Magnetoelastic coupling of compressively stressed FeÕGaAs„001…
G. Wedler and B. Wassermann
Institut fu ¨r Experimentalphysik, Freie Universita ¨t Berlin, Arnimallee 14, D– 14195 Berlin, Germany
R. Koch
Paul-Drude-Institut fu ¨r Festko ¨rperelektronik, Hausvogteiplatz 5 – 7, D– 10117 Berlin, Germany
Received 12 March 2002; published 14 August 2002
The magnetoelastic coupling, a property of major importance in heteroepitaxy, describes the dependence of
the free energy of magnetic materials on strain and stress. Using our versatile UHV cantilever beam magne-
tometer we have investigated the magnetoelastic coupling constants B
1
and B
2
of Fe/GaAs001, a system
yielding compressed Fe001 films. Both constants exhibit a strong dependence on the compressive film stress,
which so far is not explained by theory: At stress values of about -0.5 GPa the sign of B
1
is positive and thus
opposite to the bulk value; B
2
is about 15% smaller than the bulk value.
DOI: 10.1103/PhysRevB.66.064415 PACS numbers: 75.70.-i, 75.80.+q, 68.60.Bs
I. INTRODUCTION
The magnetoelastic ME coupling interrelates the state of
magnetization of ferromagnetic materials with lattice distor-
tions. It is responsible for the well-known phenomenon of
magnetostriction and can be quantified either via the ME
coupling constants B
i
Ref. 1 or the magnetostriction con-
stants
i
Ref. 2. For typical magnetostrictive distortions of
10
-3
–10
-5
the changes in the ME energy are negligibly
small compared with the other magnetic contributions to the
free energy. In heteroepitaxial thin films, however, intrinsic
distortions due to lattice mismatch between film and sub-
strate usually amount to a strain of several percent (
10
-2
), thus affecting the ME energy substantially. It may
even become comparable to the magnetocrystalline energy,
which offers the challenging opportunity to stabilize new
magnetic anisotropies different from the bulk
3,4,5
in het-
eroepitaxial thin films.
In the recent years it was convincingly shown by various
experimental
6–11
and theoretical
12–17
studies that at strain
values of a few percent a linear description of the ME energy
( F
ME
B ) is no longer sufficient. It is convenient to use a
series expansion in strain for the experimentally determined
ME coupling constants B
i
expt
:
B
i
expt
=B
i
+D
i
+••• . 1
Here B
i
is a ME coupling constant of the unstrained state,
whereby good agreement with the corresponding bulk con-
stants was found.
7–9
D
i
is a second-order constant. In the
case of Fe the second-order expansion of B
1
is valid for a
strain of at least 4% according to theoretical
calculations.
15
Experimental studies, however, point to the
necessity of third- or higher-order terms already at a tensile
strain above +0.5%,
9
whereas films under compression have
not been investigated so far.
In this study we report on the ME coupling constants B
1
and B
2
of Fe/GaAs001, a technologically important het-
eroepitaxial system, where the influence of compressive
strain on the ME coupling can be investigated. The misfit is
-1.36% ( a
Fe
=0.2866 nm, a
GaAs
/2=0.2827 nm); i.e.,
ideal coherent growth is accompanied by a compressive
stress of -2.8 GPa. Being one of the promising candidates
for spin injection into semiconductors,
18
growth, epitaxy, and
magnetism of Fe/GaAs001 have been studied intensively in
the past. It is well established that Fe grows epitaxially in its
bcc modification with the 001 plane parallel to the
substrate
19
both at elevated temperatures 420–450 KRefs.
20–22 and at room temperature Refs. 23–26. However,
due to diffusion of As and Ga into the Fe film, no sharp
interface is formed.
22,27,28
Recent stress investigations
29
re-
vealed that interdiffusion is reduced in films deposited at 300
K, but definitely not negligible.
II. EXPERIMENT
The experiments were performed in a multiple chamber
UHV system base pressure 1 10
-10
mbar) equipped
with a sensitive cantilever beam magnetometer CBM for in
situ stress and magnetic measurements
30
and a four-grid low-
energy electron diffraction LEED optics for in situ control
of the substrate and film quality as well as a homebuilt UHV
scanning tunneling microscope STM for in situ structural
investigations. The substrates were cut from 100- m-thick
As-capped GaAs001 wafers, which were prepared in a
separate molecular beam epitaxy MBE chamber by re-
moval of the oxide, deposition of an 0.5- m-thick
GaAs001 buffer layer, and coating with a thin As layer.
Prior to the Fe deposition the As capping layer was desorbed
at about 650 K until 001 LEED spots were obtained; some
of the substrates were additionally annealed for 1 h at 810 K,
which exhibit also faint (2 4) and c (4 4) superstructure
spots. Fe was electron beam evaporated from a Knudsen-
type tungsten source at a pressure better than 2
10
-9
mbar; the deposition rate determined by a quartz
crystal microbalance was 0.0080.001 nm/s. Immediately
after the film preparation the magnetic measurements were
performed in situ. For the measurements of B
1
the substrate
orientation has to be chosen so that the 100 and 010 di-
rections of the Fe001 film are parallel to the length and
width of the cantilever beam; the difference in magnetostric-
tive stress upon saturation magnetization along 100 and
PHYSICAL REVIEW B 66, 064415 2002
0163-1829/2002/666/0644155/$20.00 ©2002 The American Physical Society 66 064415-1