Negative magnetoresistance in Fe
3
O
4
/Au/Fe spin valves
Sebastiaan van Dijken, Xavier Fain, Steven M. Watts, and J. M. D. Coey
SFI Trinity Nanoscience Laboratory, Physics Department, Trinity College, Dublin 2, Ireland
(Received 26 February 2004; revised manuscript received 10 May 2004; published 31 August 2004)
The structural, electrical, and magnetic properties of Fe
3
O
4
/Au/Fe spin valves on MgO001 are presented.
In contrast to more conventional spin valve structures, the current-in-plane resistance of the Fe
3
O
4
/Au/Fe spin
valves is found to be smallest for an antiparallel alignment of the magnetization of the Fe
3
O
4
and Fe layer.
Since the electrical current is transported through the low resistance Au and Fe layers, the negative magne-
toresistance effect is attributed to an inverse electron spin scattering asymmetry at the Fe
3
O
4
/ Au interface.
DOI: 10.1103/PhysRevB.70.052409 PACS number(s): 75.70.-i, 73.50.-h, 75.70.Cn
The incorporation of Fe
3
O
4
films into magnetoresistance
(MR) devices has attracted much scientific attention in recent
years. The main reason for this is its high Curie temperature
of 858 K and the prediction that Fe
3
O
4
exhibits full negative
spin polarization at the Fermi level.
1,2
Although experiments
have not confirmed the complete half-metallicity of Fe
3
O
4
,
they do show that the number of minority electrons is larger
than the number of majority electrons at the Fermi level.
3–7
Since the spin polarization of Fe
3
O
4
is opposite to that of
most other magnetic materials commonly used in magnetic
tunnel junctions (MTJs) and magnetic spin valves, the use of
one Fe
3
O
4
electrode in these devices is expected to yield
negative MR effects, i.e., the electrical resistance is lowest
for an antiparallel alignment of the magnetization direction
of the two ferromagnetic layers. For MTJs negative tunnel-
ing magnetoresistance (TMR) has indeed been observed,
5–7
but until now only positive giant magnetoresistance (GMR)
effects have been reported for magnetic spin valves with one
Fe
3
O
4
layer.
8,9
. In this paper we show that Fe
3
O
4
/Au/Fe
spin valves exhibit negative GMR in the current-in-plane
geometry, due to opposite electron spin scattering asymme-
tries at the Fe
3
O
4
/ Au spacer layer interface and in the Fe
layer and at the Fe/Au interface.
The Fe
3
O
4
/Au/Fe spin valves were grown by dc-
magnetron sputtering on MgO001 substrates in a Leybold
Z550-S system with a base pressure of 10
-7
mbar. The
Fe
3
O
4
films were reactively sputtered from a pure Fe target
in 3 10
-3
mbar Ar and 4 10
-5
mbar O
2
at a substrate tem-
perature of 673 K. Deposition of Fe
3
O
4
on MgO001 under
these optimized conditions results in single crystalline films
with a (001) orientation. In addition, resistance measure-
ments on these films show a clear Verwey metal-insulator
transition around 120 K, which is characteristic for Fe
3
O
4
and an indication of good quality film growth.
10,11
The Au
and Fe films were grown at substrate temperatures below
373 K. The deposition rates were determined by small angle
x-ray diffraction reflectivity measurements. The magne-
totransport properties of the spin valves were measured in
the van der Pauw configuration. After contacting the 10 mm
square samples in the four corners, the samples were placed
in an electromagnet with a maximum field of 160 mT. The
MR of the spin valves was measured in the longitudinal
H
I and transverse HI geometry. Superconducting
quantum interference device (SQUID) magnetometry was
used to study the magnetization reversal processes in the spin
valve structures.
Figure 1 shows a -2 x-ray diffraction scan of a
MgO001 /30 nm Fe
3
O
4
/5 nm Au/10 nm Fe/2 nm Au spin
valve. Since the lattice parameter of Fe
3
O
4
a =8.396 Å is
approximately twice that of the MgO001 substrate a
=4.213 Å, the absence of any distinct Fe
3
O
4
reflections in
Fig. 1 indirectly indicates the growth of an (001)-oriented
Fe
3
O
4
film on MgO001. X-ray diffraction scans of Fe
3
O
4
films on SrTiO
3
001 substrates do indeed confirm the
growth of epitaxial Fe
3
O
4
001 films under the selected
deposition conditions. Figure 1 reveals a good
MgO001
Au111 out-of-plane epitaxial relationship. The
absence of any clear Fe reflections indicates that Fe grows on
the Au111 spacer layer without a well-defined crystal ori-
entation. From transmission electron microscopy and small
angle x-ray diffraction reflectivity measurements the rough-
ness of the interfaces in the spin valve is estimated to be
smaller than 1 nm.
Figure 2 shows the resistance of a 30 nm Fe
3
O
4
/5 nm
Au/10 nm Fe/2 nm Au spin valve as a function of the ap-
plied magnetic field in the longitudinal and transverse geom-
etry at 90 and 300 K. Two different MR effects can be dis-
tinguished. In small magnetic fields the resistance of the spin
valve abruptly decreases in the longitudinal measurement,
while it increases in the transverse measurements. This an-
isotropic magnetoresistance (AMR) is due to magnetization
reversal in the Fe layer through which part of the electrical
current is transported. In addition to the AMR effect, the
FIG. 1. X-ray diffraction scan of a 30 nm Fe
3
O
4
/5 nm
Au/10 nm Fe/2 nm Au spin valve on a MgO001 substrate.
PHYSICAL REVIEW B 70, 052409 (2004)
1098-0121/2004/70(5)/052409(4)/$22.50 ©2004 The American Physical Society 70 052409-1