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A cross-type amorphous video head
~H ELSEVIER
Journalof magnetism and magnetic materials
Journal of Magnetism and Magnetic Materials 159 (1996) 256-268
A cross-type amorphous video head Shigekazu Otomo a,*, Takeo Yamashita b, Noriyuki Kumasaka b a Central Research Laborator3', Hitachi Ltd., Kokubunji, Tokyo 185, Japan b Video and Personal Media System Dil,ision. Hitachi Ltd., 1410-1nada, Hitachinaka-shi, Ibaraki-ken 312, Japan
Received 13 April 1995; revised 17 October 1995
Abstract A cross-type video head composed of cross-shaped amorphous sputtered films and ferrite that can be used with high coercive tapes is described. A three-dimensional magnetic field calculation shows that the cross-type head has a strong recording field and high reproduction efficiency compared to a conventional metal-in-gap head (MIG) or a plate-type head. No large bumps in the output spectrum are observed during recording and reproduction under normal conditions with the cross-type head. The amorphous sputtered film deposited on the inclined plane of the ferrite substrate used for the cross-type head shows a homogeneous structure and high permeability similar to that of a film deposited on a horizontal plane. A cross-type head using a C o - N b - Z r amorphous film annealed in a magnetic field under optimum conditions shows excellent recording and reproduction characteristics when used with a high coercivity metal tape. Keywords: Amorphous sputtered film; Cross-type head; High coercive tape; MIG head; Permeability; Pseudo-gap effect ; Recording field; Reproduction efficiency; Three-dimensional field calculation; Video head
1. Introduction Magnetic heads that can effectively record on magnetic media that have a high coercive force, H~., are needed to develop high density magnetic recording systems. Various types of video heads that use magnetic films with high saturation magnetic flux density, Bs, have been developed for use with high H c media. Among them, the metal-in-gap (MIG) head [1-5], in which a high B~ film is deposited on the gap surfaces of a ferrite core, is easier to produce than the plate-type head [6-8] because of its simple manufacturing process. However, bumps in the output spectrum have been observed when recording * Corresponding author. Fax: + 81-285-84-1550.
and reproducing with the M I G head because the interface between the magnetic film and the ferrite core acts as a pseudo-gap. Although several efforts to solve this problem have been reported [2-5], it is difficult to completely eliminate the pseudo-gap effect. In this paper, we investigate the properties of various types of video heads by using a three-dimensional field calculation. We find that a cross-type head [9], in which a high B s film is deposited on a triangular projection of a ferrite core, has a strong recording field and high reproduction efficiency while showing no pseudo-gap effect. We fabricated a cross-type head using a Co-based amorphous sputtered film and examined the recording and reproduction characteristics.
0304-8853/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. SSD10304-8853(95)00942-6
S. Otomo et al /Journal of Magnetism and Ma~lletic Mat,:,riaLs lSv ¢19~)~51 2.% 20<~'
2. Calculations and experimental procedures Fig. 1 shows various structures of video heads that use magnetic films. A plate-type head has a magnetic core composed only of a magnetic film, and the thickness of the core is equal to the track width. A metal-in-gap (MIG) head is composed of a ferrite core and a magnetic film is deposited on the gap surfaces of the ferrite core; the interface between the ferrite and the magnetic film is parallel to the gap. On the other hand, a cross-type head has a structure in which the magnetic film is deposited on a triangular projection of ferrite; thus, the interface between the ferrite and the film is inclined relative to the gap. The thicknesses of the magnetic cores are larger than the track widths in both the cross-type and the MIG head. The cross-type head can be easily produced because its fabrication process is similar to that of the MIG head [9]. The cross-type head has a complicated structure composed of multiple magnetic materials that lie across the track, as shown in Fig. 1. Therefore, three-dimensional field calculations using the finite
~
tw!.iilmil~~ magnetic film
gap
!!nmagnet,c ctor '
nonmagnetic substrate
(a) Plate-type head
257
element method are required to forecast the properties of the head accurately; however, these calculations generally take a very long time due to the large number of elements. In this work, we use lhe T---(_~ method based on the current vector potential (T) and the magnetic scalar potential (.(2). This method reduces the calculation time since there is only a small number of unknown variables [10]. As the magnetic film, we used a Co~4Nbl~Zr3 amorphous film that had a saturation magnetic flux density B~ of 0.9 T, a crystallization temperature, T~, of 520°C, and a magnetostriction, A~, of - 0 . 3 × 10 -6 [11], and we used a M n - Z n ferrite single crystal as the ferrite block. A Pb-based glass was used at a working temperature of 470°C to fill the V-shaped grooves and bond the blocks. Cross-type heads with a track width of 25 to 30 # m were obtained from blocks in which the amorphous film on the inclined plane of the ferrite core was 15 to 20 ,am thick. The effective gap length, ge, was about 0.3 /xm. We also prepared a MIG head using the same C o - N b - Z r amorphous film for comparison. The film in the MIG head was 6 /xm thick. The relative permeability, tx, of the amorphous films was measured with a vector impedance meter and a network analyzer. The morphology of the films was observed with a scanning electron microscope (SEM). The recording and reproduction characteristics were measured with a C o - y - F e 2 0 3 tape having a coercive force, H~, of 55 k A / m and a metal particle tape having an H~ of 116 k A / m .
3. Results and discussion .-l-3 2222
tin).!.
magnetic film d (=
glass ferrite
(b) MIG head glass
~iiC77777
magnetic film
.Z
j~: 0 ~'m d .i i. (c) Cross-type head
ferrite
Fig. 1. Structures of video heads using magnetic films.
3.1. Head properties determined by three-dimensional field calculations Fig. 2 shows the recording field, H x, obtained by three-dimensional field calculation for various types of heads. The recording field, H X, in the figure denotes the field in the direction of the tape motion at the center of the track and the gap. The spacing, Sp, between the calculation point and the head surface is 0.15 /xm. Parameters for the calculation are as follows: the gap length, g, is 0.3 /xm; the gap depth, g~, is 20 /xm; the track width, t w, is 28 /xm; the core thickness, t c, is five times tw; and the
S. O m m o et al. / , h m r m E qf ~hlgm, ti,~m and Magnetic Materials 159 (1996) 256 268
258 480 400
100 Magnetic film Bs =1 T, ,u=1000
I
I
I
Cross-typehead
Ferrite ,u=500
, Cross-type head /
Ferrite Bs =0.5T. F=500 60
320
o~ 4O
240 160
/j~
// 80
20
g =0.3~,m
~0'
I
0'.2
0.4
0
0'.8
' 500
~ 1500
I 0'00
2000
,u
I
0.6
go= 20 ,u m
0
gd = 20 )Lm Sp=0.15,um
V 0, 0.0
g =0.3~m
"" MIG head
.0
Magnetemotive force (AT) Fig. 2. Recording field, H~, obtained by a three-dimensional field
calculation.
distance between the gap and the ferrite, d, is 5 /xm for the MIG and the cross-type head. The film thickness, tm, which depends on tw, d and 0, the angle of the top of the ferrite (60°), is 15 /xm for the cross-type head. On the other hand, t m equals tw for the plate-type head and d for the MIG head, as shown in Fig. 1. The saturation magnetic flux density, B~, and the relative permeability, /x, are 1.0 T and 1000, respectively, for the magnetic film, and 0.5 T and 500 for the ferrite. The recording field, H×, of the cross-type head increases more rapidly with increasing magnetomotive force than that of the plate-type head, and then saturates to the same value as that of the plate-type head. We attribute the quick increase in the H× of the cross-type head to the low reluctance of the head core due to its large core thickness. On the other hand, the saturation field of the MIG head is much lower than those of the cross-type or the plate-type heads. This is because the ferrite adjacent to the high B~ film saturates easily at a high motive force. Fig. 3 shows the changes in the reproduction efficiency, r/r, due to the relative permeability, /~, of the magnetic film for each head. The reproduction efficiency, r/r, was obtained by calculating the ratio of the flux passing through the coil area to the total flux passing through the head, when a small magnet is set in front of the gap. The relative permeability, /x, of ferrite is set to 500. The reproduction effi-
Fig. 3. Changes in the reproduction efficiency,
T/r, with the
relative permeability, >, of the magnetic film.
ciency, r/i., increases as the /x of the film in each head increases; higher r/r is obtained in the low-p~ region for the cross-type head compared to the plate-type head because of the large core thickness of the cross-type head. The change in the r/r of the MIG head as /x of the film increases is small because the film in the MIG head is thin. These results show that the cross-type head has great potential for use with high H c tapes due to its strong recording field and high reproduction efficiency. The recording and reproduction characteristics of the cross-type head using an amorphous film are examined in the following sections. 3.2. Pseudo-gap effect in the MIG and the cross-o,pe heads" 3.2.1. MIG head Fig. 4 shows the output spectra when a MIG head is used for recording and reproduction with a metal recording current
150mA /
s
'1 '
200mA /
~ ~'~//
V
O -60 0
I
I
I
2
4
6
Frequency
I
8 (MHz)
I
I
10
12
Fig. 4. Output spectra when a MIG head is used for recording and reproduction with a metal tape (tape speed = 5.8 m / s ) .
S. Otomo et a l . / Journal of Magnetism and Magm, lic MateriaLs 159 (1996) 256-26S
(a) Recording current: 40 mA O-
/
A REC/REP:MIG head Br
-20 ~ "
/
~
-
~ k
EC:M,G.ead P: gerritehead
~
1313
-40
C, [ REC : Ferritehead v kREP:MiGhead
O
~ . \.
V -60 i
I
2
4
i
i
i
i
6 8 10 Frequency (MHz)
12
(b) Recording current: 200 mA 0 P: MIG head : MIG head : Ferritehead
-20
259
current of 200 mA, which is about five times the optimum current. Large bumps are observed when recording with the MIG head and reproducing with the ferrite head (curve B), as in the case of recording and reproducing with the MIG head (curve A). On the other hand, small bumps similar to those of curve C in Fig. 5a are observed when recording with the ferrite head and reproducing with the MIG head. Therefore, the large bumps at the high recording current are caused by recording with the MIG head. These bumps in the output spectrum of the MIG head are thought to be caused by interference between the signals reproduced by the gap and those from a pseudo-gap formed at the interface between the film and the ferrite [2]. Taking the pseudo-gap into account, the gap-loss function, Lg, is expressed
1313
as
45 O_ -40 ~5 O
Lg = s i n ( ~ - g / a ) / ( r r g / a )
+ 2g'/( g + 2g')sin(rrg'/a) / ( ~r g ' / A ) c o s ( 2 ~ - t ' / a ) ,
-60 i
0
2
4
6 8 10 Frequency (MHz)
12
Fig. 5. Output spectra when a MIG head and a ferrite head are used for recording and reproduction with a metal tape at a recording current of (a) 40 mA and (b) 200 mA (tape speed - 5.8 m/s).
tape. Bumps are observed in all output spectra, which were obtained at various recording currents. The more the recording current increases, the greater the amplitude of the bumps. The origin of the bumps was analyzed by separating the recording and the reproduction procedures. Fig. 5a shows the output spectra at a recording current of 40 mA, which is close to the optimum recording current. When a ferrite head is used for recording and the MIG head is used for reproduction (curve C), bumps are as common and as large as those observed when recording and reproducing with the MIG head (curve A). On the other hand, the spectrum is very smooth when recording with the MIG head and reproducing with the ferrite head (curve B). These results imply that the bumps occur during reproduction with the MIG head. Fig. 5b shows the output spectrum at a recording
(1)
where t' = I m Jr- ( g Jr- g ' ) / 2 , g' is the pseudo-gap length, and 3, is the wavelength. When ,~ equals g or g', the output reaches a minimum according to Eq. (1). However, these A are smaller than the region of wavelength examined in this experiment. The bumps observed in this work are caused by the term cos(2rrt'/A) which is the function of the phase difference between the signals reproduced by the gap and the pseudo-gap. Therefore, the maximum and minimum output will occur at wavelengths
a .... = t ' / n ,
ami n = t ' / ( n - 1 / 2 ) ,
n = l , 2 .....
(2) In the recording process, the phase difference between the signals recorded on the tape by the gap and the pseudo-gap is also expressed as cos(2~-t'/A). Therefore, the maximum and minimum output will also occur at the wavelengths of Eq. (2). Fig. 6 shows the relations between the number of bumps n and the reciprocal of the wavelengths '~max and ~min" According to Eq. (2), t' is obtained from the slope of the lines in the figure. Almost identical values of t' are obtained when recording and when reproducing with the MIG head. Also, the value of t' obtained from the bumps coincides well with that observed with an optical microscope. Therefore, the
260
S. Ommo et at./.Io ,'hal q/ M~ ~1 erich and Magnetic Materials 159 (1996) 256-268 2.0 REP: MIG head ]
t,=641 °
1.5
maximum output
E v 1.0
j/~
0.5
~.,'"""
~"~- minimumoutput 1/2-- (n- 1/2)/t'
~ "
I
~/ 2 2.0
~""•"•
=n /t'
1/2
,':&37
4
6 n
8
10
12
f REC: MIG head / (recording current = 25O mA)
1.5
maximum
~'E
~
"~'"
1/,1 = n / t'°utput. . / J ' " / ' " " t'= 6.1 tz m~,'"'"
0.5
imp2 i ~ 2 ~ t t ,
~ 0
occur when the cross-type head is used for reproduction, as shown in Fig. 8a. This is because the output from the pseudo-gap, if one exists at the interface between the ferrite and the film in the cross-type head, is reduced by azimuth loss due to the large inclination (60 °) of that interface to the gap. On the other hand, bumps occur when the cross-type head is used for recording at high recording currents, as shown in Fig. 8b. Two types of bumps are observed when recording with the cross-type head, as shown in Fig. 7b and Fig. 8b: one type occurs at low frequencies (long wavelengths) and has a large amplitude; the other occurs at high frequencies (short wavelengths) and has a rather small amplitude. Fig. 9 shows the relation between the number of bumps n and the reciprocal of the wavelength A,m. at which the output reaches a minimum in Fig. 7b and Fig. 8b. Two kinds of lines with different inclinations are obtained
,""
.
-
I 2
' I 4
~
= 6.0/z m I 6
I 8
I 10
t
12
n Fig. 6. Relations between the number of bumps, n, and the reciprocal of the wavelengths Am.x and hmi..
~"
o bumps observed when recording with the MIG head are also caused by the pseudo-gap that exists at the interface between the ferrite and the magnetic film.
a) Metaltape
-20
250 mA
-40
-60 0
i
i
i
i
I
i
2
4
6
8
10
12
t 10
12
Frequency (MHz)
3.2.2. C r o s s - O ' p e h e a d
Fig. 7 shows the output spectra when the crosstype head is used for recording and reproduction with the metal tape and C o - y - F e 2 0 3 tape. Smooth output spectra with only small bumps are observed when the high H~ metal tape is used, as shown in Fig. 7a. On the other hand, as shown in Fig. 7b, complicated bumps in the output spectra are observed when a C o - 7 - F e 2 0 3 tape with low H c is used at recording currents of 75 mA or above; however, virtually no bumps occur at the recording current of 20 mA, which is close to the optimum current. Fig. 8 shows the output spectra when the C o - 7 Fe20 3 tape is recorded with the cross-type head and reproduced with the ferrite head. No large bumps
recording current 40 mA / 75 mA
(b) Co- Y Fe203tape
0
recording current |[ ~ \
~o.
~,
~
20 mA
-40
o -60 0
2
4
6
8
!
Frequency (MHz) Fig. 7. Output spectra when a cross-type head is used for recording and reproduction with a metal tape and a Co-T-Fe203 tape (tape speed = 5.8 m/s).
S. Otomo et al. / Journal of Magnetism and Magm, tic Mmeriol.v 15v ( 199~ ) 256 2h5, 0
(a' r REC: Ferrite head ) [ REP: Cross-type head
261
short wavelength 1.0
-20 ~
recordingcurrent "t3
long w
~
d
0.5 /u-
8
1' 3m, n = ( n - 1 / 2 ) ' t ' I
50
-60 ]
,
,
2
4
i
i
~
i
I
I
100 150 Recording current (mA)
I
200
I
0
~
6 8 10 Frequency (MHz)
12
Fig. 10. Change in the distance, t', with the recording current.
(b) r BEC: Gross4ypehead R EP: Ferrite head
~'/~
"~.~
shows the change in the distance t' with the recording current. The distance t' is normalized to the distance d between the gap and the top of the fmxite as shown in the inset. The bumps observed at short
recordingcurrent
O -60 I
O
2
4
6 Frequency
8 10 (MHz)
(a) I- REC:Cross-typehead L REP: Cross-type head
O"
I
12
Fig. 8. Output spectra when a C o - y - F e 2 0 3 tape is separately recorded and reproduced with a cross-type head and a ferrite head (tape speed = 5.8 m / s ) .
20 ¸
- 1MHz
R
"5 -4o 0
as shown in Fig. 9. According to Eq. (2), the distance between the gap and the pseudo-gap t' can be obtained from the inclination of those lines. Fig. 10
-60 10MHz I
O1.S
75mA
100 mA
I
I
I
300
REC: Ferrite head REP: Cross-type head frequency
125 mA
E ::k v 1.0
v
_ 2MHz
E ~
I
100 150 200 250 Recording current (mA)
(b) [
m
recording current
I
50
0.5 -oo
0
0
I 2
f 4
I 6
I 8
I 10
I
12
n
Fig. 9. Relation between the number of bumps, n, and the reciprocal of the wavelength )tmin at which the output is at a minimum in Fig. 7b and Fig. 8b.
0
50
10 MHz I I 1 I I 100 150 200 250 300 Recording current (mA)
Fig. l 1. Curves of output versus recording current when a C o - y Fe203 tape is recorded with a cross-type head and a ferrite head (tape speed = 5.8 m / s ) .
262
S. ()lomo et al. / , I o m m d r~/Mawtetism and Magnetic Materials 159 (1996) 256-268
wavelengths occur when the recording current is more than three to four times the optimum current, and the distance t' does not change with the recording current. This distance t' coincides with the distance d between the gap and the top of the ferrite. We think these bumps observed at short wavelengths occur when the top of the ferrite saturates at the high recording current, and then acts as a pseudogap where the recording field leaks. The reason no bumps are observed when using the metal tape is that the recording field from the pseudo-gap is so weak that it cannot record on the high /q. tape. On the other hand, the bumps observed in the long wavelengths also occur at recording currents three to four times higher than the optimum current, and the distance t' increases with the recording current. In this type of bump, the wavelengths at which the maximum and minimum output occur change with the recording current; therefore, bumps are also observed in the curves of the output versus the recording current, as shown in Fig. l la. The bumps in the curves of the output versus the record-
ing current are also observed when the ferrite head is used for recording, as shown in Fig. 1 lb. Therefore, this type of bump generally occurs when the recording field is much stronger than the optimum recording condition. This phenomenon is known to occur when a circular magnetization mode is produced by a strong field [12]. The circular magnetization mode when recording with the ferrite head occurs only in the short wavelength region (high frequency), as shown in Fig. 1 lb. On the other hand, the bumps are also observed in the long wavelength region (low frequency) when recording with the cross-type head, as shown in Fig. l la. This suggests that the recording field of the cross-type head is strong enough to record the deep part of the tape, so a circular magnetization mode at a long wavelength is produced. The result of the measurement is summarized in Table 1. Consequently, the cross-type head appears to have excellent potential because no bumps occur in the output spectrum when reproducing or recording with high H~ metal tape or low H,. C o - T - F % O 3 tape under normal recording conditions.
Table 1 Summary of measurements tbr pseudo-gap effect (a) MIG head with metal tape Recording MIG
Ferrite
i rec: Reproduction
Optimum (40 mA)
High (200 mA)
Optimum (40 mA)
High (200 mA)
MIG Ferrite
Small bumps No bumps
Large bumps Large bumps
Small bumps
Small bumps
(b) Cross-type head with metal tape Recording Cross i rec: Reproduction
Optimum (40 mA)
High (200 mA)
Cross
No bumps
No bumps
(c) Cross-type with Co-y-Fe203 tape Recording Cross i rec: Reproduction Cross Ferrite
Ferrite
Optimum (20 mA)
High (125 mA)
Optimum (20 mA)
High (125 mA)
No bumps No bumps
Large bumps Large bumps
No bumps
No bumps
S. Otomo et al. / J o u r n a l q f Magnetism and MaL, neti ' Malerictl,~ 159 t1990} 250 26,~'
2(~ ~
amorphous film
\
iiiiiiiiiiiiiiii~i~iii~!:SUUb[lrd[r~ iiiiiiii!iiiiiiii:!iiil
(a)
5~m
amorphous film
MnO-NiO substrate
(b)
10~m
Fig. 12. Scanning electron micrographs of cross-sections of C o - N b Zr amorphous films deposited (a) on the inclined plane of the triangular substrate, and (b) on the horizontal and vertical planes of a substrate.
264
S. Otomo et a/. /,hmrmd qf Ma~,netism and Magnetic Materials 159 (1996) 256-268
3.3. Magnetic properties q[ the anunT)hous.fi/m used in the cross-07)e head
10000
To obtain a cross-type head with high reproduction efficiency, the magnetic film deposited on the inclined plane of the triangular projection must have high permeability. However, crystalline magnetic films such as permalloy which are deposited obliquely from a vapor are known to have large anisotropy due to their oriented crystalline structure [13]. Thus, the magnetic properties of films deposited on the inclined plane are expected to be degraded compared to those of conventional magnetic films deposited on the horizontal plane. We thought the degradation of the magnetic properties in the amorphous film would be minimal, however, because it has no crystalline structure. To examine this problem, we investigated the magnetic properties of amorphous films on the horizontal and the inclined planes.
1000
3.3.1. Structure of amorphous films Fig. 12 shows scanning electron micrographs of cross-sections of C o - N b - Z r amorphous films deposited on the inclined plane of a triangular substrate, and on the horizontal and vertical planes of a substrate made of non-magnetic MnO-NiO. The samples were fractured and viewed as cleaved. Although the film deposited on the horizontal plane has a homogeneous structure, the film deposited on the vertical plane exhibits a heterogeneous columnar structure. This columnar structure is caused by a self-shadowing effect [13] which occurs when sputtered particles do not accumulate in depressions on the film surface because the sputtered particles are obliquely incident. The film deposited on the plane inclined at an angle of 60 ° to the horizontal has the same homogeneous structure as a film deposited on the horizontal plane, as shown in Fig. 12a. Although the film on the vertical plane has twice the H c of the film on the horizontal plane, the H c of the film on the inclined plane is only 15 to 30% greater than that of the horizontal film due to the homogeneous structure.
3.3.2. Permeabili~' of amorphous films Fig. 13 shows the frequency characteristic of the permeability of a C o - N b - Z r amorphous film de-
tm=24,um / tg =1.5 MHz
(a) Hardaxis "~
"
~
~
,
"~ 100
lC .001
i
i
,
ca cu ated
.....y"""~" ~ ) ~
.01
.
~"~.~,,~,
Easyaxis
.1 1 Frequency (MHz)
10
100
Fig. 13. Frequency characteristic of the permeability of a C o - N b Zr amorphous film deposited on a horizontal plane.
posited on a horizontal plane. Amorphous films are known to have induced anisotropy, and the direction of anisotropy can be controlled by annealing in a magnetic field. The hard axis of sample (a) and the easy axis of sample (b) are arranged to lie in the measuring direction of permeability by magnetic annealing. Permeability is much higher in the hard axis than the easy axis, especially at high frequencies. This is because magnetization changes in the hard axis are caused by rotation of the magnetic moment which occurs easily at high frequencies, while magnetization changes in the easy axis are caused by wall motion which is very weak at high frequencies. The real part of the permeability /x' in the hard axis keeps its value from a low frequency to a limit frequency Jg, then decreases at frequencies above f~ because of eddy current loss. The frequency characteristics of the real part of permeability /z' and the imaginary part /2' in the hard axis coincide well with the calculated curves obtained by the theory of eddy current loss [14], as shown in Fig. 13. Here, the limit frequency,
Sg =
2),
(3)
is the frequency at which /x' decreases to 0.678 times /zd, the value of /z' at low frequency, where p is the specific resistivity, t is the film thickness, and /% is the permeability of a vacuum. Fig. 14 shows the permeability of the amorphous film on the inclined plane of the triangular substrate. The film was annealed at 400°C for 30 min in a 800 k A / m magnetic field applied in the direction of the
S. Otomo et al. / J o u r n a l o f Magnetism and Mak, nelic Materiulv 1,59 (1996)
measuringdirectionof permeability directionof magneticfield
m
Mano°iPi~iill ~ate--
l
Constructionof specimen I ] I
10000
"' ~' "<
~
1000
i
I ~g = 2.7 M~z
+----..~
a.
L
,..........-..> > ,,'"'
100
.
tm=14/zm
. "i ..... I
1
A
........
I
10 Frequency (MHz)
.
i ....
100
Fig. 14. Permeabilityof a Co-Nb-Zr amorphous film on the inclined plane of a triangularsubstrate. top of the substrate to position the hard axis in the measuring direction. The permeability at low frequencies is almost the same as that of the film on the horizontal plane shown in Fig. 13. The frequency characteristics of permeability fit the calculated curves if fg is assumed to be 2.7 MHz. On the other hand, the calculated fg from the film thickness according to Eq. (3) is 3.7 MHz; that is, the curves of permeability shift slightly towards lower frequencies compared to theoretical curves. This is attributable to larger anisotropy dispersion of the film deposited on the inclined plane than that of the film on the horizontal plane. Consequently, the amorphous sputtered film appears to be attractive for use in the cross-type head because the film deposited on the inclined plane has almost the same soft magnetic properties as a film on the horizontal plane.
3.4. Changes in output of the cross-~pe head due to magnetic annealing Since the permeability of amorphous films is very different from one direction to another, it is important to control the direction of anisotropy of amorphous films used in heads. Thus, we investigated the
2,56 2&~, '
2(~5
effect of the direction of anisotropy on the output of the cross-type head. Fig. 15 shows the output of cross-type heads in which amorphous films are annealed in a magnetic field applied in the directions of tape movement, track width, and gap depth. The output is highest when the magnetic field is applied in the direction of tape movement, and is lowest when applied in the direction of the gap depth. Fig. 16 shows Bitter patterns and magnetic domain structures of films on the inclined plane annealed under the above conditions. When the magnetic field is applied in the direction of tape movement, magnetic walls lining up in the direction of the short side of the film are observed, as shown in Fig. 16a. From this pattern, the magnetization of the film is assumed to point in the direction of the short side of the film along the inclined plane. When the magnetic field is applied in the direction of gap depth, magnetic walls parallel to the long side of the film are observed, as shown in Fig. 16c. This means that the magnetization in the film points in the direction of the long side of the film. On the other hand, a complicated domain is observed as shown in Fig. 16b when the magnetic field is applied in the
-2 ~
~ 03
~ -6
.
magne~icDfield',
~o.
~
-12 -14
magneticannealing:400°C,30 min. 7 5 m/s
[]
~.
tape
_v~ ~
I II-I I I gapdepth direction I 1
I 2
I I I 3 4 5 Frequency (MHz)
I 6
7
Fig. 15. Outputof cross-typeheads in whichamorphousfilmsare annealed in a magnetic field applied in the directions of tape movement,track width,and gap depth.
8. Otomo ut al. / , I o m m d q/'Maqnetism and Magnetic Materials 159 (1996) 256-268
266
direction of track width. This complex domain is thought to be caused by the magnetization component perpendicular to the fihn surface, which may be
produced due to the large angle (60 °) between the applied field and the inclined plane [15]. These domain structures suggest that the permeability of
..--", amorphous 1
~ t,,, ,,i:::-" '
.
~
~
film
~
observedplane
~~w-.~
///I
~"
~ MnO-NiO substrate
(c) gap depthdirection
(a) tape movementdirection (b) trackwidthdirection direction of field
Bitterpattern
domainstructure wall
(a) tape motion 200/~ m I
directionof magnetization
®z ®
® ,~
I
wall
(b) track
width 200 # I
m
i
directionof magnetization
~
'Z- Per$;2diSn~lar
I
wall
directionof magnetization
(c)
gap depth 200,/zm
9
Fig. 16. Bitter patterns and magnetic domain structures of a Co N b - Z r amorphous film deposited on an inclined plane.
S. Ommo et al. / Journal of Magnetism and Mat,,m, ti¢- MateriaLs 159 (1996) 256 26~¢ 15
4. Conclusions
10Cross-type head with Co-Nb-Zr amorphous film
v~ 5 0
~ s ) Mn-Zn ferrite
head
-1 0
I "%
tape speed = 3.75 m/s gap length = 0.3 ,um
-15 0
267
r 1
I 2
I 3 Frequency
I 4
I 5
I 6
(MHz)
Fig. 17. Output of a cross-type head and a ferrite head with a metal tape.
the film in the head when the field is applied in the direction of tape movement is high in the direction of the gap depth and low in the direction of tape movement. This high permeability in the direction of gap depth is believed to produce high reproduction efficiency and high output because reproduction flux can easily reach the coil area. Inversely, low permeability in the direction of the gap depth is thought to cause the low output of the head when the field is applied in the direction of the gap depth. On the other hand, the decrease in the output of the head when the field is applied in the direction of the track width is attributable to the difficulty in achieving rotation of magnetization due to the complicated domain shown in Fig. 16b. Fig. 17 shows the output of the cross-type head compared with that of a ferrite head. The output of the cross-type head using a C o - N b - Z r amorphous film is 8 dB higher at 5 MHz than that of a ferrite head with the same gap length when using high H c metal tape. Therefore, the cross-type head shows excellent recording and reproduction characteristics for high H, tapes.
The properties of various types of video heads were investigated using a field calculation. A crosstype head composed of amorphous film and ferrite was fabricated and its recording and reproduction characteristics were examined. The results are as follows: (1) A cross-type head, in which a magnetic film with high saturation magnetic flux density B is deposited on the inclined planes of a triangular projection of ferrite, has the potential to generate a strong recording field and show high reproduction efficiency according to a three-dimensional field calculation. (2) Bumps in the output spectrum occur during reproduction and recording at high recording current with a metal-in-gap (MIG) head whose interfaces between magnetic film and ferrite are parallel. The cross-type head, in which the interfaces between the film and the ferrite are inclined, generates no large bumps during recording and reproduction with high H c metal tape or low H, Co-'y-Fe203 tape under normal recording conditions. (3) C o - N b - Z r amorphous film deposited on the inclined plane of the cross-type head shows a homogeneous structure and high permeability almost equal to that of a film deposited on a horizontal plane. (4) The output of the cross-type head using a C o - N b - Z r amorphous film is highest when the film is annealed in a magnetic field applied in the direction of tape movement, and lowest when the field is applied in the direction of the gap depth. The output of the cross-type head is 8 dB higher at 5 MHz than that of a ferrite head when using a high H~ metal tape.
Acknowledgements We would like to thank Prof. T. Nakata and Dr. K. Fujiwara of Okayama University for their offer of the T-~Q calculation program and valuable comments. We also wish to thank Mr. N. Saito for his help preparing the amorphous film and Mr. M. Nagasawa for his collaboration in fabricating the video heads. We are also grateful to Drs. H. Fujiwara, M. Kudo, J. Morikawa, A. Iwama, and T. Tamura for their helpful discussions and encouragement.
268
S. Otomo et al. /Jo,rnal :?/Magt~eti.sm and Magnetic Materials 159 (1996) 256-268
References [l] F.J. Jefl~rs, R.J. McClure, W.W. French and N.J. Griffith. IEEE Trans. Magn. 18 (1982) 1146. [2] J.J.M. Ruigrok, IEEE Trans. Magn. 20 (1984) 872. [3] C.W.M.P. Sillen, J.J.M. Ruigrok, A. Broese van Grocnou and U. Enz, IEEE Trans. Magn. 24 (1988) 1802. [4] P.V. Koeppe and M.H. Kryder, IEEE Trans. Magn. 25 (1989) 3701. [5] K. Kajiwara, M. Hayakawa, Y. kunito, Y. Ikeda, K. Hayashi, K. Aso and T. Ishida, IEEE Trans. Magn. 26 (1990) 2978. [6] K. Matsuura, K. Oyamada and T. Yazaki, IEEE Trans. Magn. 19 (1983) 1623. [7] K. Takahashi, K. Ihara, S. Muraoka, H. Yohda, E. Sawai and N. Kaminaka, IEEE Trans. Magn. 23 (1987) 2928. [8] N. Kaminaka, H. Sakakima, K. Takahashi, K. Osano and H. Hasegawa, IEEE Trans. Magn. 26 (1990) 2936.
[9] N. Kumasaka, H. Fujiwara, S. Otomo, T. Yamashita, S. Takayama, N. Saito and M. Kudo, United States Patent, No. 4,656,547 (1987). [10] T. Nakata, N. Takahashi, K. Fujiwara and Y. Okada, IEEE Trans. Magn. 24 (1988) 94. [11] S. Otomo, N. Kumasaka, H. Fujiwara. S. Takayama, T. Yamashita, N. Saito and M. Kudo, United States Patent, No. 4,972,285 (1990). [12] S. Iwasaki and K. Takemura, IEEE Trans. Magn. 11 (1975) 1173.
[13] D.O. Smith, M.S. Cohen and G.P. Weiss, J. Appl. Phys. 31 (1960) 1755. [14] M. Matsumoto, Magnetic Recording (Kyoritsu Syuppan, 1977) p. 186 (in Japanese). [15] I.B. Puchalska, A. Hubert, S. Winkel and B. Mirecki, IEEE Trans. Magn. 24 (1988) 1787.
Journalof magnetism and magnetic materials
Journal of Magnetism and Magnetic Materials 159 (1996) 256-268
A cross-type amorphous video head Shigekazu Otomo a,*, Takeo Yamashita b, Noriyuki Kumasaka b a Central Research Laborator3', Hitachi Ltd., Kokubunji, Tokyo 185, Japan b Video and Personal Media System Dil,ision. Hitachi Ltd., 1410-1nada, Hitachinaka-shi, Ibaraki-ken 312, Japan
Received 13 April 1995; revised 17 October 1995
Abstract A cross-type video head composed of cross-shaped amorphous sputtered films and ferrite that can be used with high coercive tapes is described. A three-dimensional magnetic field calculation shows that the cross-type head has a strong recording field and high reproduction efficiency compared to a conventional metal-in-gap head (MIG) or a plate-type head. No large bumps in the output spectrum are observed during recording and reproduction under normal conditions with the cross-type head. The amorphous sputtered film deposited on the inclined plane of the ferrite substrate used for the cross-type head shows a homogeneous structure and high permeability similar to that of a film deposited on a horizontal plane. A cross-type head using a C o - N b - Z r amorphous film annealed in a magnetic field under optimum conditions shows excellent recording and reproduction characteristics when used with a high coercivity metal tape. Keywords: Amorphous sputtered film; Cross-type head; High coercive tape; MIG head; Permeability; Pseudo-gap effect ; Recording field; Reproduction efficiency; Three-dimensional field calculation; Video head
1. Introduction Magnetic heads that can effectively record on magnetic media that have a high coercive force, H~., are needed to develop high density magnetic recording systems. Various types of video heads that use magnetic films with high saturation magnetic flux density, Bs, have been developed for use with high H c media. Among them, the metal-in-gap (MIG) head [1-5], in which a high B~ film is deposited on the gap surfaces of a ferrite core, is easier to produce than the plate-type head [6-8] because of its simple manufacturing process. However, bumps in the output spectrum have been observed when recording * Corresponding author. Fax: + 81-285-84-1550.
and reproducing with the M I G head because the interface between the magnetic film and the ferrite core acts as a pseudo-gap. Although several efforts to solve this problem have been reported [2-5], it is difficult to completely eliminate the pseudo-gap effect. In this paper, we investigate the properties of various types of video heads by using a three-dimensional field calculation. We find that a cross-type head [9], in which a high B s film is deposited on a triangular projection of a ferrite core, has a strong recording field and high reproduction efficiency while showing no pseudo-gap effect. We fabricated a cross-type head using a Co-based amorphous sputtered film and examined the recording and reproduction characteristics.
0304-8853/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. SSD10304-8853(95)00942-6
S. Otomo et al /Journal of Magnetism and Ma~lletic Mat,:,riaLs lSv ¢19~)~51 2.% 20<~'
2. Calculations and experimental procedures Fig. 1 shows various structures of video heads that use magnetic films. A plate-type head has a magnetic core composed only of a magnetic film, and the thickness of the core is equal to the track width. A metal-in-gap (MIG) head is composed of a ferrite core and a magnetic film is deposited on the gap surfaces of the ferrite core; the interface between the ferrite and the magnetic film is parallel to the gap. On the other hand, a cross-type head has a structure in which the magnetic film is deposited on a triangular projection of ferrite; thus, the interface between the ferrite and the film is inclined relative to the gap. The thicknesses of the magnetic cores are larger than the track widths in both the cross-type and the MIG head. The cross-type head can be easily produced because its fabrication process is similar to that of the MIG head [9]. The cross-type head has a complicated structure composed of multiple magnetic materials that lie across the track, as shown in Fig. 1. Therefore, three-dimensional field calculations using the finite
~
tw!.iilmil~~ magnetic film
gap
!!nmagnet,c ctor '
nonmagnetic substrate
(a) Plate-type head
257
element method are required to forecast the properties of the head accurately; however, these calculations generally take a very long time due to the large number of elements. In this work, we use lhe T---(_~ method based on the current vector potential (T) and the magnetic scalar potential (.(2). This method reduces the calculation time since there is only a small number of unknown variables [10]. As the magnetic film, we used a Co~4Nbl~Zr3 amorphous film that had a saturation magnetic flux density B~ of 0.9 T, a crystallization temperature, T~, of 520°C, and a magnetostriction, A~, of - 0 . 3 × 10 -6 [11], and we used a M n - Z n ferrite single crystal as the ferrite block. A Pb-based glass was used at a working temperature of 470°C to fill the V-shaped grooves and bond the blocks. Cross-type heads with a track width of 25 to 30 # m were obtained from blocks in which the amorphous film on the inclined plane of the ferrite core was 15 to 20 ,am thick. The effective gap length, ge, was about 0.3 /xm. We also prepared a MIG head using the same C o - N b - Z r amorphous film for comparison. The film in the MIG head was 6 /xm thick. The relative permeability, tx, of the amorphous films was measured with a vector impedance meter and a network analyzer. The morphology of the films was observed with a scanning electron microscope (SEM). The recording and reproduction characteristics were measured with a C o - y - F e 2 0 3 tape having a coercive force, H~, of 55 k A / m and a metal particle tape having an H~ of 116 k A / m .
3. Results and discussion .-l-3 2222
tin).!.
magnetic film d (=
glass ferrite
(b) MIG head glass
~iiC77777
magnetic film
.Z
j~: 0 ~'m d .i i. (c) Cross-type head
ferrite
Fig. 1. Structures of video heads using magnetic films.
3.1. Head properties determined by three-dimensional field calculations Fig. 2 shows the recording field, H x, obtained by three-dimensional field calculation for various types of heads. The recording field, H X, in the figure denotes the field in the direction of the tape motion at the center of the track and the gap. The spacing, Sp, between the calculation point and the head surface is 0.15 /xm. Parameters for the calculation are as follows: the gap length, g, is 0.3 /xm; the gap depth, g~, is 20 /xm; the track width, t w, is 28 /xm; the core thickness, t c, is five times tw; and the
S. O m m o et al. / , h m r m E qf ~hlgm, ti,~m and Magnetic Materials 159 (1996) 256 268
258 480 400
100 Magnetic film Bs =1 T, ,u=1000
I
I
I
Cross-typehead
Ferrite ,u=500
, Cross-type head /
Ferrite Bs =0.5T. F=500 60
320
o~ 4O
240 160
/j~
// 80
20
g =0.3~,m
~0'
I
0'.2
0.4
0
0'.8
' 500
~ 1500
I 0'00
2000
,u
I
0.6
go= 20 ,u m
0
gd = 20 )Lm Sp=0.15,um
V 0, 0.0
g =0.3~m
"" MIG head
.0
Magnetemotive force (AT) Fig. 2. Recording field, H~, obtained by a three-dimensional field
calculation.
distance between the gap and the ferrite, d, is 5 /xm for the MIG and the cross-type head. The film thickness, tm, which depends on tw, d and 0, the angle of the top of the ferrite (60°), is 15 /xm for the cross-type head. On the other hand, t m equals tw for the plate-type head and d for the MIG head, as shown in Fig. 1. The saturation magnetic flux density, B~, and the relative permeability, /x, are 1.0 T and 1000, respectively, for the magnetic film, and 0.5 T and 500 for the ferrite. The recording field, H×, of the cross-type head increases more rapidly with increasing magnetomotive force than that of the plate-type head, and then saturates to the same value as that of the plate-type head. We attribute the quick increase in the H× of the cross-type head to the low reluctance of the head core due to its large core thickness. On the other hand, the saturation field of the MIG head is much lower than those of the cross-type or the plate-type heads. This is because the ferrite adjacent to the high B~ film saturates easily at a high motive force. Fig. 3 shows the changes in the reproduction efficiency, r/r, due to the relative permeability, /~, of the magnetic film for each head. The reproduction efficiency, r/r, was obtained by calculating the ratio of the flux passing through the coil area to the total flux passing through the head, when a small magnet is set in front of the gap. The relative permeability, /x, of ferrite is set to 500. The reproduction effi-
Fig. 3. Changes in the reproduction efficiency,
T/r, with the
relative permeability, >, of the magnetic film.
ciency, r/i., increases as the /x of the film in each head increases; higher r/r is obtained in the low-p~ region for the cross-type head compared to the plate-type head because of the large core thickness of the cross-type head. The change in the r/r of the MIG head as /x of the film increases is small because the film in the MIG head is thin. These results show that the cross-type head has great potential for use with high H c tapes due to its strong recording field and high reproduction efficiency. The recording and reproduction characteristics of the cross-type head using an amorphous film are examined in the following sections. 3.2. Pseudo-gap effect in the MIG and the cross-o,pe heads" 3.2.1. MIG head Fig. 4 shows the output spectra when a MIG head is used for recording and reproduction with a metal recording current
150mA /
s
'1 '
200mA /
~ ~'~//
V
O -60 0
I
I
I
2
4
6
Frequency
I
8 (MHz)
I
I
10
12
Fig. 4. Output spectra when a MIG head is used for recording and reproduction with a metal tape (tape speed = 5.8 m / s ) .
S. Otomo et a l . / Journal of Magnetism and Magm, lic MateriaLs 159 (1996) 256-26S
(a) Recording current: 40 mA O-
/
A REC/REP:MIG head Br
-20 ~ "
/
~
-
~ k
EC:M,G.ead P: gerritehead
~
1313
-40
C, [ REC : Ferritehead v kREP:MiGhead
O
~ . \.
V -60 i
I
2
4
i
i
i
i
6 8 10 Frequency (MHz)
12
(b) Recording current: 200 mA 0 P: MIG head : MIG head : Ferritehead
-20
259
current of 200 mA, which is about five times the optimum current. Large bumps are observed when recording with the MIG head and reproducing with the ferrite head (curve B), as in the case of recording and reproducing with the MIG head (curve A). On the other hand, small bumps similar to those of curve C in Fig. 5a are observed when recording with the ferrite head and reproducing with the MIG head. Therefore, the large bumps at the high recording current are caused by recording with the MIG head. These bumps in the output spectrum of the MIG head are thought to be caused by interference between the signals reproduced by the gap and those from a pseudo-gap formed at the interface between the film and the ferrite [2]. Taking the pseudo-gap into account, the gap-loss function, Lg, is expressed
1313
as
45 O_ -40 ~5 O
Lg = s i n ( ~ - g / a ) / ( r r g / a )
+ 2g'/( g + 2g')sin(rrg'/a) / ( ~r g ' / A ) c o s ( 2 ~ - t ' / a ) ,
-60 i
0
2
4
6 8 10 Frequency (MHz)
12
Fig. 5. Output spectra when a MIG head and a ferrite head are used for recording and reproduction with a metal tape at a recording current of (a) 40 mA and (b) 200 mA (tape speed - 5.8 m/s).
tape. Bumps are observed in all output spectra, which were obtained at various recording currents. The more the recording current increases, the greater the amplitude of the bumps. The origin of the bumps was analyzed by separating the recording and the reproduction procedures. Fig. 5a shows the output spectra at a recording current of 40 mA, which is close to the optimum recording current. When a ferrite head is used for recording and the MIG head is used for reproduction (curve C), bumps are as common and as large as those observed when recording and reproducing with the MIG head (curve A). On the other hand, the spectrum is very smooth when recording with the MIG head and reproducing with the ferrite head (curve B). These results imply that the bumps occur during reproduction with the MIG head. Fig. 5b shows the output spectrum at a recording
(1)
where t' = I m Jr- ( g Jr- g ' ) / 2 , g' is the pseudo-gap length, and 3, is the wavelength. When ,~ equals g or g', the output reaches a minimum according to Eq. (1). However, these A are smaller than the region of wavelength examined in this experiment. The bumps observed in this work are caused by the term cos(2rrt'/A) which is the function of the phase difference between the signals reproduced by the gap and the pseudo-gap. Therefore, the maximum and minimum output will occur at wavelengths
a .... = t ' / n ,
ami n = t ' / ( n - 1 / 2 ) ,
n = l , 2 .....
(2) In the recording process, the phase difference between the signals recorded on the tape by the gap and the pseudo-gap is also expressed as cos(2~-t'/A). Therefore, the maximum and minimum output will also occur at the wavelengths of Eq. (2). Fig. 6 shows the relations between the number of bumps n and the reciprocal of the wavelengths '~max and ~min" According to Eq. (2), t' is obtained from the slope of the lines in the figure. Almost identical values of t' are obtained when recording and when reproducing with the MIG head. Also, the value of t' obtained from the bumps coincides well with that observed with an optical microscope. Therefore, the
260
S. Ommo et at./.Io ,'hal q/ M~ ~1 erich and Magnetic Materials 159 (1996) 256-268 2.0 REP: MIG head ]
t,=641 °
1.5
maximum output
E v 1.0
j/~
0.5
~.,'"""
~"~- minimumoutput 1/2-- (n- 1/2)/t'
~ "
I
~/ 2 2.0
~""•"•
=n /t'
1/2
,':&37
4
6 n
8
10
12
f REC: MIG head / (recording current = 25O mA)
1.5
maximum
~'E
~
"~'"
1/,1 = n / t'°utput. . / J ' " / ' " " t'= 6.1 tz m~,'"'"
0.5
imp2 i ~ 2 ~ t t ,
~ 0
occur when the cross-type head is used for reproduction, as shown in Fig. 8a. This is because the output from the pseudo-gap, if one exists at the interface between the ferrite and the film in the cross-type head, is reduced by azimuth loss due to the large inclination (60 °) of that interface to the gap. On the other hand, bumps occur when the cross-type head is used for recording at high recording currents, as shown in Fig. 8b. Two types of bumps are observed when recording with the cross-type head, as shown in Fig. 7b and Fig. 8b: one type occurs at low frequencies (long wavelengths) and has a large amplitude; the other occurs at high frequencies (short wavelengths) and has a rather small amplitude. Fig. 9 shows the relation between the number of bumps n and the reciprocal of the wavelength A,m. at which the output reaches a minimum in Fig. 7b and Fig. 8b. Two kinds of lines with different inclinations are obtained
,""
.
-
I 2
' I 4
~
= 6.0/z m I 6
I 8
I 10
t
12
n Fig. 6. Relations between the number of bumps, n, and the reciprocal of the wavelengths Am.x and hmi..
~"
o bumps observed when recording with the MIG head are also caused by the pseudo-gap that exists at the interface between the ferrite and the magnetic film.
a) Metaltape
-20
250 mA
-40
-60 0
i
i
i
i
I
i
2
4
6
8
10
12
t 10
12
Frequency (MHz)
3.2.2. C r o s s - O ' p e h e a d
Fig. 7 shows the output spectra when the crosstype head is used for recording and reproduction with the metal tape and C o - y - F e 2 0 3 tape. Smooth output spectra with only small bumps are observed when the high H~ metal tape is used, as shown in Fig. 7a. On the other hand, as shown in Fig. 7b, complicated bumps in the output spectra are observed when a C o - 7 - F e 2 0 3 tape with low H c is used at recording currents of 75 mA or above; however, virtually no bumps occur at the recording current of 20 mA, which is close to the optimum current. Fig. 8 shows the output spectra when the C o - 7 Fe20 3 tape is recorded with the cross-type head and reproduced with the ferrite head. No large bumps
recording current 40 mA / 75 mA
(b) Co- Y Fe203tape
0
recording current |[ ~ \
~o.
~,
~
20 mA
-40
o -60 0
2
4
6
8
!
Frequency (MHz) Fig. 7. Output spectra when a cross-type head is used for recording and reproduction with a metal tape and a Co-T-Fe203 tape (tape speed = 5.8 m/s).
S. Otomo et al. / Journal of Magnetism and Magm, tic Mmeriol.v 15v ( 199~ ) 256 2h5, 0
(a' r REC: Ferrite head ) [ REP: Cross-type head
261
short wavelength 1.0
-20 ~
recordingcurrent "t3
long w
~
d
0.5 /u-
8
1' 3m, n = ( n - 1 / 2 ) ' t ' I
50
-60 ]
,
,
2
4
i
i
~
i
I
I
100 150 Recording current (mA)
I
200
I
0
~
6 8 10 Frequency (MHz)
12
Fig. 10. Change in the distance, t', with the recording current.
(b) r BEC: Gross4ypehead R EP: Ferrite head
~'/~
"~.~
shows the change in the distance t' with the recording current. The distance t' is normalized to the distance d between the gap and the top of the fmxite as shown in the inset. The bumps observed at short
recordingcurrent
O -60 I
O
2
4
6 Frequency
8 10 (MHz)
(a) I- REC:Cross-typehead L REP: Cross-type head
O"
I
12
Fig. 8. Output spectra when a C o - y - F e 2 0 3 tape is separately recorded and reproduced with a cross-type head and a ferrite head (tape speed = 5.8 m / s ) .
20 ¸
- 1MHz
R
"5 -4o 0
as shown in Fig. 9. According to Eq. (2), the distance between the gap and the pseudo-gap t' can be obtained from the inclination of those lines. Fig. 10
-60 10MHz I
O1.S
75mA
100 mA
I
I
I
300
REC: Ferrite head REP: Cross-type head frequency
125 mA
E ::k v 1.0
v
_ 2MHz
E ~
I
100 150 200 250 Recording current (mA)
(b) [
m
recording current
I
50
0.5 -oo
0
0
I 2
f 4
I 6
I 8
I 10
I
12
n
Fig. 9. Relation between the number of bumps, n, and the reciprocal of the wavelength )tmin at which the output is at a minimum in Fig. 7b and Fig. 8b.
0
50
10 MHz I I 1 I I 100 150 200 250 300 Recording current (mA)
Fig. l 1. Curves of output versus recording current when a C o - y Fe203 tape is recorded with a cross-type head and a ferrite head (tape speed = 5.8 m / s ) .
262
S. ()lomo et al. / , I o m m d r~/Mawtetism and Magnetic Materials 159 (1996) 256-268
wavelengths occur when the recording current is more than three to four times the optimum current, and the distance t' does not change with the recording current. This distance t' coincides with the distance d between the gap and the top of the ferrite. We think these bumps observed at short wavelengths occur when the top of the ferrite saturates at the high recording current, and then acts as a pseudogap where the recording field leaks. The reason no bumps are observed when using the metal tape is that the recording field from the pseudo-gap is so weak that it cannot record on the high /q. tape. On the other hand, the bumps observed in the long wavelengths also occur at recording currents three to four times higher than the optimum current, and the distance t' increases with the recording current. In this type of bump, the wavelengths at which the maximum and minimum output occur change with the recording current; therefore, bumps are also observed in the curves of the output versus the recording current, as shown in Fig. l la. The bumps in the curves of the output versus the record-
ing current are also observed when the ferrite head is used for recording, as shown in Fig. 1 lb. Therefore, this type of bump generally occurs when the recording field is much stronger than the optimum recording condition. This phenomenon is known to occur when a circular magnetization mode is produced by a strong field [12]. The circular magnetization mode when recording with the ferrite head occurs only in the short wavelength region (high frequency), as shown in Fig. 1 lb. On the other hand, the bumps are also observed in the long wavelength region (low frequency) when recording with the cross-type head, as shown in Fig. l la. This suggests that the recording field of the cross-type head is strong enough to record the deep part of the tape, so a circular magnetization mode at a long wavelength is produced. The result of the measurement is summarized in Table 1. Consequently, the cross-type head appears to have excellent potential because no bumps occur in the output spectrum when reproducing or recording with high H~ metal tape or low H,. C o - T - F % O 3 tape under normal recording conditions.
Table 1 Summary of measurements tbr pseudo-gap effect (a) MIG head with metal tape Recording MIG
Ferrite
i rec: Reproduction
Optimum (40 mA)
High (200 mA)
Optimum (40 mA)
High (200 mA)
MIG Ferrite
Small bumps No bumps
Large bumps Large bumps
Small bumps
Small bumps
(b) Cross-type head with metal tape Recording Cross i rec: Reproduction
Optimum (40 mA)
High (200 mA)
Cross
No bumps
No bumps
(c) Cross-type with Co-y-Fe203 tape Recording Cross i rec: Reproduction Cross Ferrite
Ferrite
Optimum (20 mA)
High (125 mA)
Optimum (20 mA)
High (125 mA)
No bumps No bumps
Large bumps Large bumps
No bumps
No bumps
S. Otomo et al. / J o u r n a l q f Magnetism and MaL, neti ' Malerictl,~ 159 t1990} 250 26,~'
2(~ ~
amorphous film
\
iiiiiiiiiiiiiiii~i~iii~!:SUUb[lrd[r~ iiiiiiii!iiiiiiii:!iiil
(a)
5~m
amorphous film
MnO-NiO substrate
(b)
10~m
Fig. 12. Scanning electron micrographs of cross-sections of C o - N b Zr amorphous films deposited (a) on the inclined plane of the triangular substrate, and (b) on the horizontal and vertical planes of a substrate.
264
S. Otomo et a/. /,hmrmd qf Ma~,netism and Magnetic Materials 159 (1996) 256-268
3.3. Magnetic properties q[ the anunT)hous.fi/m used in the cross-07)e head
10000
To obtain a cross-type head with high reproduction efficiency, the magnetic film deposited on the inclined plane of the triangular projection must have high permeability. However, crystalline magnetic films such as permalloy which are deposited obliquely from a vapor are known to have large anisotropy due to their oriented crystalline structure [13]. Thus, the magnetic properties of films deposited on the inclined plane are expected to be degraded compared to those of conventional magnetic films deposited on the horizontal plane. We thought the degradation of the magnetic properties in the amorphous film would be minimal, however, because it has no crystalline structure. To examine this problem, we investigated the magnetic properties of amorphous films on the horizontal and the inclined planes.
1000
3.3.1. Structure of amorphous films Fig. 12 shows scanning electron micrographs of cross-sections of C o - N b - Z r amorphous films deposited on the inclined plane of a triangular substrate, and on the horizontal and vertical planes of a substrate made of non-magnetic MnO-NiO. The samples were fractured and viewed as cleaved. Although the film deposited on the horizontal plane has a homogeneous structure, the film deposited on the vertical plane exhibits a heterogeneous columnar structure. This columnar structure is caused by a self-shadowing effect [13] which occurs when sputtered particles do not accumulate in depressions on the film surface because the sputtered particles are obliquely incident. The film deposited on the plane inclined at an angle of 60 ° to the horizontal has the same homogeneous structure as a film deposited on the horizontal plane, as shown in Fig. 12a. Although the film on the vertical plane has twice the H c of the film on the horizontal plane, the H c of the film on the inclined plane is only 15 to 30% greater than that of the horizontal film due to the homogeneous structure.
3.3.2. Permeabili~' of amorphous films Fig. 13 shows the frequency characteristic of the permeability of a C o - N b - Z r amorphous film de-
tm=24,um / tg =1.5 MHz
(a) Hardaxis "~
"
~
~
,
"~ 100
lC .001
i
i
,
ca cu ated
.....y"""~" ~ ) ~
.01
.
~"~.~,,~,
Easyaxis
.1 1 Frequency (MHz)
10
100
Fig. 13. Frequency characteristic of the permeability of a C o - N b Zr amorphous film deposited on a horizontal plane.
posited on a horizontal plane. Amorphous films are known to have induced anisotropy, and the direction of anisotropy can be controlled by annealing in a magnetic field. The hard axis of sample (a) and the easy axis of sample (b) are arranged to lie in the measuring direction of permeability by magnetic annealing. Permeability is much higher in the hard axis than the easy axis, especially at high frequencies. This is because magnetization changes in the hard axis are caused by rotation of the magnetic moment which occurs easily at high frequencies, while magnetization changes in the easy axis are caused by wall motion which is very weak at high frequencies. The real part of the permeability /x' in the hard axis keeps its value from a low frequency to a limit frequency Jg, then decreases at frequencies above f~ because of eddy current loss. The frequency characteristics of the real part of permeability /z' and the imaginary part /2' in the hard axis coincide well with the calculated curves obtained by the theory of eddy current loss [14], as shown in Fig. 13. Here, the limit frequency,
Sg =
2),
(3)
is the frequency at which /x' decreases to 0.678 times /zd, the value of /z' at low frequency, where p is the specific resistivity, t is the film thickness, and /% is the permeability of a vacuum. Fig. 14 shows the permeability of the amorphous film on the inclined plane of the triangular substrate. The film was annealed at 400°C for 30 min in a 800 k A / m magnetic field applied in the direction of the
S. Otomo et al. / J o u r n a l o f Magnetism and Mak, nelic Materiulv 1,59 (1996)
measuringdirectionof permeability directionof magneticfield
m
Mano°iPi~iill ~ate--
l
Constructionof specimen I ] I
10000
"' ~' "<
~
1000
i
I ~g = 2.7 M~z
+----..~
a.
L
,..........-..> > ,,'"'
100
.
tm=14/zm
. "i ..... I
1
A
........
I
10 Frequency (MHz)
.
i ....
100
Fig. 14. Permeabilityof a Co-Nb-Zr amorphous film on the inclined plane of a triangularsubstrate. top of the substrate to position the hard axis in the measuring direction. The permeability at low frequencies is almost the same as that of the film on the horizontal plane shown in Fig. 13. The frequency characteristics of permeability fit the calculated curves if fg is assumed to be 2.7 MHz. On the other hand, the calculated fg from the film thickness according to Eq. (3) is 3.7 MHz; that is, the curves of permeability shift slightly towards lower frequencies compared to theoretical curves. This is attributable to larger anisotropy dispersion of the film deposited on the inclined plane than that of the film on the horizontal plane. Consequently, the amorphous sputtered film appears to be attractive for use in the cross-type head because the film deposited on the inclined plane has almost the same soft magnetic properties as a film on the horizontal plane.
3.4. Changes in output of the cross-~pe head due to magnetic annealing Since the permeability of amorphous films is very different from one direction to another, it is important to control the direction of anisotropy of amorphous films used in heads. Thus, we investigated the
2,56 2&~, '
2(~5
effect of the direction of anisotropy on the output of the cross-type head. Fig. 15 shows the output of cross-type heads in which amorphous films are annealed in a magnetic field applied in the directions of tape movement, track width, and gap depth. The output is highest when the magnetic field is applied in the direction of tape movement, and is lowest when applied in the direction of the gap depth. Fig. 16 shows Bitter patterns and magnetic domain structures of films on the inclined plane annealed under the above conditions. When the magnetic field is applied in the direction of tape movement, magnetic walls lining up in the direction of the short side of the film are observed, as shown in Fig. 16a. From this pattern, the magnetization of the film is assumed to point in the direction of the short side of the film along the inclined plane. When the magnetic field is applied in the direction of gap depth, magnetic walls parallel to the long side of the film are observed, as shown in Fig. 16c. This means that the magnetization in the film points in the direction of the long side of the film. On the other hand, a complicated domain is observed as shown in Fig. 16b when the magnetic field is applied in the
-2 ~
~ 03
~ -6
.
magne~icDfield',
~o.
~
-12 -14
magneticannealing:400°C,30 min. 7 5 m/s
[]
~.
tape
_v~ ~
I II-I I I gapdepth direction I 1
I 2
I I I 3 4 5 Frequency (MHz)
I 6
7
Fig. 15. Outputof cross-typeheads in whichamorphousfilmsare annealed in a magnetic field applied in the directions of tape movement,track width,and gap depth.
8. Otomo ut al. / , I o m m d q/'Maqnetism and Magnetic Materials 159 (1996) 256-268
266
direction of track width. This complex domain is thought to be caused by the magnetization component perpendicular to the fihn surface, which may be
produced due to the large angle (60 °) between the applied field and the inclined plane [15]. These domain structures suggest that the permeability of
..--", amorphous 1
~ t,,, ,,i:::-" '
.
~
~
film
~
observedplane
~~w-.~
///I
~"
~ MnO-NiO substrate
(c) gap depthdirection
(a) tape movementdirection (b) trackwidthdirection direction of field
Bitterpattern
domainstructure wall
(a) tape motion 200/~ m I
directionof magnetization
®z ®
® ,~
I
wall
(b) track
width 200 # I
m
i
directionof magnetization
~
'Z- Per$;2diSn~lar
I
wall
directionof magnetization
(c)
gap depth 200,/zm
9
Fig. 16. Bitter patterns and magnetic domain structures of a Co N b - Z r amorphous film deposited on an inclined plane.
S. Ommo et al. / Journal of Magnetism and Mat,,m, ti¢- MateriaLs 159 (1996) 256 26~¢ 15
4. Conclusions
10Cross-type head with Co-Nb-Zr amorphous film
v~ 5 0
~ s ) Mn-Zn ferrite
head
-1 0
I "%
tape speed = 3.75 m/s gap length = 0.3 ,um
-15 0
267
r 1
I 2
I 3 Frequency
I 4
I 5
I 6
(MHz)
Fig. 17. Output of a cross-type head and a ferrite head with a metal tape.
the film in the head when the field is applied in the direction of tape movement is high in the direction of the gap depth and low in the direction of tape movement. This high permeability in the direction of gap depth is believed to produce high reproduction efficiency and high output because reproduction flux can easily reach the coil area. Inversely, low permeability in the direction of the gap depth is thought to cause the low output of the head when the field is applied in the direction of the gap depth. On the other hand, the decrease in the output of the head when the field is applied in the direction of the track width is attributable to the difficulty in achieving rotation of magnetization due to the complicated domain shown in Fig. 16b. Fig. 17 shows the output of the cross-type head compared with that of a ferrite head. The output of the cross-type head using a C o - N b - Z r amorphous film is 8 dB higher at 5 MHz than that of a ferrite head with the same gap length when using high H c metal tape. Therefore, the cross-type head shows excellent recording and reproduction characteristics for high H, tapes.
The properties of various types of video heads were investigated using a field calculation. A crosstype head composed of amorphous film and ferrite was fabricated and its recording and reproduction characteristics were examined. The results are as follows: (1) A cross-type head, in which a magnetic film with high saturation magnetic flux density B is deposited on the inclined planes of a triangular projection of ferrite, has the potential to generate a strong recording field and show high reproduction efficiency according to a three-dimensional field calculation. (2) Bumps in the output spectrum occur during reproduction and recording at high recording current with a metal-in-gap (MIG) head whose interfaces between magnetic film and ferrite are parallel. The cross-type head, in which the interfaces between the film and the ferrite are inclined, generates no large bumps during recording and reproduction with high H c metal tape or low H, Co-'y-Fe203 tape under normal recording conditions. (3) C o - N b - Z r amorphous film deposited on the inclined plane of the cross-type head shows a homogeneous structure and high permeability almost equal to that of a film deposited on a horizontal plane. (4) The output of the cross-type head using a C o - N b - Z r amorphous film is highest when the film is annealed in a magnetic field applied in the direction of tape movement, and lowest when the field is applied in the direction of the gap depth. The output of the cross-type head is 8 dB higher at 5 MHz than that of a ferrite head when using a high H~ metal tape.
Acknowledgements We would like to thank Prof. T. Nakata and Dr. K. Fujiwara of Okayama University for their offer of the T-~Q calculation program and valuable comments. We also wish to thank Mr. N. Saito for his help preparing the amorphous film and Mr. M. Nagasawa for his collaboration in fabricating the video heads. We are also grateful to Drs. H. Fujiwara, M. Kudo, J. Morikawa, A. Iwama, and T. Tamura for their helpful discussions and encouragement.
268
S. Otomo et al. /Jo,rnal :?/Magt~eti.sm and Magnetic Materials 159 (1996) 256-268
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[9] N. Kumasaka, H. Fujiwara, S. Otomo, T. Yamashita, S. Takayama, N. Saito and M. Kudo, United States Patent, No. 4,656,547 (1987). [10] T. Nakata, N. Takahashi, K. Fujiwara and Y. Okada, IEEE Trans. Magn. 24 (1988) 94. [11] S. Otomo, N. Kumasaka, H. Fujiwara. S. Takayama, T. Yamashita, N. Saito and M. Kudo, United States Patent, No. 4,972,285 (1990). [12] S. Iwasaki and K. Takemura, IEEE Trans. Magn. 11 (1975) 1173.
[13] D.O. Smith, M.S. Cohen and G.P. Weiss, J. Appl. Phys. 31 (1960) 1755. [14] M. Matsumoto, Magnetic Recording (Kyoritsu Syuppan, 1977) p. 186 (in Japanese). [15] I.B. Puchalska, A. Hubert, S. Winkel and B. Mirecki, IEEE Trans. Magn. 24 (1988) 1787.