- Email: [email protected]
Hyper frequency behavior of the GMR effect in a single spin valve sensor
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 316 (2007) 340–343 www.elsevier.com/locate/jmmm
Hyper frequency behavior of the GMR effect in a single spin valve sensor N. Biziere, C Fermon, G. Le Goff Service de Physique de l’Etat Condense´ (CNRS URA 2464), DSM/DRECAM/SPEC, CEA Saclay, 91191 Gif sur Yvette Cedex, France Available online 6 March 2007
Abstract In the last decade, new techniques to increase the data storage density have been one of the major axes of research in magnetism. One direction explored consists of increasing the reading speed of read heads. This implies to understand the behavior of the giant magneto resistive (GMR) effect at high frequency. We offer an original method to isolate the GMR of a spin valve sensor, designed as a yoke shape, by combination of classical ferromagnetic resonance technique and demodulation measurement of the CIP voltage of the sensor. We show that the classical description of the GMR effect, based on a mean field theory, is kept valid in a frequency range from DC to 10 GHz. We also show the direct correlation between the ferromagnetic resonance of the free layer sensor and the GMR amplitude. r 2007 Elsevier B.V. All rights reserved. Keywords: Magnetic sensors; Microwaves; Giant magneto-resistance; Spin waves
1. Introduction
2. Experimental setup
Since its experimental discovery in 1988 [1], much attention has been paid to the static properties of the giant magneto resistive (GMR) effect. Whereas a lot of noise studies have used the GMR effect to probe the dynamics of the magnetization [2,3], very few papers, to our knowledge, have dealt with the problem of the hyper-frequency behavior of the GMR effect [4]. In this paper, we offer an original way to probe the GMR of a spin valve sensor in a frequency range from DC to 10 GHz. The idea is first to use a micro-antenna to produce a local precession of the free layer of the spin valve and second to clearly separate the GMR effect from various direct coupling by using the spin valve as an in situ demodulator. In Section 2, we describe the system and the experimental setup. In Section 3, we briefly present the magnetic dynamics of the spin valve sensor. Experimental measurements about the electrical properties from DC to 10 GHz are presented in Section 4.
The spin valve sensor used is made of a free magnetic layer (Ni80Fe20/CoFe) separated from a hard magnetic layer (CoFe/IrMn) by a thin copper film. The stack (Ni80Fe20 (3.5 nm)/CoFe (1.2 nm)/Cu (2.9 nm)/CoFe (3.5 nm)/IrMn (10 nm)) is deposited by sputtering on a thick glass substrate. The terms free and hard refer to the ability of the magnetization to align in the direction of a small external magnetic field. The free and hard layers are grown such as their anisotropies are perpendicular. Conventional UV lithography technique combined with ion milling in argon atmosphere is used to design the sensor as a yoke shape. Its dimensions are given in Fig. 1. This geometry allows getting a uniform magnetization in the area of interest (Fig. 1). The magnetization of the free layer is in the direction of the yoke length. A four-contact measurement is made possible by lift-off of two Ti (10 nm)/Cu (400 nm)/Au (10 nm) slotline type waveguides (Fig. 1). Both waveguide dimensions have been calculated using the TXLINE freeware to be closely adapted 50 O in a large frequency range (from DC to 10 GHz). The current pads can be connected via standard picoprobes either to a DC current source or to an hyper
Corresponding author.
E-mail address: [email protected] (N. Biziere). 0304-8853/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2007.03.010
ARTICLE IN PRESS N. Biziere et al. / Journal of Magnetism and Magnetic Materials 316 (2007) 340–343
25
25
500
5 20
20
20
500
delta signal in V/sqrt(Hz)
50
4.0x10-5
Hres = 371.5 Oe +/- 1.62 Oe delta H = 77.8 Oe +/- 3 Oe
2.0x10-5
depha sed lorentzian fit
341
0.0 -2.0x10-5 -4.0x10-5 -6.0x10-5 -8.0x10-5
I (w2)
-1.0x10-4 300
35 80
I (w1)
6
10
300
Fig. 1. (Top) Dimensions of the sensor designed as a yoke shape (gray) and the contact pads (delimited by black lines). (Bottom) Dimensions of the shorted antenna. All dimensions are given in micrometers.
frequency source (0.01–20 GHz). The voltage pads are connected, also via picoprobes, either to a multimeter for DC measurement or to a spectrum analyzer (9 kHz– 26.5 GHz) for hyper-frequency measurement. On top of the sensor, a coplanar waveguide, shorted at one end, is fabricated by UV lithography and a lift-off of Ti (10 nm)/Cu (400 nm)/Au (10 nm) (Fig. 1). It works as a micro-antenna. It is electrically isolated from the sensor by a 300 nm thick Si3N4 insulating layer. The central conductor of the antenna is roughly aligned with the bar of the sensor. The precision is about half a micron due to the resolution of the optical alignment. In the experiment, a hyper-frequency source injects a current in the antenna through a circulator and a picoprobe. The amplitude of the reflected signal is measured using the spectrum analyzer. The sample and the picoprobes are placed into the gap of an electromagnet providing a static magnetic field (0–1 T with a precision of 3 Oe) parallel to the length of the yoke. 3. Dynamics of the spin valve magnetization The magnetization dynamics of the sensor is studied using ferromagnetic resonance technique with the microantenna [5]. The micro-antenna described above creates at the surface of the active area of the sensor a hyperfrequency magnetic field that we will call the pumping field. Its amplitude can be evaluated in a good approximation by the value I/2w, where I is the current amplitude in the antenna and w the width of the central conductor. In the geometry described in Fig. 1, the pumping field is
400 external field in Oe
500
Fig. 2. Absolute variation of the reflected signal on the antenna as a function of the external field value. The frequency of the current injected in the antenna is 7 GHz. The power delivered to the antenna is 0 dbm.
perpendicular to the static magnetic field and can be considered uniform all over the active area. The experiment consists of sweeping the value of the static field while keeping the frequency of the current in the antenna constant. When the static field reaches the resonant value, a part of the electromagnetic power is absorbed by the sample so the amplitude of the reflected signal decreases. In terms of impedance, the coupling between the antenna and the sample is of inductive nature. The relation between the susceptibility of the sample and the antenna response can be written as w¼
DZw , 2pf m0 tL
(1)
where DZ is the change of the antenna complex impedance, f the frequency, t the thickness of the magnetic film, and L the length of the active area. Fig. 2 shows an example of the measurement of the absolute variation of the reflected signal at 7 GHz as function of the external magnetic field amplitude. The measured signal is the sum of two signals. The first one is a background signal coming from the leak of our circulator and the second one is the reflected signal from the antenna. The spectrum analyzer measures the modulus of this sum. As the phase between the two signals is different, it induces a mixing of the real and imaginary part of the antenna impedance. At the first order, experimental data can be fitted with a function of the form: Aðw0 cos y þ w00 sin yÞ.
(2)
This resonance corresponds to the uniform precession of the free-layer stack. Indeed, considering the uniformity of the pumping field over the active area and the orientation of the free-layer magnetization (aligned with the applied external field), the first mode possibly excited is the uniform mode. We have measured the resonance field in a frequency range from 1 to 10 GHz (Fig. 3). Results are in good
ARTICLE IN PRESS N. Biziere et al. / Journal of Magnetism and Magnetic Materials 316 (2007) 340–343
342
2.0x10-5 amplitude in sqrt(V²/Hz)
resonance frequency in GHz
8 7 6 5
experimental data Kittel fit with Nz = 0 Ny=0.99674 +/- 0.00045 Nx=1-Ny Ms=1.4802 +/- 0.03
4 3
105 MHz 1005 MHz 2005MHz 5005 MHz
1.5x10-5 1.0x10-5 5.0x10-6 0.0
2 0 1 0
100
200 300 400 resonance field in Oe
500
1000 2000 3000 4000 frequency of the pumping field in MHz
5000
600 Fig. 4. Measured voltage at the frequency w1–w2 as a function of the pumping field frequency in zero external field.
Fig. 3. Resonance frequency as a function of the external field.
absolute variation in sqrt(V²/Hz)
agreement with the Kittel resonance model for a very thin ellipsoid. The calculation of the demagnetizing factor shows that the active area can be considered almost infinite in all directions except thickness. This is in accordance with the very small value of the aspect ratio (t/wE105). The value of the saturation magnetization calculated by fitting the experimental data (1.48 T) is in quite good accordance with neutron reflectivity measurements made on continuous films which gave Ms about 1.51 T for the free layer.
4.0x10-7
1.6x10-5
2.0x10-7
1.2x10-5
0.0
8.0x10-6 10000
12000
4.0x10-6
0.0
4. GMR behavior at gigahertz The hyper-frequency behavior of the GMR effect is probed by a demodulation method. The pumping field is fixed at a frequency w1. The power applied on the microantenna is 0 dbm, which corresponds to a field amplitude of about 10 Oe. As the GMR of the sensor is cosine-like dependant on the angle between the free and hard layer magnetizations, the GMR resistance varies as cos(w1t+f1). We also feed the sensor with an oscillating current of the form I0 cos(w2t). When considering the Ohm’s Law, we obtain signals at frequencies (w17w2), which only depend on the GMR effect. Let us notice that the anisotropic magneto resistance (AMR) is very small for our sensor. Fig. 4 shows the measurement at the frequency (w1w2) in zero external field as a function of the frequency of the pumping field w1 and for different frequencies of the feeding current w2. For each frequency w2, we measure an increase of the signal when w1 reaches the resonance frequency measured with the micro-antenna. We explained this peak by the fact that at the resonance, the amplitude of the free-layer magnetization diverges and so does the angle between both the layers. Fig. 5 shows measurement done at w2 kept constant (2005 MHz). We have measured the variation of the GMR
0
5000 frequency of the pumping field in MHz
10000
Fig. 5. Measured voltage at the frequency w1–w2 as a function of the pumping field frequency for different values of the external field (’, 0 Oe; J, 50 Oe; m, 100 Oe; and %, 150 Oe). The sensor is fed with a current at 2005 MHz. In the inset is shown the measurement for H ¼ 1000 Oe.
for different value of the external field. The position of the peak in the GMR demodulated signal corresponds to the position of the ferromagnetic resonance frequency. Its amplitude roughly decreases as 1/fresonance as expected for ferromagnetic resonance precession. At low frequency, the signal gets lower and lower as the external field increases. This effect is probably linked with the behavior of the real part of the susceptibility of the free layer, known to decrease as the static susceptibility, when increasing the external field. We have been able to follow the GMR signal until 10 GHz. In conclusion, we have been able to measure the GMR effect up to pumping frequencies of 10 GHz and for current frequencies injected in the GMR sensor up to 5 GHz. The limitation of the response of the GMR sensor is mainly given by the position of the main ferromagnetic resonance. This result has potential repercussions for future data storage applications since read heads are led to reach the gigahertz range in the next decade.
ARTICLE IN PRESS N. Biziere et al. / Journal of Magnetism and Magnetic Materials 316 (2007) 340–343
Acknowledgments
References
We acknowledge the European commission who has partially funded this work through the RTN program Dynamics, the STREPS project biomagsens NMP4-CT2005-07210 and the SFI Nanoscience Laboratory of the Trinity College Dublin, Ireland, for providing the spin valve films.
[1] [2] [3] [4] [5]
M.N. Baibich, et al., Phys. Rev. Lett. 61 (1988) 2472. W.H. Rippard, et al., Phys. Rev. Lett. 92 (2004) 027201-1. H.W. Schumacher, et al., J. Magn. Magn. Mater. 286 (2005) 362. T. Rausch, et al., J. Appl. Phys. 85 (1999) 314. M. Bailleul, et al., Phys. Rev. Lett., APS 91 (2003) 137204.
343
Journal of Magnetism and Magnetic Materials 316 (2007) 340–343 www.elsevier.com/locate/jmmm
Hyper frequency behavior of the GMR effect in a single spin valve sensor N. Biziere, C Fermon, G. Le Goff Service de Physique de l’Etat Condense´ (CNRS URA 2464), DSM/DRECAM/SPEC, CEA Saclay, 91191 Gif sur Yvette Cedex, France Available online 6 March 2007
Abstract In the last decade, new techniques to increase the data storage density have been one of the major axes of research in magnetism. One direction explored consists of increasing the reading speed of read heads. This implies to understand the behavior of the giant magneto resistive (GMR) effect at high frequency. We offer an original method to isolate the GMR of a spin valve sensor, designed as a yoke shape, by combination of classical ferromagnetic resonance technique and demodulation measurement of the CIP voltage of the sensor. We show that the classical description of the GMR effect, based on a mean field theory, is kept valid in a frequency range from DC to 10 GHz. We also show the direct correlation between the ferromagnetic resonance of the free layer sensor and the GMR amplitude. r 2007 Elsevier B.V. All rights reserved. Keywords: Magnetic sensors; Microwaves; Giant magneto-resistance; Spin waves
1. Introduction
2. Experimental setup
Since its experimental discovery in 1988 [1], much attention has been paid to the static properties of the giant magneto resistive (GMR) effect. Whereas a lot of noise studies have used the GMR effect to probe the dynamics of the magnetization [2,3], very few papers, to our knowledge, have dealt with the problem of the hyper-frequency behavior of the GMR effect [4]. In this paper, we offer an original way to probe the GMR of a spin valve sensor in a frequency range from DC to 10 GHz. The idea is first to use a micro-antenna to produce a local precession of the free layer of the spin valve and second to clearly separate the GMR effect from various direct coupling by using the spin valve as an in situ demodulator. In Section 2, we describe the system and the experimental setup. In Section 3, we briefly present the magnetic dynamics of the spin valve sensor. Experimental measurements about the electrical properties from DC to 10 GHz are presented in Section 4.
The spin valve sensor used is made of a free magnetic layer (Ni80Fe20/CoFe) separated from a hard magnetic layer (CoFe/IrMn) by a thin copper film. The stack (Ni80Fe20 (3.5 nm)/CoFe (1.2 nm)/Cu (2.9 nm)/CoFe (3.5 nm)/IrMn (10 nm)) is deposited by sputtering on a thick glass substrate. The terms free and hard refer to the ability of the magnetization to align in the direction of a small external magnetic field. The free and hard layers are grown such as their anisotropies are perpendicular. Conventional UV lithography technique combined with ion milling in argon atmosphere is used to design the sensor as a yoke shape. Its dimensions are given in Fig. 1. This geometry allows getting a uniform magnetization in the area of interest (Fig. 1). The magnetization of the free layer is in the direction of the yoke length. A four-contact measurement is made possible by lift-off of two Ti (10 nm)/Cu (400 nm)/Au (10 nm) slotline type waveguides (Fig. 1). Both waveguide dimensions have been calculated using the TXLINE freeware to be closely adapted 50 O in a large frequency range (from DC to 10 GHz). The current pads can be connected via standard picoprobes either to a DC current source or to an hyper
Corresponding author.
E-mail address: [email protected] (N. Biziere). 0304-8853/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2007.03.010
ARTICLE IN PRESS N. Biziere et al. / Journal of Magnetism and Magnetic Materials 316 (2007) 340–343
25
25
500
5 20
20
20
500
delta signal in V/sqrt(Hz)
50
4.0x10-5
Hres = 371.5 Oe +/- 1.62 Oe delta H = 77.8 Oe +/- 3 Oe
2.0x10-5
depha sed lorentzian fit
341
0.0 -2.0x10-5 -4.0x10-5 -6.0x10-5 -8.0x10-5
I (w2)
-1.0x10-4 300
35 80
I (w1)
6
10
300
Fig. 1. (Top) Dimensions of the sensor designed as a yoke shape (gray) and the contact pads (delimited by black lines). (Bottom) Dimensions of the shorted antenna. All dimensions are given in micrometers.
frequency source (0.01–20 GHz). The voltage pads are connected, also via picoprobes, either to a multimeter for DC measurement or to a spectrum analyzer (9 kHz– 26.5 GHz) for hyper-frequency measurement. On top of the sensor, a coplanar waveguide, shorted at one end, is fabricated by UV lithography and a lift-off of Ti (10 nm)/Cu (400 nm)/Au (10 nm) (Fig. 1). It works as a micro-antenna. It is electrically isolated from the sensor by a 300 nm thick Si3N4 insulating layer. The central conductor of the antenna is roughly aligned with the bar of the sensor. The precision is about half a micron due to the resolution of the optical alignment. In the experiment, a hyper-frequency source injects a current in the antenna through a circulator and a picoprobe. The amplitude of the reflected signal is measured using the spectrum analyzer. The sample and the picoprobes are placed into the gap of an electromagnet providing a static magnetic field (0–1 T with a precision of 3 Oe) parallel to the length of the yoke. 3. Dynamics of the spin valve magnetization The magnetization dynamics of the sensor is studied using ferromagnetic resonance technique with the microantenna [5]. The micro-antenna described above creates at the surface of the active area of the sensor a hyperfrequency magnetic field that we will call the pumping field. Its amplitude can be evaluated in a good approximation by the value I/2w, where I is the current amplitude in the antenna and w the width of the central conductor. In the geometry described in Fig. 1, the pumping field is
400 external field in Oe
500
Fig. 2. Absolute variation of the reflected signal on the antenna as a function of the external field value. The frequency of the current injected in the antenna is 7 GHz. The power delivered to the antenna is 0 dbm.
perpendicular to the static magnetic field and can be considered uniform all over the active area. The experiment consists of sweeping the value of the static field while keeping the frequency of the current in the antenna constant. When the static field reaches the resonant value, a part of the electromagnetic power is absorbed by the sample so the amplitude of the reflected signal decreases. In terms of impedance, the coupling between the antenna and the sample is of inductive nature. The relation between the susceptibility of the sample and the antenna response can be written as w¼
DZw , 2pf m0 tL
(1)
where DZ is the change of the antenna complex impedance, f the frequency, t the thickness of the magnetic film, and L the length of the active area. Fig. 2 shows an example of the measurement of the absolute variation of the reflected signal at 7 GHz as function of the external magnetic field amplitude. The measured signal is the sum of two signals. The first one is a background signal coming from the leak of our circulator and the second one is the reflected signal from the antenna. The spectrum analyzer measures the modulus of this sum. As the phase between the two signals is different, it induces a mixing of the real and imaginary part of the antenna impedance. At the first order, experimental data can be fitted with a function of the form: Aðw0 cos y þ w00 sin yÞ.
(2)
This resonance corresponds to the uniform precession of the free-layer stack. Indeed, considering the uniformity of the pumping field over the active area and the orientation of the free-layer magnetization (aligned with the applied external field), the first mode possibly excited is the uniform mode. We have measured the resonance field in a frequency range from 1 to 10 GHz (Fig. 3). Results are in good
ARTICLE IN PRESS N. Biziere et al. / Journal of Magnetism and Magnetic Materials 316 (2007) 340–343
342
2.0x10-5 amplitude in sqrt(V²/Hz)
resonance frequency in GHz
8 7 6 5
experimental data Kittel fit with Nz = 0 Ny=0.99674 +/- 0.00045 Nx=1-Ny Ms=1.4802 +/- 0.03
4 3
105 MHz 1005 MHz 2005MHz 5005 MHz
1.5x10-5 1.0x10-5 5.0x10-6 0.0
2 0 1 0
100
200 300 400 resonance field in Oe
500
1000 2000 3000 4000 frequency of the pumping field in MHz
5000
600 Fig. 4. Measured voltage at the frequency w1–w2 as a function of the pumping field frequency in zero external field.
Fig. 3. Resonance frequency as a function of the external field.
absolute variation in sqrt(V²/Hz)
agreement with the Kittel resonance model for a very thin ellipsoid. The calculation of the demagnetizing factor shows that the active area can be considered almost infinite in all directions except thickness. This is in accordance with the very small value of the aspect ratio (t/wE105). The value of the saturation magnetization calculated by fitting the experimental data (1.48 T) is in quite good accordance with neutron reflectivity measurements made on continuous films which gave Ms about 1.51 T for the free layer.
4.0x10-7
1.6x10-5
2.0x10-7
1.2x10-5
0.0
8.0x10-6 10000
12000
4.0x10-6
0.0
4. GMR behavior at gigahertz The hyper-frequency behavior of the GMR effect is probed by a demodulation method. The pumping field is fixed at a frequency w1. The power applied on the microantenna is 0 dbm, which corresponds to a field amplitude of about 10 Oe. As the GMR of the sensor is cosine-like dependant on the angle between the free and hard layer magnetizations, the GMR resistance varies as cos(w1t+f1). We also feed the sensor with an oscillating current of the form I0 cos(w2t). When considering the Ohm’s Law, we obtain signals at frequencies (w17w2), which only depend on the GMR effect. Let us notice that the anisotropic magneto resistance (AMR) is very small for our sensor. Fig. 4 shows the measurement at the frequency (w1w2) in zero external field as a function of the frequency of the pumping field w1 and for different frequencies of the feeding current w2. For each frequency w2, we measure an increase of the signal when w1 reaches the resonance frequency measured with the micro-antenna. We explained this peak by the fact that at the resonance, the amplitude of the free-layer magnetization diverges and so does the angle between both the layers. Fig. 5 shows measurement done at w2 kept constant (2005 MHz). We have measured the variation of the GMR
0
5000 frequency of the pumping field in MHz
10000
Fig. 5. Measured voltage at the frequency w1–w2 as a function of the pumping field frequency for different values of the external field (’, 0 Oe; J, 50 Oe; m, 100 Oe; and %, 150 Oe). The sensor is fed with a current at 2005 MHz. In the inset is shown the measurement for H ¼ 1000 Oe.
for different value of the external field. The position of the peak in the GMR demodulated signal corresponds to the position of the ferromagnetic resonance frequency. Its amplitude roughly decreases as 1/fresonance as expected for ferromagnetic resonance precession. At low frequency, the signal gets lower and lower as the external field increases. This effect is probably linked with the behavior of the real part of the susceptibility of the free layer, known to decrease as the static susceptibility, when increasing the external field. We have been able to follow the GMR signal until 10 GHz. In conclusion, we have been able to measure the GMR effect up to pumping frequencies of 10 GHz and for current frequencies injected in the GMR sensor up to 5 GHz. The limitation of the response of the GMR sensor is mainly given by the position of the main ferromagnetic resonance. This result has potential repercussions for future data storage applications since read heads are led to reach the gigahertz range in the next decade.
ARTICLE IN PRESS N. Biziere et al. / Journal of Magnetism and Magnetic Materials 316 (2007) 340–343
Acknowledgments
References
We acknowledge the European commission who has partially funded this work through the RTN program Dynamics, the STREPS project biomagsens NMP4-CT2005-07210 and the SFI Nanoscience Laboratory of the Trinity College Dublin, Ireland, for providing the spin valve films.
[1] [2] [3] [4] [5]
M.N. Baibich, et al., Phys. Rev. Lett. 61 (1988) 2472. W.H. Rippard, et al., Phys. Rev. Lett. 92 (2004) 027201-1. H.W. Schumacher, et al., J. Magn. Magn. Mater. 286 (2005) 362. T. Rausch, et al., J. Appl. Phys. 85 (1999) 314. M. Bailleul, et al., Phys. Rev. Lett., APS 91 (2003) 137204.
343