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DECAY OF THE REMANENT MAGNETIZATION IN
CoCr FILMS
D. Lottis, E. Dan Dahlberg, J. Christner, J. Lee, R. Peterson, R. White
To cite this version:
JOURNAL DE PHYSIQUE
Colloque C8, Suppl6ment au no 12, Tome 49, d6cembre 1988
DECAY
OF
THE REMANENT MAGNETIZATION IN CoCr FILMSD. K. Lottis, E. Dan Dahlberg, J. A. Christner
',
J. I. Lee',
R. L. Peterson'
and R. M. White'
School of Physics and Astronomy, University Of Minnesota, Mpls., MN 55455, U.S.A.
Abstract. - We have performed measurements of the decay of a signal recorded on single layer CoCr films using a thin film ring head. These results are compared with data obtained by magnetometry of magnetization decays in uniformly saturated samples. The importance of the demagnetizing fields on these measurments is discussed briefly.
1. Introduction
Previously we have reported the results of studies of magnetization decay in single-layer CoCr films on A1 substrates [I, 21. These are films prepared in such a manner as to have the easy magnetization axis per- pendicular t o the film plane, taking advantage of the c-axis alignement of the rod-shaped grains normal t o the plane [3]. Webb, Schultz and Oseroff [4] have per- formed similar studies, with similar experimental re- sults, on films deposited on glass. In reference [l] we concentrated on the temperature dependence of the de- cay rate of the remanent magnetization in zero applied field, while reference [2] was devoted to understanding the relationship between the applied field, the internal demagnetizing field, and the decay rate.
In the present paper we discuss the results of mag- netization decays observed in the same samples us- ing conventional magnetic recording procedures, which rely on alternating magnetization directions in the film. We compare this data to the remanent magne- tization decay measurements performed using SQUID
magnetometry where the samples are uniformly mag- netized prior to measurements.
2. Experimental results
The data of references [I, 21 were taken using a com- mercial SQUID magnetometer [5], beginning about a minute after the removal of a saturating field and con-
the magnetization M when the time (t) has the value
t = 1 (in whatever units are employed), while S is the decrease of M per "e-cade" of time. Fits were quite satisfactory for the 2 t o 3 decades of time measured.
The measurements we now report were performed on the same films using an entirely different method. The films of 0.5 to 1.5 microns in thickness were pre- pared by RF sputtering on blank A1 alloy hard disks, as described in reference [I]. The microstructure of the films is characterized by rod-shaped grains which grow along the normal direction, having diameters of about
0.05 to 0.1 microns in these films [8].
After an initial DC erase of the entire disk, a square wave signal was written to the films using a thin film ring head having a 0.71 micron gap. Frequencies var- ied from 1.37 t o 10 MHz, with a relative head to media velocity of 1390 cm/s. This signal density corresponds t o wavelengths varying from about 0.5 to 5 microns. The track width was 25 microns in all cases. The de- cay of the readback signal was then measured using the same head, beginning about 1 second after recording, for times up t o several thousand seconds. The signal decayed steadily throughout the experiment. An off- sett anomaly in the data at t
=
5 s is thought to be due to initiation of radial track scans, not reflecting an effect in the magnetization of the film. These data are once again satisfactorily characterized by fitting to equation (1) above. Figure 1 shows a few typical de- cays, and compares the recording data to the SQUIDmagnetometry data. tinuing for several hours. Measurements were taken
both in zero field [l] and in the presence of applied
fields [2], with sample magnetizations having been ini- 3. Discussion tially saturated by applying fields of at least 4 kOe.
The slow decays of the magnetization were quantified The most obvious feature of the data is the decrease by fitting data t o the logarithmic form in the decay rate with increasing recording density. One notes also that for the lowest recording density M = MI
-
S In (t),
(1) shown, which corresponds t o a wavelength f about 5 microns, the rate for the decay of the recorded signal as used in existing phenomenological models for the amplitude has nearly reached the value observed in a magnetic aftereffect [6, 71. Here MI gives the value of simple magnetized uniformly.'control Data Corporation, Minneapolis, M N 55435, U.S.A.
C8 - 1990 JOURNAL DE PHYSIQUE
t
L G U I S vs. densitydci
I
Fig. 1. - A few typical FtEAD/WRITE; decays, a t 3 record- ing densities, are compared t o the results obtained using SQUID magnetometry for a uniformly saturated sample. Inset shows variation of rate S, obtained from fits t o equa- tion (I), with the recording density. Values of M are nor- malized t o MI from fit.
That a relation exists between the decay rate and the recording density may be understood if we first note that demagnetizing fields appear to drive the de- cay of the remanent magnetization in the absence of
o S vs. H
x S vs. (H - DM)
X
-4000 -2000 0 2000
H - Appl. Field (Oe)
Fig. 2. - The logarithmic decay rate S obtained by fitting data t o equation (1) shows a very broad, slow variation with the externally applied field, but is sharply peaked near the sample's coercivity when plotted against the effective de- magnetization field Hi = Ha - DM1, where Mi is obtained from fit t o equation (1) and D is the demagnetizing factor, taken here as 4 [8, 111.
an applied field [2]. This claim is supported by consid- ering the data displayed in figure 2, which shows the variation of the rate S with the applied field. The slow variation of S over a range which is broad on a scale of the coercivity
H,
seems to be at odds with exist- ing models for the effect, and contrasts with what has been observed in other systems [6, 7, 9, lo]. However, when plotted against an effective internal field which takes account of the demagnetization effects, the data peaks sharply at a value near the sample's coercivity, as expected [6, 71.Thus, although the recorded track has zero net mag- netic moment, the magnetization within a single bit is expected to decay just as for a uniformly saturated film, provided its area is large compared to the film thickness and grain size. The demagnetizing field ex- perienced by material within a bit will decrease as the bit size decreases, and the resulting decay rates fall off correspondingly. Even at the highest densities possible (bit length being limited by grain size) there will be residual demagnetizing fields due t o dipole-dipole in- teractions within the width of the bit, which is about 25 microns here. This agrees with the observed persis- tence of decays even to the highest densities.
[l] Lottis, D. K., Dahlberg, E. D., Christner, J., Lee, J. I., Peterson, R. and White, R. W., J. Appl.
Phys. 63 (1988) 2920.
[2] Lottis, D. K., Dahlberg, E. D., Christner, J., Lee,
J. I., Peterson, R. and White, R. W., to be pub- lished.
[3] Iwasaki, S. and Ouchi, K., IEEE Trans. Magn.
Mag-14 (1978) 849.
[4] Webb, B. C., Schultz, S. and Oseroff, S., J. Appl. Phys. 63 (1988) 2923.
[5] Quantum design, Inc/; San Diego, CA.
[6] Gaunt, P., Philos. Mag. 34 (1976) 775, and earlier references cited therein.
[7] Berkowitz, A. E., IEEE Trans. Magn. Mag-22
(1986) 466.
[8] Khan, M. R., Lee, J. I., Seagle, D. J. and Fer- nelius, N. C., J. Appl. Phys. 63 (1988) 833. [9] Kloepper, R. M., Finkelstein, B. and Braunstein,
D . P., IEEE Trans. Magn. Mag-20 (1984) 757.
