IEEE TRANSACTIONS ON MAGNETICS, VOL. 37, NO. 4, JULY 2001
2417
Kerr Effect of Ordered and Disordered Fe1 xPtx(001)
Alloy Films
Y. M. Hu, J. C. A. Huang, S. Y. Huang, and T. H. Wu
Abstract—The (001) oriented Fe1 Pt alloy films have been
grown on Pt seeding layers on MgO (001) substrates by MBE. At
growth temperature of 500 C, rather ordered structures were
formed for FePt, FePt2 , and FePt3 films, which exhibit large perpendicular magnetic anisotropy and Kerr rotations. In contrast,
Fe2 Pt and Fe3 Pt films were poorly ordered and did not display
perpendicular magnetization for similar growth conditions. The
optimal growth temperature of the FePt ( = 0 5) alloys occurs
at about 500 C, and the surface order–disorder transition
occurs above 650 C. We demonstrate that chemical ordering is
important for perpendicular magnetic anisotropy and Kerr effects
for Fe1 Pt alloy films, which in turn depend strongly on the
alloy composition and growth temperature.
I. INTRODUCTION
T
HE DISCOVERY of large perpendicular magnetic
anisotropy (PMA) and magneto-optical (MO) Kerr effect
in Fe–Pt alloys and multilayers has stimulated considerable
work because of the basic research and technological importance as MO or magnetic recording media [1]–[3]. Binary
Fe–Pt alloys have several ordered phases: cubic Cu Au (L1 )
structure for Fe Pt and FePt , and CuAu (L1 ) structure for
FePt. [4], [5] The equiatomic FePt may form chemically
ordered compounds of tetragonal L1 phase which possess a
modulate structure consisting of pure Fe and Pt planes stacked
alternately along the tetragonal c (easy) axis, thus resulting in
an extremely high uniaxial magnetic anisotropy energy (K
10 erg/cm ) [6]. -axis oriented epitaxial FePt films with
PMA have been successfully prepared by annealing the (001)
oriented Fe/Pt multilayers [7] and directly grown by molecular
beam epitaxy (MBE) technique [8]. On the other hand, due
to the isotropic characteristic of the L1 structure, no PMA
property was expected in the Fe Pt and FePt alloy films.
Pt alloy films
In this study, we have synthesized the Fe
with different composition by MBE. We present here the structural and magnetic properties of five distinct alloy compositions:
Fe Pt, Fe Pt, FePt, FePt and FePt . To study the chemical ordering upon the PMA and Kerr effects, we also prepared FePt
alloy films with distinct growth temperatures.
Manuscript received October 13, 2000.
This work was supported by the ROC NSC under Grant 89-2112-M-006-037.
Y. M. Hu is with the Electric Engineering Department, Nan-Jeon Junior College of Technology and Commerce, Tainan, Taiwan.
J. C. A. Huang and S. Y. Huang are with the Department of Physics, National
Cheng-Kung University, Tainan, Taiwan (e-mail: jcahuang@mail.naku.edu.tw).
T. H. Wu is with the Department of Humanities and Science, National Yunlin
University of Science and Technology, Touliu, Taiwan.
Publisher Item Identifier S 0018-9464(01)06721-8.
II. EXPERIMENTAL PROCEDURES
The epitaxial growth of the Fe
Pt (0.25
0.75)
alloy films was carried out in a Vacuum Product made molecular
beam epitaxy system (MBE-930) equipped with five e-beam
evaporators. The base pressure of the MBE system was of about
torr. Epitaxial grade MgO (001) substrate was out2 10
gased at 1000 C for about one hour in UHV condition prior
Pt
to the sample growth. Before initial depositon of the Fe
alloys, 150 Å thick Pt seeding layers were grown at 700 C on
Pt alloy films 1000 Å thick
the MgO (001) substrates. Fe
were subsequently co-deposited on the Pt seeding layer at temperatures ranging from 250 C to 650 C. The growth pressures
Pt films were controlled at
and deposition rates for the Fe
about 5 10 torr and 0.2 Å/s, respectively. No post-annealing
treatment was employed in this work. To retain the sample uniformity the sample holder was rotated with a constant speed of
30 rpm.
Pt films were
The surface and bulk structures of the Fe
characterized in-situ by reflection high-energy electron diffraction (RHEED) and ex-situ by x-ray diffraction (XRD). The
Pt
magnetic hysteresis loops and Kerr rotations of the Fe
alloys were investigated by polar magneto-optical Kerr effect
(PMOKE). The PMOKE measurement was carried out at
up to 15 kOe. The
room temperature in a magnetic field
632.8 nm) for
penetration depth of the He–Ne laser (
PMOKE experiment is about 200 Å.
III. RESULTS AND DISCUSSIONS
Fig. 1 shows the
XRD scans (using Cu K radiation) for
Pt alloys (
0.25, 0.33, 0.5, 0.67 and 0.75) grown
the Fe
at substrate temperature of 500 C. The Pt seeding layer 150 Å
thick, was grown as fcc (001) structure with small amount of
(111) phase, as evidenced by the XRD and RHEED studies. The
Pt films were also grown as (001) oriented
subsequent Fe
structures with a minor component of (111) phase. For the sam0.25 and 0.33), the intensities of
ples with less Pt content (
Pt are relatively
(001) and (003) superstructure peaks of Fe
small or almost disappeared, as shown in Fig. 1. This suggests
that the Fe Pt and Fe Pt samples were poorly ordered at growth
temperature of 500 C. Note that the bulk order–disorder transition temperature of Fe Pt is about 840 C [4], which is much
lower than those of FePt (1300 C) and FePt3 (1350 C). It is
possible that the (surface) disordering of Fe Pt films occurs at
much lower temperature ( 500 C), above which an ordered
Fe Pt alloy cannot be formed.
In contrast, rather ordered structures of FePt, FePt and FePt
films were obtained, as indicated by the (001) peaks of x-ray
0018–9464/01$10.00 © 2001 IEEE
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IEEE TRANSACTIONS ON MAGNETICS, VOL. 37, NO. 4, JULY 2001
Fig. 3. Specular x-ray scans for equiatomic FePt films of 1000 Å thick grown
at various temperatures.
Fig. 1. X-Ray scans from 1000 Å Fe
temperature of 500 C.
Fig. 2.
PMOKE results for Fe
Pt /150 Å Pt/MgO(001) with growth
Pt films grown at 500 C.
diffraction shown in Fig. 1. The FePt and FePt films possess the
chemically ordered L1 and L1 structure, respectively. In addition, the FePt film contains comparable amount of ordered FePt
and FePt structures, as indicated by the splitting of (002) peaks.
By calculation of the x-ray diffraction integrated intensities of
the (001), (002) and (003) peaks, the degrees of long-range ordering parameters (S) for FePt, FePt and FePt films are estimated as 0.92, 0.57 and 0.39, respectively.
Fig. 2 shows the corresponding polar MOKE loops of these
Pt films. No PMA signals have been detected for Fe Pt
Fe
and Fe Pt films. In contrast, the FePt–BFePt and FePt samples exhibit large PMA effect with Kerr rotations of 0.83 , 0.73
and 0.35 , respectively. It seems that there is a good correlation between the PMA and the degree of chemical ordering
Pt (001) films. The PMA and Kerr effects are
of the Fe
enhanced for chemically ordered or partially ordered samples
(FePt, FePt and FePt ) and suppressed for disordered ones
Pt alloys
(Fe Pt and Fe Pt). Similar trend occurs for Fe
grown at lower temperatures (200 C–500 C). Neither Fe Pt
nor Fe Pt samples exhibit PMA, probably due to disorder and
the increase of Fe–Fe ferromagnetic coupling.
We also studied the temperature dependence of the order parameter and Kerr rotations for the equiatomic FePt films. The
FePt alloys of 1000 Å thick were prepared on 150 Å Pt seeding
layers on MgO (001) substrates at temperature ranging from
200 C to 650 C. The FePt (001) and (003) peaks are observed
for FePt films grown at temperature between 400 C and 600 C,
as shown in Fig. 3. All the FePt films were mainly grown as ordered or partially ordered (001) structures, except for the one
grown at 200 C with a disordered fcc (111) dominated structure. The calculated order parameters for the FePt films grown
at 200 C, 350 C, 400 C, 500 C, 600 C and 650 C are 0,
0.51, 0.49, 0.92, 0.93 and 0.46, respectively. The optimal growth
temperature for chemical ordering of FePt film is about 500 C.
Moreover, the chemical ordering of the FePt film is much deteriorated for growth temperature above 650 C. This temperature is much lower than that of the bulk order–disorder transition temperature (1300 C) of FePt. This indicates that the
order–disorder transition for FePt films is likely dominated by
surface diffusion during epitaxy. These results are similar with
the previous report by Farrow et al. [6].
The temperature dependence of magnetic properties of FePt
films was also examined. The 500 C grown sample displays
0.98) and Kerr rotation (
the best loop squareness (
0.84 ), as shown in Fig. 4. For growth temperature higher than
650 C, insignificant PMA and Kerr effects were found. With
growth temperature down to 200 C, the loop squareness and
Kerr rotation were also decreased to zero. It is clear that for FePt
films the growth temperature plays a critical role for the chemical ordering of the L1 phase, which in turn strongly influence
the PMA and Kerr effects.
IV. CONCLUSIONS
We have studied the (001) oriented Fe
Pt films prepared
on Pt seeding layers on MgO (001) substrates by MBE. DepenPt films on alloy compodence of chemical ordering of Fe
sition ( ) was observed at growth temperature of 500 C. Rather
HU et al.: KERR EFFECT OF ORDERED AND DISORDERED Fe
Pt (001) ALLOY FILMS
2419
ordering is important for the PMA and Kerr effects, which in
turn depend strongly on the alloy composition and growth temperature for the Fe
Pt alloy films.
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Fig. 4. PMOKE hysteresis loops for 1000 Å FePt films grown at various
temperatures.
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Kerr effects. In contrast, Fe Pt and Fe Pt films are poorly ordered and display no PMA effect. In addition, study of FePt alloys at different growth temperature reveals that the optimal ordering temperature occurs about 500 C, and the order–disorder
transition occurs above 650 C. We conclude that the chemical
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