Electromotive force and huge magnetoresistance induced by
zinc-blende MnAs nanomagnets in magnetic tunnel junctions
P. N. Hai1, S. Ohya1,2, S. E. Barnes3, S. Maekawa4,5 and M. Tanaka1,2
1
Department of Electrical Engineering and Information Systems, University of Tokyo, Japan
2
Japan Science and Technology Agency
3
Physics Department, University of Miami, USA
4
Institute for Materials Research, Tohoku University, Japan
5
CREST, Japan Science and Technology Agency
For nanostructures such as magnetic nanowires and spin valves, it is theoretically predicted
that an electromotive force (emf) arises from a time-varying magnetization, such as the
motion of magnetic domains, even in a static magnetic field [1][2]. This reflects the
conversion of magnetic to electrical energy. Here we show that magnetization reversal of
zinc-blende (ZB) MnAs nanomagnets in magnetic tunnel junctions (MTJs) can induce an emf
in a static magnetic field for a time scale of 102~103 sec. Our results strongly suggest that
Faraday’s Law of induction must be generalized in order to account for purely spin effects in
magnetic nanostructures [3].
The studied MTJs consist of ZB MnAs nanomagnets coupled to a NiAs-structure hexagonal
MnAs top electrode through an AlAs barrier, and to a GaAs:Be bottom electrode through a
GaAs barrier. The ZB MnAs nanoparticles, with a 2-3 nm diameter, are fabricated by the lowtemperature annealing of GaMnAs at 480°C. Figure 1 shows the current (I) – voltage (V)
characteristics of a round 200 m diameter MTJ measured at 3 K with (dashed curve) and
without (solid curve) a 10 kG magnetic field. The polarity of the applied bias voltage is
defined as the voltage of the top MnAs film with respect to the substrate. The I – V curve, for
10 kG, is shifted toward the positive bias voltage region by 21 mV. This shift corresponds to
an emf of 21 mV generated by the MTJ. The inset in Fig. 1 shows the resistance of the MTJ
increased sharply when |V| < 50 mV, implying a Coulomb blockade (CB) energy of ~50 meV.
The emf is induced by a co-tunnelling process
of electrons and magnetization of ZB MnAs
nanomagnets subject to this strong CB [3]. Due
to this CB and the emf, a huge
magnetoresistance of up to 100,000% or even
higher is observed for certain bias voltages. The
huge magnetoresistance and emf may find
potential applications in a completely new kind
of magnetic sensors with ultra high sensitivity,
as well as in new active devices such as “spin
batteries”.
References
[1] S. E. Barnes, J. Ieda and S. Maekawa, Appl.
Phys. Lett. 89, 122507 (2006).
[2] S. E. Barnes and S. Maekawa, Phys. Rev.
Lett. 98, 246601 (2007).
[3] P. N. Hai, S. Ohya, M. Tanaka, S.E. Barnes
and S. Maekawa, Nature, doi:10.1038/nature
07879 (2009).
Fig.1 Current (I) – voltage (V) characteristics
of a 200 m-diameter MTJ, measured at 3 K with
(dashed curve) and without (solid curve) a
magnetic field of 10 kG applied in plane along
the GaAs[110] azimuth. A magnetic field of 10
kG induced an emf of 21 mV. The inset shows the
voltage dependence of the MTJ resistance at
zero-field.