The magnetorefractive effect has been used in infrared reflection spectroscopy to study the magnetotransport properties of synthetic spin valves. This optical noncontact technique shows excellent correlation with the electrical giant magnetoresistance data

Matthew, J A D

McNeill, K A

Thompson, S M

Vopsaroiu, M

English

Vopson, Melvin

Matthew, J.

McNeill, K.

Thompson, S.

Name not available

Noncontact GMR measurements of synthetic spin valves using IR reflection spectroscopy

Portsmouth University Research Portal (Pure)

M. Vopsaroiu

J.A.D. Matthew

K.A. McNeill

S.M. Thompson

Crossref

White Rose Research Online

This is a repository copy of Noncontact GMR measurements of synthetic spin valves using
IR reflection spectroscopy.
White Rose Research Online URL for this paper:
http://eprints.whiterose.ac.uk/652/
Article:
Vopsaroiu, M, Matthew, J A D, McNeill, K A et al. (1 more author) (2003) Noncontact GMR 
measurements of synthetic spin valves using IR reflection spectroscopy. IEEE 
Transactions on Magnetics. pp. 2830-2832. ISSN 1941-0069 
https://doi.org/10.1109/TMAG.2003.815726
eprints@whiterose.ac.uk
https://eprints.whiterose.ac.uk/
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indicated otherwise. They may be downloaded and/or printed for private study, or other acts as permitted by 
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2830 IEEE TRANSACTIONS ON MAGNETICS, VOL. 39, NO. 5, SEPTEMBER 2003
Noncontact GMR Measurements of Synthetic Spin
Valves Using IR Reflection Spectroscopy
M. Vopsaroiu, J. A. D. Matthew, K. A. McNeill, and S. M. Thompson
Abstract—The magnetorefractive effect has been used in
infrared reflection spectroscopy to study the magnetotransport
properties of synthetic spin valves. This optical noncontact
technique shows excellent correlation with the electrical giant
magnetoresistance data.
Index Terms—Giant magnetoresistance (GMR), infrared (IR)
reflection spectroscopy, magnetorefractive effect (MRE), synthetic
spin valves.
I. INTRODUCTION
I
N THIS paper, we describe an optical noncontact method
for the measurement of the magnetotransport effects in giant
magnetoresistance (GMR) materials, which has been applied
to spin valve samples. The method is based on the magnetore-
fractive effect (MRE) [1]. The infrared dielectric properties of
metals are well described by the Drude model in which the com-
plex dielectric function is related to the complex conductivity
and the frequency of the incident light. Hence, changing its con-
ductivity modifies the dielectric function of a metal. In the case
of GMR materials, spin-dependent phenomena are based on the
principle that the conductivities for spin-up and spin-down elec-
trons are different. As a consequence, altering the microscopic
magnetic configuration through the application of an external
magnetic field can control the transport properties of GMR ma-
terials, which in turn affects the dielectric properties through the
changes in the conductivity. Transport properties depend mainly
on the electron transitions inside the conduction band, the intra-
band transitions, which dominate the dielectric properties in the
infrared (IR) spectral region (2–20 m). Determining its reflec-
tivity/transmissivity coefficients can therefore perform a direct
measure of the changes in the dielectric properties of a material.
Hence, IR transmission/reflection spectroscopy as a function of
the applied magnetic field can provide a direct tool for probing
the spin-dependent conductivities in GMR materials. The spin
properties of the conduction electrons and the effects of the ex-
ternal magnetic field are contained in an extended Drude model
of the form
(1)
Manuscript received December 1, 2002. This work was supported by the En-
gineering and Physical Sciences Research Council and Seagate Technology.
M. Vopsaroiu, J. A. D. Matthew, and S. M. Thompson are with the
Department of Physics, University of York, York, YO10 5DD, U.K. (e-mail:
mv4@york.ac.uk; jadm1@york.ac.uk; smt4@york.ac.uk).
K. A. McNeill is with Seagate Technology, R&D Londonderry, N. Ireland,
BT48 0BF, U.K. (e-mail: Kevin.A.McNeill@seagate.com).
Digital Object Identifier 10.1109/TMAG.2003.815726
where is the complex dielectric function, is the frequency
independent contribution to , is the plasma frequency,
the frequency of the incident light, is the normal-
ized magnetization, is the conduction electron scattering
rate in the self-averaging limit (SAL) [2], and is the spin
asymmetry constant also obtained in the SAL formalism [1].
Relation (1) describes the MRE and was first theoretically and
experimentally demonstrated in 1995 [1]. The MRE is defined
as the variation of the refractive index (or dielectric function) of
a material due to a change in its conductivity when a magnetic
field is applied. Since the MRE is related to the intraband tran-
sitions of the conduction electrons, it is fundamentally different
from the magnetooptic Kerr effect or Faraday effect in which the
interband transitions dominate rather than the intraband transi-
tions. It is important to stress that only GMR materials show the
MRE and this is also clear from the relation (1), where if either
or is zero the relation reduces to the Drude dielectric
function. The MRE has previously been used to study granular
GMR materials such as (Co) Ag [3], (CoFe) (Al O
[4] and exchange-biased GMR multilayers [5]. In this paper,
the MRE has been used for the first time to measure magne-
totransport properties of synthetic spin valves. The results re-
ported here are compared with the electrical magnetotransport
data and show an excellent correlation between the electrical
GMR measurements and optical MRE, leading the way to non-
contact GMR measurements of synthetic spin valves.
II. EXPERIMENT
Synthetic spin valve systems have been experimentally in-
vestigated in this paper. They are different from conventional
spin valves, in that the pinned FM layer is replaced by a pinned
trilayer structure [synthetic antiferromagnet (SAF)] consisting
of two antiferromagnetically coupled FM layers separated by
a nonmagnetic layer (usually Ruthenium). This design has the
advantage of improved thermal and magnetic performances [6]
compared to conventional spin valves. The synthetic spin valve
samples studied in this paper were sputtered by Seagate Tech-
nology. They consisted of a pinned layer of an antiferromagnet-
ically coupled CoFe–Ru–CoFe trilayer in direct contact with a
layer of AFM PtMn. The pinned layer is separated from the free
CoFe–NiFe bilayer by a layer of Cu. The total thickness of the
resulting stack is around 450 Å, which is well within the limits
of the IR skin penetration depth (around 600 Å for good con-
ducting metals). Two variations of this structure were studied:
a top spin valve (TSV) with the pinned layer at the top of the
stack and a bottom spin valve (BSV) with the pinned layer at
the bottom of the stack (see the inserted diagrams in Figs. 4 and
5). Current in plane (CIP) dc magnetotransport measurements
0018-9464/03$17.00 © 2003 IEEE
VOPSAROIU et al.: NONCONTACT GMR MEASUREMENTS OF SYNTHETIC SPIN VALVES 2831
Fig. 1. MRE as a function of the wavelength in a BSV sample for decreasing
positive applied fields (9000 to 0 Oe).
were made using a four-point probe with a 1-mA applied cur-
rent in a maximum applied magnetic field of 9 kOe (Figs. 4
and 5). The electrical GMR was calculated using the relation
GMR (2)
where is the resistivity of the sample in an applied mag-
netic field and is the maximum resistivity that corresponds
to zero magnetization of the sample.
IR spectra were acquired between 2.5 and 20 m using a
Fourier transform IR reflection spectrometer with 0.25- m
resolution and a liquid nitrogen-cooled HgCdTe detector.
Although polarized light gives the highest reflectivity, the op-
timum MRE was obtained for polarized light using a KRS-5
grid polarizer and for an incidence angle of 65 with respect
to the surface normal. The spectra were collected at room
temperature in an applied magnetic field up to a maximum of
9 kOe. The measurements were made in decreasing field
order following saturation in a positive applied field in order to
make a direct comparison with the GMR measurements. The
MRE infrared spectrum for a particular applied field shows
the percentage change in the reflection of IR light due to the
application of the magnetic field as a function of wavelength
and it was calculated according to the relation
MRE (3)
are three consecutive spectra where and are
taken in zero applied field and is the spectrum acquired at an
applied magnetic field. and were then averaged in order to
take in account any time variations in the background. For both
the electrical and optical measurements, the pinning field was
always parallel to the applied positive magnetic field direction.
III. RESULTS AND DISCUSSIONS
IR reflection spectroscopy has been performed at different
positive (Fig. 1) and negative (Fig. 2) applied magnetic fields.
Fig. 2. MRE as a function of the wavelength in a BSV sample for increasing
negative applied fields (0 to  9000 Oe).
The MRE spectra for both the BSV (Figs. 1 and 2) and TSV
show a broadly similar shape to those observed in CoAg gran-
ular GMR films [3] in that there is a broad positive peak in the
MRE followed by a crossover into a negative reflection region
at higher wavelengths. This is consistent with magnetically in-
duced modification of the scattering of Drude like free electrons
responsible for the GMR. The crossover occurs at much longer
wavelengths in the synthetic spin valves, typically 15 m com-
pared to 10 m in the CoAg. This is attributed to the different
spin-independent resistivities in the two types of material [3].
The other significant feature is the presence of sharp positive
and negative excursions around 8 m, which are likely to be due
to Si–O–Si vibration modes in the substrate. Close inspection of
the MRE curves reveals that they follow the same asymmetric
behavior as the electrical resistance under an applied magnetic
field.
The rapid switching of the free layer is observed as a sudden
change in both the electrical resistance and the IR reflection.
The slow magnetic reversal of the synthetic antiferromagnet is
observed as the smoother variation with field of both the GMR
and MRE.
Although the absolute value of the MRE depends on the
choice of the wavelength, there is a high degree of correlation
between MRE and GMR, which is also insensitive to the
precise choice of the wavelength. Usually, the magnitude of
the MRE is taken as the absolute value between the lowest
and the highest point of the MRE spectrum and that normally
corresponds to the highest MRE. However, in this case, we
chose to quantify the MRE values between 10 m and the
lowest point of the spectra that corresponds to a 20- m wave-
length (Fig. 3). By plotting the MRE values as a function of the
applied magnetic field, direct correlation between the MRE and
the GMR is obtained (Figs. 4 and 5). The strong asymmetry in
the electrical properties is reproduced by the MRE, with very
similar behavior for the TSV and BSV. Although the two curves
correlate very well, the constant of proportionality is different
than unity. Alternative measures of the MRE (e.g., only the
absolute values at one wavelength) show similar correlations.
The maximum GMR values obtained are 7.2% for the BSV
sample and 6.6% for the TSV sample, while the maximum MRE
values are 1.8% for the BSV and 1.7% for the TSV.
2832 IEEE TRANSACTIONS ON MAGNETICS, VOL. 39, NO. 5, SEPTEMBER 2003
Fig. 3. Example of an MRE reflection spectrum taken at H = 6000 Oe for
a TSV sample. Magnitude of the MRE effect is taken as the absolute value
between the value at 10 m and the lowest value at 20 m.
Fig. 4. GMR and MRE data for a BSV measured as a function of the applied
field, after saturation in positive field.
Fig. 5. GMR and MRE data for a TSV measured as a function of the applied
field, after saturation in positive field.
Moreover, the IR reflection spectroscopy is highly sensitive
to the changes in conductivity when the permalloy switches and
these changes were detected in the IR spectra with a precision as
small as 2 Oe (the free layer of the BSV switches at 55 and
70 Oe for the TSV). The field step was varied more slowly for
the MRE, resulting in a sharper transition at the switching of the
free layer. A closer look at the correlation diagrams (Figs. 4 and
5) shows that the MRE follows not only the main peak in the
GMR data, which is related to the reversal of the free layer, but
also increases around the small broad peak (around 3.5 kOe)
that is associate d with the spin flop transition occurring in the
SAF. Spin flop of the magnetization in the SAF occurs when an
external magnetic field is applied parallel to SAF layers. Both
theoretical [7] and experimental studies [6] have demonstrated
that under a certain applied field the minimization of the total
energy of the system occurs when the SAF orients orthogonally
to the applied magnetic field. Increasing the external magnetic
field further, the flopped spins scissor toward the axis of the
magnetic field orienting parallel when a high magnetic field is
applied.
IV. CONCLUSION
IR reflection spectroscopy on synthetic spin valves has
demonstrated that this technique is suitable for high-resolution
noncontact measurements of GMR with the ability to detect
electric conductivity variations under an applied magnetic field
with a precision as small as 2 Oe. The technique has been
shown to be sensitive to the GMR intraband scattering and
the effects on the GMR due to spin flop transitions occurring
in the SAF. IR spectroscopy therefore has the potential for
detailed experimental studies including the ability to perform
in situ measurements, making it a very attractive and powerful
investigation tool for both academic research and industry.
REFERENCES
[1] J. C. Jacquet and T. Valet, “A new magnetooptical effect discovered on
magnetic multilayers: The magnetorefractive effect,” in Magnetic Ultra-
thin Films, Multilayer and Surfaces, E. Marinero, Ed. Pittsburgh, PA:
Materials Res. Soc., 1995, pp. 477–490.
[2] S. Zhang, “Theory of giant magnetoresistance in magnetic granular
films,” Appl. Phys. Lett., vol. 61, p. 1855, 1992.
[3] V. G. Kravets, D. Bozec, J. A. D. Matthew, S. M. Thompson, H. Menard,
A. B. Horn, and A. F. Kravets, “Correlation between the magnetorefrac-
tive effect, giant magnetoresistance and optical properties of C–Ag gran-
ular magnetic films,” Phys. Rev. B., vol. 65, p. 054 415, Jan. 2002.
[4] D. Bozec, V. G. Kravets, J. A. D. Matthew, S. M. Thompson, and A.
F. Kravets, “Infrared reflectance and magnetorefractive effects in metal-
isolator CoFe–Al O granular films,” J. Appl. Phys., vol. 91, no. 10, pp.
8795–8797, 2002.
[5] J. van Driel, F. R. de Boer, R. Coehoorn, G. H. Rietjens, and
E. S. J. Heuvelmans-Wijdenes, “Magnetorefractive and mag-
netic-linear-dichroism effect in exchanged-biased spin valves,” Phys.
Rev. B., vol. 61, pp. 15 321–15 326, 2000.
[6] Z. Lu, G. Pan, A. Al-Jibouri, and M. Hoban, “Effect of Ru thickness on
spin flop in synthetic spin valves,” J. Appl. Phys., vol. 91, no. 10, pp.
7116–7118, 2002.
[7] J. G. Zhu, “Spin valve and dual spin valve heads with synthetic antifer-
romagnets,” IEEE Trans. Magn., vol. 35, pp. 655–660, Feb. 1999.
[8] H. C. Tong, C. Qian, L. Miloslavsky, S. Funada, X. Shi, F. Liu, and S.
Dey, “The spin flop of synthetic antiferromagnetic films,” J. Appl. Phys.,
vol. 87, no. 9, pp. 5055–5057, 2000.


This is a repository copy of Noncontact GMR measurements of synthetic spin valves usingIR reflection spectroscopy.White Rose Research Online URL for this paper:https://eprints.whiterose.ac.uk/652/Article:Vopsaroiu, M, Matthew, J A D, McNeill, K A et al. (1 more author) (2003) Noncontact GMR measurements of synthetic spin valves using IR reflection spectroscopy. IEEE Transactions on Magnetics. pp. 2830-2832. ISSN 1941-0069 https://doi.org/10.1109/TMAG.2003.815726eprints@whiterose.ac.ukhttps://eprints.whiterose.ac.uk/Reuse Items deposited in White Rose Research Online are protected by copyright, with all rights reserved unless indicated otherwise. They may be downloaded and/or printed for private study, or other acts as permitted by national copyright laws. The publisher or other rights holders may allow further reproduction and re-use of the full text version. This is indicated by the licence information on the White Rose Research Online record for the item. Takedown If you consider content in White Rose Research Online to be in breach of UK law, please notify us by emailing eprints@whiterose.ac.uk including the URL of the record and the reason for the withdrawal request. 2830 IEEE TRANSACTIONS ON MAGNETICS, VOL. 39, NO. 5, SEPTEMBER 2003Noncontact GMR Measurements of Synthetic SpinValves Using IR Reflection SpectroscopyM. Vopsaroiu, J. A. D. Matthew, K. A. McNeill, and S. M. ThompsonAbstract—The magnetorefractive effect has been used ininfrared reflection spectroscopy to study the magnetotransportproperties of synthetic spin valves. This optical noncontacttechnique shows excellent correlation with the electrical giantmagnetoresistance data.Index Terms—Giant magnetoresistance (GMR), infrared (IR)reflection spectroscopy, magnetorefractive effect (MRE), syntheticspin valves.I. INTRODUCTIONIN THIS paper, we describe an optical noncontact methodfor the measurement of the magnetotransport effects in giantmagnetoresistance (GMR) materials, which has been appliedto spin valve samples. The method is based on the magnetore-fractive effect (MRE) [1]. The infrared dielectric properties ofmetals are well described by the Drude model in which the com-plex dielectric function is related to the complex conductivityand the frequency of the incident light. Hence, changing its con-ductivity modifies the dielectric function of a metal. In the caseof GMR materials, spin-dependent phenomena are based on theprinciple that the conductivities for spin-up and spin-down elec-trons are different. As a consequence, altering the microscopicmagnetic configuration through the application of an externalmagnetic field can control the transport properties of GMR ma-terials, which in turn affects the dielectric properties through thechanges in the conductivity. Transport properties depend mainlyon the electron transitions inside the conduction band, the intra-band transitions, which dominate the dielectric properties in theinfrared (IR) spectral region (2–20 m). Determining its reflec-tivity/transmissivity coefficients can therefore perform a directmeasure of the changes in the dielectric properties of a material.Hence, IR transmission/reflection spectroscopy as a function ofthe applied magnetic field can provide a direct tool for probingthe spin-dependent conductivities in GMR materials. The spinproperties of the conduction electrons and the effects of the ex-ternal magnetic field are contained in an extended Drude modelof the form(1)Manuscript received December 1, 2002. This work was supported by the En-gineering and Physical Sciences Research Council and Seagate Technology.M. Vopsaroiu, J. A. D. Matthew, and S. M. Thompson are with theDepartment of Physics, University of York, York, YO10 5DD, U.K. (e-mail:mv4@york.ac.uk; jadm1@york.ac.uk; smt4@york.ac.uk).K. A. McNeill is with Seagate Technology, R&D Londonderry, N. Ireland,BT48 0BF, U.K. (e-mail: Kevin.A.McNeill@seagate.com).Digital Object Identifier 10.1109/TMAG.2003.815726where is the complex dielectric function, is the frequencyindependent contribution to , is the plasma frequency,the frequency of the incident light, is the normal-ized magnetization, is the conduction electron scatteringrate in the self-averaging limit (SAL) [2], and is the spinasymmetry constant also obtained in the SAL formalism [1].Relation (1) describes the MRE and was first theoretically andexperimentally demonstrated in 1995 [1]. The MRE is definedas the variation of the refractive index (or dielectric function) ofa material due to a change in its conductivity when a magneticfield is applied. Since the MRE is related to the intraband tran-sitions of the conduction electrons, it is fundamentally differentfrom the magnetooptic Kerr effect or Faraday effect in which theinterband transitions dominate rather than the intraband transi-tions. It is important to stress that only GMR materials show theMRE and this is also clear from the relation (1), where if eitheror is zero the relation reduces to the Drude dielectricfunction. The MRE has previously been used to study granularGMR materials such as (Co) Ag [3], (CoFe) (Al O[4] and exchange-biased GMR multilayers [5]. In this paper,the MRE has been used for the first time to measure magne-totransport properties of synthetic spin valves. The results re-ported here are compared with the electrical magnetotransportdata and show an excellent correlation between the electricalGMR measurements and optical MRE, leading the way to non-contact GMR measurements of synthetic spin valves.II. EXPERIMENTSynthetic spin valve systems have been experimentally in-vestigated in this paper. They are different from conventionalspin valves, in that the pinned FM layer is replaced by a pinnedtrilayer structure [synthetic antiferromagnet (SAF)] consistingof two antiferromagnetically coupled FM layers separated bya nonmagnetic layer (usually Ruthenium). This design has theadvantage of improved thermal and magnetic performances [6]compared to conventional spin valves. The synthetic spin valvesamples studied in this paper were sputtered by Seagate Tech-nology. They consisted of a pinned layer of an antiferromagnet-ically coupled CoFe–Ru–CoFe trilayer in direct contact with alayer of AFM PtMn. The pinned layer is separated from the freeCoFe–NiFe bilayer by a layer of Cu. The total thickness of theresulting stack is around 450 Å, which is well within the limitsof the IR skin penetration depth (around 600 Å for good con-ducting metals). Two variations of this structure were studied:a top spin valve (TSV) with the pinned layer at the top of thestack and a bottom spin valve (BSV) with the pinned layer atthe bottom of the stack (see the inserted diagrams in Figs. 4 and5). Current in plane (CIP) dc magnetotransport measurements0018-9464/03$17.00 © 2003 IEEEVOPSAROIU et al.: NONCONTACT GMR MEASUREMENTS OF SYNTHETIC SPIN VALVES 2831Fig. 1. MRE as a function of the wavelength in a BSV sample for decreasingpositive applied fields (9000 to 0 Oe).were made using a four-point probe with a 1-mA applied cur-rent in a maximum applied magnetic field of 9 kOe (Figs. 4and 5). The electrical GMR was calculated using the relationGMR (2)where is the resistivity of the sample in an applied mag-netic field and is the maximum resistivity that correspondsto zero magnetization of the sample.IR spectra were acquired between 2.5 and 20 m using aFourier transform IR reflection spectrometer with 0.25- mresolution and a liquid nitrogen-cooled HgCdTe detector.Although polarized light gives the highest reflectivity, the op-timum MRE was obtained for polarized light using a KRS-5grid polarizer and for an incidence angle of 65 with respectto the surface normal. The spectra were collected at roomtemperature in an applied magnetic field up to a maximum of9 kOe. The measurements were made in decreasing fieldorder following saturation in a positive applied field in order tomake a direct comparison with the GMR measurements. TheMRE infrared spectrum for a particular applied field showsthe percentage change in the reflection of IR light due to theapplication of the magnetic field as a function of wavelengthand it was calculated according to the relationMRE (3)are three consecutive spectra where and aretaken in zero applied field and is the spectrum acquired at anapplied magnetic field. and were then averaged in order totake in account any time variations in the background. For boththe electrical and optical measurements, the pinning field wasalways parallel to the applied positive magnetic field direction.III. RESULTS AND DISCUSSIONSIR reflection spectroscopy has been performed at differentpositive (Fig. 1) and negative (Fig. 2) applied magnetic fields.Fig. 2. MRE as a function of the wavelength in a BSV sample for increasingnegative applied fields (0 to  9000 Oe).The MRE spectra for both the BSV (Figs. 1 and 2) and TSVshow a broadly similar shape to those observed in CoAg gran-ular GMR films [3] in that there is a broad positive peak in theMRE followed by a crossover into a negative reflection regionat higher wavelengths. This is consistent with magnetically in-duced modification of the scattering of Drude like free electronsresponsible for the GMR. The crossover occurs at much longerwavelengths in the synthetic spin valves, typically 15 m com-pared to 10 m in the CoAg. This is attributed to the differentspin-independent resistivities in the two types of material [3].The other significant feature is the presence of sharp positiveand negative excursions around 8 m, which are likely to be dueto Si–O–Si vibration modes in the substrate. Close inspection ofthe MRE curves reveals that they follow the same asymmetricbehavior as the electrical resistance under an applied magneticfield.The rapid switching of the free layer is observed as a suddenchange in both the electrical resistance and the IR reflection.The slow magnetic reversal of the synthetic antiferromagnet isobserved as the smoother variation with field of both the GMRand MRE.Although the absolute value of the MRE depends on thechoice of the wavelength, there is a high degree of correlationbetween MRE and GMR, which is also insensitive to theprecise choice of the wavelength. Usually, the magnitude ofthe MRE is taken as the absolute value between the lowestand the highest point of the MRE spectrum and that normallycorresponds to the highest MRE. However, in this case, wechose to quantify the MRE values between 10 m and thelowest point of the spectra that corresponds to a 20- m wave-length (Fig. 3). By plotting the MRE values as a function of theapplied magnetic field, direct correlation between the MRE andthe GMR is obtained (Figs. 4 and 5). The strong asymmetry inthe electrical properties is reproduced by the MRE, with verysimilar behavior for the TSV and BSV. Although the two curvescorrelate very well, the constant of proportionality is differentthan unity. Alternative measures of the MRE (e.g., only theabsolute values at one wavelength) show similar correlations.The maximum GMR values obtained are 7.2% for the BSVsample and 6.6% for the TSV sample, while the maximum MREvalues are 1.8% for the BSV and 1.7% for the TSV.2832 IEEE TRANSACTIONS ON MAGNETICS, VOL. 39, NO. 5, SEPTEMBER 2003Fig. 3. Example of an MRE reflection spectrum taken at H = 6000 Oe fora TSV sample. Magnitude of the MRE effect is taken as the absolute valuebetween the value at 10 m and the lowest value at 20 m.Fig. 4. GMR and MRE data for a BSV measured as a function of the appliedfield, after saturation in positive field.Fig. 5. GMR and MRE data for a TSV measured as a function of the appliedfield, after saturation in positive field.Moreover, the IR reflection spectroscopy is highly sensitiveto the changes in conductivity when the permalloy switches andthese changes were detected in the IR spectra with a precision assmall as 2 Oe (the free layer of the BSV switches at 55 and70 Oe for the TSV). The field step was varied more slowly forthe MRE, resulting in a sharper transition at the switching of thefree layer. A closer look at the correlation diagrams (Figs. 4 and5) shows that the MRE follows not only the main peak in theGMR data, which is related to the reversal of the free layer, butalso increases around the small broad peak (around 3.5 kOe)that is associate d with the spin flop transition occurring in theSAF. Spin flop of the magnetization in the SAF occurs when anexternal magnetic field is applied parallel to SAF layers. Boththeoretical [7] and experimental studies [6] have demonstratedthat under a certain applied field the minimization of the totalenergy of the system occurs when the SAF orients orthogonallyto the applied magnetic field. Increasing the external magneticfield further, the flopped spins scissor toward the axis of themagnetic field orienting parallel when a high magnetic field isapplied.IV. CONCLUSIONIR reflection spectroscopy on synthetic spin valves hasdemonstrated that this technique is suitable for high-resolutionnoncontact measurements of GMR with the ability to detectelectric conductivity variations under an applied magnetic fieldwith a precision as small as 2 Oe. The technique has beenshown to be sensitive to the GMR intraband scattering andthe effects on the GMR due to spin flop transitions occurringin the SAF. IR spectroscopy therefore has the potential fordetailed experimental studies including the ability to performin situ measurements, making it a very attractive and powerfulinvestigation tool for both academic research and industry.REFERENCES[1] J. C. Jacquet and T. Valet, “A new magnetooptical effect discovered onmagnetic multilayers: The magnetorefractive effect,” in Magnetic Ultra-thin Films, Multilayer and Surfaces, E. Marinero, Ed. Pittsburgh, PA:Materials Res. Soc., 1995, pp. 477–490.[2] S. Zhang, “Theory of giant magnetoresistance in magnetic granularfilms,” Appl. Phys. Lett., vol. 61, p. 1855, 1992.[3] V. G. Kravets, D. Bozec, J. A. D. Matthew, S. M. Thompson, H. Menard,A. B. Horn, and A. F. Kravets, “Correlation between the magnetorefrac-tive effect, giant magnetoresistance and optical properties of C–Ag gran-ular magnetic films,” Phys. Rev. B., vol. 65, p. 054 415, Jan. 2002.[4] D. Bozec, V. G. Kravets, J. A. D. Matthew, S. M. Thompson, and A.F. Kravets, “Infrared reflectance and magnetorefractive effects in metal-isolator CoFe–Al O granular films,” J. Appl. Phys., vol. 91, no. 10, pp.8795–8797, 2002.[5] J. van Driel, F. R. de Boer, R. Coehoorn, G. H. Rietjens, andE. S. J. Heuvelmans-Wijdenes, “Magnetorefractive and mag-netic-linear-dichroism effect in exchanged-biased spin valves,” Phys.Rev. B., vol. 61, pp. 15 321–15 326, 2000.[6] Z. Lu, G. Pan, A. Al-Jibouri, and M. Hoban, “Effect of Ru thickness onspin flop in synthetic spin valves,” J. Appl. Phys., vol. 91, no. 10, pp.7116–7118, 2002.[7] J. G. Zhu, “Spin valve and dual spin valve heads with synthetic antifer-romagnets,” IEEE Trans. Magn., vol. 35, pp. 655–660, Feb. 1999.[8] H. C. Tong, C. Qian, L. Miloslavsky, S. Funada, X. Shi, F. Liu, and S.Dey, “The spin flop of synthetic antiferromagnetic films,” J. Appl. Phys.,vol. 87, no. 9, pp. 5055–5057, 2000.

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Noncontact GMR measurements of synthetic spin valves using IR reflection spectroscopy

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