Interface magnetic properties of epitaxial Fe-InAs heterostructures

Abstract

The growth and interface magnetic properties of epitaxial Fe films grown on InAs [100]-4×2 have been studied using low-energy electron diffraction, in situ magneto-optical Kerr effect, and X-ray magnetic circular dichroism. The magnetic properties at room temperature were found to proceed via three phases with thickness; a nonmagnetic phase, a superparamagnetic phase, and a ferromagnetic phase. The initial ferromagnetic phase might be stabilized by interparticle interactions. The films show bulk-like spin moments of 1.90 ± 0.15 μ_B with the thickness above about 20 ML and a large enhancement ∼ 260% of the ratio of orbital versus spin moment in the ultrathin region.

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2652IEEE TRANSACTIONS ON MAGNETICS, VOL. 38, NO. 5, SEPTEMBER 2002
Interface Magnetic Properties of Epitaxial
Fe–InAs Heterostructures
Y. B. Xu, M. Tselepi, J. Wu, S. Wang, J. A. C. Bland, Y. Huttel, and G. van der Laan
Abstract—The growth and interface magnetic properties of epi-
taxial Fe films grown on InAs (100)-4
low-energyelectrondiffraction,insitumagneto-opticalKerreffect,
and X-ray magnetic circular dichronism. The magnetic properties
at room temperature were found to proceed via three phases with
thickness; a nonmagnetic phase, a superparamagnetic phase, and
a ferromagnetic phase. The initial ferromagnetic phase might be
stabilized by interparticle interactions. The films show bulk-like
spin moments of 1.90
0.15 with the thickness above about 20
MLandalargeenhancement
260?oftheratiooforbitalversus
spin moment in the ultrathin region.
2 have been studied using
Index Terms—Fe–InAs, interface magnetism, spin electronics,
ultrathin films.
I. INTRODUCTION
I
important topic for the study of fundamental magnetic prop-
erties of ultrathin films and for the development of the next
generation of spin-electronic devices. While Fe–GaAs has
received much attention [1]–[5], Fe–InAs offers excellent
opportunities for controlling electrical as well as magnetic
properties [6], [7]. InAs has a higher low-field mobility
than GaAs and InP, which makes it a suitable candidate for
high-speed field effect transistors. The low-contact resistance
between metals and the narrow gap semiconductors such as
InAs can reduce thermal dissipation in devices. The fabrication
of ever-smaller devices leads to higher current densities, which
in turn need low-resistance contacts. The magnetic properties
of the first few monolayers are expected to be determined
by both possible intermixing at the interface and the growth
structures. Previous studies of Fe–GaAs found that there are
“magnetically” dead layers due to the formation of nonfer-
romagnetic compounds at the interface, which is detrimental
to the spin-dependent transport in spin electronic devices.
NTERFACE magnetism in ferromagnetic metal (FM)/semi-
conductor (SC) heterostructures continues to be an
Manuscript received February 14, 2002; revised May 20, 2002. This work is
supported by EPSRC, by IRPF (York), and by the Visiting Scholar Foundation
of Key Laboratory, China
Y. B. Xu is with the Department of Electronics, The University of York, York
YO105DD,U.K.,andalsowiththeStateKeyLabforMaterialsModificationby
Laser, Ion and Electronics, Fudan University, Shanghai 200433, China (e-mail:
yx2@york.ac.uk).
M. Tselepi and J. A. C. Bland are with the Cavendish Laboratory, University
of Cambridge, Cambridge CB3 0HE, U.K.
J. Wu is with Department of Physics, The University of York, York YO10
5DD, U.K.
S. Wang is with Department of Electronics, The University of York, York
YO10 5DD, U.K.
Y. Huttel and G. van der Laan are with Daresbury Laboratory, Warrington
WA4 4AD, U.K.
Digital Object Identifier 10.1109/TMAG.2002.801981.
Recent studies [2], however, demonstrated that there are no
“magnetic dead” layers at the interface when Fe is grown on
Ga-rich 4
6 GaAs(100) substrate at room temperature and
the Fe films grows via three phases: a nonmagnetic phase, a
superparamagnetic phase, and a ferromagnetic phase. In this
paper, we report a combined low-energy electron diffraction
(LEED), in situ magneto-optical Kerr effect (MOKE), and
X-ray magnetic circular dichronism (XMCD) study of the
interface magnetic properties of the Fe–InAs(100) system.
II. SAMPLE FABRICATION AND MEASUREMENTS
The Fe films were grown on InAs(100) at a rate of approxi-
mately one monolayer (ML) per minute with the substrate held
at room temperature. The InAs(100) substrates were cleaned
using a combination of oxygen plasma etching and wet etching
(HCl
H O14) before loading into the UHV system
and annealed in the chamber at 510 C for half an hour be-
fore growth. The MOKE loops were collected during growth in
the longitudinal geometry using an electromagnet with a max-
imum field of 2 kOe. For ex situ measurements, the samples
were capped with 20 ML of Au. The XMCD experiment was
performed at beam line 1.1, CLRC Daresbury Laboratory, with
80% circularly polarized X-rays. The experimental details were
given in a previous publication on Fe–GaAs [8].
III. RESULTS AND DISCUSSIONS
Fig. 1 shows the LEED patterns of (a) the InAs substrate
before growth and (b)–(f) after Fe deposition. The clear and
sharp pattern from the substrate indicates that the InAs surface
is very flat and well crystallized. Auger spectroscopy measure-
ments show that the substrate is free of O, but has a tiny C peak.
No Fe LEED pattern was observed for the first 5 ML as shown
in Fig. 1(b). After about 8ML faint LEED spots from the Fe
film appear. The clear LEED patterns in Fig. 1(e) and (f) show
that the Fe grows epitaxially on InAs(001) at room temperature.
The lack of LEED patterns for the first five monolayers indi-
cates that the growth proceeds via the three dimensional (3-D)
Volmer–Weber growth mode as in Fe–GaAs [2].
Fig. 2 shows a detailed in situ MOKE study of the evolution
of the magnetic phase. The magnetic field is applied along the
011 direction. Our previous MOKE measurements at certain
thicknesses showed that the easy axis of the uniaxial anisotropy
in Fe–InAs (100)-4
2 is along the 011 direction [7]. A
MOKE signal was first detected at a thickness of 2.5 ML, with
the intensity linearly proportional to the applied magnetic field.
With further Fe deposition the MOKE-loop curves become
s-shaped at about 3 ML. The MOKE loops after 3.5 ML clearly
0018-9464/02$17.00 © 2002 IEEE

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XU et al.: INTERFACE MAGNETIC PROPERTIES OF EPITAXIAL Fe–InAs HETEROSTRUCTURES2653
Fig. 1.
68 eV and (b)–(f) after Fe deposition, 120 eV.
LEED patterns of (a) the InAs (001)-4?2 substrate after annealing,
Fig.2.
Fe thicknesses.
InsituMOKEhysteresisloopsfortheFe–InAs(100)-4?2ofdifferent
show hysteresis, indicating the onset of the ferromagnetic
phase. The square loops in a thickness range of about 4–10 ML
further confirm that the uniaxial anisotropy dominates in the
ultrathin region [7].
Fig. 3(a) is typical normalized X-ray absorption spectra
(XAS), as shown by an 8-ML film, with two opposite magne-
tizations. The spectra were collected at normal incidence to
minimize possible saturation effects in the total electron yield
[9]. A linear background has been subtracted from the raw data.
The XMCD spectrum (solid line) and its integration (dotted
line) are shown in Fig. 3(b). The spin and orbital moments are
Fig. 3.
(dotted line) XMCD spectra of an 8-ML film with both the external magnetic
field and the photon beam applied perpendicular to the film plane.
(a) Normalized XAS and (b) normalized (solid line) and integrated
Fig. 4.
moment versus spin moment as a function of film thickness determined from
the XMCD measurements.
(a) Spin moment, (b) orbital moment, and (c) the ratio of orbital
determined by applying the XMCD sum rule with
[9], [10] and by subtracting a simple two-step background from
the XAS spectra. Fig. 4 shows the thickness dependences of
(a) spin moments
, (b) orbital moments
the ratios of orbital versus spin moment
moments of the 25 and 40 ML films are about 1.90
which are very close to the bulk value of 1.98
decreasing the thickness, the spin moment is reduced. The spin
moments of the 8 and 15 ML films are about 1.22
and 1.750.15 , respectively. The orbital moment of the
films is enhanced by about 70% compared with the bulk value,
but shows little variation with the thickness. The
however, increases with decreasing thickness and is enhanced
by about 260% compared with the bulk value in the 8 ML film.
6.61
, and (c)
. The spin
0.15
[10]. With
,
0.12
ratio,

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2654IEEE TRANSACTIONS ON MAGNETICS, VOL. 38, NO. 5, SEPTEMBER 2002
The lack of magnetic signal for the first 2.5 ML might
be due to the smaller initial cluster size, which prevents the
development of magnetic ordering or ordering above room
temperature. The lack of LEED patterns from the Fe suggests
that the films are not continuous and that clusters are formed in
the initial stage of growth as shown by our STM studies [11].
As more Fe is deposited, the islands grow to form bigger clus-
ters. The exchange interaction within these clusters becomes
stronger and leads to internal ferromagnetic ordering, so giving
rise to the well-known superparamagnetic phase. We have
demonstrated in Fe–GaAs [2] that a superparamagnetic phase
develops within a narrow thickness range of about 3.5–5 ML
before the onset of long range ferromagnetic ordering. The
MOKE loops in Fig. 2 show that the superparamagnetic phase
also develops in Fe–InAs, but in an even narrower thickness
range of about 2.5–3 ML. The s-shaped loop from the 3-ML
film is consistent with the Langevin function used to describe
the magnetization of superparamagnetic clusters [12], [13]. By
fitting the curve, the effective magnetic moment per cluster is
obtained to be (1.6
0.2) 10
the coverage, the islands grow and coalesce and long-range
ferromagnetic order develops. The films have a well-defined
magnetic coercivity and remanence ratio after 3.5–4 ML. We
should note that the long range ordering might develop before
the complete coalescence of the islands due to interparticle
interactions. In this aspect, there is an important difference
between Fe–GaAs and Fe–InAs. In Fe–GaAs, the onset of the
long-range ferromagnetic phase and the LEED pattern of Fe
were observed at about the same critical thickness of around
5 ML. This suggests that the long-range ferromagnetism occurs
when the film becomes continuous. In Fe–InAs, however,
the long-range ferromagnetism in a thickness range of about
3.5–5 ML might be stabilized by interparticle interactions, as
the films are not continuous in this thickness range.
The large spin moments for the 25- and 40-ML films demon-
strate that the films have a bulk like moment. This also con-
firms that it is the growth morphology rather than Fe–InAs in-
termixing which plays a dominant role in determining the evo-
lution of the ferromagnetic phases. The spin moment is reduced
in the ultrathin films as shown by the 8-ML film. This might be
due to a decrease of the Curie temperature in the ultrathin films
and/or some of the clusters are still not ferromagnetic. We also
would like to point out that while we have demonstrated that
the Fe films have a bulk like moment, the possibility of having a
submonolayer of “magnetic dead” layer at the interface cannot
be excluded due to the resolution limit.
An enhancement of the orbital moment is expected at sur-
faces and interfaces due to a reduction of the symmetry. First
principle calculations on bcc Fe surfaces have predicted a 100%
enhancement of orbital moment as compared to the bulk value
[14], which should be partially responsible for the observed or-
bital moment enhancement. However, the giant enhancement in
ratio in the 8-ML film suggests an additional mecha-
nism. As pointed out by van der Laan et al. [15], the presence of
surface roughness, interdiffusion, steps, or terraces will lead to
morelocalizedatomic-like3-Dwavefunctionsandanenhanced
orbital moment. One of the distinct features of a reconstructed
semiconductor surface is the formation of regular atomic scale
. With further increase in
structures, such as the In dimer row along the 011 direction
in the InAs (100)-4
2 surface [7]. We thus propose that this
atomicscalestructureleadstomorelocalizedwavefunctionsfor
theFeatoms closetotheinterface and acorrespondinglargeen-
hancement in
as we found in Fe–GaAs [8].
IV. CONCLUSION
We have studied the interface magnetic and structural prop-
erties of epitaxial bcc Fe films grown on InAs (001)-4
evolution of the magnetic phase was found to be dominated by
the film structure rather than possible intermixing at the inter-
face. The Fe clusters formed at the initial growth stage show
both superparamagnetism and ferromagnetism depending on
the thickness. The films show bulk-like spin moments of about
1.90
0.15for the thicknesses above about 20 ML, which
is important for spin electronic devices. A large enhancement
of the ratio of orbital moment versus spin moment in the
ultrathin region might be due to more localized wavefunctions
at the interface.
2. The
ACKNOWLEDGMENT
Y. B. Xu and J. Wu thank K. O’Grady for his support to this
work.
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H compounds: Mag-