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/R_o = [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 90^o. 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 the device).

Figure illustrates current lines in a semiconductor [yellow] with an
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 Hg_xCd_1-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 Hg_xCd_1-xTe done in collaboration with M. Kawano, N. Oda and M. Sano, Material Development Center, NEC Corp, Kanagawa, Japan.
Tineke Thio et al., Phys. Rev. B 57, 12239 (1998).
Tineke Thio and S.A. Solin, Applied Physics Letters 72, 3497 (1998).

Zero-field offset

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 as indicated.

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, 1530 (2000).

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
  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 devices:
  • 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.
• Integrable
  These devices can be integrated on semiconductor substrates in a straightforward way.

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 processing temperatures.