Abstract - The coercivity mechanism of (Co/Pt) multilayers with high Hc and high squareness fabricated by sputtering is mainly due to the wall-pinning rather ...
IEEE TRANSACTIONSON MAGNETICS, VOL. 28, NO. 5 , SEPTEMBER 1992
2754
Coercivity Mechanism and Microstructure of (Co/Pt) Multilayers Takao Suzuki, Harris Notarys, Darrell C. Dobbertin, Chien-Jung Lin, Dieter Weller, Doroles C. Miller and Grace Gorman
IBM Research Division, Almaden Research Center 650 Harry Road, San Jose, California 95120-6099
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Abstract The coercivity mechanism of (Co/Pt) multilayers with high Hc and high squareness fabricated by sputtering is mainly due to the wall-pinning rather than the nucleation process. In order to estimate the size of a wall-pinning site, a study of the temperature dependence of the coercivity, perpendicular magnetic anisotropy, and saturation magnetization for multilayers fabricated under various substrate conditions has been carried out. The size ro of the pinning sites of about 4A was obtained for the multilayer film deposited onto an etched SIN, film. The size ro increases with less interface-sharpness, confinned by X-ray low angle diffraction. This implies that the pinning site is closely related to the interfacial region. The result is compared to the case of Tb,,(FeCo),, films in which case the size is found to be about 60A 250A.
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as shown in Fig.1. The CO and Pt were deposited by rf and dc magnetron-sputtering, respectively. Four different conditions were chosen, namely (Co/Pt) multilayers were deposited onto a) Si(l11) substrate (Type l), b) Si (111) etched(-80A) by Ar (Type 2),, c) un-etched SIN, film which was deposited onto Si(ll1) (Type 3) and d) etched SIN, film on Si(ll1) (Type 4). The thickness of SIN, film was 850A. For all the cases, each layer thickness of COand Pt was 3 and 10A, respectively, and the substrate temperature during the deposition was ambient. The Pt layer was the first #l (CO/Pt)Xll
/2 (CO/Pt)Xl3
I4
/3 (CO/fl)XZO
(CO/Pt)dO
R
I. INTRODUCTION
R Multilayer films of Co/Pt have attracted much attention as a possible candidate for the next generation magneto-optical recording media, since they exhibit a strong magneto-optic effect at short wavelengths for higher recording density In spite of voluminous work on magnetic properties of this system, however, very little work has been made on the coercivity mechanism. The coercivity plays an important role in the recording process, since it governs the domain size, shape and read-stability Any fluctuation in coercivity may cause an inhomogeneity in domain shape, which in turn contributes to recording noise. Therefore, it is important to understand the origin of the coercivity in order to further improve recording performance. Zeper et aL6 discussed the coercivity and the magnetic hystersis loops in Co/Pt multilayers based on a stripe domain model developed for magnetic bubbles. They explained the saturation field as a function of number of bilayers with some fitting parameters, but did not explicitly address the coercivity mechanism, in particular the influence of a wall-pinning. It is known that the coercivity in Co/Pt multilayers strongly depends on fabrication conditions, sputtering/electron beam deposition, substrate conditions, type of an underlayer and so forth'. All those factors influence microstmctures such as interface, growth orientation, column size and thus magnetic properties and magnetization hysteresis loops. The aim of the present study is to understand the coercivity mechanism in Co/Pt multilayers through both magnetic measurements and structural analysis. For this purpose, several different substrate conditions were prepared in the present study.
.
.
'
11. EXPERIMENTAL Multilayers of (Co/Pt) were fabricated by sputtering in Ar at a pressure of 2x10-, Torr onto various substrate conditions,
pl0"
dch
F?
Subd.
Fig.1 Substrate conditions used in the present study.
deposited onto the substrate. The number of (Co/Pt) pairs, N was 13 for the types 1,2 and 20 for the 3,4 types. Such a difference in N was found not to cause any significant change in Hc g . Measurements of magnetic properties were carried out in a temperature range from 5K to 500K by a vibrating sample magnetometer, an alternating gradient force magnetometer and a torque magnetometer in fields up to 20 kOe. Structural analysis was made by high and low angle X-ray diffraction analysis (XRD), X-ray photo-emission spectroscopy (XPS), electron transmission microscopy (TEM) and atomic force microscopy (AFM). Observations of cross sections which were thinned by ion-milling were made by high resolution electron microscopy. 111. EXPERIMENTAL RESULTS AND DISCUSSION
A. Structural Analvsis Figure 2(a) shows the high angle X-ray diffraction pattern for the types 1 through 4 (R=1.54A). As clearly seen in all the types, the < 111 > texture along the film normal is present. The degree of the texture increases from the type 1 through 4. The d-spacing for (111) is about 2.220f0.005A for all the samples ,which is closer to the (1 1l)R spacing(= 2.265A) than CoFcc ( = 2.047A), reflecting a thicker pt ( = 10A) than CO ( = 3A). A high resolution electron micrograph for the type 4-sample clearly indicates a well defined (111) lattice image ,the spacing of which is 2.2f0.1A (Fig.3). Notice that a column
0018-9464/92$03.00 0 1992 IEEE
n55 gets wider as the film grows. The column width around the middle of the thickness is about 100A, and near the top surface is about 130A. Such a change in column width with thickness could be related to those measured by X-ray diffraction, and have been interpreted as "grain-size'). As can be seen in this photograph, the < 11 1 > orientation changes its direction slightly from column to column, but within the column the good lattice coherency is present throughout the thickness. The dispersion angle of < 11 1 > obtained by X-ray rocking curves for the type 4-sample is 5O. For the type 1-, 2and 3-samples, the dispersion angles are 17, 15 and 1l0, respectively. Figure 2@) shows the low angle X-ray diffraction pattern ( A 1.54A). It is clearly seen that the types 3 and 4 samples exhibit a well defined periodic structure, while the samples of type 1 and 2 show little evidence of it. This result is found to be consistent with the observation obtained by transmission electron microscopy. From the main peak (marked by an arrow) which corresponds to the first order diffraction of the
-
1 1'"
a
i
r __
Fig.3 High resolution electron micrograph of the type4 sample.
28
.... -.
.,,,->
1 .
Fig.2(a) High angle X-ray diffraction patterns (CuKa radiation). (b) Low pngle X-ray diffraction patterns (CuKa). (the arrow shows the first order peak.) (CO+ Pt) layer, one can gets the periodicity of 12.9A, as compared to the nominal of 13A. For the type-1, observations by XPS and TEM indicate that the surface region (-40A) of the unetched Si became a silicide (Si@)) which is amorphous. (Note that the Pt layer was the drst deposited onto the substrate.) On top of this silicide layer, the Co/R multilayer was grown with much less < 1 1 1 > texture, as consistent with the X-ray data. It is of interest to note that the Curie point for the type-1 ( -450K) is indeed close to that (-49OK) of Pt75Co,5alloys. The type-2 sample, on the other hand, was found to have two silicide layers. The one is the same as found in the type-1 and the other is the initial layer of CoPt(Si) which is also amorphous. The thickness of the second silicide layer is about 20A, above which the < 11 1 > texture is developed. The effect of etching SIN, on the < 111 > orientation alignment Is found to be related to the 'knoothnes? of the SiN surface before growing the Co/Pt multilayers. Observations by AFM indicate that the surface roughness gets improved as seen
C
,
Fig.4 AFM micrographs for a) unetched Si substrate, b) unetched SiN, underlayer film and c) etched SiN, underlayer film. Note that both the unetched and etched Si surfaces have nearly the same roughness.
2756
,
in Fig.4. The surface roughness of an unetched and etched Si is 1.0 -1.5A. The roughness of the unetched and etched SIN, films which were deposited onto the unetched Si substrates are 16A and 2.5A, respectively. In other words, the surface for the etched SiN, film is as smooth as for the unetched Si substrate. It is important to note that despite of the rougher surface of the type-3 than the type-2, the type-3 has the higher < 111 > texture, and also the higher perpendicular magnetic anisotropy, as will be discussed next.
B. Mametic Prouerties Fig.5 shows the polar Kerr hysteresis loops for all the samples, measured from the film-surface side. The loop of the type 1-sample is sheared, as compared to the types 2 through 4. Little difference in loop squareness is found among the type 2-,3- and 4-types except for the tail near the saturation shown by the arrows (B). Notice that the initial magnetization curve (A) of the type 1 shows no evidence for the wall pinning, since the magnetization increases at very low fields. This should be compared to the case of the type 4, as discussed later on (Fig.9). The intrinsic perpendicular magnetic anisotropy,Ku is plotted in Fig.6. Here, the perpendicular magnetic anisotropy constant Ku is defined as Ku = KO + 21cMs2 (Ms: saturation magnetization, KO: effective magnetic anisotropy constant.) The values of KO were estimated based on the field dependence of the torque for the field direction at 4 5 O from the film normal, and the saturation magnetization Ms was obtained from VSM. (The values of all Ms,KO and Ku are given per cm3 of the total
200 300 Temperature, T(K)
100
400
Fig.6 Intrinsic perpendicular magnetic anisotropy constant Ku as a function of temperature.
.-
L
Dispersion Angle, A@ (Des.)
d .
-
10
b]
9 h
a-
8 7E. 6 -
b
g.- 5 -
Fig.5 Kerr hystersis loops for the types 1, 2, 3 and 4.
2
9
b
-.-m-
432 -
b
1 -
(CO+ Pt) volume.) The trend of the change in Ku is nearly the same for all the types, decreasing with temperature. The type-4 sample exhibits the highest Ku values over the entire temperature range while the type-1 sample shows the smallest values. As mentioned above, though the type-3 sample has the rougher substrate surface than the type-2, it exhibits the higher perpendicular magnetic anisotropy over the entire temperature range. The KU is plotted in Fig.7(a) as a function of dispersion angle, together with the data of (Fe/Pt) multilayers . The Ku decreases with increasing dispersion angle. This suggests that the origin of the magnetic anisotropy is clearly related to the magnetocrystalline anisotropy, probably in a similar way to the case of (Co/Pd) multilayers as calculated by Daalderop and others'O. However, Hc behaves in a different way as shown in Fig.7@). Although Hc at 300K decreases with A@, it doesn't change significantly at 77K. The reason for the decrease with A 6 at 300K is due to the decrease in Tc for those different samples. Far below Tc, the coercivity is independent of the
0' 0
I
I
2
4
I
'
I
I
I
I
'
'
6 8 10 12 14 16 18 20 Dispersion Angle, A0 (Des.)
Fig.7 (a) Intrinsic perpendicular magnetic anisotropy Ku against dispersion angle, and (b) the coercivity vs. dispersion angle at 300 and 77K. dispersion angle. That is to say, the coercivity is not sensitive to the < 111 > orientation, but rather to nano-scale inhomogeneities of micormagnetics, much different from the perpendicular magnetic anisotropy mechanism. It may be of interest to note that the magnetic anisotropy values of (Fe/Pt) multilayers are higher at about A 0 -16O than for the (Co/Pt) case, though the origin of the anisotropy could be entirely different from each other. In Fig.8, the temperature dependence of He is given. The coercivity Hc decreases monotonically with increasing temper-
1
2757 ature. The rapid decrease of Hc for the type-1 is consistent with the magnetization change, in which case the Curie point is the lowest (-45OK). The feature of such a monotonic decrease in HC wlth temperature suggests that a single mechanism is responsible for Hc, which is contrast to permanent magnets12. Mg.9 (a) shows a magnetization hysteresis loop with the initial magnetization curves in the type-4 sample. This initial magnetization curve suggests the strong pinning for a wall motion in the Hc mechanism. The pinning field Hp is found to be about 0.8Hc for the type-4 sample. Also is shown in Fig.9 (b), depending upon the maximum applied field H,,, the "apparis much larger than the '/real// ent" Hc changes unless the H, Hc (-3.5kOe). It is also pointed out that even though the remanence Mr/Ms is unity at fileds just above Hc, the "apparent" Hc is not equal to /#real" Hc. This fact suggests that there Still exist the unreversed domains even when the applied field is above Hc. Those unreversed domains must be pinned at various sorts of the pinning sites, magnetic/nonmagnetic regions and they are the ones to expand when the applied field is reduced I
81
j
0
7 t
2
I
2.5 3 3.5 4 Maximum Applied Field,, , H ,
t
4.5 (kOe)
5
Fig3 (a) A magnetization curve of a (SACo/lOAPt)xZO multilayer film at 300K. (b) "Apparent" Hc vs. maximum applied field.
0
50
100 150 200 250 300 350 400 Temperature, T(K)
could be larger than 4 ~ as , demonstrated in some permanent magnetsI2. (Here, we use the CGS unit.) The model predicts the dependence of a on an inhomogeneity size r,. Depending on ro which is either smaller or larger than the wall width 6,, one can write the coercivity Hc as follows:
Fig.8 The temperature dependence of coercivity.
and then increased in the reversed directions, thus lowering "apparent" Hc values. Based on those observations, we can say that the wall pinning must be the major contributor to the Hc mechanism in such high Hc (Co/Pt) multilayers. On the other hand, in the case of the type-l sample (Fig.5) in which case Hc is not so high and the rapid increase of the magnetization at low flelds ((Hc) takes place, the nucleation mechanism is predominant, rather than wall-pinning. Kronmuller and others 1-13 have developed the theory for the coerdvity mechanism. The model assumes a domain wall pinnlng by a magnetically inhomogeneous region which is described by toln size. Such an inhomogeneous region is defined as the region which has a different exchange constant A' and the anisotropy constant K' than those for the matfix homogeneous region with A and K, respectively. Generally, the coercivity Hc(T) is given by
where a is a parameter responsible for a microstructure determined by the coupling strength between the wall and the wall N is a demagnetization factor, which consists of pinning site. , the macroscopic and microscopic demagnetizing effects. The macroscopic factor is given by the sample geometry and the microscopic one relates to the size and type (magnetk/nonmagnetic) of the pinning site. Therefore, Ne,
where K and 'K are both related to the variation of the exchange coupling constant and the anisotropy constant from the matrix to an inhomogeneous region. The wall width 6, is given by n(A/K)" ,which is temperature-dependent as well. By using the measured values of all Ku, Ms and Hc, and by assuming the exchange constant A for the host region is 1x10-6 erg/", all the samples were tested for both equations (2) and (3). One example for the type 4-sample is given in Fig.10 (a) and (b) for ro4dB and robsB, respectively. It is clearly seen that the case of ro4S, is fitted linearly in the entire temperature range to the observed values, whereas in the case of r,bS, it
Table 1 Summary of the inhomogeneity #
Film-structure
1
(Co/Pt)xl3 (Co/Pt)xl3 (Co/Pt)xZO (Co/Pt)xZO
2 3 4
Substrate unetched Si etched Si unetched SiN onto Si etched SiN onto Si
_r,cA)
---
--_
,31 16 4
13.8 '
4.9 3.6
.
2758
.
does not. From the fitting, one can obtain the values of ro and Ne, by assuming the numerical parameter H. which involves A' and K'. We assumed that the exchange constant A' and the the adsotropy constant K' for the pinning site to be %A and 0 , respectively. Under this condition, one obtains the size ro and the demagnetizing factor N,, as shown in Table 1. The inhomogeneity ro decreases from the type 2 through 4, in which order the dispersion angle becomes smaller and the perpendicular magnetic anisotropy Ku increases. The demagnetizing factor Ne, is accordingly decreasing in going from the type 2 through 4. No linear fitting was found for either cases (eqs.(2) and (3)) for the type 1-sample over the entire temperature range under consideration. This is consistent with the observation that no indication of the wall pinning from the initial magnetization curve for the type-1 sample is found, as mentioned before. Given the assumption made here, it may be worthwhile discussing more about such a small pinning region. It should be first noted that a size of the pinning site increases as the interfacial sharpness gets worse as seen in Fig.Z(b). This implies that the such a pinning site is closely related to a roughness of the interface between Pt and CO layers. It is known that CO atoms induce the magnetic moment on the nearest neighbor P t atoms (for example, ~ 0 . 3p B for ~ the second COR,) l4 ,and it decays fast in distance ( 0 . 1 3 , ~for nearest). This means, as the first approximation, that the Pt atoms can be polarized near the interface, but hardly inside the Pt layer. Such an argument has been already discussed by Crangle et al. l5 who discussed the magnetic moment of Pt in CO-Pt alloys. Therefore, the wall may only be localized in CO layers and in a region nearby the interface. Such walls can be coupled through the magnetostatic force, as discussed by Draaisma and de Jonge 1 6 . Under the influence of the external field, the walls which start to move can be influenced by pinning sites which are present only in and nearby CO layers, but NOT
I
120 1 110 100 90
50 40 30
20 0
10 20 30 40 50 60 70 80 90 100 (2Ku/Ms2)SB(1O-4cm)
Fig. 11 A linear !Wing for the data of Th,,(FeCo),,
film after eq.(3)-
Pt regions! Therefore, any type of fluctuation around the interface between CO and Pt layers, which is of the order of a few angstroms should play a role as a pinning site for the wall motion. In this view, the results of 4A and 16A for the type 4 and 3 are not so surprising. The value of 31A for the type-2, on the other hand, might be reflecting an inhomegeneity nature in the film plane, in addition to the variation along the film normal. The smaller Ne, than 4n implies that the contribution of the nonmagnetic region to the demagnetizing effect. It is noted that the sharper the interfacial region, the smaller the Ne, becomes. This resuIt suggests that each Pt layer acts as the nonmagnetic layer as far as the coercivity mechanism goes. This idea is consistent with the model proposed by Zeper et al.6.
The work of the computer simulation on the nucleation and wall pinning mechani~m'~.'~ by Mansuripur and Giles has shown that the size of the wall pinning should be around or larger than about 100 A for a typical TbFeCo film used for magnto-optical recording. A recent experiment by Hurst on a jitter measurement of recording performance supports the similar size of the inhomogeneity in TbFeCo 19. The result of (Co/Pt) multilayers is much smaller than this value. To clarify this matter, an experiment was made for a sample of Tb,,(FeCo),,, which has the high squareness of the hysteresis loop and Hc = 4.9kOe at 300K. The initial magnetization curve suggests that the wall pinning is the major factor for this high Hc. Measurements of Hc, Ms and Ku were made in a temperature range from 77K to 420K. As shown in Fig.11, the fitting is made possible for eq.(3), but not for eq.(2). However, depending on the temperature range, the values of ro and Ne, would change as (ro, Ne,) = (64A, 10) (250A, 30). (Here, we used the exchange constant A' to be O.lA, as the computer simulation work2" did.) The observed size in TbFeCo is favorably compared to the work by Fu et al. who showed that the patch size ( = inhomogeneity size) responsible for Hc should be larger than about lOOA for TbFeCo*". The larger value of Ne, than 4% suggests that the inhomogeneous region is magnetic. It is ttue that those values of ro and Ne, are subject to the assumption of A,K and other parameters, and also to the temperature range applied. Nevertheless, even with this uncertainty, the important difference between TbFeCo and (Co/Pt) multilayers is clearly demonstrated in the pinning size. This difference in ro and Ne, implies the different nature of the wall pinning site to each other.
6z 5
.8
.9
1
1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 (ZKU/MS*)/~~ (I o%m)
-
6 b 5
1
1.1
1.2 (ZKU/MS~)S,(
1.3 1.4 I0-4cm)
1.5
Fig.10 (a) A linear fitting to the experimental data after eq.(2), and h) after eq.(3).
2759
IV. SUMMARY It is shown that the wall pinning is a major factor for high coercivity in Co/R multilayers. Such pinning site is about 4A in size for the case of an etched SiN, film asanunderlayer, in which case the coercivity is between 3 and 4 kOe at room temperature, and a loop is rectangular. With less interfacial sharpness, the pinning size increases. An inhomogeneity near the interface between COand Pt is believed to play a major role as pinning for a wall motion. The present result is compared to a Tb2,(FeCo),, film which is found to have a 60-250A pinning
size. ACKNOWLEDCMENT The authors are grateful for Professor Masud Mansuripur at University of Arizona for his valuable comment. Also, they acknowledge Mr.Te-ho Wu at the University of Arizona for his magnetic measurements of the Tbz6(FeCo),, film. They thank Mr. Tony Logan of IBM Almaden Research Center for his AFM observations.
REFERENCES 1. W.B.Zeper, FGreidanus and P.F.Carcia, "Evaporated Co/Pt layered structure for magneto-optical recording: IEEE Trans.MAG25,3764-3766 (1989); P.F.Carcia, W.B.Zeper and F.J.A.M.Greidanus, "Pt/Co multilayers-a new magneto-optical recording media" MRS Proceeding 150, 115-120(1989). W.B.Zeper, AP.J.Jongenelis, B.A.J.Jacobs, H.W. van Kesteren and P.F.Carcia, "Magneto-optocal recording in Co/Pt multilayer and GdTbFe- based disks at 820, 647 and 458-nm wavelength": presented at the Intermag'92, #DA-03. 2. S.Hashimoto, Y.Ochiai and K.Aso, "Light wavelength dependence of magneto-optical properties in ultrathin Co/Pt and Co/Pd multilayeredfilms: Japanese J.AppLPhys. 28,L1824- 1826(1989). 3. C.-J.Lin and H.V.Do, "Magnetic and structural properties of Co/Pt multilayers": IEEE Trans. MAG-26, 1700-1702 (1990). 4. S.Hashimoto, A.Maesaka and Y.Ochiai, "Recording on Co/Pt magneto- optical disks using a 488-nm wavelength laser" J.Appl.Phys. 70, 5133-5135(1991). 5. H.W.van Kesteren, A.J.den Boef, W.B.Zeper, J.H.M.Sprint and B.A.J.Jacobs, "Scanning magnetic force microscopy on Co/Pt magneto- optical disks": J.Appl.Phys. 70, 2413-2422 (199 1). 6. W.B.Zeper, H.W.van Kesteren, B.A.J.Jacobs and J.H.M.Spruit, "Hysteresis, microstructure and magneto-optical recording in Co/Pt and Co/Pd multilayers": J.Appl.Phys. 70, 2264-2271 ( 1991). 7. Z.G.Li and P.F.Carcia, "Microstructural dependence of magnetic properties of Pt/Co multilayer thin films" : J.Appl.Phys. 71, 842-848 (1992). D.Weller, H.Notarys, T.Suzuki and G.Gorman, "Thickness de8. pendent coercivity in sputtered Co/Pt multilayers" presented at INTERMAG'92,#DA-02. 9. S.Iwata, S.Parkin, T.Suzuki and D.Weller, "Perpendicular magnetic anisotropy in FeCo/Pt multilayer films": presented at the Intermag'92, #JQ-01. 10. G.H.O.Daalderop, P.J.Kelly and M.F.H.Schuurmans, "First principle calculation of the magnetic anisotropy energy of (Co)n/(X)m multilayers: Phys.Rev.B, 7270-7273 (1990). 11. H.R.Hilzinger, "The influence of planar defects on the coercivity field of hard magnetic materials": AppLPhys. 12,253-260 (1977).
12. H.Kronmuller, K.-D.Durst and M.Sagawa, "Analysis of the magnetic hardening mechanism in RE-FeB permanent magnets": , J.Magn. Mag.Mat. 74,291-302 (1988). 13. H.Kronmuller, "Theory of nucleation fields in inhomogeneous ferromagnets": phys. stat. sol. (b) 144, 385-396 (1987). 14. M.C.Cadeville, C.E.Dahmani and F.Kern, "Magnetism and spatial order in Ni-Pt and CO-Pt alloys": J.Magn.Mag.Mat.54-57, 1055-1056 (1986). 15. J.Crangle and D.Pearson, "The magnetization of ferromagnetic binary alloys of cobalt or nickel with elements of the palladium and platinum groups": Proc. Royal. Soc. A25, 509-519 (1960); J.Crangle and W.R.Scott," Dilute ferromagnetic alloys": J.Appl. Phys. 36,921 (1965). 16. H.J.G.Draaisma and W.J.M. de Jonge, "Magnetization curves of Pd/Co multilayers with perpendicular anisotropy": J.Appl.Phys. 62(8), 3318-3322 (1987). 17. M.Mansuripur, R.C.Giles and G.Patterson, "Coericivity of domain wall motion in thin films of amorphous rare earth-transition metal alloys": J.Mag.Soc.Japan, 15, 17-30 (1991). 18. R.Giles and M.Mansuripur, "Possible source of coercivity in thin films of amorphous rare earth-transition metal alloys":Comp.Phys. 5, 204-219 (1991). 19. J.Hurst, D.Cheng, C.R.Davis and M.Chen, "Characterization of MO recording media on the micron scale": Proceeding of Optical Data Storage Topical Meeting (#WA4, San Jose, Feb.1992). 20. H.Fu, M.Mansuripur, GPatterson and R.Giles, "Investigations of effects of nanostructures on the observable behaviors of thin film magnetic media using large scale computer simulation": Proceeding of Data Storage Topical Meeting (#TuD3, San Jose, Feb.1992).
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