Thermoelectric Properties of Fe0.2Co3.8Sb12-xTex ... - Physics

Confinement of Spin Waves in Grids of Permalloy Nanowires
D. Venkateswarlu
Department of Physics, Indian Institute of Science, Bangalore 560012, India
Key words—electron beam lithography, shape anisotropy, nanowires, FMR, demagnetization fields, micromagnetics.
Recent advances in magnonics highlight its importance
in microwave frequency applications [1] . This involves the
use of spin waves for device applications. Here one needs
to look for better control over the spin waves and their band
structure [2-3] . The periodic variation in the effective
magnetic field in ferromagnetic materials is the basis for
the spin wave confinement and their propagation. Since the
effective field is the combination of exchange, external,
anisotropy and demagnetization (dipolar/ stray) fields; one
can achieve the periodic condition in many ways. In our
study, we employed geometry engineering by which the
shape anisotropy was tailored in obtaining the periodicity in
demagnetization fields. Soft magnetic material permalloy
(Ni 80 Fe 20 ) was made into grid like structure using a top
down approach.
Ferromagnetic resonance (FMR) measurements give the
information about the non travelling spin waves (k=0
modes). In general, one can see only single resonance
mode in FMR spectra in the case of thin films of permalloy.
This is due to its weak crystalline anisotropy even in
epitaxial conditions unlike the Fe thin films [4] . When the
permalloy film is made into nanowires with high aspect
ratio, it still gives the single mode but resonance condition
varies with applied field angles. This is due to the fact that
the shape anisotropy governs the condition for resonance.
This advantage of the shape anisotropy points towards the
usefulness of engineering the network structures.
Fundamentally one needs to understand the dynamics of
these network structures in order to meet the required
conditions for spin wave band structures in order to use
them in magnonics devices.
The grid structures were fabricated using electron beam
lithography followed by DC magnetron sputtering and liftoff
technique. The permalloy deposited on Si wafers was
20nm thick and is capped with 4nm gold to avoid
oxidation. The thickness of the deposited materials was
confirmed with the help of pre calibrated digital thinness
monitor. The lateral dimensions of the grid structures were
obtained using Scanning Electron Microscopy (SEM). The
permalloy wires in the grids found with widths about 140-
160nm. To understand the spin wave confinement effect
we varied the periodicities along horizontal and vertical
directions in the grid: (i) 560x560 (G1), (ii) 800x400 (G2)
and (iii) 1000x500 (G3) (all in nm units).
The FMR spectra on all the three grids were compared
with a reference sample, a continuous thin film grown
under same conditions during the fabrication. The
continuous film showed an uni-axial anisotropy. This was
induced due to the in-situ applied field during the growth.
But the magnitude is very small when compared to that of
structured samples. The quantitative comparison of
anisotropy constants was obtained with the help of fitting
done with theoretical equations involved in FMR
phenomena.
Two well resolved modes were observed in all three
grids whereas the reference sample gave only one mode.
These two modes corresponds to the spin waves confined
in horizontal and vertical sections of the grids. Spin wave
confinement was understood with the help of the mode
dependency on the in-plane angle of the applied magnetic
field with the grid. There were multiple peaks observed at
some angles in the G2, G3 which is not seen in G1.
The origin for these multiple peaks is understood with
the help of static micromagnetic simulations (MMS).
Dynamics of G1 using MMS followed by Discrete Fast
Fourier Transformations were used to correlate FMR
spectra. Our simulations methodology gave more insight
into the understanding of spin waves confinement. The
implementation of 2D-PBC [5] in our MMS made it possible
to study the real systems in realistic time.
Fig.1 Spin wave confinement in the horizontal and vertical
sections of the G1.
REFERENCES
[1] V. V. Kruglyak et al., J. Phys. D: Appl. Phys., vol. 43, p.
264001, 2010.
[2] Z. K. Wang et al., Acs Nano, vol. 4, p. 643, 2010.
[3] K. S. Lee et al., Phys. Rev. Lett., v. 102, p. 127202, 2009.
[4] S. Sakshath et al., J. Supercond. Nov. Magn.,
DOI:10.1007/s10948-011-1269-3
[5] D. Venkateswarlu et al., IEEE Trans. Magn., vol. 48, no.
11, Nov. 2012.
FMR experiments were done in collaboration with Prof. S. V. Bhat
D.Venkat@physics.iisc.ernet.in