Here we present wavelength dependence measurements of Co/Pd and Co/Pt superlattice samples with different compositions. We explore the relationship between the composition and the magneto-optical spectra. The induced magnetization in the Pt of Co/Pt or in the Pd of Co/Pd samples plays an important role in the magneto-optical activity, and is discussed for the samples measured. The experimental set-up and the samples used are described. The measurement results of one Co/Pt sample and a series of Co/Pd samples are discussed

Erwin, J. Kevin

Mansuripur, M.

Zhou, Feng Lei

English

NASA Technical Reports Server

Appendix B I _ 71
N92- o4..
WAVELENGTH DEPENDENCIES OF THE KERR ROTATION
AND ELLIPTICITY FOR THE MAGNETO-OPTICAL RECORDING MEDIA
Feng Lei Zhou, r. Kevin Erwin, and M. Mansuripur
,7
Optical Sciences Center, University of Arizona, Tucson, AZ 85721
(
I. INTRODUCTION
Amorphous rare earth-transition metal alloys are the conventional media of erasable
optical storage. Recently there has been a flurry of activity in the area of thin film
superlattice-type structures, such as Co/Pt and Co/Pd, suggesting that these materials
may be used in second generation devices. In both cases, understanding the wavelength
dependence of the megneto-optical Kerr rotation and eilipticity is important for the
application of these media, as magneto-optical recording has the potential for higher data
density at short wavelengths. Also understanding the physics of the magnetooptical
interactions is dependent on the availability of accurate measurements of the spectral
characteristics of the Kerr rotation and ellipticity t,l. In this paper we will present
wavelength dependence measurements of Co/Pd and Co/Pt superlattice samples with
different compositions. We will explore the relationship between the composition, and
magneto-optical spectra. The induced magnetization in the Pt of Co/Pt or in the Pd of
Co/Pd samples plays an important role in the magneto-optical activity 3 and it will be
discussed for the samples we measured. In section II we present the experimental set up
and describe the samples we used. In section III the measurement results of one Co/Pt
https://ntrs.nasa.gov/search.jsp?R=19920005686 2020-03-17T14:13:28+00:00Z
72 Appendix B
sample and a series of Co/Pd samples are discussed. Finally v,e present ,_oncluslons tn
section IV.
II. Experimental Procedure and Samples
Kerr rotation, eilipticity and reflectivity are measured simultaneously y a novel
method of Kerr spectroscopy 1. The spectral range of measuring system is from 350 nm
to 1100 nm and the whole operation is fully automatic. The resolution of the system is
about _.005 degrees for Kerr rotation and _.01 degrees for ellipticity.
The Co/Pd samples we used here are made by a sputtering process and have film
thicknesses from 5.5 nm to 33 rim. They have a layer structure of 0.2 nm Co and 0.9
nm Pd. All of them are sputtered on glass substrates and have no overcoating. The
measurements are done from both the film side and substrate side. All the samples have
square hysteresis loops and have coercivity fields betweeno.3 KOe and 2.5 KOe. A
domain can be written on the 33rim thick samples. Table 1 shows the full list of all the
Co/Pd samples. For the Co/Pt sample the composition of the sample is .4 nm Co and 1
nm Pt with a total thickness of 19.1 nm. It has no dielectric overcoating. The hysteresis
loop is square for this sample and the coecivity field is 1.5 KOe. Good domains can be
written on the sample. Disks made of this sample have a CNR value of 53.5 dB (with
dielectric coating). The substrate material is glass.
III. Measurement Results and Discussions
Appendix B 73
The wavelength dependence of the Kerr rotation, ellipticity and reflectivity al_ays
consist of two contributions: one is the interference of the overcoating, the film and the
substrate and the other is the electronic structure of the material. The interference effect
has been known for years 4 and it plays an important role in enhancement of the magneto-
optical signal in practice. To investigate the effect of the electronic structure of the
materials, we need to separate these two effects. The material properties are determined
by the dielectric tensor while the interference depends on the overcoating, the film and
the substrate. For superlattice Co/Pt and Co/Pd samples without dielectric overcoattng,
it is well known that for samples thinner than 50 nm, there is enough light transmitting
through the superlattice film, and the interference effect due to the substrate needs to be
taken into account. The spectra for these types of samples are a combination of the
substrate interference and the electronic structure of the materials. While for samples
with more than 50 nm thickness, the absorption of the materials allows little or no light
to transmit through the substrate. Therefore there is no interference effect from the
substrate, and the spectra is determined by the electronic structure of the materials only.
The wavelength dependence curves of Kerr rotation, ellipticity and reflectivity for
sample 4 of Co(2)/Pd(9) samples are shown in the Fig. 1. This sample is quite thin and
the interference has a strong effect on the spectra. This is a typical figure for all the
Co(2)/Pd(9) samples we measured in this paper. From this curve we can see some
interesting characteristics. The reflectivity curve has a dip between 750 nm and 850 nm
range while the Kerr rotation aud the ellipticity curves also have a bump and dip
respectively in the same range. These phenomena arise from the fact that when more
light is Irappod in the film, it gives more internal bouncing back and forth, and reflection
and the magneto-optical Kerr rotation grows bigger. The ellipticity is quite large and it
is negative for very short wavelengths and it becomes positive for longer wavelengths.
The error bars on the curves are generated by standard deviation calcualation of several
measurements. Fig. 2 is the Kerr rotation curves for all the 6 Co/Pd samples. There are
74 Appendix B
several points v,e can draw from this figure. All the samples have similar wavelength
dependent shapes and the difference is that the peak Kerr rotation poslt_on shifts from
about 400 nm for the 33 nm thick film to about 540 nm for the 5.5 nm thick film. This
peak shift _s purely interference effect and it only depends on the refractive index and
absorption coefficient of the material. For 150 nm thick film, there is no interference
effect andthe Kerr rotation peak is located at a wavelength less than 350 nm as shown in
the Fig. 7 of Ref. I. The Kerr rotation vs. film thickness for each wavelength can also be
seen in this figure. For wavelength below 560 nm the Kerr rotation peak is at thickness
of 16.5 nm (curve 3) while for wavelength above 560 nm value, the peak is at thickness
of 11 nm (curve 2). The two curves in the Fig. 3 is the Kerr rotation vs. film thickness
curve for wavelength of 450 nm and 750 nm respectively. The phenomenon that the
peak of Kerr rotation vs. film thickness shifts to thicker film thickness as the wavelength
decreases can be explained quanlitatively by interference. For a fixed refractive index n
and absortion coefficient k material, as film thickness increases, the corresponding anti-
reflection wavelength will decrease and the anti-reflection wavelength is also the peak
Kerr rotation wavelength for the reason that the Kerr rotation is inversely proportional to
the reflectivity.
Fig. 4 is the ellipticity curves for all the Co/Pd samples. For film thickness increase
from 5.5 nm to 33 rim, the ellipticity curve shifts upwards with a little tilt downwards on
the long wavelength side. Fig. 5 is the reflectivity curves for all the Co/Pd samples. It
is clear that the reflectivity curves are similar for all the different thickness samples and
the value increases as the film thickness increases. The interference effect is less
pronounced in the reflectivity curves, and only the small drop of relative values on the
short wavelength side gives the indication of interference effect. For applications the
parameter with more direct relation to the signal-noise-ratio is called the Figure Of Merit
(FOM,} and it is defined as V_V'_ _Ek22._Thg w_gavelength dependence of the FOM
for all the Co/Pd samples are plotted in the Fig. 6. From the curves we can see that they
all havethesameshapeandall the FOM peak at about the same wavelength of 480 nm.
The highest FOM for all the wavelength between 350 nm and 1050 nm is the film with
16.6 nm thickness.
We also measured all the Co/Pd samples from the substrate side. The Kerr rotation,
ellipttctty, reflectr,'ity, and figure of merit curves are shown in Fig. 7, 8, 9, and 10
respectively. From these figures we can see clearly that the interference effect is much
smaller than the results from the film side. The reason is that from the substrate side
there is a much better index match between the glass and the first layer of the M-O film
than from the film side measurement case, in which the matching is between the air and
the first layer of the M-O film. The Kerr rotations are higher from the substrate side
due to the lower reflectivities from this side. The above differences are purely optical
effect and magnetic properties are the same.
Fig. 11 is the Kerr rotation, ellipticity and reflectivity measurement for the Co/Pt
sample. Compared with the Co/Pd staples, the Kerr rotation wavelength dependence
shape is quite different, which indicates the different inter-layer interactions for these
two types of materials. The Kerr rotation values of the Co/Pt sample are much larger
than the Co/Pd samples with the same Co content of Co/Pd sample 6. It is very
interesting to notice that the wavelength dependence of reflectivity is very similar for the
Co/Pt and Co/Pd samples, which indicates that their refractive index and absorption
coefficients have similar wavelength dependence.
IV. Conclusions
The wavelength dependence of Kerr rotation, ellipticity and reflectivity for Co/Pd and
Co/Pt superlattice samples are measured with a new technique. The series results of
differernt thickness of Co/Pd samples clearly show the interference effect on the spectra.
76 Appendix B
The clear understanding of ¢,¢ the interference effect cleared the way for further
investigating the material properties. By getting the wavelength dependence of the
refractive index and absorption coefficient and together with the Kerr rotation and
ellipticity results, the dielectric tensor can be calculated and the physical properties can be
studied directly. Comparing the spectral dependence of the two Co/Pd samples and the
Co/Pt samples, we can see that the interactions between the Co layers and the Pd layers
are very strong and the magneto-optical properties for the superlattice samples are
determined by these inter-layer interactions.
Appendix B 77
References
1. F.L. Zhou, J. K. Erwin and M. Mansuripur, to appear in J. Appl. Phys (1991)
2. D. Weller and W. Reim, Appl. Physics A, 4..._9,599 (1989)
3. E.R. Moog, .1'. Zak, and S. D. Bader, to appear in J. Appl. Phys. ( 1991)
4. G.A.N. Connell, Appl. Phys. Lett., 40(3), 212 (1982)
78 Appendix B
Table i
Sample
Composition
Co/Pd
Number of
Layers
Total
Thickness (nm)
2/9
5
5.5
2/9
10
2/9
15
2/9
20
2
11
3
16.5
4
22
5
2/9
25
27.5
6
2/9
30
33
79
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Appendix B
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wavelength (nrn)
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}-
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o
Figure 1. Wavelength dependence of Kerr rotations, ellipticity and reflectivity for Co/Pd sample 4.
line is Kerr rotation curve, dashed line is the eLlipticity curve, and the .... is the reflectivity curve.
Solid
Figure 2.
A Q
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Wavelength (nm)
Wavelength dependence of Kerr rotations for the 6 Co/Pd samples, measured from the film side.
: -. .
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Appendix B
FIlM _[[)E
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750 nm
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Thickness (nm)
Figure 3. Thickness dependence of Co/Pd samples. Solid line is for 450 rim wavelength and dashed line
is for the 750 nm wavelength.
z
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Wavelength (nm)
Wavelength dependence of ellipticity for the 6 Co/Pd samples, measured from the film side.
GA;{_;av:.d, :':',f'.f-: ;:,_
OF POOR QUALI'i"V
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Appendix B
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- _ __ --+--
rTITrITIIII
o350 450 550 _50 750 850 950 tO_O
Wavelength (nm)
8l
Figure 5. Wavelength dependence of reflectivity for the 6 Co/Pd samples, measured from the film side.
t-
c_
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L.
Li.
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j _
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Wavelength (nm)
Figure 6. Wavelength dependence of figure of merit for the 6 Co/Pd samples, measured from the film
side.
00_ g.,._l_'..J I YOF P • "" " -
82 Appendix B
(',)(2)/l'd(9).SUIISTI{ATESIDE
Figure 7.
side.
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Wavelength (nm)
Wavelength dependence of Kerr rotation for the 6 Co/Pd samples, measured from the substrate
t,o
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Wavelength (rim)
Figure 8. Wavelength dependence of ellipticity for the 6 Co/Pd samples, measured from the substrate side.
ORIGINAL PAGE IS
OF POOR QUALITY
Appendix B 83
Figure 9.
side.
L-,
c_
o
(i,)(21/l'd(!)), SI)USTRATE SIDE
k
i
@
-- z
Wavelength dependence of reflectivity for the 6 Co/Pd samples, measured from the substrate
c-
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/
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_5o ,_o _o _o ?_o _o _o ,o_o
Wavelength (nm)
Figure 10. Wavelength dependence of figure of merit for the 6 Co/Pd samples, measured from the
substrate side.
84
L
i
Appendix B
('()( 1 J/l't( IDI, l!i.l IIIII
Korr I_ol ;11 i_11
_ _ Ellillli(,lly
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i #= ,k ,k
it
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_351) 450 550 650 750 8_0 950 1050
WavelengLh (nm)
Figure l 1. Wavelength dependence of Kerr rotation, ellipticity and reflectivity for Co/Pt sample, Solid
line is Kerr rotation curve, dashed line is the ellipticity curve, and the .... is the reflectivity curve.
ORIGINAL PAGE IS
OF POOR QUALITY


Wavelength dependencies of the Kerr rotation and ellipticity for the magneto-optical recording media

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