilayer structure,
locks the spin back flow leading to enhancement of 𝜶 and
𝒈↑↓ .
Keywords: spin back flow, spin mixing conductance, Gilbert
damping constant
FMR Signal (V)
Abstract
H (TPT)
2.0
= 2.93 mT
-1.0
90
100
110
H (mT)
TPT= 0.0.0062
P (12)/T(2)
T(6)/P(12)/T(2)
T(6)/F(2)/P(12)/T(2)
Fit
1.5
P(12)/T(2)
T(6)/F(2)/P(12)/T(2)
T(6)/P(12)/T(2)
(a)
120
130
1.0
2
(b)
4
6
(GHz)
8
10
Fig.1. (a) FMR spectra of Py(12nm)/Ta(2nm), Ta(6nm)/
Py(12nm)/Ta(2nm), Ta(6nm)/Fe3O4(2nm)/Py(12nm)/Ta(2nm)
recorded by using LIA-FMR detection technique at constant
frequency, ƒ = 10GHz. (b) Line width (ΔH) vs frequency
(ƒ) data of same samples fitted with red solid lines.
References
[1]
[2]
[3]
[4]
S. Mizukami et al., Phys. Rev. B 66, 104413 (2002).
O. Mosendz et al., Appl. Phys. Lett. 96, 022502 (2010).
H. Nakayama et al., Key Eng. Mater. 508, 347 (2012).
Y. Tserkovnyak et al, Phys. Rev. B 66, 224403 (2002).