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Transmission electron microscopy investigation of stripe domain walls in cobalt foils in in-plane fields
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Storage:
__ g ED
Basic and Applied
JLk!I! ELSEVIER
Journal
of Magnetism
and Magnetic
Materials
129 (1994) 410-414
A
journal of magnetism
IH
ttgnetic materials
A
Transmission electron microscopy investigation of stripe domain walls in cobalt foils in in-plane fields A. Gemperle Institute of Physics, ASCR, (Received
*, R. Gemperle,
P. Novotn?
Na Slovance 2, 180 40 Prague 8, Czech Republic
6 April 1993; in revised form 16 August
1993)
Abstract The process of the magnetization reversal of domain walls in cobalt foils with basal plane orientation has been investigated by Lorentz electron microscopy. External fields up to 500 Oe were applied in the plane of the sample along the direction of the parallel stripe domains. The magnetization reversal of the individual walls was found to be a one step process corresponding probably to the abrupt shift of vertical Bloch lines along the whole observed wall length. In contrast to the magnetization switching in the individual walls the reversal process of every observed domain wall set was found to have a double-jump character being similar to the behaviour of garnet films in the low
field region.
1. Introduction
Our previous papers [1,2] deal with the investigation of the wall magnetization reversal in garnet films using an inductive method. For a domain structure of parallel stripes the wall hysteresis loops were found to have mostly a double-jump character in low in-plane fields and always a single-jump behaviour in large ones. The interpretation was based on the presence and motion of nuclei of the magnetization reversal inside the walls, i.e. vertical Bloch lines (VBLs). The presence of VBLs in garnet films was observed optically by the anisotropic dark field illumination method [3]. Utilizing this method, wall hysteresis loops of the bubble domain lattice structure were investigated and interpreted [41. To obtain better
* Corresponding
2. Experiment The experimental investigations were performed on thin foils made from a cobalt single-
author.
0304.8853/94/$07.00 0 1994 Elsevier SSDI 0304-8853(93)E0523-F
insight into the wall reversal mechanism of the stripe domain structure it is necessary to know the distribution of the magnetization in individual parts of the domain walls during the reversal process. For this purpose the dark field method is not applicable. The Lorentz transmission electron microscopy (TEM) is considered to be one of the most powerful tools in this field. It is very difficult to prepare electron-transparent garnet samples for TEM observations. In addition, it can be expected that the removal of the sample substrate will change the original parameters of the bubble film. This was the principal reason why cobalt, having a similar magnetic behaviour, was chosen for TEM observations.
Science
B.V. All rights reserved
Information
A. Gemperle et al. /Journal
Storage: Basic and Applied
411
of Magnetism and Magnetic Materials 129 (1994) 410-414
crystal with basal plane orientation having the easy axis normal to the sample surface. These were prepared by the usual procedures [5] and in the final stage thinned by jet polishing in an electrolyte of 10% perchloric acid in methanol at -40°C. Foils having slowly varying thickness h were produced by this process. Due to the small ‘quality factor’ Q = Ku/ 27rA42 (Q = 0.5; KU-uniaxial anisotropy constant, MS-saturation magnetization) of this material a complicated surface domain structure in the thicker parts of the foil is formed (see e.g. [61X The simple magnetic domains with the magnetization orientation either “up” or “down” are realized in the parts of the sample thicker than 0,l km and thinner than 5 Frn [7-91. Only the thicknesses from 0.05 to 0.25 km were suited to TEM observations at 100 kV accelerating voltage. The domain structure of parallel stripes was generated by demagnetizing the sample from saturation in a strong field H = 20 kOe which was applied in the plane of the foil. It was verified that the orientation of the stripes was in the direction of the generating field and that the direction of the wall magnetization was also the same. The TEM observations were carried out in a JEOL 6A microscope equipped with a special magnetizing attachment. This very small triplegap electromagnet makes it possible to compensate for the inclination of the electron beam in the added magnetic field. The simplified scheme of the magnetizing circuit is shown in Fig. 1. In the case of the simple “up and down” domain configuration the principal deflection of the electrons comes from the magnetization within the domain wall itself. This effect accounts for the black-white contrast which shows up the direction of the magnetization inside the wall (see fig. 3 in [71XThe sharp wall contrast testifies their localised character. Since direct observations on the microscope screen did not afford enough resolution, details of the wall structure were analyzed on photographs having an original enlargement 8000 + 1000. The external field Ha was changed continuously and the photographs were made at fields He after about 60 Oe increments up to 250 Oe.
electron
beam
_H
t
f Fig. 1. A simplified
scheme
‘sample
of the magnetization
circuit.
The fields Ha ranging from 250 to 500 Oe could be applied for a relatively short time only and thus photographs were taken after raising the field to the desired value and decreasing it to the value of 250 Oe.
3. Results Bloch walls of the parallel stripes pointing in the direction of decreasing foil thickness just after generation are shown in Fig. 2 (Ha = He = 0). The magnetization inside the walls is everywhere in the direction of the original, i.e. generating field. The contrast reversal in the stripe head indicates the position of a VBL. Applying an in-plane field in the opposite (negative) direction to the generating field causes
Fig. 2. A typical TEM contrast photograph of a parallel stripe domain structure in a cobalt foil. Applied field H, = 0, field at exposure H, = 0. The arrows represent the wall magnetization direction. The inset shows enlarged schematic detail of domain head containing VBL.
information Storage: Basic and Applied
412
A. Gemperle et al. /Journal
of Magnetism and Magnetic Materials 129 (1994) 410-414
Fig. 3. Photograph of the domain structure of the same place as in Fig. 2 (H, = H, = -252 Oe). Detail of the domain termination ‘without VBL (see inset).
magnetization switching in individual walls. The development of the reversal process in the same part of the foil is illustrated in Fig. 3, where the Bloch walls in a field H, = EZ, = - 252 Oe are shown. Here one can see that there are areas where the applied in-plane field reversed the magnetization in every second wall. The maximum available in-plane field, H, = -441 Oe, saturated most of the observed walls into its direction (see Fig. 4, H, = - 252 Oe). The VBLs were observed at the stripe domain heads only. Other pinning points, if they exist, are very rare in the observed area of the foil. Apart from the domain walls, dislocations and bend extinction contours are observed in Figs. 2-4. Dislocations (e.g. labelled by white arrows in the figures) can be used for exact location of the observed area of the foil. Bend contours appear due to bending of the foil as a result of the combined effect of build-up of a contamination layer and heating by the electron beam. Their
lltlltltlltlllll!liltl~~t~tl~~l~!lltl. Fig. 4. Photograph of the same place as in Fig. 2 (H, = - 441 Oe, H, = -252 Oe).
density increases with the time of observation (compare Figs. 2 and 4). The difference in the number of walls having their magnetization in one and the opposite direction, relative to the total number of walls, i.e. (n’- n->/(n’+ n-1 = M,/M,, versus applied field H, is shown in Fig. 5. This dependence corresponds to the wall magnetization curve of the observed region. In contrast to the single-jump magnetization switching in the individual walls the reversal process of every observed domain wall set was found to have a double-jump character (see Fig. 5) being similar to the behaviour of the parallel stripes in garnet films in small inplane fields. However the transition from a double-jump wall hysteresis loop to the single-jump one, induced in the garnets by a large in-plane field. was not observed in cobalt.
co
1
oc
t-
t f +
I
MwlMws
Fig. 5. Magnetization reversal curve of the walls of parallel stripes in a cobalt foil. Various symbols (+, 0, 0, A, A) denote various samples. Every point is an average of measurements in several places.
Information Storage: Basic and Applied
A. Gemperle et al. /Journal
of Magnetism and Magnetic Materials 129 (1994) 410-414
4. Discussion
The strong in-plane field generates the domain structure of parallel stripes in the cobalt foil. The direction of the wall magnetization is retained in the same direction even after decreasing this field to zero (see Fig. 2). Stray fields produced by the domains and mainly by the walls themselves act against this initial ordered state. Due to this effect the reversed magnetization in every second wall should correspond to the energy minimum in zero field. However this state was reached experimentally in a low negative field -H,, (i.e. opposite to the generating field) due either to the small effect of the stray fields or due to hysteresis. In this field, nuclei of the reversal process WBLs) start to move and a stable zero magnetization wall net state is formed in the sample. The reversal process of the remaining walls, being caused by the external field, is finished in the field - Hc2. The magnetization reversal of the individual walls is considered to be a one step process corresponding to the abrupt shift of VBLs along the whole observed wall length even though VBLs were observed only at the stripe domain heads (see Figs. 2-4). It is assumed that the first jump of magnetization is due to the movement of the VBLs which are present in the stripe heads. There are a lot of stripe domain terminations in the observed region because of the increasing foil thickness and corresponding change of the stripe domain period. At the second jump of the magnetization further VBLs are nucleated either at the stripe domain heads or at the free ends of the walls at the thin edge of the sample [8]. Comparing garnet films and cobalt foils there is a difference in the obtained experimental results. For garnets a sufficiently strong in-plane field changes the double-jump character of the wall hysteresis loops to the single-jump behaviour 121.In the case of cobalt this transition was not observed. This may be caused by the difference in the energy barriers which have to be overcome during the process of the VBLs nucleation, annihilation and getting free of their pinning points to move inside the garnet and cobalt walls.
413
Double-jump wall magnetization reversal curves in stripe domains were also obtained from ac susceptibility measurements by Maartense [lO,ll] for the garnet films in the case of small fields (H < 277&f,). For applied fields strong enough (H > 27r&fJ, the magnetization reversal curve changed into the single-jump one. The interpretation of the reversal mechanism presented in [lO,ll] was based mainly on the nucleation and annihilation, near the film surface, of horizontal Bloch line (HBL) inside every wall at the critical fields f Zf,. After the first jump in the case of double-jump curve a zero magnetization wall is explained by the HBLs being located at all wall centres. Due to this magnetic wall structure the deflection of the transmitted electrons and the corresponding contrast of every wall should disappear after the first magnetization jump (i.e. for zero total wall magnetization) during the reversal process. This conclusion is in contradiction to our experimental observations obtained in cobalt foils, where contrast of the walls was observed at every applied in-plane field. Moreover, the presence of HBL along the whole wall length may not be anticipated due to energetic reasons.
5. Conclusion
Transmission electron microscopy has been used to investigate the process of magnetization reversal of stripe domain walls in cobalt foils. The wall hysteresis loops of parallel stripes were found to have a double-jump character. Experimental results are similar to those obtained on garnets by an inductive method and are in agreement with our hypothesis of the mechanism of the reversal process being connected with the motion of vertical Bloch lines.
Acknowledgement
The authors would like to thank .I. KaczCr for reading the manuscript and valuable remarks.
Information Storage: Basic and Applied
414
A. Gemperle et al. /Journal
ofMagnetism and Magnetic Materials 129 (1994) 410-414
References [ll P. Novotny, R. Gemperle, L. Murtinova and J. KaczCr, J. Magn. Magn. Mater. 68 (1987) 379. [2] P. Novotny and R. Gemperle, J. Magn. Magn. Mater. 102 (1991) 18. 131 A. Thiaville, J. Ben Youssef, Y. Nakatani and J. Miltat, J. Appl. Phys. 69 (1991) 6090. [4] K. Patek, R. Gemperle, L. Murtinova and J. Kacztr, J. Magn. Magn. Mater. 123 (1993) 223. 151 A. Gemperle, Czech. J. Phys. B 13 (1963) 62.
161J. Kaczer,
R. Gemperle and Z. Hauptman, Czech. J. Phys. 9 (1959) 606. and A. Gemperle, Phys. Status Solidi 26 [71 R. Gemperle (1968) 207. [81 P.J. Grundy and B. Johnson, Brit. J. Appl. Phys. 2 (1969) 1279. J. Phys. D 191P.J. Grundy, R.S. Tebble and D.C. Hothersall, 4 (1971) 174. [lOI I. Maartense and C.W. Searle, J. Appl. Phys. 50 (1979) 1043. [ill I. Maartense, J. Magn. Magn. Mater. 54-57 (1986) 1.571.
Storage:
__ g ED
Basic and Applied
JLk!I! ELSEVIER
Journal
of Magnetism
and Magnetic
Materials
129 (1994) 410-414
A
journal of magnetism
IH
ttgnetic materials
A
Transmission electron microscopy investigation of stripe domain walls in cobalt foils in in-plane fields A. Gemperle Institute of Physics, ASCR, (Received
*, R. Gemperle,
P. Novotn?
Na Slovance 2, 180 40 Prague 8, Czech Republic
6 April 1993; in revised form 16 August
1993)
Abstract The process of the magnetization reversal of domain walls in cobalt foils with basal plane orientation has been investigated by Lorentz electron microscopy. External fields up to 500 Oe were applied in the plane of the sample along the direction of the parallel stripe domains. The magnetization reversal of the individual walls was found to be a one step process corresponding probably to the abrupt shift of vertical Bloch lines along the whole observed wall length. In contrast to the magnetization switching in the individual walls the reversal process of every observed domain wall set was found to have a double-jump character being similar to the behaviour of garnet films in the low
field region.
1. Introduction
Our previous papers [1,2] deal with the investigation of the wall magnetization reversal in garnet films using an inductive method. For a domain structure of parallel stripes the wall hysteresis loops were found to have mostly a double-jump character in low in-plane fields and always a single-jump behaviour in large ones. The interpretation was based on the presence and motion of nuclei of the magnetization reversal inside the walls, i.e. vertical Bloch lines (VBLs). The presence of VBLs in garnet films was observed optically by the anisotropic dark field illumination method [3]. Utilizing this method, wall hysteresis loops of the bubble domain lattice structure were investigated and interpreted [41. To obtain better
* Corresponding
2. Experiment The experimental investigations were performed on thin foils made from a cobalt single-
author.
0304.8853/94/$07.00 0 1994 Elsevier SSDI 0304-8853(93)E0523-F
insight into the wall reversal mechanism of the stripe domain structure it is necessary to know the distribution of the magnetization in individual parts of the domain walls during the reversal process. For this purpose the dark field method is not applicable. The Lorentz transmission electron microscopy (TEM) is considered to be one of the most powerful tools in this field. It is very difficult to prepare electron-transparent garnet samples for TEM observations. In addition, it can be expected that the removal of the sample substrate will change the original parameters of the bubble film. This was the principal reason why cobalt, having a similar magnetic behaviour, was chosen for TEM observations.
Science
B.V. All rights reserved
Information
A. Gemperle et al. /Journal
Storage: Basic and Applied
411
of Magnetism and Magnetic Materials 129 (1994) 410-414
crystal with basal plane orientation having the easy axis normal to the sample surface. These were prepared by the usual procedures [5] and in the final stage thinned by jet polishing in an electrolyte of 10% perchloric acid in methanol at -40°C. Foils having slowly varying thickness h were produced by this process. Due to the small ‘quality factor’ Q = Ku/ 27rA42 (Q = 0.5; KU-uniaxial anisotropy constant, MS-saturation magnetization) of this material a complicated surface domain structure in the thicker parts of the foil is formed (see e.g. [61X The simple magnetic domains with the magnetization orientation either “up” or “down” are realized in the parts of the sample thicker than 0,l km and thinner than 5 Frn [7-91. Only the thicknesses from 0.05 to 0.25 km were suited to TEM observations at 100 kV accelerating voltage. The domain structure of parallel stripes was generated by demagnetizing the sample from saturation in a strong field H = 20 kOe which was applied in the plane of the foil. It was verified that the orientation of the stripes was in the direction of the generating field and that the direction of the wall magnetization was also the same. The TEM observations were carried out in a JEOL 6A microscope equipped with a special magnetizing attachment. This very small triplegap electromagnet makes it possible to compensate for the inclination of the electron beam in the added magnetic field. The simplified scheme of the magnetizing circuit is shown in Fig. 1. In the case of the simple “up and down” domain configuration the principal deflection of the electrons comes from the magnetization within the domain wall itself. This effect accounts for the black-white contrast which shows up the direction of the magnetization inside the wall (see fig. 3 in [71XThe sharp wall contrast testifies their localised character. Since direct observations on the microscope screen did not afford enough resolution, details of the wall structure were analyzed on photographs having an original enlargement 8000 + 1000. The external field Ha was changed continuously and the photographs were made at fields He after about 60 Oe increments up to 250 Oe.
electron
beam
_H
t
f Fig. 1. A simplified
scheme
‘sample
of the magnetization
circuit.
The fields Ha ranging from 250 to 500 Oe could be applied for a relatively short time only and thus photographs were taken after raising the field to the desired value and decreasing it to the value of 250 Oe.
3. Results Bloch walls of the parallel stripes pointing in the direction of decreasing foil thickness just after generation are shown in Fig. 2 (Ha = He = 0). The magnetization inside the walls is everywhere in the direction of the original, i.e. generating field. The contrast reversal in the stripe head indicates the position of a VBL. Applying an in-plane field in the opposite (negative) direction to the generating field causes
Fig. 2. A typical TEM contrast photograph of a parallel stripe domain structure in a cobalt foil. Applied field H, = 0, field at exposure H, = 0. The arrows represent the wall magnetization direction. The inset shows enlarged schematic detail of domain head containing VBL.
information Storage: Basic and Applied
412
A. Gemperle et al. /Journal
of Magnetism and Magnetic Materials 129 (1994) 410-414
Fig. 3. Photograph of the domain structure of the same place as in Fig. 2 (H, = H, = -252 Oe). Detail of the domain termination ‘without VBL (see inset).
magnetization switching in individual walls. The development of the reversal process in the same part of the foil is illustrated in Fig. 3, where the Bloch walls in a field H, = EZ, = - 252 Oe are shown. Here one can see that there are areas where the applied in-plane field reversed the magnetization in every second wall. The maximum available in-plane field, H, = -441 Oe, saturated most of the observed walls into its direction (see Fig. 4, H, = - 252 Oe). The VBLs were observed at the stripe domain heads only. Other pinning points, if they exist, are very rare in the observed area of the foil. Apart from the domain walls, dislocations and bend extinction contours are observed in Figs. 2-4. Dislocations (e.g. labelled by white arrows in the figures) can be used for exact location of the observed area of the foil. Bend contours appear due to bending of the foil as a result of the combined effect of build-up of a contamination layer and heating by the electron beam. Their
lltlltltlltlllll!liltl~~t~tl~~l~!lltl. Fig. 4. Photograph of the same place as in Fig. 2 (H, = - 441 Oe, H, = -252 Oe).
density increases with the time of observation (compare Figs. 2 and 4). The difference in the number of walls having their magnetization in one and the opposite direction, relative to the total number of walls, i.e. (n’- n->/(n’+ n-1 = M,/M,, versus applied field H, is shown in Fig. 5. This dependence corresponds to the wall magnetization curve of the observed region. In contrast to the single-jump magnetization switching in the individual walls the reversal process of every observed domain wall set was found to have a double-jump character (see Fig. 5) being similar to the behaviour of the parallel stripes in garnet films in small inplane fields. However the transition from a double-jump wall hysteresis loop to the single-jump one, induced in the garnets by a large in-plane field. was not observed in cobalt.
co
1
oc
t-
t f +
I
MwlMws
Fig. 5. Magnetization reversal curve of the walls of parallel stripes in a cobalt foil. Various symbols (+, 0, 0, A, A) denote various samples. Every point is an average of measurements in several places.
Information Storage: Basic and Applied
A. Gemperle et al. /Journal
of Magnetism and Magnetic Materials 129 (1994) 410-414
4. Discussion
The strong in-plane field generates the domain structure of parallel stripes in the cobalt foil. The direction of the wall magnetization is retained in the same direction even after decreasing this field to zero (see Fig. 2). Stray fields produced by the domains and mainly by the walls themselves act against this initial ordered state. Due to this effect the reversed magnetization in every second wall should correspond to the energy minimum in zero field. However this state was reached experimentally in a low negative field -H,, (i.e. opposite to the generating field) due either to the small effect of the stray fields or due to hysteresis. In this field, nuclei of the reversal process WBLs) start to move and a stable zero magnetization wall net state is formed in the sample. The reversal process of the remaining walls, being caused by the external field, is finished in the field - Hc2. The magnetization reversal of the individual walls is considered to be a one step process corresponding to the abrupt shift of VBLs along the whole observed wall length even though VBLs were observed only at the stripe domain heads (see Figs. 2-4). It is assumed that the first jump of magnetization is due to the movement of the VBLs which are present in the stripe heads. There are a lot of stripe domain terminations in the observed region because of the increasing foil thickness and corresponding change of the stripe domain period. At the second jump of the magnetization further VBLs are nucleated either at the stripe domain heads or at the free ends of the walls at the thin edge of the sample [8]. Comparing garnet films and cobalt foils there is a difference in the obtained experimental results. For garnets a sufficiently strong in-plane field changes the double-jump character of the wall hysteresis loops to the single-jump behaviour 121.In the case of cobalt this transition was not observed. This may be caused by the difference in the energy barriers which have to be overcome during the process of the VBLs nucleation, annihilation and getting free of their pinning points to move inside the garnet and cobalt walls.
413
Double-jump wall magnetization reversal curves in stripe domains were also obtained from ac susceptibility measurements by Maartense [lO,ll] for the garnet films in the case of small fields (H < 277&f,). For applied fields strong enough (H > 27r&fJ, the magnetization reversal curve changed into the single-jump one. The interpretation of the reversal mechanism presented in [lO,ll] was based mainly on the nucleation and annihilation, near the film surface, of horizontal Bloch line (HBL) inside every wall at the critical fields f Zf,. After the first jump in the case of double-jump curve a zero magnetization wall is explained by the HBLs being located at all wall centres. Due to this magnetic wall structure the deflection of the transmitted electrons and the corresponding contrast of every wall should disappear after the first magnetization jump (i.e. for zero total wall magnetization) during the reversal process. This conclusion is in contradiction to our experimental observations obtained in cobalt foils, where contrast of the walls was observed at every applied in-plane field. Moreover, the presence of HBL along the whole wall length may not be anticipated due to energetic reasons.
5. Conclusion
Transmission electron microscopy has been used to investigate the process of magnetization reversal of stripe domain walls in cobalt foils. The wall hysteresis loops of parallel stripes were found to have a double-jump character. Experimental results are similar to those obtained on garnets by an inductive method and are in agreement with our hypothesis of the mechanism of the reversal process being connected with the motion of vertical Bloch lines.
Acknowledgement
The authors would like to thank .I. KaczCr for reading the manuscript and valuable remarks.
Information Storage: Basic and Applied
414
A. Gemperle et al. /Journal
ofMagnetism and Magnetic Materials 129 (1994) 410-414
References [ll P. Novotny, R. Gemperle, L. Murtinova and J. KaczCr, J. Magn. Magn. Mater. 68 (1987) 379. [2] P. Novotny and R. Gemperle, J. Magn. Magn. Mater. 102 (1991) 18. 131 A. Thiaville, J. Ben Youssef, Y. Nakatani and J. Miltat, J. Appl. Phys. 69 (1991) 6090. [4] K. Patek, R. Gemperle, L. Murtinova and J. Kacztr, J. Magn. Magn. Mater. 123 (1993) 223. 151 A. Gemperle, Czech. J. Phys. B 13 (1963) 62.
161J. Kaczer,
R. Gemperle and Z. Hauptman, Czech. J. Phys. 9 (1959) 606. and A. Gemperle, Phys. Status Solidi 26 [71 R. Gemperle (1968) 207. [81 P.J. Grundy and B. Johnson, Brit. J. Appl. Phys. 2 (1969) 1279. J. Phys. D 191P.J. Grundy, R.S. Tebble and D.C. Hothersall, 4 (1971) 174. [lOI I. Maartense and C.W. Searle, J. Appl. Phys. 50 (1979) 1043. [ill I. Maartense, J. Magn. Magn. Mater. 54-57 (1986) 1.571.