- Email: [email protected]
Magnetotransport in microstripes patterned in ultrathin cobalt films
Journal of Magnetism and Magnetic Materials 165 (1997) 349-351
ELSEVIER
~i ~a ~a
journalof magnetism and magnetic materials
Magnetotransport in microstripes patterned in ultrathin cobalt films J. Caulet *, B. Bartenlian, V. Kottler, C. Chappert, A. Anane, P. Veillet lnstitut d'Electronique Fondamentale, CNRS URA 022, Unicersitd Paris-Sud, B&. 220, 91405 Orsay Cedex, France
Abstract The magnetotransport properties are an attractive alternative to standard observation techniques for the characterization of the magnetization reversal in structures with reduced lateral dimensions (stripes). In particular, magnetic films exhibit the so-called anomalous Hall effect which is very sensitive to the local magnetic domain structures. We began to work with the model system C o / A u ( l l l ) grown on silicon dioxide. We present here a new method for microstructuring magnetic multilayers, that avoid the use of technological processes on the multilayers. Preliminary magnetotransport results in 50 p,m wide stripes are presented and discussed. Keywords: Multilayers - metallic; Silicon technology; Magnetoresistance; Hall effect - anomalous
There is a growing interest in the realization and study of micro- and nanostructures made from magnetic multilayers. The C o / A u ( l l l ) model system has been extensively studied in our laboratory because of its exceptional magnetic behaviour as for instance perpendicular magnetic anisotropy [1], specific magnetization reversal process [2] and giant magnetoresistance effect [3]. Recently, the magnetization reversal in submicronic dots patterned on such films was characterized using the magneto-optical Kerr effect [4]. We are now interested in the study of the magnetization reversal in A u / C o / A u stripes. This particular geometry allows the use of magnetotransport properties such as magnetoresistance and the anomalous Hall effect as characterization tools. These properties may act as very sensitive magnetization probes [5-7]. In this paper, we present an original way to pattern magnetic films, together with preliminary magnetotransport measurements made in A u / C o / A u stripes of 50 txm width. The different layers are grown by evaporation in ultrahigh vacuum on a silicon dioxide substrate [4]. We have chosen to start our study with the stacking: Au(7.5 n m ) / C o ( l nm)/Au(2.6 nm)/Co(0.6 nm)/Au(28 nm). With these different thicknesses, the magnetizations of both cobalt layers are oriented perpendicularly to the plane of the film and since there is negligible interlayer coupling,
* Corresponding author. Email: [email protected]; fax: + 33-1-6019-2593.
each layer is expected to reverse with its own intrinsic coercivity [3]. The magnetic properties of the Co/Au(111) films depend in a crucial way on the flatness of the 28 nm thick gold buffer layer [1]. A good, (11 l)-textured buffer layer is obtained after annealing at 180°C for an hour. The very low adhesion between gold and silicon dioxide allows this structural rearrangement, but is also a limiting factor for the processing of these films, especially for 'wet' treatments. To pattern our samples, we first tried to use classical optical lithography to form a resist mask, followed by ion milling. The resist is then removed in an 0 2 plasma. This process resulted in a serious degradation of the magnetic properties of the multilayers, maybe because of the stresses coming from different thermal expansions between the film and the thick polymeric resist during the annealing and etching steps. We then designed an original method that allows the formation of patterns without any processing of the magnetic multilayer. With this method, we process only the substrate and the patterns thus defined will be reproduced onto the film. The different steps of the process are presented in Fig. 1. We begin by oxidizing the silicon substrate to form a thick SiO 2 layer with a very flat surface. Then, we use optical lithography to define the pattern followed by successive SiO 2 and silicon etching. The key idea is to etch the silicon substrate around the pattern in an isotropic way so that a silicon dioxide 'ledge' is formed (see Fig. 2) [8]. So, after deposition of the different layers, the part of the film that has grown on
0304-8853/97/$17.00 Copyright © 1997 Elsevier Science B.V. All rights reserved. Pll S0304-8853(96)00552-5
350
J. Caulet et al.// Journal of Magnetism and Magnetic Materials 165 (1997) 349-351
(D
(3)
(2)
&
FI (5)
(4)
(6) Photoresist
[ - - - - ' - ' ] Silicon substrate
I
sio~
l
Av/Cofilm
Fig. 1. Sketch of the different steps of the process. (1) Thermal oxidization of a silicon wafer; (2) optical lithography; (3) chemical etch (in buffered HF) of the silicon dioxide through a photoresist mask; (4) isotropic etch of the silicon in a SF6 + 02 plasma; (5) removal of the resist in an O 2 plasma; (6) growth of the magnetic film.
silicon dioxide is separated from the one that has grown on etched silicon. Fig. 3 shows the mask that we used to test the process: it represents a stripe with different contacts to perform magnetoresistance and Hall effect measurements. Fig. 4 shows the hysteresis loop measured in the middle of the contact dots, using the polar magneto-optical Kerr effect. One sees the separate reversal of the two cobalt layers but there is no clear step as was expected from the difference in coercive force of the two cobalt layers (500 and 900 Oe). A slight damage of the surface and the presence of impurities coming from lithographic steps may have had an effect on the quality of the gold buffer layer. However, the loop obtained on an unpatterned substrate that was grown at the same time shows the same behaviour, indicating that the processed substrate had no significant effect on the structural quality of the multilayers. The magnetotransport measurements were performed at room temperature with an ac bridge operating at low frequency (32 Hz), in a magnetic field applied perpendicu-
3.5mm
1
Fig. 3. Mask pattern of the stripe and different contacts. MR measurements are done by injecting current from electrical contact dot 1 to dot 3 and by measuring the difference of potential between dots 6 and 4. Potential between 5 and 2 corresponds to the Hall effect. The stripes are 50 txm wide and 450 Ixm long.
larly to the interfaces and to the current lines. Fig. 5a represents the magnetoresistance curve. The sharp peaks show that the reversal of the two cobalt layers is close but separate, thus giving rise to a spin valve effect. The amplitude of this effect, approximately 1.5%, corresponds to what was measured in unpattemed samples. The Hall resistivity On of the multilayer is given by [9]: PH = t V H / I = R o l l + Rs4"rrM ± , where R 0 and R s are the ordinary and anomalous Hall coefficients, t is the thickness of the structure, VH is the Hall voltage, l is the sample current, H is the applied field and M j_ is the component of magnetization perpendicular to the film plane. The Hall voltage connector lines have the same width as the stripe and so, the current lines extend laterally in these connections. This means that the area contributing to the Hall effect is larger than the 50 x 50 ixm 2 surface corresponding to the intersection of the connector lines with the stripe. Fig. 5b represents the Hall effect curve obtained on the sample. The two contributions to the Hall effect are clearly present: the slope in the curve comes from the ordinary effect (mainly from the gold layers) and the loop comes from the anomalous effect arising from the two cobalt layers. The change in resistance due to the reversal of the magnetic layers is 0.4 m I l . This leads to an anomalous Hall resistivity of 0.8 × 10 -3 WlI cm. Vavra et al. measured a resistivity of 10 -1 IxII cm in similar epitaxial
40
i
Fig. 2. Scanning Electron Microscope image of the cross section of the patterned substrate showing the SiO2 'ledge'. The white dashed line shows the separation between the SiO2 layer and the silicon substrate.
-40 -1.5
Applied Field (kOe)
1.5
Fig. 4. Hysteresis loop of bilayer Co/Au measured on a contact dot by the polar magneto-optical Kerr effect after the process.
J. Caulet et al. / Journal of Magnetism and Magnetic Materials 165 (1997) 349-351 10.8 -
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10.65.
10.6 .
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351
Hail effect. Still, this effect alone cannot explain the fluctuations because a 'misalignment magnetoresistance' is expected to only increase the Hail signal. However, the large dimension (50 ixm) of the Hall cross is about the order of magnitude of the size of the reversed domains that develop in this kind of Co ultrathin films [2]. On a local scale, around the coercive field, Hall effect and magnetoresistance can fluctuate rapidly and differently depending on the exact domain structure and its time evolution, thus inducing the observed signal fluctuations. This hypothesis is now being tested by simultaneous magnetotransport and Kerr microscopy measurements, in collaboration with J.-P. Jamet and P. Meyer of the Laboratoire de Physique des Solides of Universit6 Paris-Sud (Orsay, France), together with magnetotransport measurements on samples with variable stripe and Hall contacts widths. In conclusion, we have succeeded in realizing microstripes by deposition on a patterned silicon dioxide substrate, thus preventing any damage to these fragile systems that could be induced by post-deposition patterning. Preliminary magnetotransport measurements on 50 Ixm wide stripes show large fluctuations of the Hall effect near the coercive field, which we attribute to the evolution of the domain structure in the Hall cross.
Applied field (kOe)
Fig. 5. (a) Magnetoresistance curve at room temperature of a 450 Ixm long and 50 I~m wide Au/Co bilayer; (b) Hall effect curve at room temperature in the same sample.
C o / A u multilayers [9]. This difference is probably due to a shunting effect of the very thick gold buffer layer in our case. When the magnetization reverses, we observe very large fluctuations of the Hail signal. As R s scales with the resistivity [10], the anomalous resistance will vary during magnetization reversal due to the spin valve effect. But, in our samples, this effect is small and cannot explain the large variations of the signal. Given the precision of our mask fabrication process, the Hall voltage connector lines are not perfectly aligned. This results in the appearance of a 'misalignment resistance' that has a value of about 14 m r / i n our case, as can be seen from the constant contribution measured on the Hall effect. This value corresponds to a misalignment of about 0.5 Ixm, which seems reasonable for an optical lithography process, and will lead to a parasitic magnetoresistance effect in the Hall voltage during magnetization reversal. The measured magnetoresistance effect in this system has an amplitude of about 1.5%, which gives a resistance variation of 0.21 mI~, a value that has the same order of magnitude as the intrinsic anomalous
References
[1] S. Ould Mahfoud, R. M6gy, N. Bardou, B. Bartenlian, B. L6cuyer, P. Beauvillain, C. Chappert, J. Corno, G. Scziegel, P. Veillet and D. Weller, Mater. Res. Soc. Proc. 313 (1993) 251. [2] J. Pommier, P. Meyer, G. P6nissard, J. Ferr6, P. Bruno and D. Renard, Phys. Rev. Lett. 65 (1990) 2054. [3] C. Dupas, P. Beauvillain, C. Chappert, J.-P. Renard, F. Trigui, P. Veillet, E. V61u and D. Renard, J. Appl. Phys. (1990) 5680. [4] N. Bardou, B. Bartenlian, C. Chappert, R. M6gy, P. Veillet, J.-P. Renard, F. Rousseaux, M.-F. Ravet, J.-P. Jamet and P. Meyer, J. Appl. Phys. 79 (1996) 5848. [5] H. Beckmann, F. Ye and G. Bergmann, Phys. Rev. Lett. 73 (1994) 1715. [6] B.C. Webb and S. Schultz, IEEE Trans. Magn. 24 (1988) 3006. [7] V. Cros, S.-F. Lee, A. Hamzi, A. Fert, G. Faini and A. Cornette, communication E-XII/P49, E-MRS Spring Meeting 1996 (this conference). [8] M.-N. Charasse, B. Bartenlian, J.-P. Hirtz, A. Peugnet, J. Chazelas and G. Amendola, J. Electron. Mater. 19 (1990) 567. [9] W. Vavra, C.H. Lee, F.J. Lamelas, H. He, R. Clarke and C. Uher, Phys. Rev. B 42 (1990) 4889. [10] S. Zhang, Phys. Rev. B 51 (1995) 3632.
ELSEVIER
~i ~a ~a
journalof magnetism and magnetic materials
Magnetotransport in microstripes patterned in ultrathin cobalt films J. Caulet *, B. Bartenlian, V. Kottler, C. Chappert, A. Anane, P. Veillet lnstitut d'Electronique Fondamentale, CNRS URA 022, Unicersitd Paris-Sud, B&. 220, 91405 Orsay Cedex, France
Abstract The magnetotransport properties are an attractive alternative to standard observation techniques for the characterization of the magnetization reversal in structures with reduced lateral dimensions (stripes). In particular, magnetic films exhibit the so-called anomalous Hall effect which is very sensitive to the local magnetic domain structures. We began to work with the model system C o / A u ( l l l ) grown on silicon dioxide. We present here a new method for microstructuring magnetic multilayers, that avoid the use of technological processes on the multilayers. Preliminary magnetotransport results in 50 p,m wide stripes are presented and discussed. Keywords: Multilayers - metallic; Silicon technology; Magnetoresistance; Hall effect - anomalous
There is a growing interest in the realization and study of micro- and nanostructures made from magnetic multilayers. The C o / A u ( l l l ) model system has been extensively studied in our laboratory because of its exceptional magnetic behaviour as for instance perpendicular magnetic anisotropy [1], specific magnetization reversal process [2] and giant magnetoresistance effect [3]. Recently, the magnetization reversal in submicronic dots patterned on such films was characterized using the magneto-optical Kerr effect [4]. We are now interested in the study of the magnetization reversal in A u / C o / A u stripes. This particular geometry allows the use of magnetotransport properties such as magnetoresistance and the anomalous Hall effect as characterization tools. These properties may act as very sensitive magnetization probes [5-7]. In this paper, we present an original way to pattern magnetic films, together with preliminary magnetotransport measurements made in A u / C o / A u stripes of 50 txm width. The different layers are grown by evaporation in ultrahigh vacuum on a silicon dioxide substrate [4]. We have chosen to start our study with the stacking: Au(7.5 n m ) / C o ( l nm)/Au(2.6 nm)/Co(0.6 nm)/Au(28 nm). With these different thicknesses, the magnetizations of both cobalt layers are oriented perpendicularly to the plane of the film and since there is negligible interlayer coupling,
* Corresponding author. Email: [email protected]; fax: + 33-1-6019-2593.
each layer is expected to reverse with its own intrinsic coercivity [3]. The magnetic properties of the Co/Au(111) films depend in a crucial way on the flatness of the 28 nm thick gold buffer layer [1]. A good, (11 l)-textured buffer layer is obtained after annealing at 180°C for an hour. The very low adhesion between gold and silicon dioxide allows this structural rearrangement, but is also a limiting factor for the processing of these films, especially for 'wet' treatments. To pattern our samples, we first tried to use classical optical lithography to form a resist mask, followed by ion milling. The resist is then removed in an 0 2 plasma. This process resulted in a serious degradation of the magnetic properties of the multilayers, maybe because of the stresses coming from different thermal expansions between the film and the thick polymeric resist during the annealing and etching steps. We then designed an original method that allows the formation of patterns without any processing of the magnetic multilayer. With this method, we process only the substrate and the patterns thus defined will be reproduced onto the film. The different steps of the process are presented in Fig. 1. We begin by oxidizing the silicon substrate to form a thick SiO 2 layer with a very flat surface. Then, we use optical lithography to define the pattern followed by successive SiO 2 and silicon etching. The key idea is to etch the silicon substrate around the pattern in an isotropic way so that a silicon dioxide 'ledge' is formed (see Fig. 2) [8]. So, after deposition of the different layers, the part of the film that has grown on
0304-8853/97/$17.00 Copyright © 1997 Elsevier Science B.V. All rights reserved. Pll S0304-8853(96)00552-5
350
J. Caulet et al.// Journal of Magnetism and Magnetic Materials 165 (1997) 349-351
(D
(3)
(2)
&
FI (5)
(4)
(6) Photoresist
[ - - - - ' - ' ] Silicon substrate
I
sio~
l
Av/Cofilm
Fig. 1. Sketch of the different steps of the process. (1) Thermal oxidization of a silicon wafer; (2) optical lithography; (3) chemical etch (in buffered HF) of the silicon dioxide through a photoresist mask; (4) isotropic etch of the silicon in a SF6 + 02 plasma; (5) removal of the resist in an O 2 plasma; (6) growth of the magnetic film.
silicon dioxide is separated from the one that has grown on etched silicon. Fig. 3 shows the mask that we used to test the process: it represents a stripe with different contacts to perform magnetoresistance and Hall effect measurements. Fig. 4 shows the hysteresis loop measured in the middle of the contact dots, using the polar magneto-optical Kerr effect. One sees the separate reversal of the two cobalt layers but there is no clear step as was expected from the difference in coercive force of the two cobalt layers (500 and 900 Oe). A slight damage of the surface and the presence of impurities coming from lithographic steps may have had an effect on the quality of the gold buffer layer. However, the loop obtained on an unpatterned substrate that was grown at the same time shows the same behaviour, indicating that the processed substrate had no significant effect on the structural quality of the multilayers. The magnetotransport measurements were performed at room temperature with an ac bridge operating at low frequency (32 Hz), in a magnetic field applied perpendicu-
3.5mm
1
Fig. 3. Mask pattern of the stripe and different contacts. MR measurements are done by injecting current from electrical contact dot 1 to dot 3 and by measuring the difference of potential between dots 6 and 4. Potential between 5 and 2 corresponds to the Hall effect. The stripes are 50 txm wide and 450 Ixm long.
larly to the interfaces and to the current lines. Fig. 5a represents the magnetoresistance curve. The sharp peaks show that the reversal of the two cobalt layers is close but separate, thus giving rise to a spin valve effect. The amplitude of this effect, approximately 1.5%, corresponds to what was measured in unpattemed samples. The Hall resistivity On of the multilayer is given by [9]: PH = t V H / I = R o l l + Rs4"rrM ± , where R 0 and R s are the ordinary and anomalous Hall coefficients, t is the thickness of the structure, VH is the Hall voltage, l is the sample current, H is the applied field and M j_ is the component of magnetization perpendicular to the film plane. The Hall voltage connector lines have the same width as the stripe and so, the current lines extend laterally in these connections. This means that the area contributing to the Hall effect is larger than the 50 x 50 ixm 2 surface corresponding to the intersection of the connector lines with the stripe. Fig. 5b represents the Hall effect curve obtained on the sample. The two contributions to the Hall effect are clearly present: the slope in the curve comes from the ordinary effect (mainly from the gold layers) and the loop comes from the anomalous effect arising from the two cobalt layers. The change in resistance due to the reversal of the magnetic layers is 0.4 m I l . This leads to an anomalous Hall resistivity of 0.8 × 10 -3 WlI cm. Vavra et al. measured a resistivity of 10 -1 IxII cm in similar epitaxial
40
i
Fig. 2. Scanning Electron Microscope image of the cross section of the patterned substrate showing the SiO2 'ledge'. The white dashed line shows the separation between the SiO2 layer and the silicon substrate.
-40 -1.5
Applied Field (kOe)
1.5
Fig. 4. Hysteresis loop of bilayer Co/Au measured on a contact dot by the polar magneto-optical Kerr effect after the process.
J. Caulet et al. / Journal of Magnetism and Magnetic Materials 165 (1997) 349-351 10.8 -
(a)
10.75 -
~
~
10.7 •
10.65.
10.6 .
10.55 -4
I -3
I -2
I -1
I 0
~ 1
t 2
I 3
0
I I
I 2
I 3
4
15
~
\
14.5
14
13.5 -4
I -3
I -2
I -I
351
Hail effect. Still, this effect alone cannot explain the fluctuations because a 'misalignment magnetoresistance' is expected to only increase the Hail signal. However, the large dimension (50 ixm) of the Hall cross is about the order of magnitude of the size of the reversed domains that develop in this kind of Co ultrathin films [2]. On a local scale, around the coercive field, Hall effect and magnetoresistance can fluctuate rapidly and differently depending on the exact domain structure and its time evolution, thus inducing the observed signal fluctuations. This hypothesis is now being tested by simultaneous magnetotransport and Kerr microscopy measurements, in collaboration with J.-P. Jamet and P. Meyer of the Laboratoire de Physique des Solides of Universit6 Paris-Sud (Orsay, France), together with magnetotransport measurements on samples with variable stripe and Hall contacts widths. In conclusion, we have succeeded in realizing microstripes by deposition on a patterned silicon dioxide substrate, thus preventing any damage to these fragile systems that could be induced by post-deposition patterning. Preliminary magnetotransport measurements on 50 Ixm wide stripes show large fluctuations of the Hall effect near the coercive field, which we attribute to the evolution of the domain structure in the Hall cross.
Applied field (kOe)
Fig. 5. (a) Magnetoresistance curve at room temperature of a 450 Ixm long and 50 I~m wide Au/Co bilayer; (b) Hall effect curve at room temperature in the same sample.
C o / A u multilayers [9]. This difference is probably due to a shunting effect of the very thick gold buffer layer in our case. When the magnetization reverses, we observe very large fluctuations of the Hail signal. As R s scales with the resistivity [10], the anomalous resistance will vary during magnetization reversal due to the spin valve effect. But, in our samples, this effect is small and cannot explain the large variations of the signal. Given the precision of our mask fabrication process, the Hall voltage connector lines are not perfectly aligned. This results in the appearance of a 'misalignment resistance' that has a value of about 14 m r / i n our case, as can be seen from the constant contribution measured on the Hall effect. This value corresponds to a misalignment of about 0.5 Ixm, which seems reasonable for an optical lithography process, and will lead to a parasitic magnetoresistance effect in the Hall voltage during magnetization reversal. The measured magnetoresistance effect in this system has an amplitude of about 1.5%, which gives a resistance variation of 0.21 mI~, a value that has the same order of magnitude as the intrinsic anomalous
References
[1] S. Ould Mahfoud, R. M6gy, N. Bardou, B. Bartenlian, B. L6cuyer, P. Beauvillain, C. Chappert, J. Corno, G. Scziegel, P. Veillet and D. Weller, Mater. Res. Soc. Proc. 313 (1993) 251. [2] J. Pommier, P. Meyer, G. P6nissard, J. Ferr6, P. Bruno and D. Renard, Phys. Rev. Lett. 65 (1990) 2054. [3] C. Dupas, P. Beauvillain, C. Chappert, J.-P. Renard, F. Trigui, P. Veillet, E. V61u and D. Renard, J. Appl. Phys. (1990) 5680. [4] N. Bardou, B. Bartenlian, C. Chappert, R. M6gy, P. Veillet, J.-P. Renard, F. Rousseaux, M.-F. Ravet, J.-P. Jamet and P. Meyer, J. Appl. Phys. 79 (1996) 5848. [5] H. Beckmann, F. Ye and G. Bergmann, Phys. Rev. Lett. 73 (1994) 1715. [6] B.C. Webb and S. Schultz, IEEE Trans. Magn. 24 (1988) 3006. [7] V. Cros, S.-F. Lee, A. Hamzi, A. Fert, G. Faini and A. Cornette, communication E-XII/P49, E-MRS Spring Meeting 1996 (this conference). [8] M.-N. Charasse, B. Bartenlian, J.-P. Hirtz, A. Peugnet, J. Chazelas and G. Amendola, J. Electron. Mater. 19 (1990) 567. [9] W. Vavra, C.H. Lee, F.J. Lamelas, H. He, R. Clarke and C. Uher, Phys. Rev. B 42 (1990) 4889. [10] S. Zhang, Phys. Rev. B 51 (1995) 3632.