- Email: [email protected]
Analysis of type-II magnetic contrast from ferromagnetic thin films in the scanning electron microscopy
356
Journal of Magnetism and Magnetic Materials 35 (1983) 356-358 North-Holland Publishing Company ANALYSIS OF TYPE-II MAGNETIC CONTRAST FROM FERROMAGNETIC THIN FILMS IN T H E S C A N N I N G E L E C T R O N M I C R O S C O P Y T. I K U T A
Department of Applied Electronics, Osaka Electro-Communication University, Neyagawa, Osaka, Japan and R. S H I M I Z U
Department of Appfled Physics, Osaka University, Suita, Osaka, Japan
Type-II magnetic contrast in the scanning electron microscopy is a new observation technique for the magnetic domain in the ferromagnetic materials. The analysis of type-lI magnetic contrast for magnetic thin films is achieved by using Monte Carlo simulation. The results show that transmitted (forward-scattered) electron image contains the magnetic contrast as well as the conventional backscattered electron image.
1. Introduction
Magnetic domain contrast in the backscattered electron image of the scanning electron microscope (type-II magnetic contrast) was first observed by Philibert and Tixier [1]. At present it is well confirmed that this contrast results from the alteration of electron trajectories in the sample due to the local Lorentz force, This alteration of the electron trajectory results in a small change in the detection yield of the backscattered electron. Although this change is usually less than 1%, clear images of the magnetic domains can be observed under carefull adjustments of the experimental condition [2-4]. According to this, it is expected that this technique is applicable to the observation of the ferromagnetic domains in thin films in a wide range of the film thickness from bulk samples to very thin films which have been usually observed by using Lorentz electron microscopy. In this paper we report an analysis of type-II magnetic contrast from ferromagnetic thin films for both the magnitude and the spatial resolution under the bases of Monte Carlo simulation. In addition to this, results of the simulation for the magnetic contrast in the transmitted (forward-scattered) electron image is also described.
loss are given by using stochastic functions corresponding to the individual process mentioned above. In the magnetic materials, an electron with velocity v is deflected by the Lorentz force F which is given by F=-(e/c)(v×B). The contribution of the Lorentz force can be taken into consideration in the computer program as exchanging the original free straight flight of the electron as the arc. The magnetic flux density B is assumed to be 4*rMs (where M s is the saturation magnetization of the sample) in the present simulation. The width of the magnetic domain wall is neglected since the actual value is usually less than 1000 ..A. 3. Results and discussion
The simulation has been applied to a Permalloy (Ni 78wt%-Fe 22wt%) film of 0.5 #m thickness. Schematic illustration for the angular distribution of the backscattered electrons and the transmitted electrons is BEAM
BEAM
Magnetic Thin Film
~Wall
I
2. Monte Carlo simulation
The quantitative analysis of type-II magnetic contrast has been achieved by Monte Carlo simulation [3,5-7]. In the present simulation, the single scattering approach (screened Coulomb potential) and the continuous slowing down approximation (Bethe's energy loss formula) are used to represent the interactions between the incident electron and the target atoms. In this simulation, an electron trajectory is assumed as a zig-zag path, whose length, scattering angle and energy
(a) Domain Contrast
(b) "'--J-.... Wall Contrast
Fig, I. Schematic illustration for the angular distribution of the backscattered electrons and the transmitted electrons from ferromagnetic thin film (obtained by Monte Carlo simulation): (a) Case of domain contrast, (b) case of domain wall contrast.
0 3 0 4 - 8 8 5 3 / 8 3 / 0 0 0 0 - 0 0 0 0 / $ 0 3 . 0 0 © 1983 N o r t h - H o l l a n d
LIkuta, R. Shimizu / Type-H magnetic contrast in S E M
a) (%)
Eo=' /,6ETECYOR r 30~V ~be"
0.0~
ww
tfO J
3.06"0.12%
YlelO
I~k45"
(~/6 sr.)
O.C-
_
Ygxx
-0.05
Sample: Nb~r-e22s. (Per malloy) 4,=Ms=7,,00(XG,)
z~v
-Ib
'
'
o!o
,.o
Domain Wall Position
¢j,m)
X
b) (%}
1
i=
O0 = .
Sample:NiT~,Fe2z,~,
{I DDDDDD ]1 D D
(Permatloy) 4¢Ms= 7000(G)
{I U
! i
-0.05'
D
•
Q
DD~OI~ e~
Yield11.g7-*0,2,% j ( Tr/6 st. > '
-Ib
'
Eof30KeV B JWall +M$t-X
®i ] ®
~e'4:3~;.
i
oo
•
-Ms0.5,um
ETE CTOR to
(pro)
Wall Position X Fig. 2. Magnetic contrast as function of beam position at beam energy of 30 keY. Sample is a 22% Fe-Permailoy (Fe 22 wt%, Ni 78 wt%) film of 0.5/~m thickness: (a) case of limited solid angle detection for the backscattered electrons, (b) case of limited solid angle detection for the transmitted electrons. Domain
shown in fig. l a and b, respectively. Fig. l a indicates that the type-II contrast of magnetic domains results from the difference of the angular distribution with the direction of the in-plane magnetization. In this case, the image of magnetic domains can be observed by using off-axially positioned detector for both images of the backscattered electrons and the transmitted electrons. On the other hand, different angular distribution is found when the electron beam is just on the domain .wall as shown in fig. lb. In the case of the backscattered electron image, white or black line-shaped images of the domain wall are observed, which are corresponding to the direction of the magnetization at each side of the domain wall. This contrast is called as domain wall contrast [6,8,9]. This type magnetic contrast is also expected for the case of transmitted electron image. The alteration of the magnitude of the contrast and its inversion with the take-off angle of the detector can be predicted from the angular distribution of the transmitted electrons. To obtain the spatial resolution of type-II magnetic contrast for the domain and the domain wall, the position of the incident beam was altered around the domain wall (180 ° wall) in the present simulation. In fig. 2
357
and b, these results are shown for both images of the backscattered electrons and the transmitted electrons with a various take-off angle of the detector. In these figures, an asymmetrical relation between the contrast and the beam position is commonly found. It is concluded that this asymmetry results from the superposition between a domain contrast and a domain wall contrast around the domain wall. As to the backscattered electron image, a strong dark wall contrast and a weak domain contrast are shown. On the other hand, a somewhat strong domain contrast of the same polarity is found in the case of transmitted electron image. The wall contrast is weak in inverted as compared with the backscattered electron image. It is generally concluded from the angular distribution of fig. 1 that both the magnitude and the resolution of type-II magnetic contrast closely depend on the take-off angle of the detector. Fig. 3 shows simulated images of the type-II magnetic contrast around the domain wall. Each photograph corresponds to a different take-off angle of the detector and combinations of the direction of the magnetization at each side of the 180 ° domain wall. In the case of the backscattered electron image, the magnitude and the resolution of the contrast are almost the same for each different take-off angle of the detector. On the other hand, the magnitude of the domain contrast is decreased with the increase of the detector take-off angle for the transmitted electron image. Domain wall contrast is, however, increased with the take-off angle in this case. For the spatial resolution of the transmitted electron image, higher resolution is expected m comparison with the backscattered electron image for all take-off angles of the detector. As to the type-II magnetic contrast of bulk samples, it is well confirmed by both theory and experiment that the contrast is increased as about the 3/2 power of E 0 Perrr~ttoy
( 221~e-711Ni)
3o~v
N~'mal Mlxle ! ~ Fig. 3. Simulated images of type-II magnetic contrast around the domain wall at beam energy of 30 keV (22% Fe-Permalloy film of 0.5 ~m thickness). Each photograph corresponds to a different take-off angle of the detector and the combinations of the direction of the in-plane magnetization at each side of the 180° domain wall.
358
L lkuta, R. Shimizu
/ T y p e - H magnetic contrast in S E M
z~r Sa.mpte- Ni.Jm.Fen'~.
*X
INCIDENT BEAM 30KEY ~, Sample- Nin~.Fe2~
i
/
•
ill
i. z
INCIDENT BEAM ISKeV t Sample- Niv~ Ferz~
0.5pm
1 Fig. 4. Electron trajectories in the 2270 Fe-Permalloy film of 0.5 /~m thickness at different incident beam energies (50 incident electrons): (a) 45 keV beam energy, (b) 30 keV and (c) 15 keV. (beam energy) whereas the spatial resolution decreased as about the 2 power of E 0. In the present case of the thin film sample, this power law is no more applicable for the difference with the diffusion process of the incident electron beam in the sample. Fig. 4a-c shows a typical example of the electron trajectories in the Permalloy film of 0.5 #m film thickness with a different beam energy of 45, 30 and 15 keV, respectively. As found in these figures, the transmitted electron of higher incident beam energy preserves much information of the initial direction in the film as compared with that of lower incident beam energy. It is also found that the spatial distribution of the escape points of the transmitted electron concentrates in the center of the escape point with the increase of incident beam energy. From these trajectories, the spatial resolution of type-II magnetic contrast in the transmitted electron image is considered to be improved by using higher incident beam
energy, whereas no improvement is expected with the resolution of the magnetic contrast in the backscattered electron image. On the other hand, as shown in fig. 4c, low incident beam energy results in an almost complete diffused state within the film, and the transmission yield becomes of a very low value or zero. The resulting low signal level makes the observation of the magnetic contrast in the transmitted electron image difficult, while the backscattered electron image is yet available to obtain the magnetic contrast and its spatial resolution in the backscattered electron image from thin film samples is identical with the case of bulk samples when maximum penetration depth of the incident beam is less than the film thickness. This value of the maximum penetration depth is expected to be about 1/2 of the diameter of the sphere of complete diffusion, which is increased with about a power of 2 of the incident beam energy. Finally, followings are concluded. Type-II magnetic contrast from magnetic thin film can be obtained by using not only the backscattered electron image but also transmitted (forward-scattered) electron image. The magnetic contrast and the spatial resolution of the transmitted electron image strongly depend on the takeoff angle of the detector. The resolution of magnetic contrast can be improved by using higher incident beam energy for the case of transmitted electron image. Since the backscattering yield and the transmission yield are complementary altered with the film thickness and incident beam energy, it is very important to select an appropriate arrangement of the detector to obtain good image quality. The authors are grateful to Professor Dr. E. Sugata of Osaka Electro-Communication University for his encouragement during the present work. References
[l] J. Philibert and R. Tixier, Micron l (1969) 174. [2] D,J. Fathers, J.P. Jakubovics and D.C. Joy, Phil. Mag. 27 (1973) 765. [3] T. Ikuta and R. Shimizu, Phys. Stat. Sol. (a) 23 (1974) 605. [4] T. Yamamoto, N. Nishizawa and K. Tsuno, Phil. Mag. 34 (1976) 311. [5] D.E. Newbury, H. Yakowitz and N.C. Yew, Appl. Phys. Lett. 24 (1975) 259. [6] R. Shimizu, T. Ikuta, M. Kinoshita, T. Murayama, H. Nishizawa and T. Yamamoto, Japan. J. Appl. Phys. 15 (1976) 967. [7] J.P. Jakubovics and D.J. Fathers, Phys. Stat. Sol. (a) 46 (1978) 291. [8] T. Yamamoto and K. Tsuno, Phil. Mag. 34 (1976) 479. [9] D.C. Joy, H.J. Leamy, S.D. Ferris, D.E. Newbury and H. Yakowits, Appl. Phys. Lett. 28 (1976) 466.
Journal of Magnetism and Magnetic Materials 35 (1983) 356-358 North-Holland Publishing Company ANALYSIS OF TYPE-II MAGNETIC CONTRAST FROM FERROMAGNETIC THIN FILMS IN T H E S C A N N I N G E L E C T R O N M I C R O S C O P Y T. I K U T A
Department of Applied Electronics, Osaka Electro-Communication University, Neyagawa, Osaka, Japan and R. S H I M I Z U
Department of Appfled Physics, Osaka University, Suita, Osaka, Japan
Type-II magnetic contrast in the scanning electron microscopy is a new observation technique for the magnetic domain in the ferromagnetic materials. The analysis of type-lI magnetic contrast for magnetic thin films is achieved by using Monte Carlo simulation. The results show that transmitted (forward-scattered) electron image contains the magnetic contrast as well as the conventional backscattered electron image.
1. Introduction
Magnetic domain contrast in the backscattered electron image of the scanning electron microscope (type-II magnetic contrast) was first observed by Philibert and Tixier [1]. At present it is well confirmed that this contrast results from the alteration of electron trajectories in the sample due to the local Lorentz force, This alteration of the electron trajectory results in a small change in the detection yield of the backscattered electron. Although this change is usually less than 1%, clear images of the magnetic domains can be observed under carefull adjustments of the experimental condition [2-4]. According to this, it is expected that this technique is applicable to the observation of the ferromagnetic domains in thin films in a wide range of the film thickness from bulk samples to very thin films which have been usually observed by using Lorentz electron microscopy. In this paper we report an analysis of type-II magnetic contrast from ferromagnetic thin films for both the magnitude and the spatial resolution under the bases of Monte Carlo simulation. In addition to this, results of the simulation for the magnetic contrast in the transmitted (forward-scattered) electron image is also described.
loss are given by using stochastic functions corresponding to the individual process mentioned above. In the magnetic materials, an electron with velocity v is deflected by the Lorentz force F which is given by F=-(e/c)(v×B). The contribution of the Lorentz force can be taken into consideration in the computer program as exchanging the original free straight flight of the electron as the arc. The magnetic flux density B is assumed to be 4*rMs (where M s is the saturation magnetization of the sample) in the present simulation. The width of the magnetic domain wall is neglected since the actual value is usually less than 1000 ..A. 3. Results and discussion
The simulation has been applied to a Permalloy (Ni 78wt%-Fe 22wt%) film of 0.5 #m thickness. Schematic illustration for the angular distribution of the backscattered electrons and the transmitted electrons is BEAM
BEAM
Magnetic Thin Film
~Wall
I
2. Monte Carlo simulation
The quantitative analysis of type-II magnetic contrast has been achieved by Monte Carlo simulation [3,5-7]. In the present simulation, the single scattering approach (screened Coulomb potential) and the continuous slowing down approximation (Bethe's energy loss formula) are used to represent the interactions between the incident electron and the target atoms. In this simulation, an electron trajectory is assumed as a zig-zag path, whose length, scattering angle and energy
(a) Domain Contrast
(b) "'--J-.... Wall Contrast
Fig, I. Schematic illustration for the angular distribution of the backscattered electrons and the transmitted electrons from ferromagnetic thin film (obtained by Monte Carlo simulation): (a) Case of domain contrast, (b) case of domain wall contrast.
0 3 0 4 - 8 8 5 3 / 8 3 / 0 0 0 0 - 0 0 0 0 / $ 0 3 . 0 0 © 1983 N o r t h - H o l l a n d
LIkuta, R. Shimizu / Type-H magnetic contrast in S E M
a) (%)
Eo=' /,6ETECYOR r 30~V ~be"
0.0~
ww
tfO J
3.06"0.12%
YlelO
I~k45"
(~/6 sr.)
O.C-
_
Ygxx
-0.05
Sample: Nb~r-e22s. (Per malloy) 4,=Ms=7,,00(XG,)
z~v
-Ib
'
'
o!o
,.o
Domain Wall Position
¢j,m)
X
b) (%}
1
i=
O0 = .
Sample:NiT~,Fe2z,~,
{I DDDDDD ]1 D D
(Permatloy) 4¢Ms= 7000(G)
{I U
! i
-0.05'
D
•
Q
DD~OI~ e~
Yield11.g7-*0,2,% j ( Tr/6 st. > '
-Ib
'
Eof30KeV B JWall +M$t-X
®i ] ®
~e'4:3~;.
i
oo
•
-Ms0.5,um
ETE CTOR to
(pro)
Wall Position X Fig. 2. Magnetic contrast as function of beam position at beam energy of 30 keY. Sample is a 22% Fe-Permailoy (Fe 22 wt%, Ni 78 wt%) film of 0.5/~m thickness: (a) case of limited solid angle detection for the backscattered electrons, (b) case of limited solid angle detection for the transmitted electrons. Domain
shown in fig. l a and b, respectively. Fig. l a indicates that the type-II contrast of magnetic domains results from the difference of the angular distribution with the direction of the in-plane magnetization. In this case, the image of magnetic domains can be observed by using off-axially positioned detector for both images of the backscattered electrons and the transmitted electrons. On the other hand, different angular distribution is found when the electron beam is just on the domain .wall as shown in fig. lb. In the case of the backscattered electron image, white or black line-shaped images of the domain wall are observed, which are corresponding to the direction of the magnetization at each side of the domain wall. This contrast is called as domain wall contrast [6,8,9]. This type magnetic contrast is also expected for the case of transmitted electron image. The alteration of the magnitude of the contrast and its inversion with the take-off angle of the detector can be predicted from the angular distribution of the transmitted electrons. To obtain the spatial resolution of type-II magnetic contrast for the domain and the domain wall, the position of the incident beam was altered around the domain wall (180 ° wall) in the present simulation. In fig. 2
357
and b, these results are shown for both images of the backscattered electrons and the transmitted electrons with a various take-off angle of the detector. In these figures, an asymmetrical relation between the contrast and the beam position is commonly found. It is concluded that this asymmetry results from the superposition between a domain contrast and a domain wall contrast around the domain wall. As to the backscattered electron image, a strong dark wall contrast and a weak domain contrast are shown. On the other hand, a somewhat strong domain contrast of the same polarity is found in the case of transmitted electron image. The wall contrast is weak in inverted as compared with the backscattered electron image. It is generally concluded from the angular distribution of fig. 1 that both the magnitude and the resolution of type-II magnetic contrast closely depend on the take-off angle of the detector. Fig. 3 shows simulated images of the type-II magnetic contrast around the domain wall. Each photograph corresponds to a different take-off angle of the detector and combinations of the direction of the magnetization at each side of the 180 ° domain wall. In the case of the backscattered electron image, the magnitude and the resolution of the contrast are almost the same for each different take-off angle of the detector. On the other hand, the magnitude of the domain contrast is decreased with the increase of the detector take-off angle for the transmitted electron image. Domain wall contrast is, however, increased with the take-off angle in this case. For the spatial resolution of the transmitted electron image, higher resolution is expected m comparison with the backscattered electron image for all take-off angles of the detector. As to the type-II magnetic contrast of bulk samples, it is well confirmed by both theory and experiment that the contrast is increased as about the 3/2 power of E 0 Perrr~ttoy
( 221~e-711Ni)
3o~v
N~'mal Mlxle ! ~ Fig. 3. Simulated images of type-II magnetic contrast around the domain wall at beam energy of 30 keV (22% Fe-Permalloy film of 0.5 ~m thickness). Each photograph corresponds to a different take-off angle of the detector and the combinations of the direction of the in-plane magnetization at each side of the 180° domain wall.
358
L lkuta, R. Shimizu
/ T y p e - H magnetic contrast in S E M
z~r Sa.mpte- Ni.Jm.Fen'~.
*X
INCIDENT BEAM 30KEY ~, Sample- Nin~.Fe2~
i
/
•
ill
i. z
INCIDENT BEAM ISKeV t Sample- Niv~ Ferz~
0.5pm
1 Fig. 4. Electron trajectories in the 2270 Fe-Permalloy film of 0.5 /~m thickness at different incident beam energies (50 incident electrons): (a) 45 keV beam energy, (b) 30 keV and (c) 15 keV. (beam energy) whereas the spatial resolution decreased as about the 2 power of E 0. In the present case of the thin film sample, this power law is no more applicable for the difference with the diffusion process of the incident electron beam in the sample. Fig. 4a-c shows a typical example of the electron trajectories in the Permalloy film of 0.5 #m film thickness with a different beam energy of 45, 30 and 15 keV, respectively. As found in these figures, the transmitted electron of higher incident beam energy preserves much information of the initial direction in the film as compared with that of lower incident beam energy. It is also found that the spatial distribution of the escape points of the transmitted electron concentrates in the center of the escape point with the increase of incident beam energy. From these trajectories, the spatial resolution of type-II magnetic contrast in the transmitted electron image is considered to be improved by using higher incident beam
energy, whereas no improvement is expected with the resolution of the magnetic contrast in the backscattered electron image. On the other hand, as shown in fig. 4c, low incident beam energy results in an almost complete diffused state within the film, and the transmission yield becomes of a very low value or zero. The resulting low signal level makes the observation of the magnetic contrast in the transmitted electron image difficult, while the backscattered electron image is yet available to obtain the magnetic contrast and its spatial resolution in the backscattered electron image from thin film samples is identical with the case of bulk samples when maximum penetration depth of the incident beam is less than the film thickness. This value of the maximum penetration depth is expected to be about 1/2 of the diameter of the sphere of complete diffusion, which is increased with about a power of 2 of the incident beam energy. Finally, followings are concluded. Type-II magnetic contrast from magnetic thin film can be obtained by using not only the backscattered electron image but also transmitted (forward-scattered) electron image. The magnetic contrast and the spatial resolution of the transmitted electron image strongly depend on the takeoff angle of the detector. The resolution of magnetic contrast can be improved by using higher incident beam energy for the case of transmitted electron image. Since the backscattering yield and the transmission yield are complementary altered with the film thickness and incident beam energy, it is very important to select an appropriate arrangement of the detector to obtain good image quality. The authors are grateful to Professor Dr. E. Sugata of Osaka Electro-Communication University for his encouragement during the present work. References
[l] J. Philibert and R. Tixier, Micron l (1969) 174. [2] D,J. Fathers, J.P. Jakubovics and D.C. Joy, Phil. Mag. 27 (1973) 765. [3] T. Ikuta and R. Shimizu, Phys. Stat. Sol. (a) 23 (1974) 605. [4] T. Yamamoto, N. Nishizawa and K. Tsuno, Phil. Mag. 34 (1976) 311. [5] D.E. Newbury, H. Yakowitz and N.C. Yew, Appl. Phys. Lett. 24 (1975) 259. [6] R. Shimizu, T. Ikuta, M. Kinoshita, T. Murayama, H. Nishizawa and T. Yamamoto, Japan. J. Appl. Phys. 15 (1976) 967. [7] J.P. Jakubovics and D.J. Fathers, Phys. Stat. Sol. (a) 46 (1978) 291. [8] T. Yamamoto and K. Tsuno, Phil. Mag. 34 (1976) 479. [9] D.C. Joy, H.J. Leamy, S.D. Ferris, D.E. Newbury and H. Yakowits, Appl. Phys. Lett. 28 (1976) 466.