Anomalous variation of coercivity with annealing in nanocrystalline NiZn
ferrite films
Mrugesh Desai, Shiva Prasad, N. Venkataramani, Indradev Samajdar, A. K. Nigam et al.
Citation: J. Appl. Phys. 91, 7592 (2002); doi: 10.1063/1.1447504
View online: http://dx.doi.org/10.1063/1.1447504
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Published by the American Institute of Physics.
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JOURNAL OF APPLIED PHYSICS
VOLUME 91, NUMBER 10
15 MAY 2002
Anomalous variation of coercivity with annealing in nanocrystalline
NiZn ferrite films
Mrugesh Desai and Shiva Prasada)
Department of Physics, Indian Institute of Technology Bombay, Mumbai 400 076, India
N. Venkataramani
Advanced Center for Research in Electronics, Indian Institute of Technology Bombay, Mumbai 400 076,
India
Indradev Samajdar
Department of Metallurgical Engineering and Materials Science, Indian Institute of Technology Bombay,
Mumbai 400 076, India
A. K. Nigam
Tata Institute of Fundamental Research, Colaba, Mumbai 400 005, India
N. Keller and R. Krishnan
Laboratoire de Magnetisme et d’Optique de Versailles, CNRS, 78935 Versailles, France
E. M. Baggio-Saitovitch, B. R. Pujada, and A. Rossi
Centro Brasileiro de Pesquisas Fisicas, Rio de Janeiro 22 290-180, Brazil
The sputter deposited NiZn ferrite thin films were studied as a function of annealing temperature.
The magnetization showed a monotonic increase with increasing annealing temperature. The
coercivity shows a minimum at annealing temperature of 400 °C and shows a value of 14 Oe.
Transmission electron microscopy study indicated that the grain size increases from ⬃3 nm for the
as-deposited case to ⬃15 nm for the film annealed at 800 °C. The observed coercivity behavior
could be attributed to the defects present in the films, the change in cation distribution, and the grain
growth. © 2002 American Institute of Physics. 关DOI: 10.1063/1.1447504兴
I. INTRODUCTION
reported in the present article were deposited with rf power
of 220 W on fused quartz and Si 共100兲 substrates. The substrates were not heated during the deposition. A mixed gas of
argon and oxygen was used as a sputtering gas at a total
pressure of 6⫻10⫺3 mbar. Oxygen partial pressure was kept
at 10% of the total pressure. The films were ⬃4500 Å thick.
The as-deposited films were introduced into a furnace, which
was maintained at the desired annealing temperature 共T a 兲
ranging from 200 to 800 °C. Every time a fresh sample was
used for annealing. The annealing was carried out for 2 h,
after which the samples were furnace cooled. X-ray diffraction 共XRD兲 study was carried on these films as a function of
annealing temperature on a Philips diffractometer with copper target. Saturation magnetization was measured at room
temperature by using a vibrating sample magnetometer. Microstructure study was carried out using a Philips CM 200
keV transmission electron microscope 共TEM兲. TEM foils
were prepared by carefully etching the Si substrate side using
a mixture of nitric acid and hydrofluoric acid in a 3:1 ratio.
The selected area diffraction patterns 共SAD兲 and the brightfield images were recorded.
It is observed that the ferrite thin films, prepared by rf
sputtering, are nanocrystalline and their magnetic properties
are quite different from the corresponding bulk. These films
show many phenomena such as high field susceptibility and
multiresonance modes in ferromagnetic resonance.1 It is possible to deposit films with certain phases stable at room temperature, which are not otherwise, stable in bulk.2 Nickel
zinc 共NiZn兲 ferrite with the spinel structure is a promising
material for high frequency applications. Attempts have been
made by researchers to deposit NiZn ferrite thin films by
different techniques such as rf sputtering, pulse laser deposition, and facing target sputtering.3–5 There are also reports on
an alternative method of preparation of NiZn ferrite thin
films by spin-spray ferrite plating.6,7 However, the magnetic
properties of their films have not been understood well. In
this article, we report the preparation of NiZn ferrite thin film
by rf sputtering and study the evolution of magnetic properties as a function of annealing temperature.
II. EXPERIMENT
III. RESULTS
NiZn ferrite target with a composition Ni0.5Zn0.5Fe2 O4
was prepared by the conventional ceramic method. The films
were prepared by rf sputtering, by varying rf power and oxygen to argon gas ratio in a Leybold Z400 system. The films
Figure 1 shows the XRD patterns of the as-deposited and
the annealed NiZn ferrite thin films. The bulk XRD pattern is
also shown in the figure for the comparison. The asdeposited films showed two broad humps 共marked with the
arrows in the figure兲. These humps are observed to shift to
higher 2␪, when the films are annealed. Upon annealing, the
a兲
Author to whom correspondence should be addressed; electronic mail:
shivap@phy.iitb.ac.in
0021-8979/2002/91(10)/7592/3/$19.00
7592
© 2002 American Institute of Physics
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Desai et al.
J. Appl. Phys., Vol. 91, No. 10, 15 May 2002
7593
FIG. 1. X-ray diffraction patterns of the NiZn ferrite thin films as a function
of annealing temperature. The bulk XRD is shown for comparison.
broad humps change to peaks. The peaks become reasonably
sharp at 600 °C of annealing temperature. At T a ⫽800 °C,
the XRD showed five sharp peaks.
Figure 2 shows the variation of saturation magnetization
and coercivity as a function of annealing temperature. As is
clear from the figure, the magnetization value shows a monotonic increase with increasing annealing temperature. Asdeposited film shows a value of saturation magnetization of
the order of 500 G which is about 10% of the bulk NiZn
ferrite. The magnetization shows a sharp rise between annealing temperature of 600 and 800 °C and shows a value of
3140 G at T a ⫽800 °C, which is only 60% of the bulk value.
The coercivity, on the other hand, shows a drop from ⬃130
Oe for the as-deposited film to 14 Oe for the film annealed at
400 °C. Above 400 °C, the coercivity starts again increasing
and shows a value of 80 Oe at T a ⫽800 °C.
In order to investigate the microstructural dependence on
observed magnetic properties, we carried out a TEM study
on all the films as a function of annealing temperature. Figure 3 shows a bright-field image of the as-deposited film. It
reveals that the film is nanocrystalline in nature. SAD pattern
of the as-deposited film is shown in inset of Fig. 3. As is
clear from the SAD, the as- deposited film shows a large
number of rings even though no sharp peaks were observed
FIG. 3. The bright-field image of the as-deposited NiZn ferrite thin film.
The inlaid figure shows the SAD pattern of the as-deposited film.
in its XRD pattern. The grain sizes were measured using the
measurement stage of TEM. About 20–25 particles were
measured to obtain an average grain size. We note that the
grain size increases from about ⬃3 nm for the as-deposited
film to ⬃15 nm for the film annealed at 800 °C.
IV. DISCUSSION
As discussed in the last section, the film annealed at
800 °C shows five sharp peaks in XRD. These peaks can be
indexed to planes ⫺共220兲, 共311兲, 共400兲, 共511兲, and 共440兲 of
spinel single phase NiZn ferrite. All the peaks corresponding
to the NiZn ferrite bulk, are not observed in the thin films.
Additionally, the intensities of the peaks are also quite different from that of the bulk; for example, 共440兲 is the lowest
intensity peak 共15%兲 in the thin films, where as, it is second
highest intensity peak in the bulk with an intensity of 41%.
Similarly, the 共400兲 plane gives an intensity of 50% in the
thin films, while bulk shows that of only 16%.
The d values correspond to the diffraction rings in Fig. 3,
obtained in the case of as-deposited film, could be indexed to
a single phase spinel NiZn ferrite. The indexed d values are
listed in Table I. SAD and bright-field images for other films
TABLE I. Indexing of the SAD rings of the as-deposited NiZn ferrite thin
films, using a camera constant (␭L)⫽32.16936 mm Å.
FIG. 2. Variation of saturation magnetization and coercivity of NiZn ferrite
thin films as a function of annealing temperature. The lines have been drawn
to aid the eye.
Ring no.
Radius 共mm兲
d value 共Å兲
共hkl兲
1
2
3
4
5
6
6.681
10.90
12.78
15.417
20.04
21.80
4.8148
2.9504
2.5165
2.086
1.6060
1.4752
共111兲
共220兲
共311兲
共400兲
共511兲
共440兲
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7594
Desai et al.
J. Appl. Phys., Vol. 91, No. 10, 15 May 2002
were also obtained and it was found that their SAD patterns
could be indexed similarly as NiZn ferrite. It is clear from
the SAD pattern shown in Fig. 3 that the rings correspond to
the planes 共311兲, 共440兲, and 共220兲 have higher intensities as
compared to the other rings. This is unlike XRD results,
where 共440兲 and 共220兲 peaks have comparatively lower intensities. This can be understood if we assume that these
films are textured to some extent. This is because in XRD we
look at the planes parallel to the film plane, while in electron
diffraction we look at the planes closer to the normal of the
film plane. Such texture was also seen in copper ferrite thin
films.2
XRD study indicated that the humps in the as-deposited
films correspond to a higher d value as compared to the
annealed films. The calculated lattice parameter is found to
be larger than that for other annealed films. Recently, such
discrepancy in lattice parameter between as-deposited and
the annealed films have also been observed in other ferrite
thin films.8,9
The increase in saturation magnetization as a function of
annealing temperature could be explained on the basis of
grain growth. As grain size increases, the magnetization also
increases. The lower value of magnetization as compared to
the bulk could be due to a large grain boundary volume
present in nanocrystalline thin films.9 Such behavior is quite
common in nanocrystalline ferrite thin films.1
The most interesting result of the present study is the
coercivity behavior as a function of annealing temperature.
The coercivity shows a minimum at the annealing temperature of 400 °C. Generally, it is known that the coercivity
increases as the single domain grain size increases. When the
size of the grain attains a value at which it becomes multidomain, the coercivity starts decreasing.10 Such behavior of
coercivity has been observed in lithium zinc ferrite thin
films, where the coercivity increases upto a grain size of ⬃12
nm and then starts decreasing.9,11 The grain sizes observed in
the presents study, between as-deposited and T a ⫽400 °C are
much smaller than this and are in the single domain region.
The increase in the coercivity observed between T a
⫽400 °C and T a ⫽800 °C could be attributed to the grain
growth. However, it is surprising to see the initial drop in
coercivity between as-deposited and T a ⫽400 °C.
The decrease in coercivity could be due to several reasons. First, the change in the defect structure can affect the
coercivity. As the sputtering process involves high quenching
rates, the as-deposited films carry large number of defects in
the form of grain boundary volume, point defects etc., which
can cause large coercivity. These defects decrease upon annealing the films, which could result in a decrease of coercivity. The second reason could be that the cation distribution, in the as-deposited films, is different from the bulk,
causing an increase in the anisotropy. For example, if a large
number of Ni2⫹ ions are on tetrahedral sites in our asdeposited films, they can cause an increase in anisotropy and
a drop in the magnetic moment. As the films are annealed,
the cation distribution can change to the one observed in the
bulk, giving rise to drop in anisotropy and, hence, in
coercivity.12 Change in the cation distribution has been observed in nanocrystalline nickel ferrite powders.13
Ferromagnetic resonance 共FMR兲 study was carried out
on our NiZn ferrite films as a function of annealing temperature. It showed that the resonance line width in the perpendicular configuration, for the sample with T a ⫽200 °C, is
⬃1300 Oe, as compared to ⬃150 Oe in the bulk NiZn
ferrite.14 The large linewidth is due to defects inside the thin
film. A larger value of anisotropy could also contribute to
this increase. The FMR linewidth is observed to decrease
upon annealing upto T a ⫽600 °C, where it achieves a value
of 720 Oe. It increases above this annealing temperature and
shows a value of 1060 Oe at 800 °C. The initial drop in the
linewidth clearly indicates a decrease in the defect density
and/or the anisotropy, which we also thought as the reason
for the reduction of the coercivity. A detailed ferromagnetic
resonance study on NiZn ferrite thin films will be reported
later.
In comparison to our thin films, plated NiZn ferrite films
prepared by Zaag et al.7 and annealed at 500 °C showed a
saturation magnetization and a coercivity of ⬃1000 G and
⬃2 Oe, respectively. However, in that case, the average grain
size observed by scanning electron micrographs was an order
of magnitude larger that that observed in our films. This indicates that the films prepared by different methods lead to
different structural and magnetic properties.
In the present study, we have shown that the defects and
the grain size play a very important role in determining the
magnetic properties of the nanocrystalline NiZn ferrite films.
The annealing of the films can lead to change in grain size
and the defect density, thus affecting the magnetic properties.
ACKNOWLEDGMENT
The authors acknowledge the financial support of Motorola Applied Research Lab, Plantation, FL-33322.
1
J. Dash, S. Prasad, N. Venkataramani, R. Krishnan, P. Kishan, N. Kumar,
S. K. Date, and S. D. Kulkarni, J. Appl. Phys. 86, 3303 共1999兲.
2
M. Desai, S. Prasad, N. Venkataramani, I. Samajdar, A. K. Nigam, and R.
Krishnan, J. Appl. Phys. 91, 2220 共2002兲.
3
P. C. Dorsey, B. J. Rappoli, K. S. Grabowski, P. Lubltz, D. B. Chrisey, and
J. S. Horwiz, J. Appl. Phys. 81, 6884 共1997兲.
4
Z. Qian, G. Wang, J. M. Silvertsen, and J. H. Judy, IEEE Trans. Magn. 33,
3748 共1997兲.
5
J.-S. Lee, B.-I. Lee, and S.-K. Joo, IEEE Trans. Magn. 35, 3415 共1999兲.
6
M. Abe and Y. Tamaura, J. Appl. Phys. 55, 2614 共1984兲.
7
P. J. van der Zaag, P. Lubitz, Y. Kitamoto, and M. Abe, IEEE Trans. Magn.
35, 3436 共1999兲.
8
L. Stichauer, G. Gavoille, and Z. Simsa, J. Appl. Phys. 79, 3645 共1996兲.
9
M. Desai, J. Dash, I. Samajdar, N. Venkataramani, S. Prasad, P. Kishan,
and N. Kumar, J. Magn. Magn. Mater. 231, 108 共2001兲.
10
B. D. Cullity, Introduction to Magnetic Materials 共Addison-Wesley, New
York, 1972兲, p. 385.
11
J. Dash et al., J. Magn. Soc. Jpn. 22, 176 共1998兲.
12
A. Broese Van Groenou, P. F. Bongers, and A. L. Stuyts, Mater. Sci. Eng.
3, 317 共1968兲.
13
C. N. Chinnasamy et al., Phys. Rev. B 63, 184108 共2001兲.
14
M. J. Patni, Ph.D. thesis, IIT Bombay, India, 1972.
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