In-plane magnetic anisotropies in Fe
3
O
4
films on vicinal MgO(100)
L. McGuigan* and R. C. Barklie
School of Physics, Trinity College, Dublin 2, Ireland
R. G. S. Sofin, S. K. Arora, and I. V. Shvets
CRANN, School of Physics, Trinity College, Dublin 2, Ireland
Received 4 January 2008; revised manuscript received 29 February 2008; published 21 May 2008
Ferromagnetic resonance was used to study the influence of vicinal miscut angle and film thickness on
in-plane fourfold and uniaxial magnetic anisotropies in epitaxial Fe
3
O
4
films grown on vicinal MgO100
surfaces. The in-plane fourfold anisotropy constant K
4
is approximately the same for all films but the dominant
in-plane uniaxial constant K
2
varies linearly with the inverse Fe
3
O
4
layer thickness and approximately qua-
dratically with the vicinal angle. A second, weaker, in-plane uniaxial term is evident for the film on a larger
miscut 10° substrate. The easy axis of the dominant in-plane uniaxial term is perpendicular to the step edges.
The dominant in-plane uniaxial anisotropy has one term inversely proportional to the film thickness that is
associated with anisotropy localized at the interface and a second term that is independent of film thickness; the
latter may arise from the preferential alignment of antiphase boundaries with the step edges.
DOI: 10.1103/PhysRevB.77.174424 PACS numbers: 75.70.Ak, 75.30.Gw, 76.50.+g
I. INTRODUCTION
The magnetic properties of ferromagnetic thin films
grown epitaxially on a vicinal substrate surface are of inter-
est because of both their technological and scientific
importance.
1–3
There have been several studies of such films
on these step arrays: Fe/stepped Ag100,
4–6
Fe/stepped
Au100,
6
Fe/stepped W001,
7,8
Fe/stepped W110,
9
Fe/
stepped Mo110,
10
Fe
1-x
Co
x
/stepped GaAs100,
11
Co/
stepped Cu100,
12,13
and CoPt
3
/stepped MgO100.
14
They
all show that the parallel step arrays induce an in-plane
uniaxial magnetic anisotropy. However, the easy axis of
magnetization associated with this anisotropy is sometimes
parallel to the step edges
3,5,10,12,13
and at other times perpen-
dicular to them.
6–8,14
As noted above, measurements have so
far been mostly confined to films of Fe or Co and there are
no reports or similar investigations for half metallic ferro-
magnetic materials. Half metallic ferromagnetic HMFM
materials with their 100% spin polarization are expected to
have an important role in spin electronic devices. Examples
of such HMFM materials are rare earth doped manganites,
double perovskites, CrO
2
, and magnetite Fe
3
O
4
. Fe
3
O
4
with its high Curie temperature of 858 K Ref. 15 is particu-
larly attractive. Epitaxial films of Fe
3
O
4
are often grown on
100 MgO because there is only a small lattice mismatch of
0.34%. However, because the Fe
3
O
4
unit cell is almost twice
the size of the MgO unit cell and because films form by the
coalescence of separately nucleated islands, such films con-
tain antiphase boundaries APBs.
16–18
Across these bound-
aries, the oxygen lattice remains unaffected but the cation
lattice is displaced and this alters the magnetic exchange in-
teractions at the boundary. As shown by Margulies et al.,
16
intrasublattice exchange interactions dominate across the
boundaries that thus separate oppositely magnetized regions.
It was also shown
16
that this accounts for the difficulty in
saturating the magnetization of magnetite films even with
magnetic fields up to several tesla. Also affected by the pres-
ence of APBs is the magnetoresistance MR behavior of the
films.
19,20
To selectively extract the contribution of APBs to
the MR, Arora et al.
20
measured the MR in directions 01
¯
1
and 011, parallel and perpendicular to the step edges
formed by the epitaxial growth of Fe
3
O
4
films on vicinal
100 MgO substrates with miscut angles of 0.5° and 2°.
They showed that the APBs would preferentially align par-
allel to the step edges and so were able to show that the
observed anisotropy in MR could be attributed to the pres-
ence of the APBs. A schematic diagram of a structure with an
APB along the step edge is shown in Fig. 1. More detailed
discussion of the formation of APBs along step edges is
given elsewhere.
20
In this paper, we present the results of a systematic study
of the in-plane magnetic anisotropy of Fe
3
O
4
films grown
epitaxially on vicinal 100 MgO substrates. Since ferromag-
netic resonance FMR is particularly sensitive to changes in
FIG. 1. Schematic diagram showing one possibility of formation
of a step induced APB. The large and small circles represent O
2-
and B-site Fe
3+
/ Fe
2+
ions, respectively. The boxes indicate the
magnetite unit cell and show the change in structure as the APB is
crossed.
PHYSICAL REVIEW B 77, 174424 2008
1098-0121/2008/7717/1744249 ©2008 The American Physical Society 174424-1