Magneto-elastic coupling in epitaxial magnetic/semiconductor heterostructures
Part I Previous Track Record
Yongbing Xu has been a new lecturer at the University of York since early last year. He was a postdoctoral
research fellow from 1997 to 2000 in the Thin Film Magnetism (TFM) group (headed by Prof. Tony Bland),
Cavendish Laboratory at Cambridge University working on magnetic ultrathin films and patterned mesoscopic
structures. He had previously worked as a PhD student on a joint Leeds-York project, “Spin-resolved
photoemission”, at the CLRC Daresbury Laboratory, from 1994 to 1996, under the joint supervision of Professors
Denis Greig (Leeds) and Jim Matthew (York). Before that he was a lecturer in the National Laboratory of Solid
State Microstructures at Nanjing University, China working on magnetic multilayers and magneto-optics.
He has worked on two EPSRC projects; “Evolution of the magnetic properties of nanoscale structures” (PI:
Prof. Bland) and “Magnetic mesostructures” (PI: Prof. Bland) in Cambridge. His work was focused on the study of
the correlation between the magnetic properties and the atomic scale structure of ultrathin ferromagnetic metal
(FM) films on III-V semiconductors (SC) grown by molecular beam epitaxy (MBE). He found that the magnetic
properties of Fe/GaAs(100)-4x6 at room temperature evolve via three phases; a non-magnetic phase for the first
three and a half monolayers, a superparamagnetic phase, and a ferromagnetic phase above about five monolayers
[1]. The realisation of bulk magnetic moments at Fe/GaAs and Fe/InAs interfaces was an important step toward the
development of the next generation spin-electronic devices [1, 2]. A combined in-situ MOKE, real-time RHEED,
and in-situ STM study of Fe/InAs(100)-4x2 demonstrated the correlation of in-plane uniaxial magnetic anisotropy
and the intrinsic atomic scale structure of the reconstructed semiconductor surface [3, 4]. Along with graduate
students, he has successfully developed two techniques, namely photon-excited spin-injection [5, 6] and in-situ
magneto-resistance [7] to probe the spin-dependent properties of ultrathin films. He also studied extensively the
micromagnetism and spin-dependent transport in patterned dot arrays and wires [8, 9, 10]. He has designed and
demonstrated a simple cross-wire structure to trap domain walls for the study of the spin-dependent transport in
mesoscopic magnetic system.
His PhD thesis in Leeds was “Spin-polarized photoemission on novel magnetic materials”, which was
mainly carried out in CLRC Daresbury Laboratory working with Prof. Elaine Seddon. He studied systematically
for the first time the spin-resolved density of states of three amorphous magnetic alloys FeB, CoB and FeY using a
synchrotron radiation source [11, 12, 13]. The Leeds-York team was the first external users of the new spinpolarized photoemission facilities including a high-energy Mott polar and a compact “micro-Mott” detector in
Daresbury. This work was introduced in the key Daresbury Laboratory annual report of 1995-1996, the one which
highlights a few major achievement and developments in the laboratory throughout the year. The results have also
been presented at several national and international conferences as talks and invited talks. His work in Nanjing was
mainly about magneto-optics in magnetic/nonmagnetic multilayers carried out in the group headed by Prof. Hongru
Zhai. He had studied systematically the thickness and substrate dependence of the magneto-optical Kerr effect in
bilayer and multilayer films, and proposed a new procedure to optimize the signal to noise ratio in layered
magneto-optical storage media [14, 15]. Using MOKE spectra and nuclear magnetic resonance (NMR), he found
the spin-polarization of nonmagnetic metals Ag and Cu in magnetic multilayers Fe/Ag and Fe/Cu [16, 17].
In short, Xu has more than ten years of research experience in the fields of magnetic nanostructures,
nanofabrication using MBE growth and lithography, and surface science. He has published over 70 papers in
leading academic journals and given several talks and invited talks in national and international conferences. He
was awarded an EPSRC advanced research fellowship, started in October 2000, to work on “Spin-electronics in
mesoscopic magnetic materials”. York magnetism group now has four permanent academic staff working on a
wide range of subjects from magnetic recording media to spin-electronic materials.
[1]
[2]
[3]
[4]
[5]
[6]
[7]
[8]
[9]
[10]
Y. B. Xu, E. T. M. Kernohan, D. J. Freeland, A. Ercole, M. Tselepi and J. A. C. Bland, Phys. Rev. B58, 890 (1998).
Y. B. Xu, D. J. Freeland, E. T. M. Kernohan, M. Tselepi, C. M. Guertler, J. A. C. Bland, S. N. Holmes, and D. A.
Ritchie, IEEE Trans. on Magn. 35, 3661 (1999).
Y. B. Xu, E. T. M. Kernohan, M. Tselepi, J. A. C. Bland, and S. N. Holmes, Appl. Phys. Lett. 73, 399 (1998);
Y. B. Xu, D. J. Freeland, M. Tselepi and J. A. C. Bland, Phys. Rev. B, B62, 1167 (2000).
A.Hirohata, Y. B. Xu, C. M. Guertler and J. A. C. Bland, Phys. Rev. B63, 104425 (2001).
J.A. C. Bland, A. Hirohata, C. M. Guertler, Y. B. Xu, J. Appl. Phys., 89, 6744 (2001) ( invited talk)
C. M. Gurtler, Y. B. Xu, J. A. C. Bland, J. Magn. Magn. Mater, 226, 655 (2001).
Y. B. Xu, C. A. F. Vaz, A. Hirohata, J. A. C. Bland, F. Rousseaux, E. Cambril and H. Launois, J. App. Phys. 85, 6178
(1999).
Y. B. Xu, C. A. F. Vaz, A. Hirohata, C. C. Yao, J. A. C. Bland, E. Cambril, F. Rousseaux and H. Launois, Phys. Rev.
B61, R14901 (2000).
Y. B. Xu, A. Hirohata, H. T. Leung, T. Tselepi, J. A. C. Bland, E. Cambril, F. Rousseaux and H. Launois, J. Appl.
2
Phys. 87, 7019 (2000).
[11] Y. B. Xu, C. G. H. Walker, D. Greig, E. A. Seddon, I. W. Kirkman, F. M. Quinn and J. A. D. Matthew, J.Phys.
Conden. Matter. 8, 1567 (1996)
[12] Y. B. Xu, D. Greig, E. A. Seddon and J. A. D. Matthew, Phys. Rev. B55, 11442 (1997)
[13] J. A. D. Matthew, E. A. Seddon, Y. B. Xu, Journal of Electron Spectroscopy and Related Phenomena 88, 171 (1998)
(invited talk).
[14] Y. B. Xu, Q. Y. Jin, Y. Zhai, M. Lu, Y. Z. Miao, Q. S. Bie and H. R. Zhai, J. Appl. Phys. 74, 3470 (1993).
[15] Y. B. Xu, H. R. Zhai and M. Lu, J. Appl. Phys. 70, 7033 (1991).
[16] Y. B. Xu, H. R. Zhai, M. Lu, and Q. Y. Jin, Phys. Lett. A, 168, 213 (1992)
[17] Q. Y. Jin, Y. B. Xu, and H. R. Zhai et al, Phys. Rev. Lett. 72, 768 (1994).
Professor M.R.J.Gibbs has held a Personal Chair within the Department of Physics and Astronomy at the
University of Sheffield since 1st October 1997. Prior to that he spent 10 years in the School of Physics at the
University of Bath, before moving to Sheffield with effect from January 1994. Throughout his career, Professor
Gibbs has studied the magnetic properties of materials. This began with studies of bulk amorphous ferromagnets
(EPSRC grants GR/F52101, GR/F52064, GR/G45878 and GR/J97618) and device applications (EPSRC grants
GR/D63448 and GR/E49562). More recently interests have extended to consider bulk nanophase soft magnetic
materials (EPSRC grants GR/K72407 and GR/M45535), magnetic force microscopy (EPSRC grants GR/K18306
and GR/L42094), and magnetic thin films and related devices (EPSRC grants GR/J96475, GR/K55905 and
GR/R00395).
A strong thread running through many of the research activities has been the study of magnetoelasticity.
Early work studied fundamental aspects of magnetostriction in amorphous ferromagnets [1,2] and its measurement
[3], together with the associated E effect [4]. Applications in magnetometry [5,6] and acoustic transducers [7]
have also been considered. Since 1992 work has been undertaken to study magnetostriction in monolithic and
multilayer films. These have been grown by magnetron sputtering in-house. Amorphous ferromagnetic films have
been characterised [8,9] building on knowledge gained from bulk ribbons and wires. The control of induced
anisotropy has been established [10], offering the possibility of applications in micro-electromechanical systems
(MEMS) [11,12]. Magnetostriction in multilayers has also been studied [13,14], and a phenomenological
interpretation of the data developed [15,16]. The measurement of magnetostriction when the sample is a thin film
on a rigid substrate has also been explored, and a significant contribution made to the understanding of the
cantilever deflection method [17]. This area forms a further part of the current proposal.
[1]
[2]
[3]
[4]
[5]
[6]
[7]
[8]
[9]
[10]
[11]
[12]
[13]
[14]
[15]
[16]
[17]
A.P.Thomas, M.R.J.Gibbs, J.H.Vincent and S.J.Ritchie, IEEE Trans.Mag. 27, 5247(1991).
A.P.Thomas and M.R.J.Gibbs, J.Magn.Magn.Mat. 103, 97 (1992).
P.T.Squire and M.R.J.Gibbs, J.Phys.E: Sci.Instrum. 20, 499 (1987).
P.T.Squire and M.R.J.Gibbs, IEEE Trans.Mag. 25, 3614 (1989).
P.T.Squire and M.R.J.Gibbs, IEEE Trans.Mag. 24, 1755 (1988).
D.Brugel, M.R.J.Gibbs and P.T.Squire, J.Appl.Phys. 63, 4249 (1988).
D.W.Rees, M.R.J.Gibbs and N.G.Pace, IEEE Trans.Ultrasonics, Ferroelectrics & Frequency Control 36, 332 (1989)
A.D.Mattingley, C.Shearwood and M.R.J.Gibbs, IEEE Trans.Mag. 30, 4806 (1994)
C.Shearwood, A.D.Mattingley and M.R.J.Gibbs, J.Magn.Magn.Mat. 162, 147 (1996)
M.Ali, R.Watts, W.J.Karl and M.R.J.Gibbs, J.Magn.Magn.Mat. 190, 199 (1998)
M.R.J.Gibbs, C.Shearwood, J.L.Dancaster, P.E.M.Frere and A.J.Jacobs-Cook, IEEE Trans.Mag. 32, 4950 (1996)
W.J.Karl, R.Watts, A.L.Powell, C.R.Whitehouse, R.B.Yates and M.R.J.Gibbs, Electro.Chem.Soc.Proc. 98-20, 354
(1999)
T.A.Lafford, M.R.J.Gibbs, R.Zuberek and C.Shearwood, J.Appl.Phys. 76, 6534 (1994)
T.A.Lafford, R.Zuberek and M.R.J.Gibbs, J.Magn.Magn.Mat. 140-144, 577 (1995)
T.A.Lafford and M.R.J.Gibbs, IEEE Trans.Mag. 31, 4094 (1995)
H.J.Hatton and M.R.J.Gibbs, J.Magn.Magn.Mat. 156, 67 (1996)
R.Watts, M.R.J.Gibbs, W.J.Karl and H.Szymczak, Appl.Phys.Lett. 70, 2607 (1997)
3
Part II Proposed project and its context
A. Introduction and background
The increasing miniaturization of storage media and electronic devices calls for magnetic materials having well
defined and controlled magnetic anisotropy, coercivity and saturation fields at thicknesses down to the
nanometer/atomic scale. Magnetic ultrathin films, with thicknesses on the nanometer/atomic scale, have become
increasingly important for high-density magnetic storage [1] and spin-electronic devices [2, 3]. Patterned ultrathin
dot arrays, for example, have the possibility of overcoming the superparamagnetic limit (or have a
superparamagnetic limit at very small lateral dimensions), as the stray field is minimized, and the combination of
anisotropies and the exchange interaction may keep the spins aligned parallel along a certain direction [1, 4, 5]. The
use of ultra-thin ferromagnetic pads in spin-electronic devices [6-8] may reduce the dipole interaction and allow
controllable switching of the pads, as well as minimizing the stray field in adjacent semiconductors.
There is growing evidence that the magnetic properties of ultrathin films may become dominated by interface
phenomena such as lattice mismatch. One of the key intrinsic magnetic parameters in this may be the magnetoelastic (ME) interaction, which links the structure and the magnetic properties of materials. The magneto-elastic
coupling caused by even a small strain can change dramatically the magnetic anisotropy and the magnetization
process of a magnetic thin film. For example, a 1% lattice mismatch in a cubic Fe system will introduce a magnetoelastic energy comparable to the magneto-crystalline energy. Recent studies [9-14] reveal that both the magnitude
and the sign of the magneto-elastic coupling coefficient in ultrathin films could be distinctly different from that of
their respective bulk materials. Sander et al [9, 10] found that the magneto-elastic coupling coefficient of Fe films
grown on W depends on the film thickness, and even the sign is changed at around a thickness of 3nm. Even in
amorphous alloys the magneto-elastic coupling was found to differ by more than a factor of three near the surface
region [13]. These results suggest strongly that one cannot take for granted the bulk magneto-elastic constants in
explaining the magnetic anisotropy observed in ultrathin films. However, understanding of the magneto-elastic
interaction in ultrathin films is still poor, and very few systematic studies have been made.
Magnetic/semiconductor heterostructures are of growing interest for the study of fundamental magnetism of
ultrathin films and for the development of spin-electronic devices. Within the context of spin-electronics, the
electrons’ spins, not just their electrical charge, are controlled for the operation in information circuits. As the
conventional solid-state electronic devices are based on semiconductors, the injection and manipulation of spin
electrons in magnetic/semiconductor heterostructures may lead to the development of next generation spinelectronic devices for data storage and processing at the same time [2, 3, 14]. The patterned ultrathin magnetic
particles on semiconductors might also be promising materials for high-density magnetic storage media [1, 4, 5].
The stabilization of epitaxial bcc Fe on GaAs has been achieved in several groups [15-20]. Recently, Xu et al [19]
demonstrated that high quality Fe films without interface magnetic dead layers can be achieved in the Fe/GaAs
system. A new epitaxial magnetic/semiconductor, namely Fe/InAs, has also been synthesized despite the large
lattice mismatch (5.6%) [21]. One of the important issues concerning the magnetic properties of the Fe/III-V
semiconductor system is the origin of a uniaxial magnetic anisotropy, which can then be controlled and exploited in
future devices. Take Fe/GaAs(100), for example: there is a strong uniaxial anisotropy, unexpected from the cubic
anisotropy of bulk bcc Fe, in the ultrathin films below about 50ML [15-20]. The cubic anisotropy of the bulk phase
can even disappear completely at a thickness around 7ML. The measurement of the magneto-elastic constant,
which determines the balance of long range strain effects and short-range chemical effects (section E.a), in this
project will give a crucial insight into this issue. Furthermore, the ferromagnetic metal/III-V semiconductor
heterostructures provide a unique “laboratory” to study directly for the first time fundamental aspects (sections E.b,
E.c, and E.d) concerning the magneto-elastic coupling in magnetic nanostructures because of the novel “tunable”
lattice mismatch in this system.
B. Project objectives
1. Determine unambiguously the origin of the uniaxial magnetic anisotropy by measuring the thickness
dependence of the magneto-elastic constants in Fe/GaAs and Fe/InAs.
2. Establish the relationship between the strain, the thickness, and the magneto-elastic coupling by exploring the
unique tunable lattice mismatch between Fe and certain III-V semiconductors.
3. Study the magneto-elastic coupling in metastable magnetic phases such as bcc Co films.
4. Study the possible effect of nonmagnetic capping layers to the magnetic-elastic coupling in ultrathin magnetic
films and it’s implication to the operation of practical devices.
4
C. Timeliness and novelty
A key aspect of this proposal is the new and close collaboration between York and Sheffield magnetism research
groups with complementary expertise in MBE growth (Dr. Xu in York) and magneto-elastic measurements (Prof.
Gibbs in Sheffield). There is growing evidence that the magneto-elastic constants in nanoscale magnetic materials
are in general different from their bulk values in not only the magnitude but also possibly the sign. There has been
little progress, however in understanding the basic physics of magnetoelasticity in nano scale materials as well as in
tailoring the magnetoelastic interaction in devices. Ferromagnetic metal/semiconductor heterostructures are
important materials for the development of spin electronic devices and the understanding of the magneto-electronic
interaction in this system is crucial to control their magnetic properties. This proposal represents an opportunity to
establish a world-class facility, ultimately with in-situ measurement capacity, for the study of magneto-elastic
interaction in epitaxial magnetic thin films. In comparison with reported studies in the Fe/W system [9],
ferromagnetic metal/semiconductor heterostructures and specifically Fe/GaIn1-xAsx provide a unique opportunity to
establish directly the strain dependence of the magnetoelastic coupling and to probe for the first time the intrinsic
size effect in nano scale magnetic materials. This proposal also provides support for a newly appointed academic
(Xu) to further develop his research base. This is his first EPSRC proposal apart from the fellowship one.
D. Experimental methodology
a. Growth of ferromagnetic metal/III-V semiconductor (FM/SC) heterostructures
With funds from EPSRC, the university and the Department, Xu has recently built up a new UHV chamber
in York especially for the growth of magnetic/semiconductor heterostructures. The growth system has three e-beam
sources, which can be used for magnetic metals, such as Fe, Co, and FeNi, and nonmagnetic metals such as Au and
Cr. The sample manipulator has incorporated an e-beam heater for the annealing of substrates and the removal of
As capping layers to ensure a clean surface for growth. There is a shutter fitted with a linear driver for step growth.
A set of samples of up to about ten different thicknesses can be fabricated in one growth run to ensure the same
growth conditions. For ex-situ magnetic and magneto-elastic constant measurements, the magnetic films will be
capped with a thin Au or Cr layer of about 3nm. With funds from this application we will purchase a RHEED
system for monitoring the growth and structural analysis, and an Ar sputtering gun for further substrate cleaning
when necessary. The RHEED system, in combination with magneto-elastic measurements will allow for a full
understanding of the relation between structure and magnetic properties in ultrathin films.
b. Ex-situ and in-situ magneto-elastic measurements.
For the last ten years Professor Gibbs has been studying the magneto-elastic properties of films on a wide range
of substrates. A number of techniques have been evaluated including small-angle-magnetisation-rotation, cantilever
and bent substrate [22-24]. Only the bent substrate technique is appropriate in this context. The present facilities in
Sheffield have been developed for the study of soft magnetic films (NiFe and FeCo) with anisotropy fields no
higher than 5kAm-1. The technique will now be adapted to handle the field requirements of Fe/GaAs and Fe/InAs
films by building a system around an electromagnet. Work on NiFe
has shown excellent signal to noise ratio down to 3nm thickness of
NiFe (see Fig.1), and thus the technique requires only the field
adaptation to be viable. As the MOKE signal from Fe and Co is
about three times larger than that from FeNi, good signal to noise
ratio down to a few monolayers will be achieved in ultrathin Fe
and Co films. Sheffield also has AFM/MFM facilities, which will
be used to evaluate surface roughness contributions to the surface
magnetism. It is important to work ex-situ (even though there will
be a capping layer) not only to evaluate the technique and ensure
correct implementation in UHV, but also to study extensively the
effects of capping layers.
In a parallel phase of the project, an appropriate form of the exsitu Villari measurement method developed in Sheffield will be
Fig.1 MOKE loops for 15nm NiFe film on
combined with York UHV growth chamber to develop an in-situ
thermally oxidised Si - film bent over
magneto-elastic measurement system. The key components for indifferent radii with strains varying from
situ measurements are a magnet compatible with UHV and a
-4
-3
4x10 to 1.25x10
5
sample holder capable of bending the sample in UHV. The magnet with field up to 240kAm-1 has been built up
already. Fig. 2 shows a schematic diagram of the proposed in-situ measurement system with a purposely-designed
sample holder based on the current low field ex-situ
measurement apparatus in Sheffield.
The sample here is held between three supports, the central
one is fixed and the two outer ones can be drawn backwards (to
bend the sample) via an UHV inchworm motor. This allows a
controllable and quantifiable surface strain to be induced. The
Burleigh UHV inchworm (model: UHVL-015) with a
resolution of 4nm, a maximum range of motion of 15mm, and a
maximum axial load of 7N is the most suitable one. We
performed some preliminary tests to determine the force and
strain that a piece of GaAs wafer could possibly sustain. A
piece of wafer of about 10 mm long, 5mm wide, and 0.5 mm
Inchworm
thick was found to be able to sustain a force of about 5-10N
Motor
and a surface strain of about 1-2x10-3 with a deflection of about
50-100m. The inchworm can thus induce large enough strain
with excellent resolution for the measurements. The sample
holder will further incorporate a small filament for the
annealing
of the substrate. The York workshop has an excellent
Fig.2 A schematic of the proposed in-situ Villari
track
record
in building UHV equipment. Xu has been working
measurement system. The sample will be bent by
on
UHV
based
experiment for about ten years and has
a UHV inchworm motor with a resolution of 4nm
considerable experiences in designing and modifying UHV
and a maximum force of 7N.
components and sample holders. Such a system will be the first
in UK to have the capacity of in-situ magneto-elastic measurement, which is important as the capping layer may
change the magnetic properties of nano scale films. It is worth noting that the lack of understanding of the
magneto-elastic interaction in ultrathin films might be partially due to the lack of in-situ characterisation
equipment in the UHV environment. As far as the applicants know, there is only one dedicated system (in Prof.
Kirschner’s group in Halle) in the world capable of carrying out the in-situ measurements.
E. Scientific Programme
a. Does ME contribute to uniaxial anisotropy in Fe/GaAs and Fe/InAs?
Two distinctly different mechanisms have been proposed [15-20] for the uniaxial magnetic anisotropy observed
in Fe/GaAs and Fe/InAs; 1) Interface anisotropy due to the unidirectional nature of Fe-As, Fe-Ga and Fe-In bonds
at the ferromagnetic metal/semiconductor interface. By examining the anisotropy of the Fe films deposited on two
different kinds of GaAs substrates showing different reconstructions, Kneedler et al [18] proposed that the
unidirectional nature of Fe-As or Fe-Ga bonds is responsible for the UMA. This might be understood as a shortrange “chemical” effect, in which the electronic structure of the Fe atoms near the interface differ distinctly from
“normal” bcc Fe. 2) Magneto-elastic interactions due to strain in the ultrathin epitaxial films caused by lattice
mismatch. Xu et al [25] proposed that a uniaxial magneto-elastic coupling contribute to the uniaxial anisotropy.
Two pieces of evidence [25] have been observed in Fe/GaAs(100) and Fe/InAs(100) of the same reconstruction to
support this picture. Firstly, the easy axis direction of the UMA in Fe/InAs(100)-4x2 differs by 90 degree from that
in Fe/GaAs(100)-4x2. The Fe film in the ultrathin region is compressed (in-plane) on GaAs whilst expanded on
InAs. This will lead to opposite strain tensor components for the two systems. Secondly, a uniaxial strain
relaxation is observed, which occurs over the same thickness range as the uniaxial magnetic anisotropy. However,
the magnetic anisotropy observed suggests that the magnetic-elastic coupling coefficient in the ultrathin Fe films is
distinctively different from that in the bulk Fe. The uniaxial anisotropy energy is much larger in Fe/GaAs than that
in Fe/InAs. If the Fe films have the same bulk magneto-elastic coupling coefficient, the uniaxial anisotropy in
Fe/InAs should be about four times larger than that in Fe/GaAs. Further more, the lattice relaxation in Fe/InAs from
the RHEED measurements [25] indicates that the easy axis in Fe/InAs should be along [0-11] rather than along
[011] if the Fe films have the bulk magneto-elastic coupling coefficient. All these imply that both the magnitude
and the sign of the magneto-elastic coupling coefficient in Fe/GaAs and Fe/InAs might be very different from the
bulk and different from each other as well. The measurement of the magneto-elastic constants will thus allow us to
determine quantitatively the contribution of the lattice relaxation to the magnetic anisotropy. This thus will be a key
experiment to establish unambiguously the origin of the uniaxial magnetic anisotropy in Fe/GaAs and Fe/InAs, an
open issue over the last ten years.
6
b. Direct determination of the strain dependence of the
magneto-elastic coupling coefficient.
The thickness dependence of the magneto-elastic coupling
coefficient observed in Fe/W [9] and Fe/MgO [12] suggested
that the change of the magnitude and sign of the magnetoelastic coupling in nanometer scale films is due to the variation
of the strain in the Fe. It is, however, a difficult task to
establish the correlation between the strain and the magnetoelastic coupling coefficient by measuring only the thickness
dependence, as the magnetisation, the crystalline anisotropy as
well as the distribution of the lattice relaxation across the layer
may change with thickness. Fe/III-V semiconductor
heterostructures provide a unique opportunity to determine
Fig.3 The fundamental energy gap against
directly the correlation between the strain and the magnetolattice constant for several III-V compounds
elastic coupling coefficient. Fig. 3 shows the lattice constants
and Si-Ge. The lattice constant (x2) is shown
as well as the fundamental energy gap of several classes of IIIfor bcc Fe.
V materials. The lattice constant (x2) is also shown for bcc Fe.
As shown by the figure, the lattice mismatch can be varied
from –1.3% in Fe/GaAs to 12% in Fe/InSb. The lattice mismatch can further be varied continuously, as the lattice
constants of the III-V semiconductors can be tailored by varying the composition. Take Fe/InxGa1-xAs for example;
where the lattice mismatch can be varied continuously from –1.3% (x=0) to 5.6% (x=1). The strain dependence of
the magneto-elastic coupling coefficient can thus be determined literally at any lattice mismatch. This should give
an important insight into the mechanism of the variation of magneto-elastic interaction in ultrathin films and allow
us to establish a comprehensive database for theoretical studies.
c. Intrinsic “Size effect” in nano scale magnets?
An issue of fundamental importance is “does the magneto-elastic coupling coefficient change in magnetic ultrathin
films without the change of the lattice constants”, or in general “does the magneto-elastic coupling coefficient
change in magnetic nanostructures”. The magneto-elastic coupling is dependent on the spin-orbital coupling and
band structures etc. The orbital moment in ultrathin films, for example, can be very different from that in bulk
materials. A giant orbital moment enhancement has been observed in Fe/GaAs [26]. It thus can be expected that the
magneto-elastic coupling coefficient will change in magnetic nanostructures. It is, however, almost impossible to
study this issue experimentally using magnetic metal/nonmagnetic metal systems as there is always strain in the
ultrathin films. Fe/III-V semiconductor heterostructures again provide a unique opportunity to address this
fundamental issue, as the lattice constants of III-V semiconductors can be tuned to match the lattice constants of
bcc Fe. At the composition, x = 0.2, the lattice constant of InxGa1-xAs is 5.73Å, which is identical to that of two unit
cells of bcc Fe. The Fe film grown on top of is expected to have the same lattice constants as that of bulk bcc Fe.
We can thus achieve a novel “quasi-home-epitaxy” with two completely different kinds of materials, but without
lattice mismatch. The nano scale “size effect” of the magneto-elastic interaction in magnetic materials will thus be
studied for the first time.
d. Magneto-elastic coupling in meta-stable bcc phases in Co/ GaAs and Co/InAs.
While the studies of ferromagnetic metal/III-V semiconductor heterostructures have been concentrated on
Fe/GaAs, other magnetic/III-V semiconductor heterostructures are also interesting and some of them may be
important for applications. As for example Co and Ni have large spin polarisation at the Fermi level, they can be
used as ferromagnetic pads with possibly higher spin-injection efficiency in the spin-electronic devices. The
stabilization of epitaxial Co on GaAs has been previous demonstrated [27-29]. The Co films were found to form a
bcc phase in the ultrathin region and an hcp phase at higher coverage. We have also found that the meta-stable bcc
Co phase can be stabilized in Co/InAs [30]. Co/GaAs and Co/InAs will allow us to study two interesting issues, 1)
the magnetic-elastic coupling in meta-stable phase, which has not been studied as far as we know, and 2) any
differences of the magnetoelastic coupling between bcc Fe and bcc Co. There have been few systematic studies of
magnetic properties in both Co/GaAs and Co/InAs systems so far. Our preliminary MOKE measurements show
that the easy axis of the uniaxial anisotropy in Co/GaAs(100) is along <011>, rotated by 90 degree as compared
with that in Fe/GaAs. This implies that the magneto-elastic coupling coefficient in bcc Co has an opposite sign to
that in bcc Fe.
7
E. Relevance to beneficiaries
The proposed research represents the frontiers in the fields of nanomaterials and nanomagnetism. It is believed
to be crucially important to both fundamental research and technical applications with the beneficiaries falling into
following categories.
a) The UK has a leading position in MBE growth and study of thin film magnetism, with several groups working
actively in this field. However, the magneto-elastic interaction, a key parameter to correlate magnetic
properties and crystal structure, has not been addressed. The results from this project will be extremely
valuable to the understanding of the magnetic properties and in particularly the magnetic anisotropy in
magnetic thin films and devices. The in-situ magnetoelastic measurement system to be developed in this
project will be just second to the one in Germany and might be crucial to help the UK magnetic community to
keep a leading position in this field.
b) This project will provide crucial and unique experimental data for theoretical calculations (see supporting
letters). There is little theoretical understanding of the magneto-elastic interaction in nano scale magnetic
materials partially due to the lack of experimental data.
c) Magnetic/semiconductor materials are the building block to develop spin electronic devices, which may be
operated for data storage and processing at the same time. The understanding of the magneto-electronic
interaction in this system is important to control the magnetic properties of the devices and is of interest to
Toshiba and Seagate for example.
F. Dissemination and exploitation:
Publications through leading scientific journals as well as reports at international conferences will be the
main route to disseminate the results to the global community of scientists working on this and related fields. Some
of the key results will be disseminated through the international computer network such as World-Wide-Web to
reach the general public. The results will be reported at CMMP, the conference on “Current Research on
Magnetism” organized by IoP, to ensure the quick dissemination to the UK magnetism community.
G. Justification of Resources
Xu has recently relocated his EPSRC advanced fellowship to York following his appointment as a lecturer.
In order to establish his research and to carry out his fellowship project in York, he has built up a UHV chamber for
the sample growth. The department has already contributed £5K, a basic UHV chamber, and one-year technician
time toward this project. The university innovation and research-priming fund has also contributed £3K. Apart
from these, the applicant has spent a major part of his EPSRC fellowship-supporting fund to the construction of this
UHV chamber. He now has a running UHV system with the main chamber pumped by a powerful Edward EXT501
turbopump and the small transfer chamber by an EXT70H turbopump, and three e-beam growth sources.
The principle resources requested in this project are:
1) Two RA1A appointments (one Sheffield one York) - this level of staffing is necessary as the UHV growth and
characterisation (York) and the magnetoelastic studies (Sheffield) require high intellectual input and are
complementary - one appointee could not cover the whole project. The design, construction, and running of
two complementary state-of-the-art ME measurement systems would require a total manpower of four man
years with two man years each in York and Sheffield. This level of support is crucial for the rapid development
of the project in both institutions. As shown in the attached work plan, during the first year, the RA in York
will work on the sample growth, structural analysis and magnetization measurements, and the RA in Sheffield
will concentrate on updating the ex-situ ME measurement system and measuring the ME properties of the
samples. The RAs will jointly design and construct the sample holder for the in-situ measurements which will
then lead to the establishment of an in-situ ME measurement system in York around the end of the first year
and the beginning of the second year. As this project has four major related but distinctive scientific objectives,
the RAs are expected to concentrate on two objectives each during the second year while exchanging ideas and
experimental results. They will grow the samples and carry out the measurements both in-situ and ex-situ. To
achieve their objectives, they will use all the necessary facilities in both York and Sheffield and will be equally
attached to both institutions in that phase of the work.
2) York: RHEED, ion gun, stabilized laser, and inchworm motor. The major equipment requested in this project is
a RHEED system. This equipment is essential to monitor the epitaxial growth and measure the lattice
relaxation of the films. Ion gun is necessary for the cleaning of the semiconductor substrates, which don’t have
an As capping layer. The stabilized laser will be used for in-situ high sensitive magneto elastic measurements.
8
3)
4)
5)
6)
7)
It can also be used for in-situ MOKE if necessary. The UHV inchworm motor rather than a mechanical linear
driver will be used to bent the sample as it has an unparallel resolution and a quantifiable motion.
Sheffield: a 4" Newport electromagnet is already available. For this ex-situ ME measurement system a laser,
polarisers, detection system, power supply and PC will have to be purchased. The current system is in heavy
demand for soft film studies.
Technician time (0.5 man year to each of Sheffield and York) to modify the UHV growth chamber and the
manipulator, and to build the apparatus for ex-situ and in-situ sample bending.
Wafers from Cambridge (letter of support attached)
The consumables budget is to cover gases, cleaning materials, targets, electronic and optical components
Two people per year to one international conference (MMM or ICM), travel for regular collaboration meetings.
The project will be managed via regular meetings between the two applicants and the two RAs. These meetings
will take place approximately monthly, alternating between institutions.
References.
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8. P. R. Hammar, B. R. Bennett, M. J. Yang, M. Johnson, Phys. Rev. Lett. 83, 203 (1999).
9. D. Sander, A. Enders, J. Kirschner, J. Magn. Magn. Mater. 200, 439 (1999).
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19. Y. B. Xu, E. T. Kernoham, D. J. Freeland, A. Ercole, M. Tselepi, J. A. C. Bland, Phys. Rev. B58, 890 (1998).
20. M. Brockmann, M. Zolfl, S. Miethaner, G. Bayreuther, J. Magn. Magn Mater. 198, 384 (1999).
21. Y. B. Xu, E. T. M. Kernohan, M. Tselepi, J. A. C. Bland and S. Holmes, Appl. Phys. Lett. 73, 399 (1998).
22. K.Narita, J.Yamasaki and H.Fukunaga, IEEE Trans Mag 16, 435 (1980)
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9
Magneto-elastic coupling in epitaxial magnetic/semiconductor heterostructures
Diagrammatic workplan
York
Year 1
Sheffield
MBE growth: Fe/GaAs and
Fe/InAs, and Fe/InxGa1-xAs
Set up apparatus for ME
measurements in air
Structural analysis using
RHEED
ME measurements:
Fe/GaAs and Fe/InAs,
and Fe/InxGa1-xAs
Magnetization measurements
using AGFM etc.
AFM/MFM analyses
Design and build sample
holder for ME
measurements in UHV
Set up in-situ ME
measurement apparatus
In-situ ME measurements
Year 2
MBE growth: Co/GaAs and
Co/InAs
RHEED and AGFM
measurements
Ex-situ measurements
Compare the results
from in-situ and ex-situ
measurements, and
understand the effect of
capping layers
AFM/MFM analysis
RA1: Objectives 1 and 2; RA2: Objectives 3 and 4