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Journal of the Korean Physical Society, Vol. 41, No. 1, July 2002, pp. L1∼L3
Letters
Detection of dc Voltages Generated in Multilayered Magnetic Thin Films
Undergoing Ferromagnetic Resonance
Eung Ryul Park, Dong Keun Oh and Cheol Eui Lee∗
Department of Physics and Institute for Nano Science, Korea University, Seoul 136-701
S. H. Kim, S. R. Lee and Y. K. Kim
Division of Materials and Engineering, Korea University, Seoul 136-701
(Received 18 May 2002)
By employing an electron spin resonance spectrometer and lock-in detection technique with microwave chopping, we directly measured the dc voltages in magnetic multilayers undergoing ferromagnetic resonance at room temperature. The measured dc voltage level was on the order of nV.
This novel detection technique is expected to be instrumental in probing phenomena in the vicinity
of interfaces in magnetic multilayers.
PACS numbers: 75.70.-i, 76.50.+g
Keywords: Magnetic multilayer, Electrical detection of magnetic resonance
In 1996, Berger predicted theoretically that a dc current crossing an interface between a nonmagnetic and
a ferromagnetic layer would induce a precession of the
magnetization in the ferromagnetic layer [1]. The presence of a sharp interface in magnetic multilayers causes
a local increase in the spin-magnon interaction. The interfaces act as a source of momentum, helping the conduction electrons jump across the momentum gap between the spin-up and the spin-down Fermi surfaces as
they generate precession. Slonczewski proposed a similar current-induced spin precession, as well as a switching of the magnetization between two different static directions [2]. These phenomena are understood by the
spin-polarized nature of the current giving rise to mutual
transference of spin angular momentum and resulting
in the excitation of spin waves in magnetic multilayers.
Tsoi et al. obtained experimental evidence of such spin
precession in a Cu/Co multilayer traversed by a current
normal to the layers at 4 K [3]. Rezende et al. also experimentally observed the excitation of magnons by using
Brillouin light scattering [4]. Recently, the inverse effect
was theoretically predicted by Berger and Slonczewski
independently [2, 5]. When an external microwave radiation is absorbed under a ferromagnetic resonance (FMR)
condition, precession of the magnetization will generate
a dc voltage across the normal/ferromagnetic multilayer
due to the spin diffusion. The dc voltage of the FMR
was first detected in 1958 by Egan and Juretschke, but
their dc voltages had quite different origins, the nonlinear electrical conduction terms in ferromagnetic systems
∗ E-mail:
and involved the anomalous Hall effect and the magnetoresistive anisotropy [6].
Since the discovery of giant magnetoresistance, magnetoelectronics has been an emerging field that combines
the electron spin state with the conventional semiconductor electronics for devices with new or enhanced functionality. While the role of the interfaces is of considerable
importance, experimental studies of the roles of the interface between a ferromagnetic layer and a normal metal
are rarely found whereas much attention and research is
concentrated on the device structures and material properties.
In this work, we report the direct measurements of
dc generation as an inverse effect of transference of the
spin angular momentum in Cox Fe1−x /Cu magnetic multilayers. To do this, we used a modified electron spin
resonance (ESR) spectrometer and a lock-in detection
technique.
The sample (Cox Fe1−x /Cu, x=0.9) was fabricated on
a Si substrate by using a metal shadow mask and rfmagnetron sputtering at a chamber base pressure of
5×10−7 Torr. The sample structure is shown in Fig.
1.
The dc voltages were detected using an ESR spectrometer and a lock-in amplifier [7]. In general, conventional
ESR spectrometers employ a magnetic field modulation
in order to increase the sensitivity with a lock-in amplifier. This method, however, is not effective enough
and has a shortcoming, i.e., an electromotive voltage induction on the detection wires by the modulation field.
Thus, we adopted a microwave chopping method, instead
of field modulation.
[email protected]
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Journal of the Korean Physical Society, Vol. 41, No. 1, July 2002
Fig. 1. Structure of the fabricated sample. Top view of
upper part is circular with a diameter of 2 mm.
The sample was located in the ESR cavity with the
magnetic field parallel to the sample surface, and pointcontacted thin copper wires were directly connected to
the lock-in amplifier in order to detect the dc bias under the FMR condition. A home-built microwave bridge
was employed for chopping the microwave drive signal,
which was used instead of magnetic field modulation.
The home-built microwave bridge synchronously generated chopped microwaves in the cavity through a waveguide with a transistor transistor logic (TTL) chopping
signal from the lock-in amplifier. The microwave frequency and the chopping frequency used were 9.7 GHz
and 1 kHz, respectively. The power of the applied microwaves was 1 W, and the magnetic field was applied
parallel to the plane of the sample to eliminate the demagnetization effect. The magnetic field sweep and the
signal detection were carried out via a home-built control
program and hardware [8]. A schematic block diagram
is illustrated in Fig. 2.
Figure 3 shows the electrically detected dc voltages
and the FMR signal from the sample at room tempera-
Fig. 2. Simplified schematic diagram of the measurement
system. The sample was loaded in the cavity using an ESRsilent quartz rod of 4 mm in diameter. The electromagnet power supply, not shown here, was controlled by a PC
through a controller module, also not shown here (Bruker
ER32M).
Fig. 3. Electrically detected dc voltage (left vertical axis)
and FMR (right vertical axis) signals. The sensitivity and
the time constant of the lock-in amplifier were 500 nV and
10 s, respectively.
ture. The FMR signal was obtained by means of the conventional magnetic field modulation method. The two
signals are shown to be in very good agreement. Similar results were obtained at 120 K. Our dc voltage level
is considerably smaller than Berger’s calculations, which
assumed Co/Cu multilayer structures and sample dimensions different from ours. The small dc voltages may be
attributed to inhomogeneous multilayer interfaces.
In the present work, the weak intensity and the low
microwave power prevented us from obtaining the signals as a function of detection parameters such as the
chopping frequency and the microwave power. The dependencies on the microwave power, the temperature,
and the chopping frequency are expected to be related
to the intensity of the spin-magnon interaction. Thus,
further refined experiments and quantitative analysis of
the data should provide us with clues to understanding
the phenomena at the interfaces of the magnetic multilayers. Furthermore, instead of magnetic field modulation and/or microwave chopping, electrical modulation
by applying an ac current to the specimen, i.e., transference of momentum through spin-polarized conduction
electrons, as well as continuous microwave and magnetic
field sweep methods, can be employed for studies of the
magnetic multilayers.
We were able to directly detect the dc voltages from a
magnetic multilayer structure undergoing ferromagnetic
resonance by means of a modified ESR spectrometer and
a new lock-in detection technique. Thus, the present
work has provided a simple and effective technique, involving some modifications of existing apparatus, which
can be instrumental for investigating phenomena in multilayered magnetic thin-film systems, especially near the
interfaces.
Detection of dc Voltages Generated in Multilayered Magnetic Thin Films· · · – Eung Ryul Park et al.
ACKNOWLEDGMENTS
This work was supported by a Korea University grant.
Support from the Korea Ministry of Science and Technology (National Research Laboratory) and the Korea
Research Foundation are also gratefully acknowledged.
REFERENCES
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Seck, V. Tosi and P. Wyder, Phys. Rev. Lett. 80, 4281
(1998).
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[4] S. M. Rezende, F. M. de Aguiar, M. A. Lucena and A.
Azevedo, Phys. Rev. Lett. 84, 4212 (2000).
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