Amorphous rare earth-transition metal (RE-TM) alloys and compositionally modulated TM/TM films were characterized in terms of their magnetic, magneto-optic, and galvanomagnetic properties. The loop tracer, vibrating sample magnetometer (VSM), and Rutherford Backscattering (RBS) facility were used to characterize and analyze the various properties of these magneto-optical storage media. Kerr effect, ellipticity, coercivity, and anisotropy at various temperatures, magnetoresistance, and resistivity are among the properties measured in Co/Pt films, Co/Pd films, and TbFeCo films

Hajjar, Roger A.

Mansuripur, M.

Wu, Te-Ho

English

NASA Technical Reports Server

PRECF.D;;_G PAS-'_-.;:i_....,..,< _;,.,i F;L.<_,ED N 9 _-
Appendix A
Characterization of Magneto-Optical Media
Roger A. Hajjar , Te-l,o Iru and M. Mansuripur
Optical Sciences Center
University of Arizona
Tucson, AZ 85721
Amorphous rare earth-transition metal (RE-TM) alloys and compositionally modulated T U T_[
films (prepared in the laboratories of the ODSC sponsors) are characterized in terms of their
magnetic, magneto-optic and galvanomagnetic properties. The loop tracer, _ibrating sampte
magnetometer (VSM), and the Rutherford Backscattering facility (RBS) have been used to
characterize and analyze the various properties of these magneto-optical storage media
The loop tracer consists of a 21 kGauss electromagnet (4" diameter pole piece, 2" gap). The
magnet has a rotating stage that allows its field to be applied both perpendicular to and in the plane
of the sample. Samples of up to 1" in diameter can be analyzed in a cryogenic dewar cycling
temperatures from 80 K to 475 K. The DC differential detection technique is used to measure the
magneto=optical Kerr effect at the 633 nm HeNe wavelength. EIIipticity can also be measured with
the insertion of a quarter-wave=plate. The magnetic properties such as coercivky and anisotropv
constants at various temperatures are determined form the perpendicular and in-plane measurements
of the polar Kerr effect x (KE). Galvanomagnetic and transport properties such as the Hall effect
(HE), magnetoresistance (MR) and resistivity are measured with two mutually orthogonal pairs of
point contacts at the sample surface 2.
a) Co/Pt Films : The Co/Pt films were prepared by Dr. C-J. Lin of IBM Almaden Research Center.
These are multilayered films consisting of alternating ultrathin Co and Pt layers. The films were
deposited by e-beam evaporation from Co and Pt sources onto glass substrates 3. The periodicity and
crystallographic structures were obtained by X-ray diffraction analyses and the composition was
measured by both XRF and RBS. The total thickness of these films was nominally set to 30 nm
where the thickness of the Pt layer was set to about !.0 nm and the thickness of the Co layer varied
from 0.2nm to 0.4 nm.
All samples show perpendicular magnetic anisotropy and rectangular hysteresis loop in
perpendicular magnetic fields. Figures-1 through 4 show the various chracteristics of these films
versus Co thickness. The remanent Kerr angle and Hall resistivity at room temperature are displayed
in Fig. 1. Figure 2 shows the saturation magnetization M, obtained from the VSM hysteresis loops
and AMp longitudinal obtained from the MR measurement when the field is in the plane of the
sample. The value of Ap/p is obtained from the peak of the longitudinal MR curve as shown in
https://ntrs.nasa.gov/search.jsp?R=19920005685 2020-03-17T14:13:15+00:00Z
54 Appendix A
Fig. 5. The linear trend of 9it, Pt-I, M, and Ap/p (longitudinal peak) correlate with the increase ©t"C,_
content in these films. From the magnetization data we find that the effective Co moment is around
1750 emu/cc which is considerably enhanced relative to that of bulk cobalt. This enhancement is
believed to be due to the stretching of the cobalt lattice which makes it more atomic like (atomic spin
polarization of Co is about twice as large as that of bulk Co) 4. There is little e_idence that P.' ix
polarized as far as we can tell from the 5fs data. Evidently, more data points are needed in order to
give a quantitative assesment of the Pt polarization.
Figure 3 displays the nucleation coercivity (He) and anisotropy field (H k) for these tilms. Both
He and H k can be determined from four different measurements, namely : Kerr, Hall, VSM and .X1R
measurements with the applied field being either perpendicular to or in the plane of the sample. The
nucleation coercivity can be obtained from KE, HE or VSM hysteresis loops. We can also obtain Hc
from the perpendicular MR measurement as illustrated in Figs. 7 and 8. The linear part of the curve
in Fig. 8 has a negative slope of 3.1 x 10-8 per Oersted, which has its origin in the s-d scattering
phenomenon as interpreted by Mott s. The peaks centered around the coercive field are caused b v the
scattering of the conduction electrons from the magnetization within the domain wails. These wails
cause the resistivity to increase provided that their magnetic moments, while in the plane of the film,
are also parallel to the direction of the current. Thus the height of the peaks in Fig. 8 is a measure of
the volume fraction covered by the domain walls, while the width of the peaks corresponds to the
transition region observed in the vicinity of H c in the hysteresis loop.
The anisotropy field is determined from the initial part of the KE or HE in-plane measurement 1
(Fig. 6). Also, H), can be roughly determined from the perpendicular and in-plane VSM
measurement by extrapolating the intercept of the in=plane measurement with the saturation part of
the hysteresis loop as shown in Fig. 9. H k is also obtained from the longitudinal MR measurement as
shown in Fig. 5. In this measurement, the sample is first saturated along the easy axis. The initial
increase of resistivity with the applied field is due to the alignment of magnetization with the field.
The maximum of Ag/p (longitudinal) is reached at H = H k. Once the magnetization and the field
have been aligned, further increases in H cause a linear decrease of Ap/; which, as mentioned before,
is related to the s=d scattering. As shown in Fig. 3, the anisotropy field decreases with increasing
cobalt layer thickness. As expected, H_ decreases from a positive value towards negative values,
corresponding to an in=plane easy axis of magnetization for thick cobalt layers. A rough
extrapolation shows that the crossing point between the perpendicular and in plane anisotropy regimes
lies at about 0.7 nm of Co. For ultrathin Co ( < 0.25 am), the anisotropy field is reduced mainly due
to the island-like formation of the Co layers that become discontinuous.
Figure 4 shows the magnitude of the slope due to s=d scattering obtained from the perpendicular
MR measurement (see Fig. 8). The data obtained correlate to a certain extent with the total thickness
of one period over the thickness of cobalt, (too + tpt)/tco. Note that for pure cobalt, where this ratio
is equal to 1, the slope obtained is 3 x 10-* per Oersted.
Appendix A 55
We have also measured the electrical resistivity using the Van der Pauw technique 6, The
resistivity reflects the character of the interfaces in superlattices. For reference, the resisti_it.v of a
thick (sputtered) cobalt sample (175 am-thick) was found to be 14 _zf2.cm, and those of pure Pd and
Pt films (sputtered) were around 17 l_f2.cm. The typical room temperature resistivity for a multilaver
film is in the range of 30- 60 IA2.cm. From this behaviour, we can assume that multilavers with
abrupt interfaces should have a resistivity intermediate to those of the constituent metals while mixed
interfaces will have resistivities as much as a factor of 10 higher. For the samples studied here, the
Co/Pt films (30 am-thick) have a typical resistivity of 45 _fl.cm, while similar Co/Pd samples (see
next paragraph) have a resistivity of 37 _f'l.cm. Adopting the above argument, we attribute the I_rger
resistivity of the Co/Pt films to interfacial mixing between platinum and cobalt layers, and conclude
that Co/Pd films have sharper interfaces.
b) Co/Pd Films : A series of Co/Pd multilayer films were fabricated by Dr. Charles Brucker of
Kodak Research Laboratories. These are sputtered Co (0.2 nm)/Pd (.9 am) films with thicknesses
ranging from 5.5 nm to 87.5 am. Figures 10 - 13 display the thickness-dependence of the magnetic,
magneto-optic, and galvanomagnetic properties of these samples at room temperature. The Kerr
rotation angle Ot measured from the film side is remarkably enhanced around 11 nm of thickness.
This enhancement is due to optical interference between the rays reflected from the film surface and
those reflected at the interface between film and substrate; the enhancement is not related to the
multilayer structure of the film. Even though the concept of Kerr effect enhancement due to optical
interference is well known, we revisit this topic for the sake of completeness.
Consider a magneto-optic film sandwiched between two mediums of indices 'h and ,r3. The MO
layer has two indices of refraction denoted by n2+ and n2-, corresponding to left and right circularly
polarized light. Knowing that linearly polarized light may be decomposed into left and right circular
polarization, we write the Fresnel coefficients as :
for reflection off the top surface of the MO layer and :
for the reflection off the bottom surface of the MO layer. Assuming multiple reflections within the
MO layer, we obtain a general expression of the total reflectivity as follows :
56
r(±) =
Appendix A
rt ±) + rt ±) exp (-i6(*-))
1 + rt-*) rt±) exp (-i6(±))
where 5(-') = ,Ix nl ±) x thickness/'),. Calculation of Kerr rotation angle and ellipticity from the abG_e
equation is straightforward 7. Figure 14 shows a calculated curve of (9k and ek ,,ersus film thickness.
The magneto-optic refractive indices (at A = 790 nm) are obtained from the literature for the filn"
structure CotO.2nm)/Pd(O.gnm) with a total thickness of 50 rim.
The electrical resistivity is displayed in Fig. 12 and shows a rapid increase for thicknesses below
16.5 nm. This increase in resistivity can be explained by the change in the film morphology, namely
the film growth from an island structure to a continuous structure 8. The change in the film
morphology can also be observed in the magnetic properties such as the anisotropy, coerci_ity and
saturation magnetization where a sharp decrease is observed below 16.5 nm (see Figs. 11 and 131.
Note that, as expected, the Hall resistivity in Fig. 10 is independent of the magneto-optical
interference effects discussed above and is closely related to the resistivity data. It can be shown 9 that
Ph _x ,,i.o2, where the coefficient 7 is related to the spin-orbit coupling and goes to zero in the absence
of spin-orbit coupling. Good agreement is obtained by fitting the data with the theory.
The s-d slope observed in these films is significantly larger than in Co/Pt films of approximately
the same structure as shown in Figs. 15 and 16. This might be attributed to three different
phenomena: The d-band structure of cobalt is becoming steeper and closer to the Fermi level tsee
reference 4), the induced magnetic polarization of Pd, or the extreme proximity of the Pd Fermi
level to the edge of the d-band.
c) TbFeCo Films : A study of the thermomagnetic properties of TbFeCo films is currently
underway. These are films of variable Tb content (ranging from 22.5% to 28%), and were prepared
at the IBM Almaden Research Center. The films are deposited onto quartz substrates and consist of a
quadrilayer structure of AICr with an MO layer sandwiched between two dielectric SiN layers.
Figure 17 displays the saturation magnetization M, versus temperature for these films. Figure 18
shows the Too,,,p and M_ as a function of Tb content. The sum of the anisotropy energy constants K,
and K: at room temperature is displayed in Fig. 19. These parameters are obtained from the in-plane
Kerr measurement x. The apparent reduction of Ku (i.e., K 1 + K,) at the compensation point is not
real, but is due to the fact that the maximum field strength in our experiment (= 21 KOe) is
insufficient to tilt the magnetization away from the perpendicular direction.
Figure 20 shows a typical set of perpendicular and in-plane Kerr effect measurement results for
the TbFeCo samples. The bottom frame in Fig. 20 shows the fitting of the normalized in-plane data
to the theoretical model (normalization is by the height of the hysteresis loop); it is from this kind of
fitting that the anisotropy constants K x and K: are determined.
O;_IGINAL ':,*. "'-
r a_: 13
OF POOR QUALITY
Appendix A 57
Plots of the remanent Kerr angle and coercivity, obtained from the hysteresis loops, are sho_ n in
Fig. 21. We are currently measuring the temperature=dependence of the various properties of the__e
films in order to determine their anisotropy constants as function of temperature.
REFERENCES
1 R.A. Hajjar, F.L. Zhou and M. Mansuripur, J.Appl. Phys. 67, 5328 (1990).
z R.A. Hajjar, M. Mansuripur and H=P.D. Shieh, to appear in J. Appl. Phys. (1991).
R.A. Hajjar, M. Mansuripur and H-P.D. Shieh, presented at MMM'90, to appear in J. Appl.
Phys., April 199 I.
C.-J. Lin and H.V. Do, IEEE MAG 26, 1700 (1990).
R. H. Victora and J.M. Maclaren, "Calculated Magnetic and Electronic Properties of Co/Pd
Superlattices", (unpublished Report).
s N.F. Mott, Proe. Roy. Soc. A, (London) 153, 699 (1936) and 56, 368 (1936).
6 L.J. Van der Pauw, Philips Rev. 20, 220 (1958).
7 M. Born and E. Wolf, "Principles of Optics", 6th edition, Pergamon Press.
8 S. Hachimoto,Y. Ochiai and K. Aso, J. Appl. Phys. 67, 4429 (1990).
9 R. Karplus and J.M. Luttinger, Phys. Rev. 95, I154 (1954).
58 Appendix A
030
022
"O
v O.15
_)" 0.08
0.00
1
• (_, • ID,d
2 3 4
Co thickness (A)
Fig. 1
200
1 .GO
1.20
O.80
0.40
O00
5
A
E
fj
C:
v
I
Z
"" 800
E
u
E 475
,,,.w
• Ms Apl¢)
,6
1 2 3 4 5
Co Thickness (A)
Fig. 2
O2O
010
000
C
'0
e,, I_
, c
_ 0
O.
Q.
<1
A
Q
0
v
v !
"I-
o
-'- Hc -'- Hk
h.
- ",e
I 2 3 4 S
Co thickness (A)
15
A
Q
Io O
v
5 a¢
Z
A
0 s.oo
W
I
uJ
v- 2 50
v
o.
Q. o.0o .....
.11 t 2
-'- Al)/p -'- p
,°°
C 100
- 0
3 4 5
Co thickness (A)
A
E
(3
o.
Pig. 3
Fig. 4
wAppendix A 59
LONGITUDINAL
2
I
I_1
x
Q,
Q,
0
-20
i J
HK
-10 0 10 20
H (KOe) Fig. 5
a
II
at
¢)
0.30
0.00 i
-0.30
-20
i
-10 0 10 20
H (KOe) Fig. 6
6O Appendix A
PERPENDICULAR B
I I I
C9
I
i.u
qFI
X
el,
Q,
,11
A
Cl
6
v
il
0
0.50 k
0.00
I
I
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-10 -5
I
0.50
0.30
i
0.10
-0.10
-o.3o _-
0
H (KOe)
I
I t
-10 -5 0 5 10
Fig.
Fig.
7
H (KOe)
Appendix A 61
E
I
0
i
7 14
H (KOe)
Fig. 9
62 Appendix A
Q
'D
v
o"
0.30
022
0.1S
0.08
0.00
• O, • p,.,
250 500 750
_mDss (A)
20
15
10
S
0
1OOO
i
O
m,
N
v
!
¢L
2
O
v
v
4t
•r 1
• HC • Hk
25
0 250 SO0 750
20
15
10
5
1000
Thickness (A)
o
w
Fig. 10 Fig. 11
• llOpe . p • MII " KU
. , ,20 S j _00 ---
, ' _-_--------- .0 $ " t///--"-- 200 2
I ........ 40 1oo [ I
0 0 0 0
0 250 SO0 750 1000 0 250 SO0 7S0 1000
Tltick hess (A) Thickness (A)
Fig. 12 Fig. 13
ORIGINAL PAGE IS
OF POOR QUALITY
0.'_5
"= 0.2-.
>,
- 0. I5 -
.,-,
= 0.1-
0.05 -
Z
<
O-
z -0,05 -
<
A,S
- -0.1 -
-0,15 0
Appendix A
\
200 _00 600 S00
@
FILM THICKNESS in A
',,)_>)
3._fi!___
: "_i,O
Fig. 14
64 App.endix A
Co(2A)/PtllOA) x 25 (300A)
3
¢,,,1
I
1,1,1
X
Q,
Q,
,,d
0
-3
-20 -10 0 10 20
Fig. 15 H (KOe)
Co(2.6.)/Pd(9.6,) x 30 (330.6.)
t'}
I
1,14
Ill
Q,
Q,
,q
3
0
-10 0 10
Fig. 16 H (KOe)
Appendix A 65
Ms Vs Temperature
-200 -100 0 100 200 300
350 r--- _ - _ T -
=I
E
V
300
250
200
150
IO0
50 _.
0
-200
'-- Tb(23)
- _- Tb(23.5)
- _- Tb(25)
+ Tb(26)
--_- Tb(27)
__ __ • --e-- Tb(28)
r: + _ _- .... ---+
_ + " - __&( "k ',
-100 0 100 200 300
Temperature ('C)
Fig. 17
66 Appendix A
T
Comp
and Ms vs Tb at. %
at.|
0
_J
200
100
0
-100
-200
20
T Coul • Ms (AT RT)
A •
\ ,
\
/&
\
22 24 26 28
150
100 ,_
0
3O
Tb at. %
Fig. 18
Appendix A 67
K
I
and K vs Tb at. %
¢d
¢d
¢m
6
4
2
0
20
• K,
i
I
z/&
I I
I
22 24 26 28
6 x
4
2
0
30
I0'
Tb at. %
Fig. 19
68 Appendix A
I ...__._jflO r._ "_'--_"IOlgr e*ot
J
.t !
J
Tb(23)FeCo
_0
PERPENDICULAR PARALLEL
Experimental and theoretical data
__ L._ ___ * experimental
i f "_- -theoretical
Oql _. __
Oql -
O ----_lO--_ 11" I
M A_ _t'_ F1EI D
Fig. 20
OF PO0_ QUALITY
- Appendix A 69
0
k
and Hc vs Tb %
• O. • Hc
A
"0
1.00
0.50
0.00
20
J
I
I I
I
t -_--------_- _e
/ &--
/ \ •
J \
/
J
I I 1
22 24 26
\
\,t
\
I
28
J
20
15
10
5
' O
30
A
41)
O
O
Tb at. %
Fig. 21
70 Appendix A -


Characterization of magneto-optical media

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