Thin Film and Multilayers

1. Uniaxial anisotropy induced in 57Fe/Co/Al multilayers
Vishal Jain, Snehal Jani, N. Lakshmi, V. Sebastian, V. R. Reddy, K. Venugopalan, and Ajay Gupta
Citation: Journal of Applied Physics 113, 233906 (2013); doi: 10.1063/1.4811535
View online: http://dx.doi.org/10.1063/1.4811535
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2. Uniaxial anisotropy induced in 57
Fe/Co/Al multilayers
Vishal Jain,1
Snehal Jani,1
N. Lakshmi,1,a)
V. Sebastian,2
V. R. Reddy,3
K. Venugopalan,1
and Ajay Gupta3
1
Department of Physics, Mohanlal Sukhadia University, Udaipur 313001, India
2
Department of Physics, Nirmalagiri College, Kannur 670701, India
3
UGC-DAE Consortium for Scientific Research, Indore 452001, India
(Received 11 February 2013; accepted 5 June 2013; published online 19 June 2013)
The magnetic properties of 57
Fe/Co/Al multilayers with 20 and 40 trilayers, deposited on Si (100)
substrate using ion beam sputtering, are reported here. X-ray reflectivity and X-ray diffraction
studies indicate the formation of good quality films with preferential growth along the (110)
direction. Conversion electron M€ossbauer spectra show considerable inter-mixing between the
layers and the formation of Fe-Al/Fe-Co-Al phases. The samples are extremely soft with
coercivities 0.48 Â 103
A/m, exhibit strong in-plane uniaxial anisotropy and calculations show
that they possess a high ferromagnetic resonance frequency of $2 GHz. The saturation
magnetization value of 1.80 Â 106
A/m is comparable with that obtained in multilayer samples with
much higher content of Co. The combination of magnetic properties in these multilayers thus
makes them ideal candidates for high frequency device applications. VC 2013 AIP Publishing LLC.
[http://dx.doi.org/10.1063/1.4811535]
I. INTRODUCTION
Fe/Co based alloys, nanowires, and thin films have high
Curie temperature, low coercivity (Hc $ 5–20 Oe), and the
highest reported saturation magnetization (Ms $ 24 kOe).1,2
These properties make this class of materials ideal for many
electronics/microwave device applications such as magnetic
write heads, highly sensitive sensors, planar inductors and
high-frequency micro-transformers. In addition, well-defined
in plane uniaxial anisotropy is also important for applications
as magnetic head materials3
while high value of ferromag-
netic resonance frequency (FMR) and 100% squareness are
desirable for many microwave and integrated magnetic devi-
ces working in the GHz frequency range4
which can be real-
ized to a good extent by addition of Al into Fe-Co alloys to
improve uniaxial anisotropy, saturation magnetization, and
high-frequency response.5
The requirements are more easily met in Fe-Co based
multilayers which have large Ms, low coercivity, and tunable
anisotropy, making them promising materials for various
high-frequency applications. This paper discusses the mag-
netic properties of a very soft 57
Fe/Co/Al multilayer system
which also exhibits strong in plane uniaxial anisotropy.
II. EXPERIMENTS
Two samples of 57
Fe/Co/Al trilayers with different indi-
vidual layer thickness (designated in this paper as S1 for the
20 trilayer stack ([57
Fe13A˚ /Co6A˚ /Al8A˚ ]20) and S2 for the 40
trilayer stack ([57
Fe7A˚ /Co3A˚ /Al4A˚ ]40)) were deposited on
Si(100) substrate using ion beam sputtering. Calibration of
deposition rate was done by individually sputtering 57
Fe, Co,
and Al for 10 min under identical conditions.
Structural parameters of the multilayers, such as the inter-
facial roughness, phase formation, crystallite size, etc., were
obtained by X-ray reflectivity (XRR) done at the ID32 beam-
line at the European Synchrotron Radiation Facility (ESRF),
Grenoble, France using 15keV incidence energy and X-ray dif-
fraction (XRD) using Bruker D8 Advance X-ray diffractome-
ter. Analysis of the reflectivity data to obtain layer thickness
and interfacial roughness was done using Parratt’s model6
(Table I). The static magnetic properties of the samples were
characterized by a Lake Shore 7304 vibrating sample magne-
tometer (VSM). To generate polar curves, VSM scans were
performed by first orienting the applied magnetic field in the
plane of the film, and taking axis of the pole pieces of the mag-
net as the initial direction (i.e., h ¼ 0
). The sample was then
rotated through steps of 10
and M-H curves were recorded for
each orientation. After plotting the variation of this (arbitrary) h
with quantities like HC, squareness (Mr/Ms), etc., we obtain the
easy axis (EA). The figures were then re-plotted by fixing the
azimuthal angle h0 ¼ 0
for the value of h along the easy axis.
57
Fe conversion electron M€ossbauer spectra (CEMS) were
recorded at room temperature using a flowing gas (95% He-5%
CH4) proportional counter and a 30mCi 57
Co (Rh) source.
III. RESULTS AND DISCUSSION
A. Structural characterization
Figures 1(a) and 1(b) show the XRD patterns of samples
S1 (20 trilayers) and S2 (40 trilayers). In both samples, in
addition to the peaks due to the substrate (Figure 1(c)), a dif-
fraction peak corresponding to FeCo(110) is present. The
jump in XRD background at 2h ¼ 66
is due to the substrate
Si(300) peak. A peak corresponding to the Fe-Co-Al alloy is
also visible in S2, because of better intermixing of the much
thinner individual layers (inset of Figure 1(b)). The crystal-
lite size (calculated using the Scherrer formula7
) for S2 is
greater than that of S1.
In Figure 2, the reflectivity is shown as a function of the
momentum-transfer vector q, where q ¼ 4psin h/k, h is thea)
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0021-8979/2013/113(23)/233906/4/$30.00 VC 2013 AIP Publishing LLC113, 233906-1
JOURNAL OF APPLIED PHYSICS 113, 233906 (2013)
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3. incident angle, and k is the radiation wavelength (0.8265 A˚ ).
Considerable interdiffusion across the interfaces of the very
thin individual layers (maximum of $13 A˚ for S1 and $7 A˚
for S2) is evident in the Bragg peaks which are broad, asym-
metric, and of low intensity which has led to the formation
of Fe-Al phase for S1 and Fe-Co-Al phases for S2.
The CEMS spectra of S1 and S2 (Figure 3) are sextets
with broad and asymmetric lines indicative of a number of
close lying sextets averaged around the hyperfine field values
of different phases. The spectra were hence fitted for hyper-
fine field distributions. Presence of a considerable nonmag-
netic part in the spectrum of sample S1 is observed, which
can be attributed to the formation of FeAl at the interfaces
due to diffusion of Fe into Al.8
Interdiffusion between Fe-Al
and Fe-Co at the respective interfaces occurs on deposition.
However, on comparing the enthalpy of formation9
of FeAl
and FeCo (À10 kJ/mol and þ13 kJ/mol for FeCo, respec-
tively), larger amount of FeAl can be expected to form at the
Fe-Al interface as compared to FeCo at the Fe-Co interface.
Thus, in sample S1, since total intermixing has not occurred,
formation of FeAl and FeCo is confined to the interfaces.
However, in sample S2, due to the very thin individual
layers, almost total inter mixing has occurred leading to for-
mation of Fe with Co as nearest neighbor (nn) in quantities
enough to be visible in the M€ossbauer spectrum as the sextet
with average field 33 T. Interdiffusion between Al and Co
layers leading to the formation of AlCo also occurs as the en-
thalpy of formation is À24 kJ/mol. However, since the peak
positions for AlCo nearly coincide with that of FeCoAl, it is
not possible to distinguish the two from the XRD spectrum.
Also, since there is no Fe, it would not be visible in the
CEMS spectrum.
In both the samples, the observed trends in M€ossbauer
spectra can be related to the very small Fe/Co/Al individual
layer thicknesses. Inter-diffusion between the Fe and Al
layers will be more than that between Fe and Co layers, due
to the greater mobility of Al. Thus, on an average, more Fe
would have Al than Co as nearest neighbors. This is reflected
in the value of the highest hyperfine field observed in S1,
which is only $31 T. In S2, the individual layers are so thin
that almost complete interdiffusion has occurred, leading to
the formation of non-stoichiometric FeCoAl alloy as
observed in the XRD pattern of this sample. The variation in
individual concentrations is evident from the close-lying sex-
tets in the CEMS spectrum. Also, since there are no clear
interfaces in this sample, no nonmagnetic Fe-Al phase has
formed in quantities enough to be detectable by XRD or
CEMS.
B. DC magnetization studies
DC magnetization measurements performed at room
temperature for in plane configuration of the applied mag-
netic field are shown in Figures 4(a) and 4(b) with the
TABLE I. Structural parameters obtained from XRD and XRR.
Sample
Crystallite
Size (nm)
(61)
Assumed
thickness
(A˚ )
Simulated
thickness
(A˚ ) (62%)
Average interfacial
roughness (r)
(A˚ ) (62%)
S1 5 560 $545 7.2
S2 8 564 $560 8.2
FIG. 1. XRD patterns of samples (a) S1, (b) S2, and (c) silicon substrate
(100).
FIG. 2. XRR patterns of S1 and S2. Red lines in the XRR patterns are exper-
imental points and the thin black lines are simulations.
FIG. 3. Room temperature M€ossbauer spectra of samples S1 and S2 and the
corresponding hyperfine field distributions.
233906-2 Jain et al. J. Appl. Phys. 113, 233906 (2013)
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4. applied field along the EA and hard axes (HA) of magnetiza-
tion. Although both samples are magnetically soft
(Hc 0.48 Â 103
A/m), the coercivity of S2 is more than that
of S1. The saturation values (Ms) corresponding to EA for
samples S1 and S2 are 1.44 Â 106
and 1.80 Â 106
A/m (18.2
and 22.6 kOe) with saturation fields of $0.95 Â 103
A/m. In
comparison, the samples do not saturate at 7.96 Â 103
A/m
along the HA, and magnetization values at this field are
0.94 Â 106
and 1.25 Â 106
A/m (11.7 and 15.7 kOe), respec-
tively, for S1 and S2. The value of Ms is lower than that for
FeCo alloys (Ms $ 1.90 Â 106
A/m) but larger than observed
for Fe-Co-Al ternary alloys ($1.20 Â 106
A/m) and is mainly
because of the presence of nonmagnetic Fe-Al layers at the
interfaces. The value of Ms obtained in S2 matches with that
reported for FeCoAl thin films5
(1.78 Â 106
A/m). In fact, the
Ms value obtained in our film is more than that obtained in
films with much larger content of Co.10,11
Calculations, validated by experimental studies in bulk
FeCo alloys and multilayers, have shown that the magnetic
moment of Fe in these materials depends on the geometry,
proximity, and Co coordination number.12
Eriksson et al.13
have pointed out that in perfect, “ideal” multilayers, since Fe
comes into contact with Co only at the interfaces, the overall,
average moment of the multilayer is lower. However, pres-
ence of Co as nearest neighbor (nn) to Fe atoms is higher for
random bulk alloys which is also the case for multilayers
with a good deal of interdiffusion thus being far from the
ideal situation. In this case, these effects lead to a narrowing
of the d bands in Fe leading to an enhancement of spin
moments.12
However, in the case of Co, since the spin-up
band is nearly completely filled, a change in the number of
Fe atoms as first nn does not affect its magnetic moment.
The difference in the values of Ms for samples S1 and S2 can
thus be attributed to the effect of Co co-ordination to Fe.
Although both samples are very thin, with the same nominal
quantities of Fe, Co, and Al, in the case of S1, the larger indi-
vidual layer thickness and the presence of non-magnetic
FeAl in addition to Al reduces the overall magnetic moment
of the multilayer (1.25 Â 106
A/m, equivalent to 1.68 lB/
formula unit) in comparison to S2, where the moment is
1.80 Â 106
A/m, equivalent to 2.42 lB/formula unit. The
highly randomized state of the multilayers in S2, in which
intense interdiffusion has occurred, leads to larger number of
Co atoms as first neighbour shell to Fe atoms in this sample.
Also, although the amount of Al is about 28%, since Al in
S2 can be considered to be interspersed rather than present
as a distinct, non-magnetic spacer layer, the coupling
between the different Fe-Co present is also greatly enhanced
than that in S1. Together, these effects have resulted in sam-
ple S2 having a moment close to that of pure FeCo.
Polar diagrams between the azimuthal angle and the
squareness/coercivity were obtained following the procedure
described earlier in the experimental section (Figure 4).
Figures 4(c)–4(f) represent the polar magnetization curves of
the samples measured along the EA and HA, respectively.
The EA curves have high squareness ratios with remanence
ratios Mr/Ms ¼ 0.974 for S1 and 0.987 for S2 and a coercive
field Hc ¼ 0.03 Â 103
A/m for S1 and 1.38 Â 103
A/m for S2.
Along the HA, a linear magnetization curve is obtained.
These results suggest a perfect in-plane; two fold magnetic
uniaxial anisotropy for both samples.
In most thin multilayers with Co as first layer, in-plane
anisotropy is generally observed due to the larger anisotropy
of Co layers.14
In the present samples, although Fe was first
deposited on the substrate followed by Co, samples S1 and
S2 both show two fold, in-plane, uniaxial anisotropy.
Various authors have speculated on different reasons and
one of the more convincing one seems to be that the strength
of ferromagnetic coupling between Co and Fe seems to be as
good as the effect obtained by application of an external field
oriented in the plane of the film. Experimentally it has been
observed that thin Fe-Co multilayer systems within the range
of thickness and concentrations of Fe and Co as in the case
for our samples show a strong, in-plane anisotropy irrespec-
tive of whether Fe or Co is the underlayer. Calculations of
the magnetic anisotropy energy for this range of thickness/
concentration show that it is negative for an in-plane config-
uration leading to strong in-plane anisotropy.12,15
Although the substrate used in the present study is Si
(100), a comparison of results of different experimental stud-
ies shows that the substrate does not have any significant
role in determining the magnetic anisotropy in Fe-Co multi-
layers and that it is influenced only by relative thickness/
roughness, etc.16,17
For example, Ciprian and Carbucicchio
have compared the magnetic properties of Fe-Co multilayers
grown on Si and glass substrates and have observed strong
in-plane anisotropy in both.18
For purposes of comparison, the anisotropy constant can
be estimated by a simplified solution of the total magnetic
FIG. 4. (a) and (b) Normalized room temperature magnetization curves
along the easy and hard directions of magnetization. (c)-(h) Polar and fitted
curves of squareness/coercivity as a function of azimuthal angle h0 (in
degrees).
233906-3 Jain et al. J. Appl. Phys. 113, 233906 (2013)
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5. energy equation given below, which consists of the anisot-
ropy energy EK, demagnetization energy ED, and the external
field energy EH.19,20
Thus the total energy, ETotal is given by
ETotal ¼ EK þ EH þ ED: (1)
However, it has been pointed out19,20
that in thin films
ED favours an in-plane sense of magnetization and so, since
the magnetization in the present samples lies in the plane of
the film, contribution to ETotal from ED is constant and so
does not vary with the azimuthal angle. The anisotropy con-
stants have, therefore, been estimated using the expression
for ETotal given below
ETotal ¼ EK þ EH
¼ K1sin2
h þ K2sin4
h À l0MSHcosðh0 À hÞ; (2)
where h is the angle between the EA and the direction of Ms,
and h0 the azimuthal angle. In the present samples, the anisot-
ropy constants K1 and K2 were estimated to be 4 Â 105
J/m3
and À3 Â 104
J/m3
for S1 and 2 Â 105
J/m3
and À4 Â 104
J/m3
for S2, respectively, by fitting the experimental Mr/Ms vs. h0
curve shown in Figures 4(g) and 4(f) and then solving for
Eq. (2). Fitting the experimental Mr/Ms curve, the squareness
values obtained for S1 and S2 are 0.98 and 1.00, respectively.
In thin magnetic films with in-plane anisotropy, the natural
resonance frequency can be calculated by the Kittel equa-
tion.21
When magnetic materials are designed for high
frequency and microwave applications, one of the most
important parameters to be considered is the cut-off frequency
which is limited by the FMR. This is because in most mag-
netic materials, there is very little change in the permeability
with frequency except in the vicinity of FMR. Once the
frequency matches the FMR, there is an abrupt decrease in the
permeability and, therefore for wide applications, the upper
cut off frequency should be high, i.e., the FMR has to be high.
An FMR of the order of GHz is, therefore, highly desirable.22
FMR of both samples reported in this study is comparable and
is $2 GHz, thus making them suitable for high frequency and
microwave applications.
IV. CONCLUSIONS
This study shows that in the limit of very thin individual
layers, multilayer stacks consisting of Fe and Co with Al as
spacer layer results in the formation of extremely soft mag-
netic multilayers with high saturation moment and strong
two-fold in plane uniaxial anisotropy with 25 at. % Al and
only 25 at. % Co. The combination of high Ms, extremely
low coercivity, high in-plane uniaxial anisotropy and fairly
high FMR along with squareness ratio $1 in the multilayers
reported in this paper thus make them good potential candi-
dates for many device applications which require these com-
binations of magnetic properties, especially in the
microwave range. Moreover, use of lesser amount of Co to
obtain better results would also make these materials more
economical.
ACKNOWLEDGMENTS
This work was supported by UGC-DAE CSR CRS,
Indore, UGC DSA and DST-FIST schemes of the
Department of Physics, M. L. Sukhadia University, Udaipur.
We are greatly indebted to Parasmani Rajput, M. Zaja˛c, and
R. R€uffer for the XRR measurements.
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233906-4 Jain et al. J. Appl. Phys. 113, 233906 (2013)
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