- Email: [email protected]
Magnetic recording simulation and its applications
~
ELSEVIER
Journal of Magnetism and Magnetic Materials 134 (1994) 382-389
journalof magnetism magnetic 4~iPematerials
Magnetic recording simulation and its applications Ikuya Tagawa
*, Y o s h i h i s a
Nakamura
Research Institute of Electrical Communication, Tohoku University, Sendai 980, Japan
Abstract We have developed a magnetic recording simulator whose algorithm is based on a finite element method for magnetic field analysis and a magnetization curling model for crystalline particles in a medium. Using this simulator, we have analyzed the recording performance for an obliquely evaporated medium and the recording mechanisms of double-coated particulate medium for a superimposed video signal. When an oblique medium like an ME tape is combined with a ring head, a better recording performance is obtained because the oblique component distribution of the ring head field is sharper than those of both longitudinal and perpendicular components. In the superimposed signal recording of a double-coated particulate medium, we found that the recording depth is less than 0.3 ixm for the" short wavelength Y-signal but the long wavelength C-signal is recorded in the middle layer of the medium. Moreover it was confirmed that a double-coated medium having a perpendicularly oriented surface layer is very effective for an advanced video recording system.
1. Introduction Modelling of the m e d i u m magnetization process is an important means for quantitative analysis of the magnetic recording process. Physically accurate simulation using a computer is very useful for design of an optimum recording system. We have introduced a novel model of the magnetization mechanism for magnetic crystalline particles, based on the incoherent spin curling model, and applied it to the magnetization process of actual media [1]. Using this model, major and minor simulated hysteresis curves agree well with those measured in any magnetizing direction. To calculate the magnetic flux density distribution around a head and a medium and the internal vectorial magnetization distribution of a medium, a finite element method was introduced
* Corresponding author.
and incorporated with the medium magnetization model [2]. We have confirmed that this magnetic recording simulator is widely applicable not only for longitudinal magnetic recording but also for perpendicular recording systems [3,4]. In this paper, the magnetic recording simulator will be briefly reviewed, and the results of analyzing the recording and reproducing processes will be presented both on a ring head recording system using an obliquely evaporated medium and on a video signal storage system using a double-coated particulate medium.
2. Magnetic recording simulator 2.1. M e d i u m magnetization model
For calculation of magnetization inside a medium, an assembly of non-interacting particles is assumed and the magnetization switching in
0304-8853/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0304-8853(94)00235-J
L Tagawa, Y. Nakamura/Journal of Magnetism and Magnetic Materials 134 (1994) 382-389
each particle is approximated by the curling model. When the external field is applied in the easy axis direction of a medium, the magnetization of the medium is calculated by [1]
M=fffMsc o s [ 0 ( o ,
H/Hk)] ,
Xp(O, ~b)q( Hk) dO dgbdHk.
,
tt
I
I y
,/',¢-
YX
/.
(1)
Here Ms, H k and (0, $) are the saturation magnetization, the anisotropy field strength and the easy axis direction in spherical coordinates, respectively, p(O, ok) and q(H k) are the standardized distributions of the easy axis direction and the anisotropy field strength, respectively. ~(0, h) is the magnetization angle to the medium easy axis direction at the reduced field of h in a particle with the easy axis of (0, $), calculated by the curling magnetization reversal theory.
2.2. Recording simulation model A finite element method is used to calculate a magnetic field distribution because the saturation effects of a recording head and a soft magnetic underlayer of a double-layer medium can be easily considered. Fig. 1 shows a flow diagram for simulations of the magnetic recording process. To
t, [ Mesh Generation [ [ Set Boundary Condition [ t, I Set Current Element for Head Coil [ [ Calculate Permeability in each Element I Calculate Magnetization Element in Medium I by Curling Particle Assembly Model I 4, [ Modify Magnetization by Iteration Method [ ~t J Calculate Vector Potential in each Node J I Calculate Magnetic Flux in each Element J
Fig. 1. Flow diagram of the magnetic recording simulation using the finite element method and the magnetization curling model.
- - - -
ECe_aured
~=3~o em,dc,:
, _ '
383
'
'
,
"."'0 "
Ck
l"
,0
Co-Ni obliquely evaporated medium
Fig. 2. M - H loops calculated by the medium magnetization model and measured by VSM.
calculate M - H curves in the direction perpendicular to the medium, a demagnetizing field H d of - M y is assumed, and an iteration method is used for the self-consistent calculation. The magnetic interaction between particles is disregarded here but it is possible to consider the interparticle interaction by introducing the mean interaction field theory [5].
3. Obliquely oriented medium
3.1. Hysteresis loops A Co-Ni obliquely evaporated medium (Sony Hi8ME) was assumed in the simulation, and its saturation magnetization, the anisotropy field and the coercive force were set at 370 emu/cc, 4000 Oe and 1030 Oe, respectively. In Fig. 2, M - H loops calculated by the medium magnetization model are compared with measured ones in various magnetizing directions, where the easy axis direction of a medium is supposed to be inclined 30° to the longitudinal direction. A good agreement is obtained between measured and calculated loops. Here O ' h k / H k ---- 0.22 and tr0 = 12 deg. We have also confirmed that the applied field angular dependence of the coercive force computed by the medium model coincided quantitatively with the measured one [6]. Recording simulations were performed for media having various easy axis directions, namely 0 (longitudinal), 30 (the regular direction of a ME tape), 60, 90 (perpendicular) and 150 (the opposite direction to a regular ME tape) degrees,
384
I. Tagawa, Y. Nakamura /Journal of Magnetism and Magnetic Materials 134 (1994) 382-389
respectively. The medium thickness, the head gap length and the h e a d - m e d i u m spacing were assumed as 0.2, 0.2 and 0.04 txm, respectively. The recording current was set at 0.35 A T so that the maximum voltage was obtained in the simulation of an isolated transition recording. We have obtained a good agreement between the simulated and experimental results not only on the reproduced pulse but also on the recording density characteristics for the medium oriented to the 30 ° direction.
3.2. Magnetization distributions Fig. 3 shows the magnetic flux contours around the head gap and the magnetization distributions in the medium in the reproducing process, when the ring head has just come to the same position at which the recording field was switched in the recording process of an isolated transition. W h e n the easy axis direction is at 30 and 60 °, the
Isolated Transition
Easy • Axis
~
m I
Odeg
~
)
( Reproduction
t
.
'i ::~
.
.
~
_..N~O.04 .
t'L
30 deg
60 dog
90 deg
'!i!i}i!iiiiiiii!i! ....
150 deg Fig. 3. Magnetization distributions and magnetic flux contours for the isolated magnetization transition in the reproducing process.
30 °
-
1
~
I/I:\'~ 0
......
TrailingSide ~" ~] "il"I_ 4000~.
.
.
.
.
9o °
,.o, LoadingSide
Fig. 4. Easy axis field distributions for a ring head at a surface layer of media having various easy axis directions.
recorded magnetization direction coincides with the field direction on the leading side in the recording process. However when it is at 90 and 150 °, the magnetization direction coincides with the trailing field direction. From both the simulated angular dependence of the switching field strength and the experimental angular dependence of the remanent coercivity [7], it is clear that the switching field strength of a medium in the easy axis direction is much smaller than those in any other directions. Therefore we can consider that the magnetization transition is mainly formed by the easy axis component of the recording head field (the easy axis field). Fig. 4 shows the easy axis field distributions of a ring head at a surface layer of media having various easy axis directions. The magnetization transition is formed by the field gradient on the trailing side. The steepest gradient of the easy axis field is obtained for the medium whose easy axis is at 30 °, and the gradient becomes gentle in the order of 60, 0, 150 and 90 °. Therefore the narrowest magnetization transition is obtained in the medium with the easy axis at 30 ° . Fig. 5 shows the magnetization distributions in the medium recorded at about 95 kFRPI. The media having the easy axes at 30 and 60 ° are magnetized through the whole thickness. When the easy axis is at 0 and 150 °, the intensity of the magnetization in the layer close to the bottom surface is weaker than that in the layer close to the top surface because the longitudinal demagnetizing field owing to the longitudinal magneti-
I. Tagawa, Y. Nakamura /Journal of Magnetism and Magnetic Materials 134 (1994) 382-389
zations facing each other becomes strong in the bottom layer.
385
95.25 kFRPI
Easy Axis
~
~
0.2 p m ~
0.0~m
3.3. Reproduced output 0.2 pm
Fig. 6 shows the reproduced voltages obtained by simulations at about 100 kFRPI for media with varying easy axis directions from 0 to 70 °. Since the reproduced output at a high recording density depends on the sharpness of the magnetization transition, the largest output is obtained for the medium having the easy axis at 30 °. Moreover, even in the case of a low recording density, the medium with easy axis at 30 ° gives larger output than media having the other easy axis directions [61.
•
/
90 deg
4. Double-coated medium
4.1. Superimposed recording signal In consumer video storage systems, a chrominance signal (C-signal) in a low frequency band up to 1 MHz is superimposed on a luminance signal (Y-signal) in a high frequency band up to 8 MHz. Therefore media for video systems require excellent recording characteristics in a wide frequency region. We have analyzed the recording performances for a single-coated medium with longitudinally oriented Co-~/-Fe20 3 particles. The thickness, Ms, Hk, H~, Orhk/H k and tr0 of the recording layer were set at 0.6 I~m, 145 e m u / c c , 2500 Oe, 900 Oe, 0.32 and 20 ° in the simulation respectively, and a M I G ring head with gap length of 0.25 Ixm was assumed. The recording wavelengths, of the Y- and the C-signals were set at 0.92 and 9.2 Ixm, respectively. Fig. 7 shows the magnetic flux contour and the magnetization distribution in the single-coated medium, obtained by the simulation of the superimposed video signal recording. The Y-signal whose wavelength is shorter is recorded in the surface layer of the medium. But the C-signal which has the longer wavelength is mainly recorded in the middle and in the bottom layers. In Fig. 8, the I c / I v ratio dependencies of the Y-, the C- and the Y - 2C spurious signals are
r
-1
b
= 0.27
pm
Fig. 5. Magnetization distributions in the medium recorded at about 95 kFRPI.
compared, which were obtained by a Fourier analysis of the measured and simulated reproduced output. Here I c and I v are the recording magnetic motive forces applied to the head for
14 12 ,-, 10, "-"
8
o
4
0
I
0
,
I
,
I
,
I
,
I
,
I
,
10 20 30 40 50 60 70 Easy axis (deg)
Fig. 6. Reproduced output characteristics versus easy axis of the medium at about 100 kFRPI, obtained by simulations.
L Tagawa, Y. Nakamura /Journal of Magnetism and Magnetic Materials 134 (1994) 382-389
386
\\// DIV. = 0.20E+ 02 (Gauss/Lrn)
~x
2 r = 0.92/tm ~c=9.20twn 4
..................
):. :: ~ 7 . ' . ' : - . . . ; ; ;
Hc =900Oe
..... ~
....................
Fig. 7. Magnetization distribution and magnetic flux contours in a single-layer medium, obtained by a simulation for a superimposed recording signal.
•
'
'~I
'
'
'
I
'
•
?"
' ' ' I
-I0
Eroq,. C C g" •
o
7' o,ima .
./
/
N -30
.7
/ e,l Ey 2cO:Ic~ /to •/
.-,
ue=5.8m/s fv=6.4MHz f c = 644kHz
ie
--40
7 P'/-,.O~/"/77,7"/-//--/'/~,~',,'7,77..'-/ Background Level , , ,:l , t .... I 0.I 0.5 1.0
Ic/ Ir Fig. 8. I c / I y versus Y-, C- and Y - 2C spurious signal output characteristics obtained by Fourier analysis of the reproduced output for a single-layer medium.
the C- and the Y-signals, respectively. The Y - 2C signal is the component of the reproduced output at a frequency which is equal to a Y-signal frequency minus twice the frequency of the C-signal. The output of the Y - 2C signal becomes larger as the waveform distortion increases. The open and the solid circles indicate simulated and experimental results respectively, and we have obtained a good agreement between them as shown in the figure. When the I c / I v ratio increases, the relative output E c of the C-signal increases but the Ysignal output E v decreases. The output of the Y-signal becomes smaller than that of the C-signal in the region of 0.6 < I c / I v . On the other hand, the Y - 2C spurious signal output becomes larger in proportion to Icz. The Y - 2C signal is caused by the waveform distortion and so the I c / I Y characteristics depend strongly on the nonlinearity in the magnetic recording system. These
L Tagawa, Y. Nakamura/Journal of Magnetism and Magnetic Materials 134 (1994) 382-389
particulate medium, the C-signal component is recorded only at a depth of around 0.15 I~m. When low coercivity particles like Co-~/-Fe20 3 are used for the medium, the recording area increases in thickness for the C-signal component. This suggests that a metal particulate medium has too high a coercive force to sufficiently record the C-signal.
120 100 80
o~ 60
\
o\
~,=~~o0o.
~
. . . . o. - - -
387
",,', \
~ 40
4.3. Double-layered medium
20
0.1
0.2
0.3
0.4
0.5
0.6
0,7
Depth (tan) Fig. 9. Characteristics of Y-signal component of the magnetization distribution versus depth from the medium surface for various coercive force of single-layer media.
phenomena due to nonlinearity are reproduced faithfully by our magnetic recording simulator.
4.2. Recording depth To estimate the recorded depth of the Y- and the C-signals, we calculated the intensity of the longitudinal component of the remanent magnetization at each depth in the recorded medium. The Y- and the C-signal components of the magnetizations are obtained by the Fourier analysis for the distribution of the longitudinal magnetization component [8]. Fig. 9 shows the characteristics of the Y-signal magnetization component versus the depth which corresponds to the distance from the top surface of the medium. Here the coercive forces of media were changed from 500 to 1100 Oe. It is clear from the figure that the Y-signal component is recorded within a layer of 0.2 wm thickness from the surface. When particles having higher coercivity are used for the medium, the Y-signal magnetization component becomes larger but the recording depth scarcely changes. On the other hand, Fig. 10 shows the depth dependence of the C-signal component in the longitudinal magnetization distributions. The Csignal component is mainly recorded in the middle layer of the medium. In case the medium has higher coercivity of 1100 Oe such as a metal
From the above results, the bottom layer at depth of greater than about 0.25 ~m should be composed of lower coercive force particles such as Co-'y-Fe20 3 to attain good recording performance of the C-signal. And a higher coercive force of the top layer whose thickness is about 0.25 ~m is very effective to increase the Y-signal output. However, if the coercive force of a medium becomes too high, the writing will become hard even using a ring head with high Bs. Therefore we have assumed a double-layered medium whose top layer was made of perpendicularly oriented Ba-ferrite particles. Fig. 11 shows the magnetization vector distribution in a double-layered medium which has a perpendicularly oriented top layer of Ba-ferrite with thickness of 0.28 I~m and a bottom layer of Co-~/-Fe20 3 with 0.32 p.m thickness. Here Ms, H k, H c, Crhk/Hk and % for the Ba-ferrite top
120 H,=I
I00
....
~ 80 ~
1O00,
9OO o .... 700 5o0
6o
~4o 20 0
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Depth(pro) Fig. 10. Characteristics of C-signal component of the longitudinal magnetization distributions versus depth from the medium surface.
L Tagawa, Y. Nakamura /Journal of Magnetism and Magnetic Materials 134 (1994) 382-389
388
II)////,
,c--..
....
_-
....
/
I
\ ....
Fig. 11. Magnetization vector distribution in a double layered medium having a Ba-ferrite top layer and a Co-y-Fe203 bottom layer.
layer are 160 emu/cc, 4500 Oe, 1700 Oe, 0.09 and 35 °, respectively, and Ms, Hk, He, O ' h k / H k and tr0 for the Co-y-Fe20 3 bottom layer are 145 emu/cc, 2500 Oe, 900 Oe, 0.32 and 20°, respectively. The Y-signal of shorter wavelength is recorded in the Ba-ferrite top layer by the perpendicular magnetization mode. On the other hand, the C-signal is recorded mainly in the Co~/-Fe20 3 bottom layer by the longitudinal magnetization mode. Therefore a kind of semicircular magnetization mode is formed in the doublecoated medium. Consequently the Y-signal component of the reproduced output became about 5 dB larger than the metal particulate single-layer medium having higher coercivity than the Ba-ferrite top layer. Moreover the C-signal output for the double-layered medium is almost as same as that for the metal particulate medium [8]. 5. Conclusions
We have developed a magnetic recording simulator whose algorithm is based on both the finite element method for magnetic field analysis and the curling magnetization switching model for vectorial magnetization mechanisms of a medium. Consequently the results obtained by the simulator quantitatively agree well with experimental
ones, not only in reproduced waveforms but also in recording characteristics for various headmedium combinations. As a result of analyzing the recording mechanisms for an obliquely oriented medium like a ME tape combined with a ring head, we have confirmed that the demagnetizing field becomes weaker even at a high recording density because an anti-parallel oblique magnetization mode is formed. Furthermore the oblique component distribution of the ring head field is sharper than those of both longitudinal and perpendicular components. Oblique magnetization has a perpendicular magnetization component so that better recording performance is obtained at a higher bit density even for ring head recording. From a computer analysis of the recording of a superimposed video signal, we found that the recording depth of the Y-signal having a short wavelength was less than 0.3 ~m but the C-signal having a long wavelength was recorded in the middle layer of the medium. Moreover it was confirmed that a double-layered medium, which consisted of a perpendicular oriented Ba-ferrite top layer and a low coercivity Co-y-Fe20 3 bottom layer, is very effective for advanced video recording systems like VCRs for HDTV which requires a much wider recording density band.
I. Tagawa, Y. Nakamura /Journal of Magnetism and Magnetic Materials 134 (1994) 382-389
Acknowledgements The authors would like to thank to Dr. Yasuo Ando and Mr. Yukiya Shimizu for their contribution in simulation analyses.
References [1] Y. Nakamura and S. Iwasaki, IEEE Trans. Magn. 23, No. 1 (1987). [2] Y. Nakamura and I. Tagawa, IEEE Trans. Magn. 25, No. 5 (1989).
389
[3] K. Itoh, I. Tagawa and Y. Nakamura, J. Magn. Soc. Jpn. 15 Suppl. No. $2 (1991). [4] M. Shinohara, H. Wakamatsu, I. Kaitsu, I. Tagawa and Y. Nakamura, J. Magn. Soc. Jpn. 15 Suppl. No. $2 (1991). [5] I. Tagawa and Y. Nakamura, IEEE Trans. Magn. 29 (1993). [6] I. Tagawa, Y. Shimizu and Y. Nakamura, J. Magn. Soc. Jpn. 15 Suppl. No. $2 (1991). [7] H. Naruse, Y. Tateno, T. Sato, K. Sato, K. Chiba and T. Sasaki, Inst. Eiec. Info. Commun. Engineers Tech. Report, MR90-7 (1990), in Japanese. [8] Y. Ando, I. Tagawa and Y. Nakamura, J. Magn. Soc. Jpn. 15 Suppl. No. $2 (1991).
ELSEVIER
Journal of Magnetism and Magnetic Materials 134 (1994) 382-389
journalof magnetism magnetic 4~iPematerials
Magnetic recording simulation and its applications Ikuya Tagawa
*, Y o s h i h i s a
Nakamura
Research Institute of Electrical Communication, Tohoku University, Sendai 980, Japan
Abstract We have developed a magnetic recording simulator whose algorithm is based on a finite element method for magnetic field analysis and a magnetization curling model for crystalline particles in a medium. Using this simulator, we have analyzed the recording performance for an obliquely evaporated medium and the recording mechanisms of double-coated particulate medium for a superimposed video signal. When an oblique medium like an ME tape is combined with a ring head, a better recording performance is obtained because the oblique component distribution of the ring head field is sharper than those of both longitudinal and perpendicular components. In the superimposed signal recording of a double-coated particulate medium, we found that the recording depth is less than 0.3 ixm for the" short wavelength Y-signal but the long wavelength C-signal is recorded in the middle layer of the medium. Moreover it was confirmed that a double-coated medium having a perpendicularly oriented surface layer is very effective for an advanced video recording system.
1. Introduction Modelling of the m e d i u m magnetization process is an important means for quantitative analysis of the magnetic recording process. Physically accurate simulation using a computer is very useful for design of an optimum recording system. We have introduced a novel model of the magnetization mechanism for magnetic crystalline particles, based on the incoherent spin curling model, and applied it to the magnetization process of actual media [1]. Using this model, major and minor simulated hysteresis curves agree well with those measured in any magnetizing direction. To calculate the magnetic flux density distribution around a head and a medium and the internal vectorial magnetization distribution of a medium, a finite element method was introduced
* Corresponding author.
and incorporated with the medium magnetization model [2]. We have confirmed that this magnetic recording simulator is widely applicable not only for longitudinal magnetic recording but also for perpendicular recording systems [3,4]. In this paper, the magnetic recording simulator will be briefly reviewed, and the results of analyzing the recording and reproducing processes will be presented both on a ring head recording system using an obliquely evaporated medium and on a video signal storage system using a double-coated particulate medium.
2. Magnetic recording simulator 2.1. M e d i u m magnetization model
For calculation of magnetization inside a medium, an assembly of non-interacting particles is assumed and the magnetization switching in
0304-8853/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0304-8853(94)00235-J
L Tagawa, Y. Nakamura/Journal of Magnetism and Magnetic Materials 134 (1994) 382-389
each particle is approximated by the curling model. When the external field is applied in the easy axis direction of a medium, the magnetization of the medium is calculated by [1]
M=fffMsc o s [ 0 ( o ,
H/Hk)] ,
Xp(O, ~b)q( Hk) dO dgbdHk.
,
tt
I
I y
,/',¢-
YX
/.
(1)
Here Ms, H k and (0, $) are the saturation magnetization, the anisotropy field strength and the easy axis direction in spherical coordinates, respectively, p(O, ok) and q(H k) are the standardized distributions of the easy axis direction and the anisotropy field strength, respectively. ~(0, h) is the magnetization angle to the medium easy axis direction at the reduced field of h in a particle with the easy axis of (0, $), calculated by the curling magnetization reversal theory.
2.2. Recording simulation model A finite element method is used to calculate a magnetic field distribution because the saturation effects of a recording head and a soft magnetic underlayer of a double-layer medium can be easily considered. Fig. 1 shows a flow diagram for simulations of the magnetic recording process. To
t, [ Mesh Generation [ [ Set Boundary Condition [ t, I Set Current Element for Head Coil [ [ Calculate Permeability in each Element I Calculate Magnetization Element in Medium I by Curling Particle Assembly Model I 4, [ Modify Magnetization by Iteration Method [ ~t J Calculate Vector Potential in each Node J I Calculate Magnetic Flux in each Element J
Fig. 1. Flow diagram of the magnetic recording simulation using the finite element method and the magnetization curling model.
- - - -
ECe_aured
~=3~o em,dc,:
, _ '
383
'
'
,
"."'0 "
Ck
l"
,0
Co-Ni obliquely evaporated medium
Fig. 2. M - H loops calculated by the medium magnetization model and measured by VSM.
calculate M - H curves in the direction perpendicular to the medium, a demagnetizing field H d of - M y is assumed, and an iteration method is used for the self-consistent calculation. The magnetic interaction between particles is disregarded here but it is possible to consider the interparticle interaction by introducing the mean interaction field theory [5].
3. Obliquely oriented medium
3.1. Hysteresis loops A Co-Ni obliquely evaporated medium (Sony Hi8ME) was assumed in the simulation, and its saturation magnetization, the anisotropy field and the coercive force were set at 370 emu/cc, 4000 Oe and 1030 Oe, respectively. In Fig. 2, M - H loops calculated by the medium magnetization model are compared with measured ones in various magnetizing directions, where the easy axis direction of a medium is supposed to be inclined 30° to the longitudinal direction. A good agreement is obtained between measured and calculated loops. Here O ' h k / H k ---- 0.22 and tr0 = 12 deg. We have also confirmed that the applied field angular dependence of the coercive force computed by the medium model coincided quantitatively with the measured one [6]. Recording simulations were performed for media having various easy axis directions, namely 0 (longitudinal), 30 (the regular direction of a ME tape), 60, 90 (perpendicular) and 150 (the opposite direction to a regular ME tape) degrees,
384
I. Tagawa, Y. Nakamura /Journal of Magnetism and Magnetic Materials 134 (1994) 382-389
respectively. The medium thickness, the head gap length and the h e a d - m e d i u m spacing were assumed as 0.2, 0.2 and 0.04 txm, respectively. The recording current was set at 0.35 A T so that the maximum voltage was obtained in the simulation of an isolated transition recording. We have obtained a good agreement between the simulated and experimental results not only on the reproduced pulse but also on the recording density characteristics for the medium oriented to the 30 ° direction.
3.2. Magnetization distributions Fig. 3 shows the magnetic flux contours around the head gap and the magnetization distributions in the medium in the reproducing process, when the ring head has just come to the same position at which the recording field was switched in the recording process of an isolated transition. W h e n the easy axis direction is at 30 and 60 °, the
Isolated Transition
Easy • Axis
~
m I
Odeg
~
)
( Reproduction
t
.
'i ::~
.
.
~
_..N~O.04 .
t'L
30 deg
60 dog
90 deg
'!i!i}i!iiiiiiii!i! ....
150 deg Fig. 3. Magnetization distributions and magnetic flux contours for the isolated magnetization transition in the reproducing process.
30 °
-
1
~
I/I:\'~ 0
......
TrailingSide ~" ~] "il"I_ 4000~.
.
.
.
.
9o °
,.o, LoadingSide
Fig. 4. Easy axis field distributions for a ring head at a surface layer of media having various easy axis directions.
recorded magnetization direction coincides with the field direction on the leading side in the recording process. However when it is at 90 and 150 °, the magnetization direction coincides with the trailing field direction. From both the simulated angular dependence of the switching field strength and the experimental angular dependence of the remanent coercivity [7], it is clear that the switching field strength of a medium in the easy axis direction is much smaller than those in any other directions. Therefore we can consider that the magnetization transition is mainly formed by the easy axis component of the recording head field (the easy axis field). Fig. 4 shows the easy axis field distributions of a ring head at a surface layer of media having various easy axis directions. The magnetization transition is formed by the field gradient on the trailing side. The steepest gradient of the easy axis field is obtained for the medium whose easy axis is at 30 °, and the gradient becomes gentle in the order of 60, 0, 150 and 90 °. Therefore the narrowest magnetization transition is obtained in the medium with the easy axis at 30 ° . Fig. 5 shows the magnetization distributions in the medium recorded at about 95 kFRPI. The media having the easy axes at 30 and 60 ° are magnetized through the whole thickness. When the easy axis is at 0 and 150 °, the intensity of the magnetization in the layer close to the bottom surface is weaker than that in the layer close to the top surface because the longitudinal demagnetizing field owing to the longitudinal magneti-
I. Tagawa, Y. Nakamura /Journal of Magnetism and Magnetic Materials 134 (1994) 382-389
zations facing each other becomes strong in the bottom layer.
385
95.25 kFRPI
Easy Axis
~
~
0.2 p m ~
0.0~m
3.3. Reproduced output 0.2 pm
Fig. 6 shows the reproduced voltages obtained by simulations at about 100 kFRPI for media with varying easy axis directions from 0 to 70 °. Since the reproduced output at a high recording density depends on the sharpness of the magnetization transition, the largest output is obtained for the medium having the easy axis at 30 °. Moreover, even in the case of a low recording density, the medium with easy axis at 30 ° gives larger output than media having the other easy axis directions [61.
•
/
90 deg
4. Double-coated medium
4.1. Superimposed recording signal In consumer video storage systems, a chrominance signal (C-signal) in a low frequency band up to 1 MHz is superimposed on a luminance signal (Y-signal) in a high frequency band up to 8 MHz. Therefore media for video systems require excellent recording characteristics in a wide frequency region. We have analyzed the recording performances for a single-coated medium with longitudinally oriented Co-~/-Fe20 3 particles. The thickness, Ms, Hk, H~, Orhk/H k and tr0 of the recording layer were set at 0.6 I~m, 145 e m u / c c , 2500 Oe, 900 Oe, 0.32 and 20 ° in the simulation respectively, and a M I G ring head with gap length of 0.25 Ixm was assumed. The recording wavelengths, of the Y- and the C-signals were set at 0.92 and 9.2 Ixm, respectively. Fig. 7 shows the magnetic flux contour and the magnetization distribution in the single-coated medium, obtained by the simulation of the superimposed video signal recording. The Y-signal whose wavelength is shorter is recorded in the surface layer of the medium. But the C-signal which has the longer wavelength is mainly recorded in the middle and in the bottom layers. In Fig. 8, the I c / I v ratio dependencies of the Y-, the C- and the Y - 2C spurious signals are
r
-1
b
= 0.27
pm
Fig. 5. Magnetization distributions in the medium recorded at about 95 kFRPI.
compared, which were obtained by a Fourier analysis of the measured and simulated reproduced output. Here I c and I v are the recording magnetic motive forces applied to the head for
14 12 ,-, 10, "-"
8
o
4
0
I
0
,
I
,
I
,
I
,
I
,
I
,
10 20 30 40 50 60 70 Easy axis (deg)
Fig. 6. Reproduced output characteristics versus easy axis of the medium at about 100 kFRPI, obtained by simulations.
L Tagawa, Y. Nakamura /Journal of Magnetism and Magnetic Materials 134 (1994) 382-389
386
\\// DIV. = 0.20E+ 02 (Gauss/Lrn)
~x
2 r = 0.92/tm ~c=9.20twn 4
..................
):. :: ~ 7 . ' . ' : - . . . ; ; ;
Hc =900Oe
..... ~
....................
Fig. 7. Magnetization distribution and magnetic flux contours in a single-layer medium, obtained by a simulation for a superimposed recording signal.
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ie
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Ic/ Ir Fig. 8. I c / I y versus Y-, C- and Y - 2C spurious signal output characteristics obtained by Fourier analysis of the reproduced output for a single-layer medium.
the C- and the Y-signals, respectively. The Y - 2C signal is the component of the reproduced output at a frequency which is equal to a Y-signal frequency minus twice the frequency of the C-signal. The output of the Y - 2C signal becomes larger as the waveform distortion increases. The open and the solid circles indicate simulated and experimental results respectively, and we have obtained a good agreement between them as shown in the figure. When the I c / I v ratio increases, the relative output E c of the C-signal increases but the Ysignal output E v decreases. The output of the Y-signal becomes smaller than that of the C-signal in the region of 0.6 < I c / I v . On the other hand, the Y - 2C spurious signal output becomes larger in proportion to Icz. The Y - 2C signal is caused by the waveform distortion and so the I c / I Y characteristics depend strongly on the nonlinearity in the magnetic recording system. These
L Tagawa, Y. Nakamura/Journal of Magnetism and Magnetic Materials 134 (1994) 382-389
particulate medium, the C-signal component is recorded only at a depth of around 0.15 I~m. When low coercivity particles like Co-~/-Fe20 3 are used for the medium, the recording area increases in thickness for the C-signal component. This suggests that a metal particulate medium has too high a coercive force to sufficiently record the C-signal.
120 100 80
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Depth (tan) Fig. 9. Characteristics of Y-signal component of the magnetization distribution versus depth from the medium surface for various coercive force of single-layer media.
phenomena due to nonlinearity are reproduced faithfully by our magnetic recording simulator.
4.2. Recording depth To estimate the recorded depth of the Y- and the C-signals, we calculated the intensity of the longitudinal component of the remanent magnetization at each depth in the recorded medium. The Y- and the C-signal components of the magnetizations are obtained by the Fourier analysis for the distribution of the longitudinal magnetization component [8]. Fig. 9 shows the characteristics of the Y-signal magnetization component versus the depth which corresponds to the distance from the top surface of the medium. Here the coercive forces of media were changed from 500 to 1100 Oe. It is clear from the figure that the Y-signal component is recorded within a layer of 0.2 wm thickness from the surface. When particles having higher coercivity are used for the medium, the Y-signal magnetization component becomes larger but the recording depth scarcely changes. On the other hand, Fig. 10 shows the depth dependence of the C-signal component in the longitudinal magnetization distributions. The Csignal component is mainly recorded in the middle layer of the medium. In case the medium has higher coercivity of 1100 Oe such as a metal
From the above results, the bottom layer at depth of greater than about 0.25 ~m should be composed of lower coercive force particles such as Co-'y-Fe20 3 to attain good recording performance of the C-signal. And a higher coercive force of the top layer whose thickness is about 0.25 ~m is very effective to increase the Y-signal output. However, if the coercive force of a medium becomes too high, the writing will become hard even using a ring head with high Bs. Therefore we have assumed a double-layered medium whose top layer was made of perpendicularly oriented Ba-ferrite particles. Fig. 11 shows the magnetization vector distribution in a double-layered medium which has a perpendicularly oriented top layer of Ba-ferrite with thickness of 0.28 I~m and a bottom layer of Co-~/-Fe20 3 with 0.32 p.m thickness. Here Ms, H k, H c, Crhk/Hk and % for the Ba-ferrite top
120 H,=I
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....
~ 80 ~
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0
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0.2
0.3
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0.5
0.6
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Depth(pro) Fig. 10. Characteristics of C-signal component of the longitudinal magnetization distributions versus depth from the medium surface.
L Tagawa, Y. Nakamura /Journal of Magnetism and Magnetic Materials 134 (1994) 382-389
388
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....
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Fig. 11. Magnetization vector distribution in a double layered medium having a Ba-ferrite top layer and a Co-y-Fe203 bottom layer.
layer are 160 emu/cc, 4500 Oe, 1700 Oe, 0.09 and 35 °, respectively, and Ms, Hk, He, O ' h k / H k and tr0 for the Co-y-Fe20 3 bottom layer are 145 emu/cc, 2500 Oe, 900 Oe, 0.32 and 20°, respectively. The Y-signal of shorter wavelength is recorded in the Ba-ferrite top layer by the perpendicular magnetization mode. On the other hand, the C-signal is recorded mainly in the Co~/-Fe20 3 bottom layer by the longitudinal magnetization mode. Therefore a kind of semicircular magnetization mode is formed in the doublecoated medium. Consequently the Y-signal component of the reproduced output became about 5 dB larger than the metal particulate single-layer medium having higher coercivity than the Ba-ferrite top layer. Moreover the C-signal output for the double-layered medium is almost as same as that for the metal particulate medium [8]. 5. Conclusions
We have developed a magnetic recording simulator whose algorithm is based on both the finite element method for magnetic field analysis and the curling magnetization switching model for vectorial magnetization mechanisms of a medium. Consequently the results obtained by the simulator quantitatively agree well with experimental
ones, not only in reproduced waveforms but also in recording characteristics for various headmedium combinations. As a result of analyzing the recording mechanisms for an obliquely oriented medium like a ME tape combined with a ring head, we have confirmed that the demagnetizing field becomes weaker even at a high recording density because an anti-parallel oblique magnetization mode is formed. Furthermore the oblique component distribution of the ring head field is sharper than those of both longitudinal and perpendicular components. Oblique magnetization has a perpendicular magnetization component so that better recording performance is obtained at a higher bit density even for ring head recording. From a computer analysis of the recording of a superimposed video signal, we found that the recording depth of the Y-signal having a short wavelength was less than 0.3 ~m but the C-signal having a long wavelength was recorded in the middle layer of the medium. Moreover it was confirmed that a double-layered medium, which consisted of a perpendicular oriented Ba-ferrite top layer and a low coercivity Co-y-Fe20 3 bottom layer, is very effective for advanced video recording systems like VCRs for HDTV which requires a much wider recording density band.
I. Tagawa, Y. Nakamura /Journal of Magnetism and Magnetic Materials 134 (1994) 382-389
Acknowledgements The authors would like to thank to Dr. Yasuo Ando and Mr. Yukiya Shimizu for their contribution in simulation analyses.
References [1] Y. Nakamura and S. Iwasaki, IEEE Trans. Magn. 23, No. 1 (1987). [2] Y. Nakamura and I. Tagawa, IEEE Trans. Magn. 25, No. 5 (1989).
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[3] K. Itoh, I. Tagawa and Y. Nakamura, J. Magn. Soc. Jpn. 15 Suppl. No. $2 (1991). [4] M. Shinohara, H. Wakamatsu, I. Kaitsu, I. Tagawa and Y. Nakamura, J. Magn. Soc. Jpn. 15 Suppl. No. $2 (1991). [5] I. Tagawa and Y. Nakamura, IEEE Trans. Magn. 29 (1993). [6] I. Tagawa, Y. Shimizu and Y. Nakamura, J. Magn. Soc. Jpn. 15 Suppl. No. $2 (1991). [7] H. Naruse, Y. Tateno, T. Sato, K. Sato, K. Chiba and T. Sasaki, Inst. Eiec. Info. Commun. Engineers Tech. Report, MR90-7 (1990), in Japanese. [8] Y. Ando, I. Tagawa and Y. Nakamura, J. Magn. Soc. Jpn. 15 Suppl. No. $2 (1991).