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Magnetic force microscopy and its application to longitudinal thin films
Journal of Magnetism and Magnetic Materials 93 (1991) 123-127 North-Holland
123
Magnetic force microscopy and its application to longitudinal thin films C. Schoenenberger, S.F. A l v a r a d o IBM Research Division, Zurich Research Laboratory, 8803 Riischlikon, Switzerland
S.E. L a m b e r t a n d I.L. S a n d e r s IBM Magnetic Recording Institute, Almaden Research Center, 650 Harry Road, San Jose, CA 95120-6099, USA
The interpretation of magnetic force microscopy (MFM) images requires knowledge of the magnetic structure within the tip. The etched polycrystalline iron, cobalt and nickel wires investigated all have their apex domain oriented along the tip axis. The image formation is found to be determined by an effective domain length L. For magnetization patterns of scales large compared to L, the point dipole approximation is applicable, provided that the tip apex can be treated as an isolated domain. While this is observed to be the case for cobalt tips, it does not apply to the most common tip materials, i.e. nickel and iron. The resolving power of a tip is given by its tip radius. At sufficiently small tip-sample distances, image forces induced by the strong tip field have been detected. MFM is used to investigate two Co-based longitudinal thin films. The roughness of the transition between head on oriented domains is compared.
1. Introduction The continuously growing needs of storage density require imaging techniques with increasing resolution powers. A promising possibility is magnetic force microscopy [1, 2]. This technique utilizes a ferromagnetic probing tip lowered into the magnetic stray field of the sample under investigation. The resulting dipolar interaction force F m acting on the tip is measured, while the sample is scanned in a raster fashion [3, 4]. We have demonstrated that the surface topography can be separated from and simultaneously measured with the magnetic stray field [4]. M F M has been applied to perpendicular as well as longitudinal storage films [5, 6]. Images typically show features on a 100 nm scale. Previous work has revealed that F m cannot be interpreted if the tip is assumed to act as a point dipole. Therefore, the tip is modeled as a single conical domain of length L, with a flat cap at its front end that represents the finite-apex radius of
real tips [7]. The question arises of whether such a model is appropriate to describe the force contrast. We have systematically studied F m on intentionally written periodic domain structures in longitudinal Co-based recording media using polycrystalline Fe, Co and Ni wires, electrochemically etched into sharp tips. The parameters of the two films are given in table 1. Here, H c is the coercive field, M r the remanence, t the film thickness, I w the transition length according to the Williams-Comstock model [8], and I 0 is the intrinsic transition length given by I 0 = M r t / 2 H c. The transition length I w accounts for extrinsic parameters like the flying height and the gap width of the writing head.
2. Different image contrast Our investigation shows that the effective magnetic moment in the tip is aligned with the tip axis. The reason is the large shape anisotropy due
0304-8853/91/$03.50 © 1 9 9 1 - Elsevier Science Publishers B.V. (North-Holland)
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C Schoenenberger et aL / Magnetic .[brce microscopy to thin fihns
Table 1 Magnetic data on the two samples studied
tt~. Sample
(kA/m)
,~4rl (10- ~A)
[ (nm)
1 (nm)
It, (nm)
Cos~Pt 2~b Co71Pt i2Crlr
96 147
20 7.8
25 25
440 190
100 27
to the small t i p - c o n e angles of y ~ 15-30 °. This result holds for any of the t h r e e tip materials. T h e fact that images can look r a t h e r different is shown in fig. 1. Both i m a g e s w e r e t a k e n on the same s a m p l e ( C o P t C r ) over a track with bits of width w = 2.5 txm. T h e t r a c k direction (x-axis) is horizontal in the two images. Fig. l a has b e e n m e a s u r e d with a Co tip a n d (b) with a Ni tip, but the l a t t e r is typical for F e tips as well. T h e force on a point d i p o l e with m o m e n t m in the stray field H is given by F m = / x 0 ( m • V ) H . This leads to F m ~ a l l , l a x for o u r g e o m e t r i c a l setup [9]. Since d << w, the i n - p l a n e field H , is roughly c o n s t a n t over the bits but c h a n g e s rapidly in the transition region, w h e r e the largest force c o n t r a s t is e x p e c t e d to occur. Such a b e h a v i o r is f o u n d in case of Co tips: fig. la. T h e a l t e r n a t i n g d a r k and white stripes define the t r a n s i t i o n b e t w e e n o p p o sitely o r i e n t e d d o m a i n s . T h e m a g n e t i c force m e a s u r e d using Co tips follows the p o i n t - d i p o l e a p p r o x i m a t i o n , at least for structures on a m i c r o n scale. T h e r e l e v a n t p a r a m e t e r s for fig. l a are d=80nm, L - - = 2 0 0 n m and R < 100nm. A n y m a g n e t i c s t r u c t u r e on the s a m p l e a p p e a r s to be c o n v o l u t e d with a G a u s s i a n - l i k e function of width A. F o r a short tip d o m a i n and small cone angles we o b t a i n A < 2 V ~ + d 2 =270nm. The apparent width of the stripes in fig. l a is a b o u t 300rim and, hence, the u n d e r l y i n g intrinsic transition width is c o n s i d e r a b l y c o n v o l u t e d by the tip response. T h e u l t i m a t e resolving p o w e r of the tip is limited by R. T h e situation is strikingly different for the most f r e q u e n t l y used Ni and Fe tips, as shown in fig. lb. T h e c o n t r a s t is no l o n g e r l o c a t e d at the domain b o u n d a r i e s . This b e h a v i o u r has its origin in a tip d o m a i n which is effectively much longer. It
Fig. 1. Dependence of the image contrast on the tipmalcrial. (a) was measured with a cobalt tip and (b) with el nickel tip on the same sample (CoPtCr). The image size is 10 x 7gin e in both cases. turns out that the single d o m a i n picture is not a p p r o p r i a t e . Besides the t i p - e n d final d o m a i n of length L = 1 - 2 Ixm, a net p o l a r i z a t i o n in the rest of the tip wire also has to be c o n s i d e r e d . This p o l a r i z a t i o n is p r o b a b l y c a u s e d by the t e n d e n c y of soft-tip m a t e r i a l s to conserve m a g n e t i c flux. This essentially leads to a single-pole tip as shown on t o p of fig. lb. T h e force F m can be a p p r o x i m a t e l y d e s c r i b e d by
Fm( x , y ) = l x o M t R 2 " r r ( s ( R ) × H
+2
tan( "),/2)L R
s(L)×H
,
(1)
w h e r e M~ is the tip m a g n e t i z a t i o n , × the convolution o p e r a t o r , H = H ( x , y, d ) the s a m p l e stray field at the d i s t a n c e d above the surface, a n d s(6) is a n o r m a l i z e d G a u s s i a n with d e c a y length ¢3. T h e first t e r m in eq. (1) arises from m a g n e t i c p o l e s at the tip apex and the s e c o n d from the c h a r g e s on the cone face. T h e l a t t e r d o m i n a t e s by 1 to 2 o r d e r s of m a g n i t u d e for a static field, and hence, F m is p r o p o r t i o n a l to a strongly low-pass filtered version of H. H o w e v e r , the resolving p o w e r of the tip is still limited by R, the first t e r m
C. Schoenenberger et al. / Magnetic .f?~rce microscopy to thin films
in eq. (1), and not by L >> R. Details of this work are described in a forthcoming article [9].
u
.
.
.
.
125
i
.
.
.
.
i
2.0
z
°
x
o aO
1.0
3. Tip stray field The tip stray field experienced on the sample approaches M t. Induced modifications in the domain pattern of the sample have to be considered at small d, but the influence of the sample stray field on the tip can be neglected in the case of thin films ( H cct). The shape anisotropy of the film controls the out-of-plane rotation of the sample magnetization M. Modeling the tip as a point charge, the rotation angle 0 is small, provided that Rt/d <<2 Mf~M~-t. Moreover, the largest in-plane field of the tip has to be smaller than the a p p r o p r i a t e film coercivity He, leading to Rt/d << 2gr~jMt. In the case of sufficiently hard magnetic samples the conditions can be fulfilled at reasonable t i p - s a m p l e distances. However, in case of a soft magnetic material, a very large d ~ 1 p,m is necessary. MFM using ferromagnetic
0 LI_
0
I
0
J
~
~
0.5 1.0 T i p - S a m p l e Distance (~Jm)
Fig. 2. M a g n e t i c force versus t i p - s a m p l e distance d derived from m e a s u r e m e n t s over a l t e r n a t i n g bits of width 1 p~m written into the CoPt film. Curve (b) shows the induced image force due to the tip stray field.
tips is most powerful when investigating hard magnetic thin films. Fig. 2 shows the magnetic force versus d measured with a Ni tip over alternating bits (w = 1 p,m, CoPt film). The two curves were extracted from a series of force measurements taken at different positions. Curve (a) shows the peak at-
2Jm I
I
Fig. 3. C o m p a r i s o n of the two different film m a t e r i a l s : (a) is t a k e n on a CoPt film and (b) on a C o P t C r film. I m a g e size is 6 :x: 7 l~m 2. The inset shows the a p p a r e n t bit s t r u c t u r e for a writing density of 5000 flux reversals per m i l l i m e t e r .
126
('. Schoenenberger et al. / Magnetic force microscopy to thin films
tractive force for an undisturbed sample magnetization. The expected exponential decay with a decay length of w / v is drawn as a dotted line. Curve (b) shows the separated image force F~ caused by the induced rotation of the sample magnetization due to the presence of the tip. F~ and 0 are small in this particular example. However, strong image forces have been found on permalloy [9, 10].
4. Application The two different films are compared in fig. 3: (a) shows l~zm wide bits on a CoPt film and (b) 670nm wide bits on CoPtCr. Both images were corrected to display the in-plane field H x. It is apparent that the peak-to-peak roughness #t of the transition line between adjacent bits is much larger than the intrinsic transition length I 0 as derived from a arctangent model (table 1). The material p a r a m e t e r that governs the intrinsic transition width is the wavelength Xr associated with longitudinal ripples [11]. The detailed ripple structure could not be resolved with MFM, but a Lorentz microscopy investigation on similar CoPt films reveals A r = 2 0 0 - 3 0 0 n m [12]. However, some individual clusters are visible in fig. 3a. Their size s c, measured in the track direction, is about 200-300nm. In addition, fit = 320nm is derived. All three parameters 6t, Ar and s c are comparable in size. The smallest possible bit should contain at least one cluster. This yields a minimum bit size Wmin = 6 t + s c = 6 0 0 n m , together with the transition roughness. The written transitions in fig. 3a are already close to the maximum writing density. For the CoPtCr film a transition roughness of 6 t ~ 100nm is found, hence wmi. = 220rim. The largest writing density is expected to be 4 5 0 0 f r / m m . M F M reveals that the periodic bit structure collapses at different sample locations, for a writing density in between 4000 and 5 0 0 0 f r / m m . The inset in fig. 3b shows a measurement at 5000 f r / m m . However, when reading
the structure with a conventional recording head, a periodic signal was still detectable up to a density of 8 0 0 0 f r / m m . This indicates that the apparent roughness 6t is limited by the flying height a n d / o r the gap width of the writing head.
5. Conclusions MFM allows the magnetic stray field of ferromagnetic samples to be measured with submicron resolution. The etched Fe, Co and Ni wires investigated all have their apex domain oriented along the tip axis. The stray field experienced on the sample surface from the tip can be rather large, which limits the applicability of MFM to the study of hard magnetic samples. This limitation might be overcome by using thin film tips. The magnetic force measured using Co tips follows the field derivative as expected from a pointdipole model, but the situation is different for the more commonly used tip materials, Fe and Ni. Here, the magnetic force is more closely related to the stray field, even for magnetic structures on a micron scale. The tip behaves as an extended single-pole probe. The resolving power obtained using any of the three tip materials is limited by the tip radius. The apparent transition roughness ¢~t iS found to be comparable to the longitudinal ripple size as measured by Lorentz microscopy, and to the cluster width as seen with MFM for the CoPt film, indicating that the boundary roughness is governed by intrinsic material parameters. In the case of the CoPtCr film, however, the roughness appears to be limited by extrinsic parameters like the flying height and the gap width of the writing head.
References [1] Y. 50 [2] G. 56
Martin and H.K. Wickramasinghe, Appl. Phys. Lett. (1987) 1455. Binnig, C.F. Quate and Ch. Gerber, Phys. Rev. Lett. (1986) 930.
C. Schoenenberger et al. / Magnetic force microscopy to thin films [3] C. Sch6nenberger and S.F. Alvarado, Rev. Sci. Instrum. 60 (1989) 3131. [4] C. Sch6nenberger, S.F. Alvarado, S.E. Lambert and I.L. Sanders, J. Appl. Phys. (1990) in press. [5] Y. Martin, D. Rugar and H.K. Wickramasinghe, App[. Phys. Lett. 52 (1988) 244. [6] H.J. Mamin, D. Rugar, J.E. Stern, B.D. Terris and S.E. Lambert, Appl. Phys. Lett. 53 (1988) 1563. [7] A. Wadas and P. Griitter, Phys. Rev. B 39 (1989) 12013.
127
[8] M.L. Williams and R.L. Comstock, AIP Conf. Proc. 5 (1972) 738. [9] C. Sch6nenberger and S.F. Alvarado. Z. Phys. B (in press). [10] H.J. Mamin, D. Rugar, J.E. Stern, R.E. Fontana, Jr. and P. Kasiraj, Appl. Phys. Lett. 55 (1989) 318. [11] T. Chen, IEEE Trans. Magn. MAG-17 (1981) 1181. [12] T.A. Nguyen, P.S. Alexopoulos, C. Hwang, S.E. Lambert and 1.L Sanders, IEEE Trans. Magn. MAG-24 (1988) 2733.
123
Magnetic force microscopy and its application to longitudinal thin films C. Schoenenberger, S.F. A l v a r a d o IBM Research Division, Zurich Research Laboratory, 8803 Riischlikon, Switzerland
S.E. L a m b e r t a n d I.L. S a n d e r s IBM Magnetic Recording Institute, Almaden Research Center, 650 Harry Road, San Jose, CA 95120-6099, USA
The interpretation of magnetic force microscopy (MFM) images requires knowledge of the magnetic structure within the tip. The etched polycrystalline iron, cobalt and nickel wires investigated all have their apex domain oriented along the tip axis. The image formation is found to be determined by an effective domain length L. For magnetization patterns of scales large compared to L, the point dipole approximation is applicable, provided that the tip apex can be treated as an isolated domain. While this is observed to be the case for cobalt tips, it does not apply to the most common tip materials, i.e. nickel and iron. The resolving power of a tip is given by its tip radius. At sufficiently small tip-sample distances, image forces induced by the strong tip field have been detected. MFM is used to investigate two Co-based longitudinal thin films. The roughness of the transition between head on oriented domains is compared.
1. Introduction The continuously growing needs of storage density require imaging techniques with increasing resolution powers. A promising possibility is magnetic force microscopy [1, 2]. This technique utilizes a ferromagnetic probing tip lowered into the magnetic stray field of the sample under investigation. The resulting dipolar interaction force F m acting on the tip is measured, while the sample is scanned in a raster fashion [3, 4]. We have demonstrated that the surface topography can be separated from and simultaneously measured with the magnetic stray field [4]. M F M has been applied to perpendicular as well as longitudinal storage films [5, 6]. Images typically show features on a 100 nm scale. Previous work has revealed that F m cannot be interpreted if the tip is assumed to act as a point dipole. Therefore, the tip is modeled as a single conical domain of length L, with a flat cap at its front end that represents the finite-apex radius of
real tips [7]. The question arises of whether such a model is appropriate to describe the force contrast. We have systematically studied F m on intentionally written periodic domain structures in longitudinal Co-based recording media using polycrystalline Fe, Co and Ni wires, electrochemically etched into sharp tips. The parameters of the two films are given in table 1. Here, H c is the coercive field, M r the remanence, t the film thickness, I w the transition length according to the Williams-Comstock model [8], and I 0 is the intrinsic transition length given by I 0 = M r t / 2 H c. The transition length I w accounts for extrinsic parameters like the flying height and the gap width of the writing head.
2. Different image contrast Our investigation shows that the effective magnetic moment in the tip is aligned with the tip axis. The reason is the large shape anisotropy due
0304-8853/91/$03.50 © 1 9 9 1 - Elsevier Science Publishers B.V. (North-Holland)
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C Schoenenberger et aL / Magnetic .[brce microscopy to thin fihns
Table 1 Magnetic data on the two samples studied
tt~. Sample
(kA/m)
,~4rl (10- ~A)
[ (nm)
1 (nm)
It, (nm)
Cos~Pt 2~b Co71Pt i2Crlr
96 147
20 7.8
25 25
440 190
100 27
to the small t i p - c o n e angles of y ~ 15-30 °. This result holds for any of the t h r e e tip materials. T h e fact that images can look r a t h e r different is shown in fig. 1. Both i m a g e s w e r e t a k e n on the same s a m p l e ( C o P t C r ) over a track with bits of width w = 2.5 txm. T h e t r a c k direction (x-axis) is horizontal in the two images. Fig. l a has b e e n m e a s u r e d with a Co tip a n d (b) with a Ni tip, but the l a t t e r is typical for F e tips as well. T h e force on a point d i p o l e with m o m e n t m in the stray field H is given by F m = / x 0 ( m • V ) H . This leads to F m ~ a l l , l a x for o u r g e o m e t r i c a l setup [9]. Since d << w, the i n - p l a n e field H , is roughly c o n s t a n t over the bits but c h a n g e s rapidly in the transition region, w h e r e the largest force c o n t r a s t is e x p e c t e d to occur. Such a b e h a v i o r is f o u n d in case of Co tips: fig. la. T h e a l t e r n a t i n g d a r k and white stripes define the t r a n s i t i o n b e t w e e n o p p o sitely o r i e n t e d d o m a i n s . T h e m a g n e t i c force m e a s u r e d using Co tips follows the p o i n t - d i p o l e a p p r o x i m a t i o n , at least for structures on a m i c r o n scale. T h e r e l e v a n t p a r a m e t e r s for fig. l a are d=80nm, L - - = 2 0 0 n m and R < 100nm. A n y m a g n e t i c s t r u c t u r e on the s a m p l e a p p e a r s to be c o n v o l u t e d with a G a u s s i a n - l i k e function of width A. F o r a short tip d o m a i n and small cone angles we o b t a i n A < 2 V ~ + d 2 =270nm. The apparent width of the stripes in fig. l a is a b o u t 300rim and, hence, the u n d e r l y i n g intrinsic transition width is c o n s i d e r a b l y c o n v o l u t e d by the tip response. T h e u l t i m a t e resolving p o w e r of the tip is limited by R. T h e situation is strikingly different for the most f r e q u e n t l y used Ni and Fe tips, as shown in fig. lb. T h e c o n t r a s t is no l o n g e r l o c a t e d at the domain b o u n d a r i e s . This b e h a v i o u r has its origin in a tip d o m a i n which is effectively much longer. It
Fig. 1. Dependence of the image contrast on the tipmalcrial. (a) was measured with a cobalt tip and (b) with el nickel tip on the same sample (CoPtCr). The image size is 10 x 7gin e in both cases. turns out that the single d o m a i n picture is not a p p r o p r i a t e . Besides the t i p - e n d final d o m a i n of length L = 1 - 2 Ixm, a net p o l a r i z a t i o n in the rest of the tip wire also has to be c o n s i d e r e d . This p o l a r i z a t i o n is p r o b a b l y c a u s e d by the t e n d e n c y of soft-tip m a t e r i a l s to conserve m a g n e t i c flux. This essentially leads to a single-pole tip as shown on t o p of fig. lb. T h e force F m can be a p p r o x i m a t e l y d e s c r i b e d by
Fm( x , y ) = l x o M t R 2 " r r ( s ( R ) × H
+2
tan( "),/2)L R
s(L)×H
,
(1)
w h e r e M~ is the tip m a g n e t i z a t i o n , × the convolution o p e r a t o r , H = H ( x , y, d ) the s a m p l e stray field at the d i s t a n c e d above the surface, a n d s(6) is a n o r m a l i z e d G a u s s i a n with d e c a y length ¢3. T h e first t e r m in eq. (1) arises from m a g n e t i c p o l e s at the tip apex and the s e c o n d from the c h a r g e s on the cone face. T h e l a t t e r d o m i n a t e s by 1 to 2 o r d e r s of m a g n i t u d e for a static field, and hence, F m is p r o p o r t i o n a l to a strongly low-pass filtered version of H. H o w e v e r , the resolving p o w e r of the tip is still limited by R, the first t e r m
C. Schoenenberger et al. / Magnetic .f?~rce microscopy to thin films
in eq. (1), and not by L >> R. Details of this work are described in a forthcoming article [9].
u
.
.
.
.
125
i
.
.
.
.
i
2.0
z
°
x
o aO
1.0
3. Tip stray field The tip stray field experienced on the sample approaches M t. Induced modifications in the domain pattern of the sample have to be considered at small d, but the influence of the sample stray field on the tip can be neglected in the case of thin films ( H cct). The shape anisotropy of the film controls the out-of-plane rotation of the sample magnetization M. Modeling the tip as a point charge, the rotation angle 0 is small, provided that Rt/d <<2 Mf~M~-t. Moreover, the largest in-plane field of the tip has to be smaller than the a p p r o p r i a t e film coercivity He, leading to Rt/d << 2gr~jMt. In the case of sufficiently hard magnetic samples the conditions can be fulfilled at reasonable t i p - s a m p l e distances. However, in case of a soft magnetic material, a very large d ~ 1 p,m is necessary. MFM using ferromagnetic
0 LI_
0
I
0
J
~
~
0.5 1.0 T i p - S a m p l e Distance (~Jm)
Fig. 2. M a g n e t i c force versus t i p - s a m p l e distance d derived from m e a s u r e m e n t s over a l t e r n a t i n g bits of width 1 p~m written into the CoPt film. Curve (b) shows the induced image force due to the tip stray field.
tips is most powerful when investigating hard magnetic thin films. Fig. 2 shows the magnetic force versus d measured with a Ni tip over alternating bits (w = 1 p,m, CoPt film). The two curves were extracted from a series of force measurements taken at different positions. Curve (a) shows the peak at-
2Jm I
I
Fig. 3. C o m p a r i s o n of the two different film m a t e r i a l s : (a) is t a k e n on a CoPt film and (b) on a C o P t C r film. I m a g e size is 6 :x: 7 l~m 2. The inset shows the a p p a r e n t bit s t r u c t u r e for a writing density of 5000 flux reversals per m i l l i m e t e r .
126
('. Schoenenberger et al. / Magnetic force microscopy to thin films
tractive force for an undisturbed sample magnetization. The expected exponential decay with a decay length of w / v is drawn as a dotted line. Curve (b) shows the separated image force F~ caused by the induced rotation of the sample magnetization due to the presence of the tip. F~ and 0 are small in this particular example. However, strong image forces have been found on permalloy [9, 10].
4. Application The two different films are compared in fig. 3: (a) shows l~zm wide bits on a CoPt film and (b) 670nm wide bits on CoPtCr. Both images were corrected to display the in-plane field H x. It is apparent that the peak-to-peak roughness #t of the transition line between adjacent bits is much larger than the intrinsic transition length I 0 as derived from a arctangent model (table 1). The material p a r a m e t e r that governs the intrinsic transition width is the wavelength Xr associated with longitudinal ripples [11]. The detailed ripple structure could not be resolved with MFM, but a Lorentz microscopy investigation on similar CoPt films reveals A r = 2 0 0 - 3 0 0 n m [12]. However, some individual clusters are visible in fig. 3a. Their size s c, measured in the track direction, is about 200-300nm. In addition, fit = 320nm is derived. All three parameters 6t, Ar and s c are comparable in size. The smallest possible bit should contain at least one cluster. This yields a minimum bit size Wmin = 6 t + s c = 6 0 0 n m , together with the transition roughness. The written transitions in fig. 3a are already close to the maximum writing density. For the CoPtCr film a transition roughness of 6 t ~ 100nm is found, hence wmi. = 220rim. The largest writing density is expected to be 4 5 0 0 f r / m m . M F M reveals that the periodic bit structure collapses at different sample locations, for a writing density in between 4000 and 5 0 0 0 f r / m m . The inset in fig. 3b shows a measurement at 5000 f r / m m . However, when reading
the structure with a conventional recording head, a periodic signal was still detectable up to a density of 8 0 0 0 f r / m m . This indicates that the apparent roughness 6t is limited by the flying height a n d / o r the gap width of the writing head.
5. Conclusions MFM allows the magnetic stray field of ferromagnetic samples to be measured with submicron resolution. The etched Fe, Co and Ni wires investigated all have their apex domain oriented along the tip axis. The stray field experienced on the sample surface from the tip can be rather large, which limits the applicability of MFM to the study of hard magnetic samples. This limitation might be overcome by using thin film tips. The magnetic force measured using Co tips follows the field derivative as expected from a pointdipole model, but the situation is different for the more commonly used tip materials, Fe and Ni. Here, the magnetic force is more closely related to the stray field, even for magnetic structures on a micron scale. The tip behaves as an extended single-pole probe. The resolving power obtained using any of the three tip materials is limited by the tip radius. The apparent transition roughness ¢~t iS found to be comparable to the longitudinal ripple size as measured by Lorentz microscopy, and to the cluster width as seen with MFM for the CoPt film, indicating that the boundary roughness is governed by intrinsic material parameters. In the case of the CoPtCr film, however, the roughness appears to be limited by extrinsic parameters like the flying height and the gap width of the writing head.
References [1] Y. 50 [2] G. 56
Martin and H.K. Wickramasinghe, Appl. Phys. Lett. (1987) 1455. Binnig, C.F. Quate and Ch. Gerber, Phys. Rev. Lett. (1986) 930.
C. Schoenenberger et al. / Magnetic force microscopy to thin films [3] C. Sch6nenberger and S.F. Alvarado, Rev. Sci. Instrum. 60 (1989) 3131. [4] C. Sch6nenberger, S.F. Alvarado, S.E. Lambert and I.L. Sanders, J. Appl. Phys. (1990) in press. [5] Y. Martin, D. Rugar and H.K. Wickramasinghe, App[. Phys. Lett. 52 (1988) 244. [6] H.J. Mamin, D. Rugar, J.E. Stern, B.D. Terris and S.E. Lambert, Appl. Phys. Lett. 53 (1988) 1563. [7] A. Wadas and P. Griitter, Phys. Rev. B 39 (1989) 12013.
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