Effect of spin-orbit coupling on magnetoresistance in organic semiconductors
Y. Sheng,
1
T. D. Nguyen,
1
G. Veeraraghavan,
2
Ö. Mermer,
1
and M. Wohlgenannt
1,
*
1
Department of Physics and Astronomy and Optical Science and Technology Center, University of Iowa, Iowa City, Iowa 52242-1479,
USA
2
Department of Electrical and Computer Engineering and Optical Science and Technology Center, University of Iowa, Iowa City, Iowa
52242-1595, USA
Received 17 July 2006; revised manuscript received 2 October 2006; published 2 January 2007
We study the recently discovered organic magnetoresistance OMAR effect in a pair of materials that have
similar chemical structures except that one contains a heavy atom to enhance spin-orbit coupling. We use
photoluminescence spectroscopy to estimate the spin-orbit coupling strength. In the material with weak spin-
orbit coupling the characteristic magnetic field scale is comparable to the hyperfine coupling strength. In the
material with strong spin-orbit coupling we find that the OMAR is strongly reduced in size and the OMAR
traces clearly exhibit a second, higher field scale which we identify with the spin-orbit coupling strength. We
model our results using the standard spin-dynamics Hamiltonian.
DOI: 10.1103/PhysRevB.75.035202 PACS numbers: 73.50.-h, 73.43.Qt
I. INTRODUCTION
There has been growing interest in magnetoelectronic
effects
1–6
in organic semiconductors. We recently
discovered
1
a large, low-field up to 10% at 10 mT and
300 K magnetoresistive effect in organic light-emitting di-
odes OLEDs, which we dubbed organic magnetoresistance
OMAR. The effect has also been observed by others.
6
In
addition to its potential applications, OMAR poses a signifi-
cant scientific puzzle since it is, to the best of our knowledge,
the only known example of large room-temperature magne-
toresistance in nonmagnetic materials with the exception of
high-mobility materials.
7,8
To the best of our knowledge the mechanism causing
OMAR is currently not known with certainty, although two
theories based on spin dynamics have been suggested very
recently.
6,9
In general, magnetoresistance in nonmagnetic de-
vices can be caused by several different physical principles:
i Lorentz force deflection, causing effects like Hall volt-
ages, classical magnetoresistance, and extraordinary
magnetoresistance,
10
ii quantum-mechanical diamagnetism,
such as effects associated with Landau levels or hopping
magnetoresistance,
11
iii interference phenomena such as
weak localization
12
that are sensitive to magnetic fields be-
cause the vector potential enters the Schrödinger equation in
a way that leads to phase shifts, and finally iv spin dynam-
ics. In our opinion, we have been able to exclude i–iii in
our earlier work
3
as possible explanations for OMAR. How-
ever, spin dynamics could be the cause of OMAR. Prior
work
6,13
has shown that OMAR is substantially reduced
upon introduction of spin-orbit coupling. No attempt was
made, however, to examine on a quantitative level whether
this observation can actually be derived from the spin-orbit
coupling Hamiltonian. In any case, a detailed comparison
between theory and experiment would not have been pos-
sible, because experimental OMAR traces in Refs. 6 and 13
in materials with strong spin-orbit coupling were below the
experimental noise level. In the present work we remedy
these shortcomings by using a modulation technique to
record OMAR traces in materials with strong spin-orbit
coupling.
For the benefit of the reader, we will briefly summarize
some of the main experimental results regarding OMAR.
We have shown
1–3,14
that i OMAR is a bulk effect rather
than an interface effect; ii the functional form of OMAR
is accurately described by the laws B
2
/ B
2
+ B
0
2
or
B
2
/ |B | + B
0
2
dependent on the material, where B
0
5 mT
in all materials we have studied; iii the effect is indepen-
dent of the magnetic field direction; iv the magnitude of the
OMAR effect is only weakly dependent on the minority car-
rier density; and v OMAR can be of positive or negative
sign, dependent on material and/or operating conditions of
the devices.
II. EXPERIMENT
The device fabrication steps were described in detail in
our earlier publications
1–3
and follow the standard OLED
fabrication recipe. The -conjugated small molecules tris8-
hydroxyquinoline aluminium Alq
3
and tris2-
phenylpyridineiridium Irppy
3
were purchased from H.
W. Sands corporation. The samples were mounted on the
cold finger of a closed-cycle helium cryostat located between
the poles of an electromagnet. The magnetoconductance ratio
I / I, for the Alq
3
devices was determined by measuring the
current I at a constant applied voltage V for different mag-
netic fields, B. Due to the much smaller magnetoresistance
effect in Irppy
3
devices, I / I could not be measured in the
same way. Instead, a modulation method was required: for
each constant voltage and B, It pulses are recorded as the
applied magnetic field is switched on and off multiple times
at a frequency of approximately 1 Hz. The ratio I / I for that
certain magnetic field is calculated using fast fourier trans-
form LABVIEW from the current pulses. By varying the ap-
plied magnetic field, I / I as a function of B is obtained. The
number of pulses was chosen such that a sufficient signal-to-
noise ratio was achieved. The modulation technique em-
ployed required the usage of a relatively small magnet B
300 mT since larger magnets have large self-inductance,
severely limiting the achievable modulation frequency. For
PHYSICAL REVIEW B 75, 035202 2007
1098-0121/2007/753/0352026 ©2007 The American Physical Society 035202-1