High frequency permeability of soft magnetic CoFeAlO films with high resistivity

Article abstract of DOI:10.1016/j.jmmm.2004.11.239

As-deposited CoFeAlO films have been prepared by reactive RF magnetron sputtering at input power of 300 W. The X-ray diffraction (XRD) results exhibit a multi-phase structure, which could be consisting of major phases: α-CoFe (1 1 0) and α-CoFe(2 1 1), and minor phase CoFe-oxides with Al ₂O₃ as the amorphous matrix were formed. Moreover, it has also been demonstrated that a nanocrystalline CoFe alloy was obtained and the evaluated sizes were below 15 nm. The thickness effects on the high-frequency characteristics of as-deposited CoFeAlO soft magnetic thin films have been studied in the range of 50 to 1200 nm. The coercivity (Hc) and anisotropy field (Hk) of these films increases largely with decrease in the film thickness. It is explained to be the result of the change in the film composition and surface roughness. The electrical resistivity (ρ) also increases with decreases in the film thickness. The effective permeability of a CoFeAlO thin film is larger than 1000 and remained constant up to 700 MHz with film thickness below 1000 nm. A CoFeAlO film exhibits excellent high-frequency characteristics, which results from softness properties, high ρ, high Hk.

Full text of DOI:10.1016/j.jmmm.2004.11.239

ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 290–291 (2005) 1571–1575
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High frequency permeability of soft magnetic CoFeAlO films
with high resistivity
Ã
N.D. Haa,c, T.S. Yoonb, H.B. Kima, J.J. Lima, C.G. Kima, C.O. Kimb,
aDepartment of Materials Science and Engineering, Chungnam National University, Daejeon, Republic of Korea
bResearch Center for Advanced Magnetic Materials, 305-764 Daejeon, Republic of Korea
cResearch Institute of Technology for Machinery, Ministry of Industry, Hanoi, Vietnam
Available online 25 December 2004
Abstract
As-deposited CoFeAlO films have been prepared by reactive RF magnetron sputtering at input power of 300 W. The
X-ray diffraction (XRD) results exhibit a multi-phase structure, which could be consisting of major phases: a-CoFe
(1 1 0) and a-CoFe(2 1 1), and minor phase CoFe-oxides with Al2O3 as the amorphous matrix were formed. Moreover, it
has also been demonstrated that a nanocrystalline CoFe alloy was obtained and the evaluated sizes were below 15 nm.
The thickness effects on the high-frequency characteristics of as-deposited CoFeAlO soft magnetic thin films have been
studied in the range of 50 to 1200 nm. The coercivity (Hc) and anisotropy field (Hk) of these films increases largely with
decrease in the film thickness. It is explained to be the result of the change in the film composition and surface
roughness. The electrical resistivity (r) also increases with decreases in the film thickness. The effective permeability of a
CoFeAlO thin film is larger than 1000 and remained constant up to 700 MHz with film thickness below 1000 nm. A
CoFeAlO film exhibits excellent high-frequency characteristics, which results from softness properties, high r; high Hk:
r 2004 Elsevier B.V. All rights reserved.
PACS: 61.68.+n; 75.50.Tt; 75.70.ꢀi
Keywords: Soft magnetic properties; High frequency; Nanogranular; X-ray diffraction
1. Introduction
magnetically ultra-soft nanocrystalline ultra-thin films
and the discovery of ultra-high-frequency permeability
in these thin films had led to burst of activity in the study
of the magnetic properties of soft thin films in the MHz
range, or those above a few GHz. At a high operating
frequency, the eddy current causes a reduction in the
permeability, which in turn reduces the efficiency of
magnetic devices. Therefore, materials with high in-
trinsic resistivity can be used to reduce the eddy current
loss. Generally, Fe-based or Co-based amorphous or
granular alloys tend to have significantly higher
electrical resistivity but these alloys have a relatively
low saturation magnetization of less than 15 kG [2] and
decreased permeability at frequencies higher than
Today, electronic devices such as power converters or
radio–communication systems require higher efficiency
and greater compactness [1]. Due to the fast miniatur-
ization of active components, inductive ones (micro-
inductor or micro-transformers) are more often the
largest. Reduction of size can be achieved thanks to new
structures and new materials. The development of
Ã
Corresponding author. Tel.: +82 42 821 6235; fax:
+82 42 822 6272.
E-mail address: magkim@cnu.ac.krXndha@cnu.ac.kr
(C.O. Kim).
0304-8853/$ - see front matter r 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.jmmm.2004.11.239

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100 MHz. Recently, nanocrystalline or nanogranular
CoFe-based oxides or nitrides, like CoFeSiO–SiO2 [3],
CoFeZrO [4] and CoFeN [5] have been used for
fabricating micro-inductors or to suppress noise in radio
frequency magnetic devices. In order to obtain soft
magnetic films for MHz to GHz frequency, two main
loss mechanisms limit the applicability of soft magnetic
films at very high frequencies: (a) Eddy currents. The
losses occur due the conductivity of materials, shape
and, especially, thickness of the films. The frequency-
dependent effective permeability due to current screen-
ing can be obtained by Maxwell’s equations for infinitely
wide films. The classical cutoff frequency f w is then
described by
where g is the gyomagnetic constant and Hk is in-plane
anisotropy field.
From Eq. (2), it is found that a first approach Hk and
Ms determine the high-frequency behavior of the
material. In addition, it turns out that different eddy
current losses (resistivity, capacitive coupling through
spacer layer) and phenomenological LL damping caused
by structural effects such as demagnetization fields,
magnetization dispersion, and domain structure-ripple
fields significantly influence the high-frequency perfor-
mance of the material.
The purpose of this work is to present a method for
fabricating the CoFeAlO thin film with high 4pMs; low
Hc; controlling Hc and determined permeability and
FMR frequency, low eddy current losses and suitable
high-frequency permeability up to the GHz range.
f w ¼ 4r=ðpm0mid2Þ.
(1)
Here r is the resistivity of the material; mi is the
intrinsic initial permeability, and d the thickness of a
single film. (b) Ferromagnetic resonance (FMR) and
Landau–Lifshitz damping (LL): FMR occurs when the
frequency of the applied RF field matches the char-
acteristic precession frequency of the spin. For thin
films, for which the demagnetizing field along the z
direction is m0Ms; the FMR frequency is expressed as
2. Experiment
CoFeAlO thin films were prepared on Si (1 0 0)
substrates in a RF reactive magnetron sputtering in
O2/Ar atmosphere of 2mTorr at an input power of
300W, and a background pressure of less than to
5 Â 10ꢀ7 Torr. For some samples, a static field in the
range of 80–120 Oe was applied parallel to the substrate
1=2
f FMR ¼ 2pgðHk m0MsÞ
;
(2)
Fig. 1. Variation of coercivity (A), anisotropy field (B), electrical resisitivity (C), saturation magnetization (D) for 600 nm CoFeAlO
thin films deposited at different O2:Ar flow ratios and input power of 300 W, 2 mTorr.

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surface during the film deposition in order to check the
effect of the field on the magnetic anisotropy. The film
composition was varied by changing the number of Al
chips on a Co70Fe30 target and oxygen partial pressure
in the range of 0–8%, and was determined by AES
analysis. The film thickness is in the range of
50–1000 nm. The microstructure of the films was
examined using X-ray diffraction (XRD). 4pMs; Hc;
and Hk were measured using a VSM with applied
magnetic field parallel to film plane. In-plane Hk was
estimated from the slope of a minor B–H loop only
when the minor loop was almost linear. The AC
complex permeability m0 and m00 were measured with
the network analyzer (HP8752C) in the range of
1–1000 MHz, and the
temperature, using the conventional four-point probe
r was measured at room
Fig. 2. X-ray diffraction patterns of the CoFeAlO thin films in
an as-deposited state at oxygen partial pressure varying from
0% to 8%, with 8 Al chips, input power of 300 W, 2 mTorr, thin
film thickness of 600 nm, applied static field during sputtering.
method.
3. Results and discussion
Figs. 1(A)–(D) show magnetic properties and elec-
trical properties of the CoFeAlO films with three
different numbers of aluminum chips (8, 10, 12) and
varying oxygen partial pressure in the range of 0–8%.
As shown in Figs. 1(A,B), Hc and Hk as functions of
oxygen partial pressure decreased with increasing PO
and reached minimum value at 4% or 3% correspond-
ing with 8 or 10, 12 aluminum chips, respectively, then
2
increased with increasing PO : Fig. 1(C) shows the
2
resistivities of the films as a function of PO : At 3% or
2
6%, the resisitivities of the films are anomalous changed.
The first point at 3% caused by increased oxide content
in the grain boundary of the thin films; the second point
at 6% may be caused by refinement of the CoFe grains
completely separated by oxide matrix (Al2O3, or Fe(Co)-
oxides) and form nanogranular structures. Fig. 1(D)
presents the relation between the saturation magnetiza-
tion and PO for magnetic thin films fabricated at 300 W
and 2 mTorr with different numbers of aluminum chips.
2
The 4pMs decreases from 23–14kG as the aluminum
chips and PO increase from 8 to 12 Al chips and from 0
2
to 8 at%, respectively. To clarify the origin of the soft
magnetic properties, the change in the microstructure of
this alloy system has been examined by selecting five as-
deposited samples, representing the different composi-
tions of the CoFeAlO component fabricated with
Fig. 3. Oxygen partial pressure dependence of effective perme-
ability (A), film thickness dependence of anisotropy field and
effective permeability (B) for CoFeAlO thin films.
different PO
:
2
Fig. 2 shows XRD patterns of CoFeAlO films. All
films done in this study show the halo pattern, and
dependence on the PO ; which indicates the existence of
2
an amorphous state. From the diffraction pattern, the
peak positions of amorphous phase of peaks were found
to decrease as the PO increases. At 3% and 8% the
peak at 2y in the range of 62–711 and 81–861
corresponding with CoFe(2 0 0) and CoFe(2 1 1) de-
creased and was more broadened. Especially at
2
XRD patterns are dramatically changed, the intensity of

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Fig. 4. Frequency dependence of permeability for CoFeAlO films as-deposited at 300 W, 2 mTorr, PO ¼ 3%.
2
PO ¼ 8%, the two peaks as mentioned above these were
4. Conclusion
2
broad similar to the amorphous state and new peaks
appeared at 2y ¼ 302331 and 34–381 corresponding to
the mixed amorphous oxides phases of CoFe-oxides and
Al2O3.
Soft magnetic nanogranular films with high resistivity,
saturation magnetization, and high Hk induced by
applied static field can be realized in CoFeAlO film
with a wide compositional range of Al and O2, b y RF
reactive magnetron sputtering. Soft magnetic nanocrys-
talline CoFeAlO films exhibit a good frequency response
of high-frequency permeability and a resonance fre-
quency as high as for the films with composition of
CoFeAlO. Soft magnetic CoFeAlO films are strong
candidates for high-frequency magnetic devices in the
GHz range. The compositional dependence of magnetic
and electrical properties is related to the formation
process of granular structures.
Fig. 3(A) shows effective permeability of CoFeAlO
films as a function of PO ; with increasing PO the mefff
2
2
increased and reached maximum values of 1024, 3562
and 9982 corresponding with 8, 10 and 12 Al
chips at 3% and then decreased continuously.
All mefff of the samples that were obtained in
this study are very high and flatten up to 700 MHz as
shown in Figs. 4(A,B,C,D), and exhibited high FRM
frequency. The values of FMR were calculated
from Eq. (2) in the range of 1.5–2 GHz. In Fig. 3(B)
shows the effect of the film thickness on Hk and
the mefff of the CoFeAlO films. The Hk decreases as a
function of the film thickness. High anisotropy field can
be obtained by controlling film thickness and applying
static field. These results suggest that the CoFeAlO thin
films have excellent high-frequency characteristics,
partly due to the high r and to the Hk; both of which
reduce the eddy current losses and increased FMR
frequency.
Acknowledgments
This work was supported by the Korean Science and
Engineering Foundation through the Research Center
for advanced Magnetic Materials at Chungnam Na-
tional University and the BK 21 program.

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