EMR -- Extraordinary magnetoresistance
in inhomogeneous semiconductors
Materials which have a large magnetoresistance (a change
in the electrical resistivity with applied magnetic field) are useful
in a wide variety of applications, including read heads for magnetic data
storage media, and motion detectors in consumer electronics and the automotive
industry. All these applications require that the magnetoresistance (MR)
be as large as possible, for a given magnetic field, H.
Why "Extraordinary"? - a historical perspective
Most metals have very small MR, which we define as MR
= DR/Ro = [R(H)-R(0)]/R(0).
For instance, copper has MR~1% at a high magnetic field, H=10 Tesla (for
comparison, the earth's magnetic field is about 0.7Gauss, or 7x10-5Tesla).
Compared to this, layered magnetic metals can be said to have "giant"
MR - hence "GMR" - because they exhibit MR~25% at H=50 Gauss and room
temperature. In these layered structures the MR arises from a difference
in carrier scattering rates, depending on the relative orientation of
the magnetisation in the adjacent layers. These GMR sensors are used in
the latest-generation read heads for magnetic hard disks.
"Collossal" magnetoresistance (or CMR) is a term applied
to a family of perovskite materials, which exhibit very large MR, up to
100,000%, but at very high magnetic fields (H=6T) and at low temperature
(T=77K) where the usefulness for applications is limited. At room temperature
and fields of order H=500 Gauss, the MR in these materials is very small.
Extraordinary magnetoresistance or EMR as
large as 100% has been demonstrated in a non-magnetic semiconductor with
an embedded metallic inhomogeneity. EMR=100% has been achieved
at room temperature and H=500 Gauss, the relevant magnetic field for high-density
data storage applications using EMR materials.
The very large MR exceeds the Corbino limit, the maximum MR observable
for a homogeneous material; the non-magnetic nature of the device brings
many advantages over conventional read-head devices. One such advantage
is that EMR materials can be operated at much higher speeds than materials
used in conventional read heads.
The physical principle behind EMR
The MR of a material contains a physical contribution from
the magnetic field dependence of the material parameters and a geometric
contribution from the dependence of the current path and output voltage
on the sample shape and electrode configuration. Either the physical or
geometrical contribution may dominate the observed MR
When a metal is embedded in a semiconductor, it acts as
a short circuit, with most of the applied current passing through
the metallic "inhomogeneity" (see figure below), and the total resistance
is smaller than that of the homogeneous semiconductor in the absence of
the metallic inhomogeneity. This is true for H=0.
However, at very high magnetic fields, H>1/m
(where m is the carrier mobility in the semiconductor),
the Hall angle approaches 90o. That is, the current density
J is perpendicular to the electric field E. Since at the surface of an
ideal metal, E is perpendicular to the surface, it follows that J must
be tangential to the surface. Therefore, at high magnetic fields the current
is constrained to flow around the metallic inhomogeneity: counterintuitively,
the metallic inhomogeneity acts as an OPEN circuit, and the total
resistance becomes very high (how high depends on the exact geometry of
Figure illustrates current lines in a semiconductor [yellow]
embedded metallic inhomogeneity [blue]. Current contacts are on the
left and right edges of the semiconductor rectangle.
The transition from the low-resistance, H=0 state to the
high-resistance, high-field state is the origin of the magnetoresistance.
It is a geometric MR, and arises even when the homogeneous semiconductor
itself has no physical MR. The cross-over field is given by Hm=1,
so it is desirable to use a semiconductor with high carrier mobility.
We have demonstrated this principle in HgxCd1-xTe
which has a zero band gap close to x=0.10 resulting in carrier mobilities
in excess of 3x104cm2/Vs in bulk material at T=300K.
At high magnetic field the physical magnetoresistance is visible, but
at low magnetic field the MR is enhanced by geometric MR as described
above. In this case the inhomogeneities are due to composition fluctuations
of the compound semiconductor. Using a Corbino device, which has concentric
current contacts, we have measured GMR as large as 28% at H=500G.
Corbino geometry (four-probe) showing concentric current
and voltage contacts.
Work on HgxCd1-xTe
done in collaboration with M. Kawano, N. Oda and M. Sano, Material Development
Center, NEC Corp,
Tineke Thio et al., Phys.
Rev. B 57, 12239 (1998).
Tineke Thio and S.A. Solin,
Applied Physics Letters 72, 3497 (1998).
Moreover, in a Corbino disk configuration, the GMR shows
a zero-field offset (ZFO). The ZFO, which can be as large as 1600G, arises
from spatial inhomogeneities (whether naturally occurring or intentionally
introduced) which contribute a Hall term to the otherwise quadratic field
dependence of the Corbino GMR. The ZFO constitutes a self-biasing
of the MR sensor.
Such a sensor can be used to detect not only a magnetic
field (300-500Gauss for magnetic data storage media) but also its sign,
which encodes the stored information.
S.A. Solin et al., Applied
Physics Letters 69, 4106 (1996).
Tineke Thio et al.,
J. Cryst. Growth 184-185, 1293 (1998).
Man-made patterned inhomogeneities
MR far larger than that observed in HgCdTe is achievable
by the introduction of carefully designed patterned inhomogeneities. We
have demonstrated this using high-mobility InSb structures embedded with
gold inhomogeneities. In a composite Van der Pauw disk with a gold inhomogeneity
at the center, we have observed room-temperature EMR as high as 100% at
500 Gauss and 9100% at 2500 Gauss (0.25T) and 1,000,000% at 5 Tesla.
Figure shows the geometry of a composite van der Pauw disk
with the metallic inhomogeneity embedded in the center (left panel); and
(right panel) EMR as a function of the size of the inhomogeneity at the
various magnetic fields indicated
Layout of van der Pauw disk showing the concentric inhomogeneity
in the center and the four bonding pads at the perimeter of the disk;
[left] a=9/16; [right] various a
The temperature coefficient dR/dT can be reduced to near
zero around room temperature by judicious design of the device.
S.A. Solin et al., Science 289,
Data storage applications
The extraordinary magnetoresistance exhibited by inhomogeneous
semiconductors has many advantages over conventional devices used as read-heads
for magnetic data storage.
EMR=100% has been demonstrated at H=500G; larger EMR values are expected
in the near future with improvements in the design of the geometry.
- Non-magnetic materials
Neither the semiconductor nor the metallic inhomogeneity is magnetic.
This has several extremely useful advantages over conventional magnetic
- No Barkhausen noise associated with magnetic domain switching
- No effect from demagnetising fields
- Device can be used in the horizontal geometry where it can be
in very close proximity to the magnetic medium where the fields
are highest. This also eliminates the need for magnetic shielding.
- EMR does not saturate with increasing magnetic field
- Zero-field offset
A non-symmetric design generates a Hall voltage which results in a built-in
bias field, required to distinguish the sign of the magnetic field which
encodes the stored information.
- Low thermal coefficient
Design variables can be easily manipulated to induce near-zero temperature
dependence of the resistance.
- Very fast response time
Response times less than a picosecond are estimated.
These devices can be integrated on semiconductor substrates in a straightforward
For completeness, we note here the potential disadvantages
of these proposed devices, which represent a new and unproven technology.
The devices require at least three contact leads, and tend to give a low
output voltage. In addition, their fabrication requires relatively low