Giant magnetoresistance in laser-deposited permalloy/Ag multilayers
Jörg Faupel, Hans-Ulrich Krebs, Andrea Käufler, Yuansu Luo, Konrad Samwer et al.
Citation: J. Appl. Phys. 92, 1171 (2002); doi: 10.1063/1.1489088
View online: http://dx.doi.org/10.1063/1.1489088
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Published by the American Institute of Physics.
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JOURNAL OF APPLIED PHYSICS
VOLUME 92, NUMBER 2
15 JULY 2002
Giant magnetoresistance in laser-deposited permalloyÕAg multilayers
Jörg Faupel and Hans-Ulrich Krebsa)
Institut für Materialphysik, University of Göttingen, Hospitalstrasse 3-7, 37073 Göttingen, Germany
Andrea Käufler, Yuansu Luo, and Konrad Samwer
I. Physikalisches Institut, University of Göttingen, Bunsenstrasse 9, 37073 Göttingen, Germany
Satish Vitta
Department of Metallurgical Engineering and Materials Science, Indian Institute of Technology, Bombay,
Mumbai 400076, India
共Received 18 January 2002; accepted for publication 6 May 2002兲
Giant magnetoresistance 共GMR兲 of 3.5% in low fields of about 10 Oe was observed at room
temperature in as-prepared laser-deposited Ni80Fe20 /Ag 共permalloy/Ag兲 multilayers. Strong
columnar growth in combination with preferential sputtering of Ag from the film surface during
deposition of Ni80Fe20 layer helps to directly create a discontinuous multilayer structure necessary
for high GMR values. The magnetoresistance was found to increase to 5.1% after annealing for just
10 min at 275 °C. This increase is attributed to structural relaxation processes such as demixing of
the intermixed interfaces, preferential diffusion of Ag to the column boundaries and reduction of
structural defects. Pulsed laser deposition appears to be a suitable technique for the preparation of
permalloy/Ag films with considerable GMR in a one-step process. © 2002 American Institute of
Physics. 关DOI: 10.1063/1.1489088兴
Multilayers of Ni80Fe20 /Ag 共Ni80Fe20 , permalloy, Py兲
are well known for a low field giant magnetoresistance
共GMR兲 effect of the order of 5%– 6% in saturation fields of
about 10 Oe at room temperature.1 These are of special interest because they can be used as highly sensitive magnetic
field sensors. Py/Ag multilayers prepared at room temperature by magnetron sputtering1 or molecular beam epitaxy2
共MBE兲 show negligible GMR in the as-prepared state. The
multilayers prepared by sputtering exhibit the GMR values
mentioned above only after annealing at 315 °C for 10 min
while those prepared by MBE require annealing at 400 °C for
40 min to show similar GMR values. The presence of magnetic Py islands in the Ag matrix in a quasimultilayer configuration is known to promote magnetostatic antiferromagnetic ordering leading to the GMR effect.3,4 Here it is
reached by the destruction of the continuous multilayer to
form a discontinuous structure.
The idea of the present work was to obtain a similar
discontinuous state of the Py/Ag multilayers essential for
observing the GMR effect already in the as-deposited state
itself. This is made possible using pulsed laser deposition
共PLD兲 because of the columnar growth tendency in many
systems.5 To attain a discontinuous structure the growth behavior of Py on Ag and Ag on Py had to be studied in detail
so that the critical value for the thickness at which a discontinuous to a continuous layer formation transition occurs was
determined. The GMR effect could be optimized on annealing at relatively low temperatures of 275 °C, when compared
to those made by sputtering or MBE. The structural and
magnetoresistive properties of the as-deposited and annealed
multilayers are discussed and compared with those of conventional deposition techniques.
The multilayers were prepared by PLD at room temperature in ultrahigh vacuum (⬍10⫺9 mbar) using a KrF excimer laser 共wavelength of 248 nm, pulse duration of 30 ns,
laser fluence of 5– 6 J/cm2兲. Details of the exact geometry are
published elsewhere.6 The multilayers were deposited onto
clean Si共111兲 substrates kept at room temperature. A typical
multilayer consists of 20 bilayers of Ag/Py with a Ag capping layer and is denoted as 关 Ag(x)/Py(y) 兴 20 . The thicknesses x and y 共given in nm兲 of the Ag and Py layers, respectively, were varied but were always less than 7.0 nm as
thicknesses larger than this value would not result in coupling of Py needed for GMR. High purity Ag foil and a
Ni80Fe20 alloy target were used for the deposition of the multilayers. GMR measurements were performed at room temperature using a four-point in-line geometry of contacts with
the current flowing in plane and the magnetic field applied
parallel to the film plane. For each sample two measurements
were performed with the applied magnetic field parallel and
perpendicular to the current flow. The maximum GMR value
is given by (R 0 ⫺R S )/R S , where R S is the resistance in saturation field and R 0 is that in zero field. The microstructure of
the multilayers was studied by wide-angle x-ray diffraction
in Bragg–Brentano geometry using Co K␣ radiation and by
low-angle 共below 3°兲 x-ray reflectometry using Cu K␣ radiation. Simulations were performed using interface and surface
roughnesses as well as bilayer periodicity as fit parameters.
On some samples, hot-stage wide-angle x-ray measurements
were performed in UHV ambient using a constant rate of
0.5 °C/min and taking scans every 10 °C. Other multilayers
were annealed at different temperatures in the range 250–
a兲
Electronic mail: [email protected]
0021-8979/2002/92(2)/1171/3/$19.00
1171
© 2002 American Institute of Physics
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1172
Faupel et al.
J. Appl. Phys., Vol. 92, No. 2, 15 July 2002
FIG. 2. 共a兲 GMR vs magnetic field H curves and 共b兲 maximum GMR values
for 关 Ag(x)/Py(1.4) 兴 20 samples with different Ag-spacer thicknesses x as a
function of annealing temperatures.
FIG. 1. In situ rate measurement at the Ag/Py interface shows that the film
thickness is reduced due to strong sputtering of the heavier Ag atoms by the
impinging Ni and Fe ions until an equilibrium rate is observed after about
170 laser pulses, 1.5 nm.
375 °C for 10 min to study the effect of annealing both on
the structure as well as the GMR effect.
The discontinuous to a continuous layer formation transition was determined by monitoring the deposition rate during growth of Py on Ag. The results are shown in Fig. 1. At
the Py/Ag interface the mass, as observed by the quartz crystal balance, is reduced first and an equilibrium rate is observed after about 170 laser pulses. The reduction of mass is
due to a combination of sputtering as well as ion implantation effects as discussed in detail in Ref. 7. The energetic Fe
and Ni ions 共100 eV兲8 present in the laser plasma plume
sputter the heavy Ag atoms from the underlying layer and
also get implanted. The equilibrium deposition rate for
Ni80Fe20 is observed as soon as the Ag film surface is completely covered by Fe and Ni atoms. At this thickness the Py
film undergoes a transition from a discontinuous state to a
continuous state. This critical thickness was determined both
from the thickness monitor and x-ray diffraction measurements and is found to be about 1.4 nm. Hence in the present
work the thickness of the Py layer is kept around this value
in all the different multilayer combinations.
The maximum GMR effect of 3.5% with a full width at
half maximum 共FWHM兲 of 10.7 Oe and a sensitivity of
0.52% Oe⫺1 at room temperature was observed in asdeposited 关 Ag(6.2)/Py(1.4) 兴 20 multilayers and is shown in
Fig. 2共a兲. It should be mentioned here that this GMR value is
similar to that observed in Py/Ag multilayers prepared by
sputtering and MBE. The main difference however is that in
the present work these values are observed in as-deposited
multilayers while those prepared by the more conventional
methods exhibit these values only after annealing at relatively high temperatures. The GMR value in multilayers with
thicknesses other than 1.4 nm for the Py layer and 6.2 nm for
the Ag layer is found to be ⬍3.5% as shown in Fig. 2共b兲. All
the samples show anisotropic magnetoresistance effects
共AMR兲 of 0.2%–0.4%, a large region, where the resistance
linearly changes with the applied magnetic field, and very
low hysteresis. Thus, the resistance values are independent of
the history of the sensor which is important for applications.
The GMR value of the as-deposited multilayers was
found to increase on annealing for 10 min in the temperature
range 275–300 °C. A maximum GMR value of 5.1% was
obtained after annealing at 275 °C, while the maximum sensitivity of 0.66% Oe⫺1 was observed at 250 °C. It should be
mentioned that the mean resistivity of the multilayer remains
almost constant during annealing. The increase of GMR is
therefore based on an increment of (R 0 ⫺R S ). In Fig. 2共b兲
the maximum GMR values are shown as a function of annealing temperature for multilayers with different Ag spacer
layer thicknesses. It can be seen that the GMR value increases systematically with increasing spacer layer thickness
and for thicknesses ⬍2.2 nm no GMR was observed even
after annealing. Notice however the rate of increase of GMR
is low compared to that observed in conventionally grown
multilayers.2,3 This is because the multilayers in the asdeposited state already exhibit considerable GMR effect due
to the special deposition conditions adopted for growth during PLD.
The structural changes accompanying annealing of the
multilayers have been studied using both low angle x-ray
reflectivity and high angle x-ray diffraction. The low angle
x-ray reflectivity scans from the 关 Ag(4.4)/Py(1.4) 兴 20
multilayer as a function of annealing temperature are shown
in Fig. 3. The presence of a clear first order bilayer period
peak together with total thickness oscillations in the asprepared multilayer indicates that the layers are well formed
and ordered along the growth direction. On annealing however the first order peak shifts to higher angles or lower
bilayer periods, decreases in intensity and becomes broad
indicating that the layered structure becomes weak. At the
same time the total thickness oscillations disappear due to
increasing surface roughness. The rms roughness ␴ at the
two interfaces, Py on Ag and Ag on Py has been determined
from a simulation of the reflectivity of the multilayer. The Py
on Ag interface roughness is found to increase from about
2.5 to 4.0 nm due to annealing at 275 °C while the Ag on Py
interface roughness is found to be constant at about 1.5 nm.
This clearly shows the asymmetric growth behavior of the
two components, Py and Ag as well as the asymmetric annealing response at the two interfaces.
The high angle x-ray diffraction spectrum as a function
of temperature from the 关 Ag(2.7)/Py(1.8) 兴 20 multilayer is
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Faupel et al.
J. Appl. Phys., Vol. 92, No. 2, 15 July 2002
FIG. 3. Low angle x-ray reflectivity as a function of annealing temperature
shows that the layered structure becomes weak and discontinuous. The clear
first order peak observed in the as-deposited multilayer indicates the presence of good layering along the growth direction.
obtained using a high vacuum (⬍10⫺6 mbar) hot stage attachment and is shown in Fig. 4共a兲. The presence of clear
satellite peaks up to about 300 °C indicates the presence of a
layered structure which breaks down for T⬎300 °C as seen
by the disappearance of satellite peaks. These results are in
close agreement with the low angle reflectivity studies. Concurrent to the disappearance of the satellite peaks the intensity of the Ag共111兲 peak increases and sharpens indicating a
grain growth phenomenon.
In order to understand the structural changes accompanying annealing in detail the Ag planar spacing d of the
strongest peak corresponding to Ag共111兲 plane reflection is
plotted, corrected for linear thermal expansion, as a function
of temperature in Fig. 4共b兲. Also the planar spacing of the
multilayers from the annealing series depicted in Fig. 2共b兲
shows the same salient features. The planar spacing decreases with increasing temperature and reaches, independent of the initial value, a quasi-steady-state value in the
temperature range 250–300 °C. This indicates that the struc-
ture reaches a state of relaxation. At this state the annealed
samples posses the highest GMR values as shown in Fig.
2共b兲.
The presence of pronounced GMR effect exhibiting
structure in the as-deposited Py/Ag multilayers prepared by
pulsed laser deposition can be understood by considering the
energetics of the deposition process. The laser generated
plasma plume has energetic ions, of the order of 100 eV,
which help in increasing the surface roughness, sputtering at
the interfaces and ion implantation. Because the sputter yield
of Py is much lower than that of Ag, in the early stages of Py
growth on Ag the interfacial roughness is increased by preferential sputtering of Ag by the impinging Fe and Ni ions.7,9
The growth of Py columns however is unhindered because of
the low sputter yields for Fe and Ni. This model accounts for
the asymmetric interface roughness observed at the two interfaces, Py on Ag and Ag on Py. The high interfacial roughness requires that the Ag layer has a certain minimum thickness so as to avoid bridging of the Py islands and thus
ferromagnetic coupling. Then, a magnetostatic coupling favors antiferromagnetic alignment which is needed for a high
GMR value and this is why the multilayers exhibit a high
GMR for a Ag layer thickness of 6.2 nm. The high kinetic
energy of the ions which is a characteristic feature of high
fluence PLD results in considerable intermixing at the interfaces, a residual compressive stress in the films, and a large
density of defects.8 Annealing results in an increased diffusion of Ag into the boundaries between Py columns and also
agglomeration of Ag within the Py layers. This reduces direct
coupling of the Py islands and promotes magnetostatic antiferromagnetic ordering. The decrease in overall defect density increases spin dependent electron scattering contribution
to the resistivity and thus aids in GMR.
In conclusion, Py/Ag multilayers which exhibit room
temperature GMR of 3.5% in the as-deposited state have
been prepared by PLD. The discontinuous growth mode of
Py together with sputtering of Ag by energetic Fe and Ni
ions result in a structure which is ideally suited for GMR
effect. Annealing the preexisting discontinuous layered structure enhances the GMR effect to 5.1% due to a combination
of Ag diffusion to isolate the Py layers and reduction of
structural defects. It should be possible to produce multilayer
structures with higher GMR value in the as-deposited condition by a further optimization of the PLD process.
1
FIG. 4. 共a兲 High angle x-ray diffraction patterns from hot-stage measurements at elevated temperatures 共spectra were taken in steps of 10 °C兲 and 共b兲
the planar spacing d of the highest peak as a function of annealing temperature from 关 Ag(x)/Py(y) 兴 20 multilayers corrected for thermal lattice expansion. The maximum GMR values are found in the marked area, when the
multilayers are in a relaxed state.
1173
T. L. Hylton, K. R. Coffey, M. A. Parker, and J. K. Howard, J. Appl. Phys.
75, 7058 共1994兲.
2
R. F. C. Farrow, R. F. Marks, T. A. Rabedeau, M. F. Toney, D. Dobbertin,
R. Beyers, and S. S. P. Parkin, J. Appl. Phys. 76, 3688 共1994兲.
3
M. A. Parker, T. L. Hylton, K. R. Coffey, and J. K. Howard, J. Appl. Phys.
75, 6382 共1994兲.
4
J. A. Borchers, P. M. Gehring, R. W. Erwin, C. F. Majkrzak, J. F. Ankner,
T. L. Hylton, K. R. Coffey, M. A. Parker, and J. K. Howard, J. Appl. Phys.
79, 4762 共1996兲.
5
S. Kahl and H. U. Krebs, Phys. Rev. B 76, 172103 共2001兲.
6
H.-U. Krebs and O. Bremert, Appl. Phys. Lett. 62, 2341 共1993兲.
7
K. Sturm and H.-U. Krebs, J. Appl. Phys. 90, 1061 共2001兲.
8
H. U. Krebs, J. Non-Equilib. Proc. 10, 3 共1997兲.
9
P. Sigmund, Phys. Rev. 184, 383 共1969兲.
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