Sensor properties of a robust giant magnetoresistance
material system at elevated temperatures
Citation for published version (APA):
Lenssen, K. M. H., Kuiper, A. E. T., Broek, van den, J. J., Rijt, van der, R. A. F., & Loon, van, A. M. (2000).
Sensor properties of a robust giant magnetoresistance material system at elevated temperatures.
Journal of
Applied Physics
,
87
(9), 6665-6667. https://doi.org/10.1063/1.372804
DOI:
10.1063/1.372804
Document status and date:
Published: 01/01/2000
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Sensor properties of a robust giant magnetoresistance material system
at elevated temperatures
K.-M. H. Lenssen,
a)
A. E. T. Kuiper, J. J. van den Broek, and R. A. F. van der Rijt
Philips Research Laboratories, Prof. Holstlaan 4, NL-5656 AA Eindhoven, The Netherlands
A. van Loon
Eindhoven University of Technology, Applied Physics Department, P.O. Box 513, NL-5600 MB Eindhoven,
The Netherlands
The temperature dependence of the giant magnetoresistance 共GMR兲 ratio, resistance and
exchange-biasing field for a spin valve comprising an Ir
19
Mn
81
-biased artificial antiferromagnet
共AAF兲 has been studied up to 325 °C. Up to 200–250 °C the temperature effects are reversible, at
higher temperatures gradual irreversible changes are observed, probably due to atomic diffusion.
The magnetoresistance effect is even at 200 °C still higher than for anisotropic magnetoresistance
sensors at room temperature. The resistance of the multilayer shows a maximum around 250°C. We
found that this is due to the peculiar behavior of Ir–Mn, which has a negative temperature
coefficient of the resistance. This provides a possibility to tune the temperature coefficient for the
complete multilayer by varying the thickness of the Ir–Mn layer. The relative decrease of the
exchange-biasing field as a function of temperature is much smaller for spin valves with AAF than
for conventional spin valves 共without AAF兲. Furthermore, it was demonstrated that the GMR ratio
can be increased to 12% at room temperature by using a dual spin valve with two AAFs. © 2000
American Institute of Physics. 关S0021-8979共00兲49008-0兴
I. INTRODUCTION
Automotive and industrial sensor applications pose very
stringent requirements on giant magnetoresistance 共GMR兲
materials. The output signal should meet the specifications
over a wide field range 共up to several tens of kiloamperes per
meter兲 and temperature range 共up to ⬃175 °C兲. More than is
the case for recording applications, the GMR properties at
elevated temperatures are very important.
Therefore, we have studied the sensor properties of our
GMR material system not only after heating, but also at high
temperatures up to 325 °C. This multilayer is based on a
Co–Fe/Ru/Co–Fe artificial antiferromagnet 共AAF兲 that is
exchange biased with Ir–Mn.
1,2
This material typically
shows a GMR effect of ⬃7% at room temperature. Recently
we have even obtained a GMR ratio of 12% in a dual spin
valve 共see Fig. 1兲; this is one of the highest values reported
so far for a spin valve comprising an AAF 共compare, for
example, Ref. 3兲.
In this article we will discuss the investigations of the
characteristics of single spin valves with exchange-biased
AAF at elevated temperatures.
II. EXPERIMENTAL SETUP
All samples were grown at room temperature by dc mag-
netron sputtering 共base pressure ⬃10
⫺ 8
mbar兲. As substrates
4⫻ 12 mm
2
Si共100兲 crystals were used, as well as similar
substrates with 0.5-
m-thick thermal oxide. During deposi-
tion, a field of ⬃20 kA/m was applied along the long axis of
the substrates.
The magnetoresistance of the spin valves was measured
in a four-terminal configuration 共current and field both along
the long axis兲, in which the temperature of the film 共in a
nitrogen atmosphere兲 could be varied between room tem-
perature and 325 °C. In this study the time period that the
film was exposed to elevated temperatures during a magne-
toresistance measurement was circa 1.5 h 共including a delay
of 0.5 h before the start of the measurement for temperature
stabilization兲.
The four-point measurements of the sheet resistance
R
sheet
as a function of temperature were carried out in a dedi-
cated setup, in which the film was heated in vacuum
(⬍ 10
⫺ 6
mbar). From these experiments the temperature co-
efficient of the resistance 兵defined as
关
⌬R/⌬T
兴
/R(25°C)其
can be determined.
The studied films consist of: 3.5 nm Ta/2 nm Ni
80
Fe
20
/10
nm Ir
19
Mn
81
/4.5 nm Co
90
Fe
10
/0.8 nm Ru/4 nm Co
90
Fe
10
/
a兲
Electronic mail: lenssen@natlab.research.philips.com
FIG. 1. Magnetoresistance curve of a dual spin valve with AAFs, consisting
of 3.5 nm Ta/2 nm Ni
80
Fe
20
/6 nm Ir
19
Mn
81
/4 nm Co
90
Fe
10
/0.8 nm Ru/4nm
Co
90
Fe
10
/2.5 nm Cu/1.6nm Co
90
Fe
10
/2.5 nm Cu/4nm Co
90
Fe
10
/0.8 nm Ru/
4nm Co
90
Fe
10
/6 nm Ir
19
Mn
81
/3.5 nm Ta.
JOURNAL OF APPLIED PHYSICS VOLUME 87, NUMBER 9 1 MAY 2000
66650021-8979/2000/87(9)/6665/3/$17.00 © 2000 American Institute of Physics
Downloaded 09 Oct 2009 to 131.155.151.77. Redistribution subject to AIP license or copyright; see http://jap.aip.org/jap/copyright.jsp
t
Cu
Cu/0.8 nm Co
90
Fe
10
/5 nm Ni
80
Fe
20
/4 nm Ta, with t
Cu
⫽ 1.5, 2.0, 2.5, 3.0, 3.5, and 5.0 nm. For comparison, some
conventional exchange-biased spin valves containing the
same materials have also been measured. Their composition
was 3.5 nm Ta/2 nm Ni
80
Fe
20
/10 nm Ir
19
Mn
81
/4 nm
Co
90
Fe
10
/t
Cu
Cu/0.8 nm Co
90
Fe
10
/5 nm Ni
80
Fe
20
/4 nm Ta, in
which the thickness of the Cu spacer was 2.5 or 3.5 nm.
Already as deposited the films showed exchange biasing.
III. TEMPERATURE DEPENDENCE
A. GMR ratio
The magnetoresistance curves at different temperatures
up to 250 °C are shown in Fig. 2 for the multilayer contain-
ing 3.0 nm Cu. It can be seen that the typical shape of the
curves remains similar for all temperatures, which is prom-
ising for application in sensors. The GMR ratio ⌬R/R
sat
共with R
sat
the saturation resistance, which is assumed to be
equal to the lowest measured resistance兲 only decreases by
about half of its value when heated up to 200 °C. Note that
the remaining effect of ⬃3% is still larger than the aniso-
tropic magnetoresistance 共AMR兲 effect at room temperature.
So this opens possibilities for a magnetic sensor that could
work up to 200 °C or higher.
The temperature dependence of the GMR ratio is shown
in Fig. 3. Over a large temperature range the GMR ratio
decreases approximately linearly. Just below 290 °C 共i.e., the
blocking temperature of our Ir–Mn films兲
4
it drops to a low
value. Despite the absence of exchange biasing above this
temperature, there is still a small GMR effect left up to
⬇350 °C. This is attributed to the different coercivities of the
AAF and the free layer. At these high temperatures the mag-
netoresistance curves become symmetric around zero field
共like in so-called hard-soft multilayers兲. The dependence of
the GMR ratio on the Cu-layer thickness is, as expected,
similar to what has been found earlier for conventional spin
valves.
5
B. Resistivity
In order to check whether any irreversible changes were
caused by the elevated temperatures, after each of the mea-
surements of Fig. 2 another magnetoresistance measurement
was done at room temperature. The resistance is a good mea-
sure for a possibly induced change in the material. The ex-
perimental results for R
sat
of the film with 3.0 nm Cu are
presented in Fig. 4. The increase in resistance at the highest
temperatures is probably due to interface diffusion. In this
temperature regime time is an important parameter. Earlier
experiments with rapid thermal processing
1
for 1 min indi-
cated that diffusion starts around 300 °C; the present inves-
tigations show that on a longer time scale 共⬃1.5 h兲 diffusion
effects can be observed at lower temperatures.
Figure 4 shows that no significant change occurs up to
200–250 °C. This indicates that the effects that we observe
in this temperature interval are reversible.
The resistance of an identical multilayer as a function of
the temperature is presented in Fig. 5. The resistance in-
creases monotonically up to ⬃250°C, but then starts to de-
crease. It has been verified that this is not due to the sub-
strate, but is a real characteristic of our multilayer. In order
to investigate this remarkable dependence further, the tem-
perature coefficients of constituting materials 共with and with-
out buffer layer兲 have been measured. The 10-nm-thick films
of Ni
80
Fe
20
and Co
90
Fe
10
showed an approximately linear
temperature dependence 共at least up to 300 °C兲 with a posi-
tive slope of, respectively, 0.14%/°C and 0.08%/°C. For 10
nm Ir
19
Mn
81
, however, a peculiar temperature dependence
was discovered 共see Fig. 6兲: the curve decreases monotoni-
cally and becomes even steeper above ⬃250 °C. Up to
250 °C the slope is around ⫺0.027%/°C, above this tempera-
ture it even almost doubles. It is this increase in the negative
FIG. 2. Magnetoresistance curves of a multilayer consisting of 3.5 nm Ta/
2nm Ni
80
Fe
20
/10 nm Ir
19
Mn
81
/4.5 nm Co
90
Fe
10
/0.8 nm Ru/4 nm
Co
90
Fe
10
/3 nm Cu/0.8 nm Co
90
Fe
10
/5 nm Ni
80
Fe
20
/4 nm Ta at, respectively,
25, 100, 125, 150, 175, 200, 225, and 250 °C 共from top down兲.
FIG. 3. The GMR ratio as a function of temperature for several thicknesses
of the Cu layer.
FIG. 4. The saturation resistance of the multilayer with 3 nm Cu at room
temperature after annealing at several temperatures.
6666 J. Appl. Phys., Vol. 87, No. 9, 1 May 2000 Lenssen
et al.
Downloaded 09 Oct 2009 to 131.155.151.77. Redistribution subject to AIP license or copyright; see http://jap.aip.org/jap/copyright.jsp
slope that causes the maximum in the R(T) curve of the total
multilayer. Repeatedly sweeping the temperature up and
down showed that this effect is reversible and fast, and there-
fore cannot be due to diffusion or a structural phase transfor-
mation.
From Fig. 5 it can be deduced that the temperature co-
efficient of the resistance of our GMR material is almost
constant up to ⬃200 °C and has a value of about ⬃0.1%/°C.
For sensor applications it is interesting to note that this is
almost three times smaller than that of a typical AMR sensor
共0.3%/°C兲.
The discovery of the negative temperature coefficient of
Ir
19
Mn
81
provides the possibility to tune the temperature co-
efficient of our multilayer by adapting the thickness of the
Ir–Mn layer. This is, however, limited in practice, since a
thicker Ir–Mn layer causes more electrical shunting and thus
reduces the GMR ratio. Moreover, the thickness of Ir–Mn
influences the blocking temperature and exchange-biasing
field.
6
C. Exchange biasing
In Fig. 7 the exchange-biasing field is plotted as a func-
tion of the temperature for exchange-biased spin valves with
and without AAF. For the multilayers with AAF this was
defined as the width between the two points where the mag-
netoresistance is half of its maximal value. Because of the
relatively large hysteresis for the spin valves without AAF
the exchange-biasing field has been determined from the
curve for increasing magnetic field. For the conventional
spin valves 共with a single 4 nm Co
90
Fe
10
layer instead of the
AAF兲 the exchange biasing decreases linearly with tempera-
ture. The temperature value that is obtained if these curves
are extrapolated to zero field is in good agreement with the
blocking temperature that was determined from magnetiza-
tion measurements.
4
It is remarkable that the relative decrease of the
exchange-biasing field is much smaller for the films with an
exchange-biased AAF. This is obviously a big advantage for
sensor applications, in particular in automotive and industrial
environments.
IV. CONCLUSIONS
We have studied the temperature dependence of GMR
ratio, resistance, and exchange-biasing field for a spin valve
comprising an Ir
19
Mn
8
-biased AAF. Up to 200–250 °C the
temperature effects are reversible, at higher temperatures
atomic diffusion starts to occur. At 200 °C the magnetoresis-
tance effect is still higher than for AMR sensors at room
temperature.
The resistance of the multilayer shows a maximum
around 250 °C. We found that this is due to the peculiar
behavior of Ir–Mn, which has a negative temperature coef-
ficient of the resistance. This provides a possibility to tune
the temperature coefficient for the complete multilayer by
varying the thickness of the Ir–Mn layer.
The relative decrease of the exchange-biasing field as a
function of temperature is much smaller for spin valves with
AAF than for conventional spin valves.
All measurements indicate that an operating range up to
200 °C seems feasible and that higher operation temperatures
up to ⬇270 °C could be possible for limited times.
1
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FIG. 5. Temperature dependence of the sheet resistance of the multilayer
with3nmCu.
FIG. 7. Temperature dependence of the exchange-biasing field for spin
valves with 共solid markers兲 and without 共open markers兲 AAF 共squares: 2.5
nm Cu, circles: 3.5 nm Cu兲.
FIG. 6. Temperature dependence of the sheet resistance of a 10.0 nm
Ir
19
Mn
81
film.
6667J. Appl. Phys., Vol. 87, No. 9, 1 May 2000 Lenssen
et al.
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