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Intrinsic hysteresis properties of ME tape
Journal of Magnetism and Magnetic Materials 120 (1993) 217-220 North-Holland
Intrinsic hysteresis properties of ME tape H.J. Richter and H. Hibst BASF AG, Ammoniak-Laboratorium, 6700 Ludwigshafen, Germany Using a biaxial vibrating sample magnetometer the magnetization reversal behaviour of obliquely evaporated Co-Ni-O layers (metal evaporated tapes) is studied. It is shown that only an appropriate rotation of the sample during measurement yields true data compensated for demagnetization. It turns out that the ME tape has a very high degree of orientation and a very narrow switching field distribution. 1. Introduction Recently, metal evaporated tape (ME tape) became commercially available. This tape consists of an obliquely oriented layer of C o - N i - O with a thickness of typically 200 nm and shows excellent recording properties. Due to the microstructure of the tape, an oblique anisotropy axis emerges leading to a recording anisotropy. The recording behaviour of the ME tape is best if the recording field at the trailing edge of the head is at a large angle to the anisotropy axis. Using the concept of the ' u p ' and 'down' particles [1], ME tape consists entirely of ' u p ' particles so that an improvement in recording behaviour can be expected. On the other hand, conventional in-plane hysteresis measurements yield very ' b a d ' magnetic data (relative remanence m r ~ 0.7, switching field distribution SFD = 0.5) so that the improvement in recording performance due to the tilt of the anisotropy axis should be far less than the deterioration caused by the ' b a d ' magnetic properties. In the present paper we show that the quantities obtained by conventional hysteresis may not be used to draw any direct conclusions concerning the magnetics of layers with a tilted anisotropy axis. Simple analysis of the remanent magnetization vector shows that ME tape carries a considerable perpendicular magnetization component [2]. This leads to a demagnetizing field opposing the perpendicular magnetization component. Consequently, the magnetic entities in the tape are subjected to an effective (internal) field given by the vector sum of the external field H a and the demagnetization field Hd: N.
'
(1)
Correspondence to: Dr. H.J. Richter, BASF AG, ZAA/FM320, 6700 Ludwigshafen, Germany. Tel.: +49-6216054805; telefax: +49-6216056153.
where Nil, N-L = demagnetization factor parallel and perpendicular to the film plane, respectively, and Mll, M L= magnetization parallel and perpendicular to the film plane, respectively. In eq. (1) Nil-- 0 and N.L = 1 is assumed because of the geometry of the thin film. Starting from conventional hysteresis, the vector diagrams of the magnetic fields for some different external fields are shown in fig. 1 for the case of an in-plane measurement of the hysteresis. Note that the internal field rotates by running through a conventional hysteresis. A special case is a measurement in the perpendicular direction where the demagnetizing field and the external field are parallel and their vectorial addition to the internal field is trivial. Therefore, the classical shearing procedure is applicable here. In order to compensate for demagnetization in the general case, the sample (or the external field) has to be rotated in such a way that the internal field comes to lie in a desired direction [3], which may, for instance, be an in-plane orientation of the internal field. Figure 1 illustrates this for some points on the hysteresis loop. 2. Results In order to be able to calculate the internal field vector, information about the magnetization in two directions is required. Therefore, a biaxial detection of the magnetization is needed. We used our vector VSM described in ref. [4]. The main difficulty in the determination of the intrinsic hysteresis is to find the external field magnitude and its orientation that makes the internal field lie in the desired direction throughout the complete measurement cycle. In the present approach, the hysteresis loop is first measured with a given sequence of values for H a and O0 using a rather small number of measuring points (typically < 100). In a subsequent step, a new sequence for H a and ~90 is calculated on the basis of the measured magnetization
0304-8853/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
H.J. Richter, H. Hibst / Intrinsic hysteresis properties of ME tape
218
-e~s~~xrsII
film plane
"-,
"..Hi/~H~ Hd
"',.. na
Ha
"x,.
~" conventional
om.nsate Fig. 1. Vector diagrams showing the vector addition of the external field and the demagnetization field at some points of the hysteresis loop. The bottom row illustrates how the sample has to be oriented to yield the internal field directing in the film plane (not to scale).
data which would make the (old) internal field point into the desired direction. Using these new values for H a and O0, the hysteresis is measured again. This procedure is continued using further steps until the internal field points into the desired direction for the entire hysteresis loop. For more detailed information the reader is referred to [5]. Another possibility for compensation for demagnetization was performed by Bernards and Cramer [3], who use a completely different approach to find the values for H a and O0 than the present one and obtain similar results. In the approach used in ref. [3], the nth point on the hysteresis loop is estimated on the basis of the two previously measured ones. By using very small steps it is assured that the loop is traced correctly, i.e. that the internal field remains in one direction. In this way the approach in ref. [3] traces the hysteresis only once using many measuring points, while the present approach takes several runs using fewer points, resulting in a roughly equal number of measuring points for both methods. The results for the compensated in-plane measurement is shown in fig. 2. Here M( II O0i) and M(_t_ O01) give the magnetization components parallel and perpendicular to the desired direction of the internal field which makes the angle O~ = 0 with the film plane (in-plane measurement). In contrast to the uncompensated hysteresis, both components reverse their magnetization at the same field strength. Figure 3 shows what has to be done with the external field in order to achieve the required condition that the internal field remains in the same direction. The shape of the curves Ha(O o) ( 0 o =
orientation of the external field with respect to the film plane) sometimes turns out to be very complicated and considerable changes occur for different values O~. In longitudinal media the hysteresis loop measured along the preferred direction is the basis for the determination of most of the magnetic data. It is therefore reasonable to compare the magnetic data of ME tape measured along its preferred direction with those of purely longitudinal media measured conventionally. Figure 4 shows the relevant hysteresis loop in direction of the easy axis (38 ° to the film plane). Since the measurement is along the easy axis, the component M(.I_ On) vanishes. From this loop it is readily seen
M
~
.
.
.
.
.
v,0i= 0"
.
Fig. 2. Compensated hysteresis loops for an in-plane measurement of the ME tape (O0j ffi 0). The two magnetization components M(llO0i) and M( _1.O0i) correspond to the magnetization components in the film plane and perpendicular to it, respectively.
1t..I. Richter, H. Hibst / Intrinsic hysteresisproperties of ME tape
219
360 °
i 1
O0i= 130°
270° 180°
.
.
.
.
,
.
.
.
.
.
.
/../" t
90° ~ o
./
100
200
300
400
~
I - 0.5 -
1
Fig. 3. External field Ha(Oo) (O0 = orientation of the external field with respect to the film plane) required to measure the loops shown in fig. 2.
Fig. 5. Compensated hysteresis loop measured along the hard axis (130" to the film plane) for the ME tape. The component M( -LO0i) is negligible.
that the magnetic data (squareness = 0.93, SFD < 0.01) are excellent so that the recording behaviour is well understood on the basis of the usual parameters. Remanence measurements compensated for demagnetization confirm that the SFD, which was determined here by the slope criterion
els it is clear that the component M(.L O0i) has to be very small for the directions close to the hard and the easy directions. In order to provide some information about the error associated with compensation, the error field Herr (see ref. [3]) after a full compensation along the intrinsic easy axis is shown in fig. 6 for Ha,max > H a > -Ha,max. For comparison, the error field for the uncompensated measurement is also shown. In principle, the error field can be made less than 2 k_A/m, which corresponds to the noise limit for a typical sample (see ref. [5]). The compensation of the demagnetizing field has to be done using eq. (1). This corresponds to the general case of shearing back the hysteresis loop vectorially. Several authors have suggested to compensate the hysteresis after conventional measurement [6-8]. As already discussed in ref. [3], this does not come close to the true intrinsic hysteresis (see also fig. 6). Furthermore, it is emphasized again here that ignoring the component of the demagnetizing field perpendicular to
,IIdMl d--H ]lHc'
SFD = ~ - i ~
(2)
is really that small [5]. The high value for the squareness is accompanied by a very high orientation factor (OF =Mr(easy axis)/M)(hard axis)ffi 11.5). This is evident from the (approximate) hard axis loop (O(~ = 130°) shown in fig. 5. Note that the measurement of the hard axis loop in exactly the hard direction can lead to problems concerning the rotation sense (noise). Therefore, it is found more useful to stay a little off the hard direction. The perpendicular component M(_L O0i) for the hard axis loop turns out to be very small and is therefore omitted here. From simple theoretical mod1
M0100i)
0.5
/
380 conventional ::~/"~ ] [ 100
..,
-260 ....
I .........
.L
I, H.i,/~km-1)
200
_|
Fig. 4. Compensated hysteresis loop measured along the easy axis (38° to the film plane) for the ME tape. The component /14"(.1. #~) is negligible.
a
f
t
O0i= 38°
~
oy i:ii Fig. 6. Error field Herr for a conventional measurement along the easy axis and for a compensated measurement after completion of the iteration.
220
H.J. Richter, H. Hibst / Intrinsic hysteresis properties o f M E tape 150
,
,
,
.
i 45
.
.
~
.
•
J
•
'
~]125
ff 100
75
135
180 25
%, o~(o)
-225 0
Oo.Ooi(o) i -2713
I 45
i
i
I 90
i 135
180
Fig. 7. Angle of the remanent magnetization vector for the compensated and uncompensated measurement as a function of the field angle for the ME tape. the external field can lead to the false conclusion that there is no compensation required for an in-plane measurement in general. Figures 7 - 9 show the results for the angle of the remanent magnetization vector, its length and coercivity as a function of the angle of the field for both compensated and uncompensated measurements. The results of the compensated measurements reveal the very simple fact that the magnetization always returns to its easy axis (except at field orientations close to the hard axis). This and the very high value for I mr I = m m 2 )1/2 can be understood in terms of a very •" r x -4 - ""rygood orientation of the magnetic entities in the tape. 1.0~
•1
,
,
~
•
.
•
compensated
0.8 0"71 ~
i
,
.
i 90
,
.
t 135
,
. 180
Fig. 9. Variation of coercivity as a function of the field angle for the compensated and uncompensated measurement for the ME tape.
The coercivity is symmetrical around the easy axis after compensation which is expected for uniaxial anisotropy. The figures show that the uncompensated data are dearly different, although the principal dependencies are similar. 3.
Conclusion
It has been shown that for recording media with a tilted anisotropy axis (ME tape) conventional hysteresis is strongly distorted due to the effect of the perpendicular magnetization component. In the general case, back shearing of the hysteresis loop can only be accomplished by continuous rotation of the sample (or the external field, respectively) during measurement. Hysteresis loop measurements compensated correctly for demagnetization reveal that the ME tape is very well oriented and has a very narrow switching field distribution. References
0.6
"~
0.5 0.4 0.3 0.2 O.1
tO,O0i(o2"' i
0"00
45
' ;l~ ..
i 135
180
Fig. 8. Length of the remanent magnetization vector normalized to saturation magnetization for compensated and uncompensated measurement as a function of the field angle for the ME tape.
[1] H.N. Bertram and I.A. Beardsley, IEEE Trans. Magn. MAG-24 (1988) 3234. [2] H.J. Richter and H. Hibst, J. Appl. Phys. 70 (1991) 5512. [3] J.P.C. Bernards and H.A.J. Cramer, IEEE Trans. Magn. MAG-26 (1991) 4873. [4] HJ. Richter, J. Magn. Magn. Mater., in press. [5] HJ. Richter, IEEE Trans, Magn., submitted. [6] K. Ouchi and I. Iwasaki, IEEE Trans. Magn. MAG-24 (1988) 3OO9. [7] D.E. Speliotis, D. Bono and P. Judge, J. Magn. Soc. Jpn. 13 (1989) 887. [8] D.E. Speliotis and J.P. Judge, J. Appl. Phys. 69 (1991) 5157.
Intrinsic hysteresis properties of ME tape H.J. Richter and H. Hibst BASF AG, Ammoniak-Laboratorium, 6700 Ludwigshafen, Germany Using a biaxial vibrating sample magnetometer the magnetization reversal behaviour of obliquely evaporated Co-Ni-O layers (metal evaporated tapes) is studied. It is shown that only an appropriate rotation of the sample during measurement yields true data compensated for demagnetization. It turns out that the ME tape has a very high degree of orientation and a very narrow switching field distribution. 1. Introduction Recently, metal evaporated tape (ME tape) became commercially available. This tape consists of an obliquely oriented layer of C o - N i - O with a thickness of typically 200 nm and shows excellent recording properties. Due to the microstructure of the tape, an oblique anisotropy axis emerges leading to a recording anisotropy. The recording behaviour of the ME tape is best if the recording field at the trailing edge of the head is at a large angle to the anisotropy axis. Using the concept of the ' u p ' and 'down' particles [1], ME tape consists entirely of ' u p ' particles so that an improvement in recording behaviour can be expected. On the other hand, conventional in-plane hysteresis measurements yield very ' b a d ' magnetic data (relative remanence m r ~ 0.7, switching field distribution SFD = 0.5) so that the improvement in recording performance due to the tilt of the anisotropy axis should be far less than the deterioration caused by the ' b a d ' magnetic properties. In the present paper we show that the quantities obtained by conventional hysteresis may not be used to draw any direct conclusions concerning the magnetics of layers with a tilted anisotropy axis. Simple analysis of the remanent magnetization vector shows that ME tape carries a considerable perpendicular magnetization component [2]. This leads to a demagnetizing field opposing the perpendicular magnetization component. Consequently, the magnetic entities in the tape are subjected to an effective (internal) field given by the vector sum of the external field H a and the demagnetization field Hd: N.
'
(1)
Correspondence to: Dr. H.J. Richter, BASF AG, ZAA/FM320, 6700 Ludwigshafen, Germany. Tel.: +49-6216054805; telefax: +49-6216056153.
where Nil, N-L = demagnetization factor parallel and perpendicular to the film plane, respectively, and Mll, M L= magnetization parallel and perpendicular to the film plane, respectively. In eq. (1) Nil-- 0 and N.L = 1 is assumed because of the geometry of the thin film. Starting from conventional hysteresis, the vector diagrams of the magnetic fields for some different external fields are shown in fig. 1 for the case of an in-plane measurement of the hysteresis. Note that the internal field rotates by running through a conventional hysteresis. A special case is a measurement in the perpendicular direction where the demagnetizing field and the external field are parallel and their vectorial addition to the internal field is trivial. Therefore, the classical shearing procedure is applicable here. In order to compensate for demagnetization in the general case, the sample (or the external field) has to be rotated in such a way that the internal field comes to lie in a desired direction [3], which may, for instance, be an in-plane orientation of the internal field. Figure 1 illustrates this for some points on the hysteresis loop. 2. Results In order to be able to calculate the internal field vector, information about the magnetization in two directions is required. Therefore, a biaxial detection of the magnetization is needed. We used our vector VSM described in ref. [4]. The main difficulty in the determination of the intrinsic hysteresis is to find the external field magnitude and its orientation that makes the internal field lie in the desired direction throughout the complete measurement cycle. In the present approach, the hysteresis loop is first measured with a given sequence of values for H a and O0 using a rather small number of measuring points (typically < 100). In a subsequent step, a new sequence for H a and ~90 is calculated on the basis of the measured magnetization
0304-8853/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
H.J. Richter, H. Hibst / Intrinsic hysteresis properties of ME tape
218
-e~s~~xrsII
film plane
"-,
"..Hi/~H~ Hd
"',.. na
Ha
"x,.
~" conventional
om.nsate Fig. 1. Vector diagrams showing the vector addition of the external field and the demagnetization field at some points of the hysteresis loop. The bottom row illustrates how the sample has to be oriented to yield the internal field directing in the film plane (not to scale).
data which would make the (old) internal field point into the desired direction. Using these new values for H a and O0, the hysteresis is measured again. This procedure is continued using further steps until the internal field points into the desired direction for the entire hysteresis loop. For more detailed information the reader is referred to [5]. Another possibility for compensation for demagnetization was performed by Bernards and Cramer [3], who use a completely different approach to find the values for H a and O0 than the present one and obtain similar results. In the approach used in ref. [3], the nth point on the hysteresis loop is estimated on the basis of the two previously measured ones. By using very small steps it is assured that the loop is traced correctly, i.e. that the internal field remains in one direction. In this way the approach in ref. [3] traces the hysteresis only once using many measuring points, while the present approach takes several runs using fewer points, resulting in a roughly equal number of measuring points for both methods. The results for the compensated in-plane measurement is shown in fig. 2. Here M( II O0i) and M(_t_ O01) give the magnetization components parallel and perpendicular to the desired direction of the internal field which makes the angle O~ = 0 with the film plane (in-plane measurement). In contrast to the uncompensated hysteresis, both components reverse their magnetization at the same field strength. Figure 3 shows what has to be done with the external field in order to achieve the required condition that the internal field remains in the same direction. The shape of the curves Ha(O o) ( 0 o =
orientation of the external field with respect to the film plane) sometimes turns out to be very complicated and considerable changes occur for different values O~. In longitudinal media the hysteresis loop measured along the preferred direction is the basis for the determination of most of the magnetic data. It is therefore reasonable to compare the magnetic data of ME tape measured along its preferred direction with those of purely longitudinal media measured conventionally. Figure 4 shows the relevant hysteresis loop in direction of the easy axis (38 ° to the film plane). Since the measurement is along the easy axis, the component M(.I_ On) vanishes. From this loop it is readily seen
M
~
.
.
.
.
.
v,0i= 0"
.
Fig. 2. Compensated hysteresis loops for an in-plane measurement of the ME tape (O0j ffi 0). The two magnetization components M(llO0i) and M( _1.O0i) correspond to the magnetization components in the film plane and perpendicular to it, respectively.
1t..I. Richter, H. Hibst / Intrinsic hysteresisproperties of ME tape
219
360 °
i 1
O0i= 130°
270° 180°
.
.
.
.
,
.
.
.
.
.
.
/../" t
90° ~ o
./
100
200
300
400
~
I - 0.5 -
1
Fig. 3. External field Ha(Oo) (O0 = orientation of the external field with respect to the film plane) required to measure the loops shown in fig. 2.
Fig. 5. Compensated hysteresis loop measured along the hard axis (130" to the film plane) for the ME tape. The component M( -LO0i) is negligible.
that the magnetic data (squareness = 0.93, SFD < 0.01) are excellent so that the recording behaviour is well understood on the basis of the usual parameters. Remanence measurements compensated for demagnetization confirm that the SFD, which was determined here by the slope criterion
els it is clear that the component M(.L O0i) has to be very small for the directions close to the hard and the easy directions. In order to provide some information about the error associated with compensation, the error field Herr (see ref. [3]) after a full compensation along the intrinsic easy axis is shown in fig. 6 for Ha,max > H a > -Ha,max. For comparison, the error field for the uncompensated measurement is also shown. In principle, the error field can be made less than 2 k_A/m, which corresponds to the noise limit for a typical sample (see ref. [5]). The compensation of the demagnetizing field has to be done using eq. (1). This corresponds to the general case of shearing back the hysteresis loop vectorially. Several authors have suggested to compensate the hysteresis after conventional measurement [6-8]. As already discussed in ref. [3], this does not come close to the true intrinsic hysteresis (see also fig. 6). Furthermore, it is emphasized again here that ignoring the component of the demagnetizing field perpendicular to
,IIdMl d--H ]lHc'
SFD = ~ - i ~
(2)
is really that small [5]. The high value for the squareness is accompanied by a very high orientation factor (OF =Mr(easy axis)/M)(hard axis)ffi 11.5). This is evident from the (approximate) hard axis loop (O(~ = 130°) shown in fig. 5. Note that the measurement of the hard axis loop in exactly the hard direction can lead to problems concerning the rotation sense (noise). Therefore, it is found more useful to stay a little off the hard direction. The perpendicular component M(_L O0i) for the hard axis loop turns out to be very small and is therefore omitted here. From simple theoretical mod1
M0100i)
0.5
/
380 conventional ::~/"~ ] [ 100
..,
-260 ....
I .........
.L
I, H.i,/~km-1)
200
_|
Fig. 4. Compensated hysteresis loop measured along the easy axis (38° to the film plane) for the ME tape. The component /14"(.1. #~) is negligible.
a
f
t
O0i= 38°
~
oy i:ii Fig. 6. Error field Herr for a conventional measurement along the easy axis and for a compensated measurement after completion of the iteration.
220
H.J. Richter, H. Hibst / Intrinsic hysteresis properties o f M E tape 150
,
,
,
.
i 45
.
.
~
.
•
J
•
'
~]125
ff 100
75
135
180 25
%, o~(o)
-225 0
Oo.Ooi(o) i -2713
I 45
i
i
I 90
i 135
180
Fig. 7. Angle of the remanent magnetization vector for the compensated and uncompensated measurement as a function of the field angle for the ME tape. the external field can lead to the false conclusion that there is no compensation required for an in-plane measurement in general. Figures 7 - 9 show the results for the angle of the remanent magnetization vector, its length and coercivity as a function of the angle of the field for both compensated and uncompensated measurements. The results of the compensated measurements reveal the very simple fact that the magnetization always returns to its easy axis (except at field orientations close to the hard axis). This and the very high value for I mr I = m m 2 )1/2 can be understood in terms of a very •" r x -4 - ""rygood orientation of the magnetic entities in the tape. 1.0~
•1
,
,
~
•
.
•
compensated
0.8 0"71 ~
i
,
.
i 90
,
.
t 135
,
. 180
Fig. 9. Variation of coercivity as a function of the field angle for the compensated and uncompensated measurement for the ME tape.
The coercivity is symmetrical around the easy axis after compensation which is expected for uniaxial anisotropy. The figures show that the uncompensated data are dearly different, although the principal dependencies are similar. 3.
Conclusion
It has been shown that for recording media with a tilted anisotropy axis (ME tape) conventional hysteresis is strongly distorted due to the effect of the perpendicular magnetization component. In the general case, back shearing of the hysteresis loop can only be accomplished by continuous rotation of the sample (or the external field, respectively) during measurement. Hysteresis loop measurements compensated correctly for demagnetization reveal that the ME tape is very well oriented and has a very narrow switching field distribution. References
0.6
"~
0.5 0.4 0.3 0.2 O.1
tO,O0i(o2"' i
0"00
45
' ;l~ ..
i 135
180
Fig. 8. Length of the remanent magnetization vector normalized to saturation magnetization for compensated and uncompensated measurement as a function of the field angle for the ME tape.
[1] H.N. Bertram and I.A. Beardsley, IEEE Trans. Magn. MAG-24 (1988) 3234. [2] H.J. Richter and H. Hibst, J. Appl. Phys. 70 (1991) 5512. [3] J.P.C. Bernards and H.A.J. Cramer, IEEE Trans. Magn. MAG-26 (1991) 4873. [4] HJ. Richter, J. Magn. Magn. Mater., in press. [5] HJ. Richter, IEEE Trans, Magn., submitted. [6] K. Ouchi and I. Iwasaki, IEEE Trans. Magn. MAG-24 (1988) 3OO9. [7] D.E. Speliotis, D. Bono and P. Judge, J. Magn. Soc. Jpn. 13 (1989) 887. [8] D.E. Speliotis and J.P. Judge, J. Appl. Phys. 69 (1991) 5157.