Low Relaxation Rate in Epitaxial Vanadium-Doped Ultrathin Iron Films
C. Scheck,
1
L. Cheng,
1
I. Barsukov,
2
Z. Frait,
2
and W. E. Bailey
1,
*
1
Materials Science, Department of Applied Physics and Applied Mathematics, Columbia University, New York, New York 10027, USA
2
Institute of Physics, Academy of Sciences of the Czech Republic, Prague, Czech Republic
(Received 29 December 2006; published 12 March 2007)
The longest relaxation time and sharpest frequency content in ferromagnetic precession is determined
by the intrinsic (Gilbert) relaxation rate
G
. For many years, pure iron (Fe) has had the lowest known value
of
G
±
57 MHz
for all pure ferromagnetic metals or binary alloys. We show that an epitaxial iron alloy
with vanadium (V) possesses values of
G
which are signi±cantly reduced to
35
²
5 MHz
at 27% V. The
result can be understood as the role of spin-orbit coupling in generating relaxation, reduced through the
atomic number
Z
.
DOI:
10.1103/PhysRevLett.98.117601
PACS numbers: 76.30.Fc, 72.25.Rb, 75.40.Gb, 75.50.Bb
Ultrafast magnetization dynamics comprise a major area
of current research in magnetism. Novel dynamical phe-
nomena have been observed recently in con±ned structures
[
1
], with programmed ±eld pulses [
2
,
3
], through interac-
tions with intense light pulses [
4
,
5
], and under the in²u-
ence of spin polarized currents [
6
,
7
].
In all cases, the observed phenomena compete against
ferromagnetic
relaxation
in
the
magnetic
material.
Relaxation aligns magnetization
M
with applied ±elds
H
, bringing dynamics to a stop. The lowest limit of the
relaxation rate is intrinsic to a given material and given by
G
±
±²M
s
, where
²
is the related dimensionless damping
constant. In metals, the damping has seen renewed theo-
retical interest [
8
] motivated particularly by its formal
relationship with spin momentum transfer torques [
9
,
10
],
or by its enhancement with impurities [
11
]. Low relaxation
rates are of particular interest for low critical currents in
spin momentum transfer [
12
], narrow band response in
frequency domain devices [
13
], and reduced thermal noise
in nanoscale magnetoresistive sensors [
14
].
Pure iron (Fe) has long been known to exhibit the lowest
measured intrinsic relaxation rate of all elemental ferro-
magnetic metals or binary alloys [
15
]. Lowest values of
57 MHz (
²
±
0
:
002
) have been found in both single-
crystal whiskers and epitaxial ±lms [
16
] at room tempera-
ture. Elemental Ni, Co, and standard alloys such as
Ni
81
Fe
19
show much higher values (
G
±
220
, 170,
114
²
10 MHz
, respectively.)
In this Letter, we show that the intrinsic relaxation rate
G
in a low-
Z
ferromagnetic alloy can be substantially lower
than that known for pure Fe. Epitaxial
MgO
³
100
´
=
Fe
1
ÿ
x
V
x
³
8nm
´³
100
´
ultrathin ±lms, deposited by UHV
sputtering, exhibit values of
G
to 35 MHz, reduced by
µ
40%
. While a comparable value has been identi±ed
recently in NiMnSb [
7
], the low damping has been attrib-
uted to the special electronic characteristics of this ordered,
half
-metallic compound [
17
], including a low orbital com-
ponent of the magnetic moment. We show that in
Fe
1
ÿ
x
V
x