MAGNETIC HEADS FOR HIGH COERCIVITY RECORDING MEDIA

Transcription

1 Journal of ptoelectronics and Advanced Materials Vol. 6, No. 3, September 4, p MAGNETIC HEADS FR HIGH CERCIVITY RECRDING MEDIA V. N. Samofalov *, E. I. Il ashenko a, A. Ramstad b, L.Z. Lub anu, T.H. Johansen c National Technical Universit Kharkov Poltechnical Institute, Frunze St., 6, Kharkov, Ukraine a Moscow State Universit, Vorob eovi Gori,9899 Moscow, Russia b Tandberg Storage, Kjelsåsveien 6, P.. Bo 96, Kjelsås, N-4 slo, Norwa c Universit of slo, Department of Phsics, Blindern, N-36 slo, Norwa The application of high coercive storage media would increase densit, realiabilit and lifetime for thermal deca of recording information. The sstems of permanent magnets generating strong magnetic fields are proposed to create the heads for recording on high coercive media (H C = 5-5 ke). n the basis of calculations made for different magnet sstems, the possibilit of dense longitudinal and perpendicular recording was grounded. (Received April 6, 4 accepted June, 4) Kewords: Permanent magnet, Stra field, Demagnetization, Giant anisotrop, Coercive force. Introduction At present, commercial application of high coercivit media for magnetic recording is complicated b the absence of recording heads capable to magnetize alternativel such highcoercive media. Finding a solution of the problem will allow to increase densit and reliabilit of recording information [,,3]. In the present work, for longitudinal and perpendicular information recording on high coercive media, we propose to use strong stra fields occurring at the edges of charged magnet surfaces with giant magnet anisotrop [4]. It was shown that such fields are attained in the sstems of hard magnetic materials based on rare-earth metals with uniaial anisotrop field H K > ke and coercive force H C > πm S (M S is the saturation magnetization of the magnet material). In Fig. a one of the magnet sstems generating strong magnetic fields is shown to illustrate the recording principle. The longitudinal field component H (,,z) is here given b H = M S + b + ln b + + ( + b) + ( b) b + + b + ( + a) ( + a) + ( b) + ( + b) b + + b + ( a) ( a) + ( b) + ( + b) () Shown in Fig. b is a surface plot of the field H (,) for the case b = a, and z =.a. As one sees, for points positioned near Y ais, the horizontal component of stra field takes large values H ~ ke, which is more than twice the magnet material induction value. With increasing distance from magnet surface, the component H decreases following to logarithmic law. The perpendicular component of the stra field H z is limited in magnitude b πm S, which is seen from Fig. showing H z () at = for z =.a,.a and.3a. * Corresponding author: Samofalov@kpi.kharkov.ua

2 9 V. N. Samofalov, E. I. Il ashenko, A. Ramstad, L. Z. Lub anu, T. H. Johansen (a) z M S (b) H (ke) YKE aa b b - -a a Fig. (a). The sstem of cored magnets generating the strong magnetic field; (b) Horizontal component H (,) calculated for points in the range -a < < a; < < b=a/ on the -plane at z =.a. A magnet material magnetization of M S = 75 G was taken for the calculations f( ) f( ) H z (ke) f( ) /a 3 Fig.. Calculated vertical component of stra field H z ( ) at = over the magnet sstem shown in Fig. a, for different distances z from the top surface: z/a =. curve, z/a =. curve, z/a =.3 curve 3. M S = 75 G was assumed for the calculations.. Eperimental test sstem In order to prove the eistence of such fields, a sstem like the one in Fig. a was made using SmCo 5 magnets. The sstem had dimensions 4 4 mm 3. The fields were measured using a giant magnetoresistive (GMR) effect sensor of size 3 mm and. µm thickness [5]. From the observed data plotted in Fig. 3 it is seen that measured H () is in good agreement with the fitted field profiles, which were calculated from eq. () assuming an M S of 75 G for the magnet material.

3 Magnetic heads for high coercivit recording media 93 8 H (ke) v. 4 3 ln.6. 4 H3( ) v,. /a.5 Fig. 3. Comparison of calculated (solid curves) and eperimentall measured (circles) values of the stra fields above magnet sstem for various /a. The upper curve corresponds to the case b >> a, and the bottom one to b = a/. M S = 75 G was assumed. 3. Head for longitudinal recording A novel head is constructed on the basis of the test sstem s abilit to generate concentrated magnetic field of high strength for the component H (). The principal scheme of the head is shown in Fig. 4a. The head consists of a pair of magnets magnetized anti-parallel, and a sstem for creating the sub-magnetizing field. The magnet size in Y direction is b. A non-magnetic laer with thickness δ is positioned between the magnets. This laer ecludes echange interaction, and thus, stabilizes the magnetic state in the magnets. The magnet size is c >> a in the direction of the z ais. The calculated H (, ) component at z =.a is plotted in Fig. 4b. For calculations, we assumed a residual magnetization of M = G for the magnet material, and a laer width of δ =.5. From the figure it is seen that the non-magnetic laer increases the recording area size and decreases the strength of strong field. So, this laer should be thin, satisfing the condition of /δ >. The scheme of the sub-magnetization sstem is shown on Fig. 4a. This sstem is necessar to generate the sub-magnetization field of same direction, i.e., according to magnetization the recording. The sub-magnetization sstem should provide magnetization of adjacent soft-magnetic laers in anti-parallel of neighboring magnets. The recording is carried out as follows: the head is positioned above the medium at such distance t, that longitudinal (or perpendicular) component of permanent magnet stra field being a bit lower than the field of nucleation H nuc (or coercive force H C ) of storage magnetic media. Then, sub-magnetisation sstem is switched on and creates an additional field H with the value corresponding to resulting field H + H > H nuc (or coercive force H C ). The additional field H should be high enough (-3 ke) to allow to optimize the recording on media with different coercive force. Additionall, the field should eceed the stra field H occurring at the interface of the medium where alternation of magnetization direction in storage medium takes place (the point of tpe, see Fig. 4a). This requirement is connected with the fact that H field direction on right hand from the point (Fig.4a) coincides with magnetization field of the magnets H. As a result, the total field H +H ma become higher than H C, and during further head movement, homogeneous medium magnetization will occur even if recorded signal is not given on the head. The possibilit to set the field H value in wide range permits to avoid rubbing out the information during the head movement. It is worth noting that in the case of a film variant of the head, one should produce nonmagnetic interlaer as well between soft-magnetic laers of sub-magnetization sstem and highcoercive magnets. Such the interlaer ecludes echange interaction between the laers and, consequentl, does not lead to decreased permeabilit of soft magnetic laer.

4 94 V. N. Samofalov, E. I. Il ashenko, A. Ramstad, L. Z. Lub anu, T. H. Johansen (a) H (ke) (b) 5 a Y K E t h.5 /a /a. Fig. 4. (a) Scheme of the head for longitudinal recording. (b) Calculated stra field longitudinal component H (, ) in the area -.a < <.a, -.5a < < on the plane XY (z = ). For the calculations, M S = G and non-magnetic laer thickness δ =.5a were taken. It is epected that due to eistence of cores in sub-magnetization sstem which are positioned along with the magnets, the decrease of stra fields in recording point will occur. Preliminar evaluation calculations have shown that the decrease of the field would not be substantial, if the material of hardmagnetic laer possesses the giant value of uniaial magnetic anisotrop field. For eample, at such anisotrop field as for SmCo 5 with H C = 3 45 ke [6], the additional stra fields from cores would be much smaller and practicall do not change the magnetization direction in permanent magnets. Consequentl, the strong field in recording point will be preserved, and its non-homogeneous properties will not change as well. Note, that on designing the sub-magnetization sstem, the decreasing effect of the magnets on core permeabilit should be taken into consideration. It is seen from Fig. b that on the magnet edges in the points with coordinates = ± a, the field component H is directed opposite to H in the point and reaches large values. In the head with core, in these points the component H enforces but will remain lower than in the point. Under magnetization of cores in the moment of recording, the component H in point will enforce, and vice versa, in the points = ± a, it will decrease. So, undesirable recording in these points will be absent. The size of the remagnetisation area on the medium ma be estimated b [ ep{ ( H + H H )/ M } ( t / ]. 5 = a a. () nuc S ) H being demagnetization field in this area, t is the distance from the medium to the head. Note that the nucleation field depends not onl on magnetic parameters of medium material, but also on the remagnetisation area size and shape. B the magnitude, it ma eceed the medium coercive force. It is seen from equation () that the area size depends on magnet sizes a, nucleation field H nuc, submagnetization field H, and saturation magnetization of head permanent magnets. Varing these parameters, it is possible to optimize the recording process. So, to obtain small sized area ( <. µm), one should use the media with high coercive force H C ke and higher. n highcoercive media, it is possible to obtain the small sized area even using the heads with rather large magnet characteristic size a. This is one of advantages of the proposal method. It worth to be noted that at small distance from the medium a/t, the normal component of stra field H z πm s takes high magnitudes as well (see Fig. 4). The field of such value is able to magnetize the medium locall in normal direction. To eclude this, one should use the media with z > πm s. In addition, the media for longitudinal recording should have crstallographic teture with EA parallel to medium plane.

5 Magnetic heads for high coercivit recording media The head for perpendicular recording The principle scheme of the head for perpendicular recording is shown on Fig. 5a. It is distinguish from longitudinal recording head in that the magnetizations in the permanent magnets are directed contrar to each other. Such sstem would be stable and not demagnetized, if magnet coercive force is higher than magnetisation of the permanent magnets, C > 4πM s. So far as the component of strong force H z occurs near the point, for preserving the homogeneousl magnetized state in the magnets it is required that their material possesses the strong field of uniaial anisotrop k > ke. The head sub-magnetization sstem consists of a core and a magnetizing coil. For ecluding the echange interaction between the magnets as well between the magnets and soft laers it is necessar to create a thin non-magnetic interlaer. Under electrical current recording, the both head cores are magnetized in the same direction and generate an additional field. There is an optimal distance to the medium t (depending on characteristic sizes, and core width b) corresponding to maimum field strength in the point of recording. Estimation calculations indicates the possibilit to retain the submagnetization field over the medium = -3 ke and even higher. (a) a M S z Y K E h t (b) Fz( H ).5 z. 4 (ke) Fz( ) F3z( ) /b Fig. 5 (a) The scheme of the head for perpendicular recording. (b) The calculated stra field perpendicular component H z () for different distances from the medium; t =.b the upper curve; t =.b- the middle curve; t =.3b the lower curve. For the calculations, M S = G and c = b were taken. n Fig. 5b the calculated stra field components H z () over the head with sizes c = b for different distances to the medium (the upper curve z =.b, the middle curve z =.b, and the lower curve z =.3b) are shown. It s worth to note, that the field strength H z () is determined b the magnet sizes c and b, and independent on a size. 5. ther tpe of the recording head Using the strong fields occurring near the edges of charged surfaces in high-anisotrop magnets, it is possible to create other models of the heads as well. We consider onl single additional model: a head for point recording. A principle scheme for perpendicular point recording without submagnetization sstem is shown on Fig. 6a. Here the Z ais is perpendicular to XY plane. In the vicinit of, the component H Z takes high magnitudes, while near four points of and tpes, the strong field being caused b the component H X. The calculated field component H z (, ) for the points in the ranges -.h < <.h, -.h < <.h (h- magnet thickness), is shown on Fig. 6b. For the calculations we took M s = G and magnet sizes >> h. So far as for such the sstem, the field components H z depend on magnet thickness h, to attain the necessar field strength magnitudes under recording, the laer thicknesses have to be more µm.

6 96 V. N. Samofalov, E. I. Il ashenko, A. Ramstad, L. Z. Lub anu, T. H. Johansen M S H z (ke) M S -.a /a -.a /a.a Fig. 6. The scheme of the head for point recording, a-the scheme of magnet sstem; b-the stra field perpendicular component H z (, ) calculated for points in the area -.a < <.a, -.a < <.a on the plane XY (z = ). M S = G was assumed for the calculations. 6. Conclusions The proposed sstem of recording on high-coercive media has a number of advantages in compared to previous ones, see e.g., [7,8]. Namel, using the proposed heads it is possible to generate strong magnetic stra fields H st > ke with ver high field gradients, up to e/nm. Since the strong field is localized in a narrow area, it opens the path to realization of super-dense recording. However, in our opinion, for realization of such sstem it is necessar to resolve a number of technical tasks. (i) First of all, it is necessar to master the technolog of preparing hard-magnetic laers needed to the magnetic heads. These laers should be snthesized on the basis of rare-earth materials, because the alone ma attain a uniaial anisotrop field eceeding ke. In addition, the technolog should provide the production of film laers with well defined easiest magnetization ais relative to its plane. (ii) For creation of media for longitudinal and perpendicular recording, it is also necessar to master the technolog of thin hard-magnetic laers with coercive force C = 5 ke and higher. It would be simpler to realize the proposed recording sstems, if the medium hard-magnetic laer has quite perfect magnetic and crstallographic structure. We suggest that, thin films based on Co-Pt, Co-Pd, Fe-Pt and also BaFe 9 will be convenient as such media, since the are more stable compared to rare-earth based materials. References [] R. Wood, IEEE Trans. Magnetics 36, 36 (). [] H. N. Bertram and M. Williams, IEEE Trans. Magnetics 36, 4 (). [3] H. Gavrila, V. Ionita, J. ptoelectron. Adv. Mater. 5(4), 99 (3). [4] V. N. Samofalov, A. G. Ravlik, D. P. Belozorov, B. A. Avramenko, Phs. Metals Metallograph 97(3), (4). [5] N. Jiang, K. Yamakawa, N. Honda, K. uchi, IEEE Trans. Magnetics 34, 339 (998). [6] E. A. Nesbitt, J. H. Wernick, Rare Earth Permanent Magnets, Academic Press, New York (973). [7] Y. Kanai, R. Matsubara, K. Fujiwara, N. Takahashi, IEEE Trans. Magnetics 38, (). [8] K. S. Kim, C. E. Lee, S. H. Lim, IEEE Trans. Magnetics 38, 3 ().