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MAGNETIC CORRELATIONS IN CrO2 TAPES
STUDIED BY NEUTRON DEPOLARISATION
R. Rosman, M. Rekveldt, H. Cramer
To cite this version:
JOURNAL DE PHYSIQUE
Colloque C8, Supplkment au no 12, Tome 49, dkembre 1988
MAGNETIC CORRELATIONS IN Cr02 TAPES STUDIED BY NEUTRON
DEPOLARISATION
R. Rosman ( I ) , M. Th. Rekveldt ( I ) and H. A. J. Cramer (2)
( I ) Interfacultair Reactor fnstituut, Technical University Delft, 2629 JB Delft, The Netherlands
(2) Philips Research Laboratories, 5600 J A Eindhoven, The Netherlands
Abstract. - Correlations between the magnetisation directions of particles and the angular distribution of the mag- netisation were studied by means of the 3-D Neutron Depolarisation technique. After any magnetisation process, large elongated volumes in which the particle magnetisations are almost parallel are found. Thermal annealing considerably changes the observed correlations.
Introduction
Noise properties of magnetic recording tapes are highly dependent on correlations between the parti- cle magnetisations [I]. Few techniques are available to measure these correlations.
This paper deals with a study of magnetic corre- lations in commercial CrO2 video tapes. Using 3-D polarisation analysis [2] of a neutron beam transmit- ted through tapes with different magnetic histories, the mean correlation length perpendicular and paral- lel to the plane of these tapes can be deduced. When interpreting the data, the tapes are assumed to con- tain superdomains consisting of particles with strongly correlated magnetisation directions. The results are compared qualitatively with noise measurements. Theory
Polarisation analysis yields a depolarisation matrix
Dij. From the determinant of thix matrix, the mean
correlation length along the neutron path, (6)
,
is ob- tained the relation:1
Dl = exp (- B ? c ~ ~ (S2)/
(6))Here, B, is the spontaneous magnetic induction, c = 4.6 1014A ~ - ' . m - ~ (A = 0.35 nm), and d the trans- mission length. The quantity S is determined by the particle size and orientation, the mean magnetisation, correlations between the particle magnetisations and local variations in the particle loading.
In the interpretation of (6)
,
positive correlations are assumed to be due to the presence of superdomains in which the particles have almost parallel magnetisation directions. Then, the correlation length S can be writ- ten aslation parameter, describing negative correlations be- tween particles or superdomains. K is defined as:
with nk the unit vector in the direction of the magneti- sation in the kth particle/superdomain [3]. The mean orientation of the superdomain magnetisation is char- acterised by the direction cosines yi = (mq)
,
which can be deduced from Dij [3].It follows that by measuring Dij for different trans-
mission directions one obtains the parameters describ- ing the angular distribution of and the correlations be- tween the particle magnetisations. Independently, the correlation along the length direction (y-axis) can be deduced from noise measurements [I]. In this paper, only a qualitative comparison between noise and de- polarisation measurements is made.
Experimental
The tapes studied consist of a polyester substrate with a coating (magnetic thickness 1.0 pm) of needle- shaped CrO2-particles of mean length 200 nm and diameter 20 nm, immersed in a polymer binder. The particles are oriented mainly along the y-axis. The magnetic states of the tapes are given in table I. Ini- tially, all tapes have been magnetised to saturation along the vaxis. After wards, the magnetisation of tb ,e 2 and 3 has been decreased by applying a field in the direction reversed to the magnetisation, tape 4 has been bulk erased and tape 5 thermal annealed. Samples consisting of 200-400 layers (yz-plane) were used in the depolarisation measurements, which were Table I. - Measured quantities and magnetic states of
the tapes.
b = ~ ( l - m ~ ) ( l + ~ ) / ( l - ~ ) (2) n x . treatment m, s ( 0 = 0 ) 7~ (0 = O ) A x
(v
-
axis) (nm)abs. errors : (rt2) :f 0.04)
with A the particle or superdomain size in which all
;
:z
122;~.
::it
;:
o.93 25
*
positive correlations are included, m the reduced mag- 3 d c
-
demag. 0.00 28 0.95 95 25 15 18 -0.55 4 bulk erased 0.00 24 0.94 82 8*
42;
z;,y
netisation of the tape (m = M/Ms) and K a corre- s annealed 0.00 12 0.8 20
-
C8
-
1984 JOURNAL DE PHYSIQUEperformed for different values of 8 (-70'
<
0<
70°, Fig. la).The noise measurements were carried out on a ro- tating tape setup.
tape 2
Fig. 1. - (a) Sketch of setup (P: polariser, S: sample, A: analyser, D: detector) (b-f) Results of depolarisation measurements (A) and fitted curves.
Results and conclusions
Figures lb-lf give the quantity ((6')
/
(6) cos (8) plotted verstcs 8, together with fitted curves. For the fits we assumed an angle-independent correlation pa- rameter K, a mean superdomain length L and thick- ness A and superdomain magnetisation orientations within a cone oriented along the y-axis with apex 2a. Infinite long superdomains parallel to the length direc- tion of the tape should yield a horizontal line. Table I gives the values of 6 i d y, for 8 = 0, following directly from the measurements, the values of K I A and a as a result of the fit and those of A and K , following from a comparison of the measurements with each other.In tape 1, for all needles the y-component of the magnetisation has the same sign (K = 0 ) . As A
>>
20 nm and L>>
200 nm it is concluded that the parti- cles are grouped in large superdomains instead of hav- ing uncorrelated magnetisation directions. From theangle dependency of (6"
/
(6) it can be deduced that the cone in which the superdomain magnetisations are oriented has an apex of 2a: = 55'. This is in good agreement with the value of y, for 8 = 0.At B = 0, the correlation lengths of tapes 2, 3 and 4 equal that of tape 1. Magnetic superdomains are ob- served also here, which are assumed to have the same thickness. The y-component of the magnetisation of some superdomains has been reversed, resulting in an anti-parallel orientation and a strong negative corre- lation. In tape 2 and 3, the angular spread of the superdomain magnetisation is smaller than in tape 1. It was impossible to determine L accurately in these tapes. In tape 4 the superdomain length has decreased in comparison with tape 1.
In the annealed tape (tape 5), the correlation length at 8 = 0 is half the correlation length in the other tapes. In this case, it is impossible to deduce both A and K. On physical grounds a negative correla- tion between individual particles is expected. As the mean particle diameter amounts to 20 nm, we find K = -0.3, which equals the value describing an anti- parallel arrangement of particles in an hexagonal struc- ture. The value of a following from the value of y, at perpendicular transmission amounts to 40°, which is in good agreement with the maximum value of the re- duced remanent magnetisation. It was not possible t o fit the measurements assuming an angle-independent correlation parameter.
The noise levels of the dc-(de)magnetised tapes ap- pear to be about equal. The noise level of the bulk erased tape is 6 dB lower. As the noise level increases with increasing superdomain volume, this is in qual- itative agreement with the results presented above. Noise measurements on annealed tapes could not be performed.
A more comprehensive analysis of the measurements will be given in [4].
In conclusion, with the ND technique we demon- strated the presence of strong magnetic correlations in particulate recording tapes, corresponding to the pres- ence of elongated superdomains.
[I] Nunneley, L. L., et al., IEEE Trans. Magn. Mag. 23 (1987) 1767.
[2] Rekveldt, M. Th., 2. Phys. 259 (1973) 391-410. [3] Rekveldt, M. Th., J. Magn. Magn. Mater. 1
(1976) 342.
