- Email: [email protected]
Au(111) films with perpendicular magnetic anisotropy
Journal of Magnetism and Magnetic Materials 148 (1995) 293-294
~ Journal of ,','U*''n ,~
ELSEVIER
magnetic
mateglals
Light diffraction effects in the magneto-optical properties of 2D arrays of magnetic dots of Au/Co/Au(111) films with perpendicular magnetic anisotropy N. Bardou a, B. Bartenlian a F. Rousseaux b, D. Decanini b, F. Carcenac b, C. Chappert a,* p. Veillet a, p. Beauvillain a R. M6gy a, y . Suzuki a j. Ferr6 ~ a lnstitut d'Electronique Fondamentale (URA CNRS 022), Universit~ Paris Sua~ 91405 Orsay Cedex, France b Laboratoire de Microstructures et Microeleetronique (UPR CNRS 020), BPI07, 92225 Bagneu.x Cedex, France ¢ Laboratoire de Physique des Solides (URA CNRS 02), Universit~ Paris Sud, 91405 Orsay Cede.x, France
Abstract Using X-ray lithography and ion beam etching we have patterned micrometer-sized square lattices o f round dots, out of Au/Co/Au(lll) films with perpendicular easy magnetization axes. The magneto-optical Kerr rotation shows strong variations versus the diffraction order, the light polarization direction and the ratio of fdm to background reflectance, particularly when the lattice spacing is high compared with the dot size. This reveals an important interplay between diffraction and magneto-optics.
The interplay between diffraction and magneto-optical effects in patterned magnetic thin films promises to b e an exciting field of research. This has been used, for instance, in regular 2D arrays of magnetic dots to investigate the magnetization reversal in a single dot [1]. We have studied the Kerr rotation at polar incidence (PMOKE) of regular square lattices of circular dots, patterned in A u / C o / ' Au(111) sandwiches with perpendicular magnetic anisotropy. The film preparation and magnetic properties have been reported elsewhere [2,3]. We report here the results obtained on two samples with 3 A L (S1) and 7 A L (S2) thick Co films. The Co layers are sandwiched between a 5 nm thick Au capping layer and a 25 nm thick (111) Au buffer layer, deposited on a thermally oxidized silicen wafer (tsioz = 100 nm). The samples are patterned using X-ray lithography and ion beam etching. These techniques have shown excellent resolution (50 nm [4]), which is promising for the future evolution o f our work. First, the samples are covered with a positive resist ( P M M A / M A A copolymer) about 600 nm thick. The resist is then annealed for 1 h at 115°C, and exposed to X-rays through a specific mask [4]. After development, we get a lattice of resist dots. The unprotected parts of the Au/Co/Au film are then removed by ion beam etching, and the resist is removed with oxygen plasma.
* Corresponding author. Fax: +33-1-60 19 25 93; e-mail: [email protected].
Four I x 1 m m 2 an'ays are patterned on each sample, with respective ratios of the dot diameter to the lattice spacing of 1/1.1, 2 / 2 . 2 , 1 / 2 and 2 / 4 (all lengths in Ixm). Care was taken to keep large areas either unetched (by masking during the ion beam etching process) or fully etched; in the following these are referred to as continuous film (CF) or background (BG). Furthermore, in sample S1 the ion etching process was completed well into the SiO 2 underlayer, while in sample $2 it was stopped before full removal of the Au buffer layer. For the PMOKE measurements we used either red (A = 632.8 nm) or green (A = 543.5 nm) HeNe lasers, with beams focused to Gaussian diameters of 0.4 ram. A Glan-Taylor polarizer is placed in front of the sample to control the polarization of the incident light. Detection is performed classically through a photoelastic modulator (PEM) and an analyzer placed on the diffracted beam. The system was calibrated before each measurement by rotating together the PEM and the analyzer. From the P M O K E measurements on the CF areas, it appears dearly that the magnetic properties of the samples have changed during the patterning process, with for instance large increases in the coercive force. However, the saturation Kerr rotation and the overall shapes o f the hysteresis loops remain the same [3], which is of primary interest in the present work. Other aspects will be discussed in a forthcoming paper. At zero diffraction order (specular reflection) we observe already two main classes of i~ehaviour, depending on
0304-8853/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0 3 0 4 - 8 8 5 3 ( 9 5 ) 0 0 2 4 3 - X
294
N. Bardou et aL /Journal of Magnetism and Magnetic Materials 148 (]995) 293-294
0.8 0.7,.~ '
~
_
~ •
0.G
~0.5
~
0,4,
....
0.03
--~
81 * 84~.ISnrn
~A--
8R - I;~2.Srtm
'1::1
0.01 .
o
0,2. 0.t, I
I
1/!.1
2/2.2
Y------~ tl2
i°
rig
,
111,1
2/2,2
112
214
pattern
Fig. 2. Kerr rotation at specular reflection for the different patterns on sample S1, the packing of the arrays. The reflectance R (Fig. 1) is much higher for the densely packed than for loosely packed arrays. In the latter case, R is even smaller than for the BG. Also, the Kerr rotation changes sign with respect to the CF for the loosely packed arrays (Fig. 2). Note that this is not entirely true for sample $2 at k = 543.5 nm, 0.08 0,06 0.04
~ 0.02 ~
t - 83Lsrtrn
---o--
p . i132 ,~nm
•- - ~ - -
p - 843.8nm
2J4-p
a
-s
0
lt1.1
a2.z
1
2
3
4
order
Fig. 4. Kerr rotation versus diffraction order far array 2 / 4 of sample S1.
probably due to the unetched Au BG layer (Fig. 3). The behaviour of sample S1 can be understood qualitatively with the following simple scheme. The optical intensities come from interferences between the light reflected on the dots (electric field Ea) and on the BG (EBG), respectively. In a first approximation, EBG is expected to undergo a dephasing of about ar upon reflection on SlOe (whose clerical index is greater than 1), .while E a should be comparatively little dephased (the optical indices of Au are snmll). The change in sign of the Kerr rotation should then appear when the absolute value o f E n o is higher than that o f E~. This can also explain the low reflectance of the loosely packed lattices. Things are more complicated for sample $2, where the unetehed A : layer drastically changes the reflection properties of the background, particularly when, going from /t -----632.8 to 543.5 nm, we approach the plasma edge of Au. Effects at high diffraction orders are also more important in loosely packed lattices. Fig. 4 displays the variation o f the saturation Kerr rotation for array 2 / 4 o f sample S1. One observes first that the Kerr rotation for non-zero orders recovers the sign of the CF one. More important, there are strong differences between OK for s and p polarizations of the incident light. A detailed calculation o f the magneto.optical effects in the frame of the diffraction theory of polarized light is in progress in our laboratory.
References
It:,
zt4
pattern
]Fig. 3. Kerr rotation ~,t specular reflection for the different patterns
on sample $2.
I "
diffraction
~ -0.04 .0,~ -0.08
CF
I
-0.02
Zl4
0,0Z
.0.08
I
-0,015.
0,08O.C.~
o.o:
I
-0.01 ,
Fig. 1. Reflectance of the different patterns at specular reflection for normal incidence. All values are relative to those calculated for the unpatterned films [5].
~ .0,04 ,O,t~
0
~,~ -0.00S ,
I)allern
GF
0.01S.
0.1005,
~. 0 . 3 .
CF
~
0.02,
8 1 - 6:32.8nm
--'-¢,--- S2.54&S:'m
0
~
0.025,
[1] [2] [3] [4] [5]
O. Geoffrey et al., J. Magn. Magn. Mater. 121 (1993) 516. C. Cesari et al., J. Magn. Magn. Mater. 78 (1989) 296. S. Ould-Mahfoud et al., MIlS Prec. Ser. 313 (1993) 251. F. Rousseaux et al., Mieroelectronie Eng. 17 (1992) 157. G. PEnissard, PhD thesis, Oreay (1993).
~ Journal of ,','U*''n ,~
ELSEVIER
magnetic
mateglals
Light diffraction effects in the magneto-optical properties of 2D arrays of magnetic dots of Au/Co/Au(111) films with perpendicular magnetic anisotropy N. Bardou a, B. Bartenlian a F. Rousseaux b, D. Decanini b, F. Carcenac b, C. Chappert a,* p. Veillet a, p. Beauvillain a R. M6gy a, y . Suzuki a j. Ferr6 ~ a lnstitut d'Electronique Fondamentale (URA CNRS 022), Universit~ Paris Sua~ 91405 Orsay Cedex, France b Laboratoire de Microstructures et Microeleetronique (UPR CNRS 020), BPI07, 92225 Bagneu.x Cedex, France ¢ Laboratoire de Physique des Solides (URA CNRS 02), Universit~ Paris Sud, 91405 Orsay Cede.x, France
Abstract Using X-ray lithography and ion beam etching we have patterned micrometer-sized square lattices o f round dots, out of Au/Co/Au(lll) films with perpendicular easy magnetization axes. The magneto-optical Kerr rotation shows strong variations versus the diffraction order, the light polarization direction and the ratio of fdm to background reflectance, particularly when the lattice spacing is high compared with the dot size. This reveals an important interplay between diffraction and magneto-optics.
The interplay between diffraction and magneto-optical effects in patterned magnetic thin films promises to b e an exciting field of research. This has been used, for instance, in regular 2D arrays of magnetic dots to investigate the magnetization reversal in a single dot [1]. We have studied the Kerr rotation at polar incidence (PMOKE) of regular square lattices of circular dots, patterned in A u / C o / ' Au(111) sandwiches with perpendicular magnetic anisotropy. The film preparation and magnetic properties have been reported elsewhere [2,3]. We report here the results obtained on two samples with 3 A L (S1) and 7 A L (S2) thick Co films. The Co layers are sandwiched between a 5 nm thick Au capping layer and a 25 nm thick (111) Au buffer layer, deposited on a thermally oxidized silicen wafer (tsioz = 100 nm). The samples are patterned using X-ray lithography and ion beam etching. These techniques have shown excellent resolution (50 nm [4]), which is promising for the future evolution o f our work. First, the samples are covered with a positive resist ( P M M A / M A A copolymer) about 600 nm thick. The resist is then annealed for 1 h at 115°C, and exposed to X-rays through a specific mask [4]. After development, we get a lattice of resist dots. The unprotected parts of the Au/Co/Au film are then removed by ion beam etching, and the resist is removed with oxygen plasma.
* Corresponding author. Fax: +33-1-60 19 25 93; e-mail: [email protected].
Four I x 1 m m 2 an'ays are patterned on each sample, with respective ratios of the dot diameter to the lattice spacing of 1/1.1, 2 / 2 . 2 , 1 / 2 and 2 / 4 (all lengths in Ixm). Care was taken to keep large areas either unetched (by masking during the ion beam etching process) or fully etched; in the following these are referred to as continuous film (CF) or background (BG). Furthermore, in sample S1 the ion etching process was completed well into the SiO 2 underlayer, while in sample $2 it was stopped before full removal of the Au buffer layer. For the PMOKE measurements we used either red (A = 632.8 nm) or green (A = 543.5 nm) HeNe lasers, with beams focused to Gaussian diameters of 0.4 ram. A Glan-Taylor polarizer is placed in front of the sample to control the polarization of the incident light. Detection is performed classically through a photoelastic modulator (PEM) and an analyzer placed on the diffracted beam. The system was calibrated before each measurement by rotating together the PEM and the analyzer. From the P M O K E measurements on the CF areas, it appears dearly that the magnetic properties of the samples have changed during the patterning process, with for instance large increases in the coercive force. However, the saturation Kerr rotation and the overall shapes o f the hysteresis loops remain the same [3], which is of primary interest in the present work. Other aspects will be discussed in a forthcoming paper. At zero diffraction order (specular reflection) we observe already two main classes of i~ehaviour, depending on
0304-8853/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0 3 0 4 - 8 8 5 3 ( 9 5 ) 0 0 2 4 3 - X
294
N. Bardou et aL /Journal of Magnetism and Magnetic Materials 148 (]995) 293-294
0.8 0.7,.~ '
~
_
~ •
0.G
~0.5
~
0,4,
....
0.03
--~
81 * 84~.ISnrn
~A--
8R - I;~2.Srtm
'1::1
0.01 .
o
0,2. 0.t, I
I
1/!.1
2/2.2
Y------~ tl2
i°
rig
,
111,1
2/2,2
112
214
pattern
Fig. 2. Kerr rotation at specular reflection for the different patterns on sample S1, the packing of the arrays. The reflectance R (Fig. 1) is much higher for the densely packed than for loosely packed arrays. In the latter case, R is even smaller than for the BG. Also, the Kerr rotation changes sign with respect to the CF for the loosely packed arrays (Fig. 2). Note that this is not entirely true for sample $2 at k = 543.5 nm, 0.08 0,06 0.04
~ 0.02 ~
t - 83Lsrtrn
---o--
p . i132 ,~nm
•- - ~ - -
p - 843.8nm
2J4-p
a
-s
0
lt1.1
a2.z
1
2
3
4
order
Fig. 4. Kerr rotation versus diffraction order far array 2 / 4 of sample S1.
probably due to the unetched Au BG layer (Fig. 3). The behaviour of sample S1 can be understood qualitatively with the following simple scheme. The optical intensities come from interferences between the light reflected on the dots (electric field Ea) and on the BG (EBG), respectively. In a first approximation, EBG is expected to undergo a dephasing of about ar upon reflection on SlOe (whose clerical index is greater than 1), .while E a should be comparatively little dephased (the optical indices of Au are snmll). The change in sign of the Kerr rotation should then appear when the absolute value o f E n o is higher than that o f E~. This can also explain the low reflectance of the loosely packed lattices. Things are more complicated for sample $2, where the unetehed A : layer drastically changes the reflection properties of the background, particularly when, going from /t -----632.8 to 543.5 nm, we approach the plasma edge of Au. Effects at high diffraction orders are also more important in loosely packed lattices. Fig. 4 displays the variation o f the saturation Kerr rotation for array 2 / 4 o f sample S1. One observes first that the Kerr rotation for non-zero orders recovers the sign of the CF one. More important, there are strong differences between OK for s and p polarizations of the incident light. A detailed calculation o f the magneto.optical effects in the frame of the diffraction theory of polarized light is in progress in our laboratory.
References
It:,
zt4
pattern
]Fig. 3. Kerr rotation ~,t specular reflection for the different patterns
on sample $2.
I "
diffraction
~ -0.04 .0,~ -0.08
CF
I
-0.02
Zl4
0,0Z
.0.08
I
-0,015.
0,08O.C.~
o.o:
I
-0.01 ,
Fig. 1. Reflectance of the different patterns at specular reflection for normal incidence. All values are relative to those calculated for the unpatterned films [5].
~ .0,04 ,O,t~
0
~,~ -0.00S ,
I)allern
GF
0.01S.
0.1005,
~. 0 . 3 .
CF
~
0.02,
8 1 - 6:32.8nm
--'-¢,--- S2.54&S:'m
0
~
0.025,
[1] [2] [3] [4] [5]
O. Geoffrey et al., J. Magn. Magn. Mater. 121 (1993) 516. C. Cesari et al., J. Magn. Magn. Mater. 78 (1989) 296. S. Ould-Mahfoud et al., MIlS Prec. Ser. 313 (1993) 251. F. Rousseaux et al., Mieroelectronie Eng. 17 (1992) 157. G. PEnissard, PhD thesis, Oreay (1993).