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MAGNETIC VISCOSITY EFFECTS IN DIGITAL
RECORDING MEDIA
S. Uren, K. O’Grady, R. Chantrell
To cite this version:
JOURNAL DE PHYSIQUE
Colloque C8, Suppl6ment au no 12, Tome 49, d6cembre 1988
MAGNETIC VISCOSITY EFFECTS IN DIGITAL RECORDING MEDIA
S. Uren, K. O'Grady and R. W. Chantrelll
Department of Physics, UCNW, Bangor, Gwynedd, LL57 BUW, G.B.
Abstract. - We study the relationship between the coefficient of magnetic viscosity (S = -dr/d In t ) and the switching field distribution (S.F.D.) for a magnetic recording tape. The SFD can be measured as the media is magnetised or demagnetised. We find that the appropriate SFD correlates with S in the corresponding parts of the hysteresis loop.
Introduction and t h e o r y curve, and can be expressed as
The time dependent behaviour of a magnetic sys- S =
xi,,
(1-
H/
~ k ) ? (4) tem is well known and is responsible for effects suchas print-through [I]. The phenomenon essentially arises from thermally activated transitions over the anisotropy energy barrier (AE)
.
In the presence of an applied field (H) A E is given byWhere K is the anisotropy energy density, Hk is the anisotropy field, Hk=2K
/
ISB, andIS^
is the bulk sat- uration magnetisation. For a single particle volume system a Neel-Arrhenius law is expected, characterised bvT-l = fo exp
-
( A E/
kT) . (2)Where the frequency factor fo LX 10' s-'. For all real systems there is a distribution of the energy barriers (AE)
,
which can lead to a logarithmic dependence of the magnetisation with time. The coefficient of mag- netic viscosity (S) can be defined as S = -d4/ d In t ,where
7
= I / I,.S varies with the applied field (H),
passing through a maximum in the region of the coer- civity (H,)
.
In a previous paper [2] we examined the validity of a relationship due t o Street and Wooley [3] who pro- posed the relationship
S = X i r r Hf. (3) Where Xi, is the irreversible susceptibility and Hf is the "fluctuation fieldn
.
xi,, is determined by differen- tiation of an appropriate remanence curve, for exarn- ple, in the 1st quadrant Xi,, = d (IRM)/
d H which can be related to the time dependence in the first quadrant of the hysteresis loop. The differnce in the behaviour in different parts of the loop is due to the differing in- teraction configuration arising after the sample is sat- urated.From the critical volume approach [2], the time dependence is related to the appropriate remanence
Since the theory is based on a system of aligned particles, the scaling factor x is introduced t o take into account the degree of alignment of the particles in the system. Y
In this paper we present data to show that equa- tion (4) holds for either
xi,,
derived from the IRM curve or the d.c. demagnetisation remanence curve when fitted to experimental time dependence data from the appropriate quadrants, and we demonstrate the effects of the interaction configuration by consid- ering the form of the Henkel plot [4] for the system.Experimental
The sample studied in this work is a Xidex UK LTd IBM 3480 data tape containing a partially aligned dis- persion of CrO2 particles.
To test the validity of equation (4) for the magnetis- ing and demagnetising cases, two sets of measurements were made using a vibrating sample magnetometer. For the demagnetising case, the investigation consisted of time dependence measurements and the d.c. demag- netisation remanence curve in the 2nd and 3rd quad- rants of the hysteresis loop as reported previously [2]. The magnetising case was investigated by the appro- priate measurements in the 1st quadrant. These were the isothermal remanent magnetisation curve and phe magnetising time dependence, for both of these cases the sample was initially demagnetised. The time de- pendence measurements were made as reported previ- ously
[Z],
but since the magnetisation in the first quad- rant increases with time in the first quadrant increases with time in the presence of a constant applied field, the coefficient of magnetic viscosity S, is of opposite sign (i.e. positive) than that of the demagnetising case.' ~ c h o o l of Physics and Astronomy, Lancashire Polytechnic, Preston, Lancashire PR1 ZTQ, G.B.
C8 - 1928 JOURNAL DE PHYSIQUE
Results and discussion
The following characterisation parameters were de- termined from the hysteresis loop. The coercivity Hc was 470 Oe and the loop squareness was 0.8.
The remanence curves were fitted using a cubic spline NAG routine which were then differentiated to give the switching field distributions (or X irr (H)) for the magnetising and demagnetising cases. Figure 1 shows the isothermal remanence curve and the result- ing S.F.D.
perimental and theoretically derived time dependence for the magnetising case (Hk = 750, X = 50). A simi- lar plot for the demagnetising case is shown previously
( 2 ) (Hk = 680, X = 50).
The Henkel plot (Fig. 3) shows how the different initial states of the system (i.e. demagnetised or mag- netised) govern the interaction behaviour.
a
bc
DEMAG
0.8-
a i
..
Fig. 3. - Henkel plot, of IRM vs. demagnetisation rema- nence for the CrO2 data tape.
I R M
* **
*
*
*
*
*
*
*
*
*
Fig. 1. - IRM(-) and S.F.D. (*) for the CrO2 data tape.
The values of SFD (or xi,) versus field were then Conclusion
""'I
*)
\.
[I] Flanders, (1987) 2918. P. and Sharrock, M., J., Appl. Phys. 62
[2] Uren, S., Walker, M., O'Grady, K. and Chantrell, I 4 R. W., IEEE Trans. Mag., Proc. Conf.
400
FIELD
Oe EMMA'87.used in equation (4) to predict the time dependence The measurements have shown that q u a - curve for the sample. Figure 2 shows a plot of the ex- tion (4) is equally valid for the magnetising and de- magnetising magnetic viscosity effects when used with the appropriate remanence curve. In the absence of in-
Fig. 2. - The solid line shows the theoretical fit (Eq. (4)) i3] Street, R' and Wooley, J., 'sy'' A 62
to the experimental time dependence data (*) for the mag- (1949) 562.
netising case. [4] Henkel, O., Phys. Status Solidi 7 919 (1964).
s
0.012 .,
teractions these two curves for Xi,, would be the same. However the two experimental time dependence curves are different, which suggests that interactions are a sig- nificant feature of the time dependence measurements. The disagreement between the predicted values of S and the experimental values of S at large field is due
