magnetoresistance

The change of electrical resistance produced in a current-carrying conductor or semiconductor on application of a magnetic field H. Magnetoresistance is one of the galvanomagnetic effects. It is observed with H both parallel to and transverse to the current flow. The change of resistance usually is proportional to H² for small fields, but at high fields it can rise faster than H², increase linearly with H, or tend to a constant (that is, saturate), depending on the material. In most nonmagnetic solids the magnetoresistance is positive. See also Galvanomagnetic effects.

In semiconductors, the magnetoresistance is unusually large and is highly anisotropic with respect to the angle between the field direction and the current flow in single crystals. When the magnetoresistance is measured as a function of field, it is the basis for the Shubnikov–de Haas effect, much as the field dependence of the magnetization gives rise to the de Haas-van Alphen effect. Measurement of either effect as the field direction changes with respect to the crystal axes serves as a powerful probe of the Fermi surface. Magnetoresistance measurements also yield information about current carrier mobilities. Important to practical applications is the fact that the geometry of a semiconductor sample can generate very large magnetoresistance, as in the Corbino disk. See also Fermi surface; Semiconductor.

Multilayered structures composed of alternating layers of magnetic and nonmagnetic metals, such as iron/chromium or cobalt/copper, can feature very large, negative values of magnetoresistance. This effect, called giant magnetoresistance, arises from the spin dependence of the electron scattering which causes resistance. When consecutive magnetic layers have their magnetizations antiparallel (antiferromagnetic alignment), the resistance of the structure is larger than when they are parallel (ferromagnetic alignment). Since the magnetic alignment can be changed with an applied magnetic field, the resistance of the structure is sensitive to the field. Giant magnetoresistance can also be observed in a simpler structure known as a spin valve, which consists of a nonmagnetic layer (for example, copper) sandwiched between two ferromagnetic layers (for example, cobalt). The magnetization direction in one of the ferromagnetic layers is fixed by an antiferromagnetic coating on the outside, while the magnetization direction in the other layer, and hence the resistance of the structure, can be changed by an external magnetic field. Films of nonmagnetic metals containing ferromagnetic granules, such as cobalt precipitates in copper, have been found to exhibit giant magnetoresistance as well. See also Antiferromagnetism; Ferromagnetism; Magnetization.

Magnetoresistors, especially those consisting of semiconductors such asindium antimonide or ferromagnets such as permalloy, are important to a varietyof devices which detect magnetic fields. These include magnetic recording headsand position and speed sensors. See also Magnetic materials; Magnetic recording.

A change in electrical resistance in metal or a semiconductor when it is subjected to a magnetic field. The property of magnetoresistance is used in reading the bits on magnetic tape and disk. Although used in earlier analog tape recorders, in 1991, IBM was the first to use a magnetoresistive (MR) read head in a computer disk drive.

Magnetoresistive (MR)

As storage capacity increases, the bit gets smaller and its magnetic field becomes weaker. MR heads are more sensitive to weaker fields than earlier inductive read coils, in which the bit on the medium induced the current across a gap. The MR mechanism is an active element with current flowing through it. The magnetic orientation of the bit increases the resistance in a thin-film nickel-iron layer, and the difference in current is detected by the read electronics. MR heads use the traditional inductive coil for writing.

Giant MR

In 1998, IBM introduced drives with giant MR (GMR) heads, which are sensitive to even weaker fields. GMR heads use additional thin film layers in the sensing element to boost the change in resistance, and "giant" refers to this larger change. Almost all modern drives use GMR read heads.

Extraordinary MR

Discovered in 1995 at the NEC Research Institute in Princeton, NJ, extraordinary MR (EMR) provides an even greater change in resistance. Quite unique in that the material is non-magnetic, EMR is expected to provide bit densities of a terabit per square inch some day. See superparamagnetic limit.

GMR Heads for Reading

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Magnetoresistance is the property of a material to change the value of its electrical resistance when an external magnetic field is applied to it. The effect was first discovered by William Thomson (more commonly known as Lord Kelvin) in 1856, but he was unable to lower the electrical resistance of anything by more than 5%. This effect was later called ordinary magnetoresistance (OMR). More recent researchers discovered materials showing giant magnetoresistance (GMR), colossal magnetoresistance (CMR) and magnetic tunnel effect (TMR).

Contents 1 Discovery 2 The Corbino disc 3 Anisotropic magnetoresistance (AMR) 4 References and notes 5 See also

Discovery

William Thomson (or Lord Kelvin) first discovered ordinary magnetoresistance in 1856. He experimented with pieces of iron and discovered that the resistance increases when the current is in the same direction as the magnetic force and decreases when the current is at 90° to the magnetic force. He then did the same experiment with nickel and found that it was affected in the same way but the magnitude of the effect was greater. This effect is referred to as anisotropic magnetoresistance (AMR).

Figure 1: Corbino disc. With the magnetic field turned off, a radial current flows in the conducting annulus due to the battery connected between the (infinite) conductivity rims. When a magnetic field along the axis is turned on, the Lorentz force drives a circular component of current, and the resistance between the inner and outer rims goes up. This increase in resistance due to the magnetic field is called magnetoresistance.

The Corbino disc

Figure 1 illustrates the Corbino disc. It consists of a conducting annulus with perfectly conducting rims. Without a magnetic field, the battery drives a radial current between the rims. When a magnetic field parallel to the axis of the annulus is applied, a circular component of current flows as well, due to the Lorentz force. A discussion of the disc is provided by Giuliani.^[1] Initial interest in this problem began with Boltzmann in 1886, and independently was re-examined by Corbino in 1911. ^[1]

In a simple model, supposing the response to the Lorentz force is the same as for an electric field, the carrier velocity v is given by:

where μ = carrier mobility. Solving for the velocity, we find:

where the reduction in mobility due to the B-field is apparent.

Anisotropic magnetoresistance (AMR)

AMR^[2] is the property of a material in which a dependence of electrical resistance on the angle between the direction of electrical current and orientation of magnetic field is observed. The effect is attributed to a larger probability of s-d scattering of electrons in the direction of magnetic field. The net effect is that the electrical resistance has maximum value when the direction of current is parallel to the applied magnetic field. AMR up to 50% has been observed in some ferromagnetic uranium compounds ^[3].

In a semiconductor with a single carrier type, the magnetoresistance is proportional to (1 + (μB)²), where μ is the semiconductor mobility (units m²·V⁻¹·s⁻¹ or T ⁻¹) and B is the magnetic field (units teslas). Indium antimonide, an example of a high mobility semiconductor, could have an electron mobility above 4 m²·V⁻¹·s⁻¹ at 300 K. So in a 0.25 T field, for example the magnetoresistance increase would be 100%.

To compensate for the non-linear characteristics and inability to detect the polarity of a magnetic field, a somewhat more complex structure is used for sensors. It consists of stripes of aluminum or gold placed on a thin film of permalloy (a ferromagnetic material exhibiting the AMR effect) inclined at an angle of 45°. This structure forces the current not to flow along the “easy axes” of thin film, but at an angle of 45°. The dependence of resistance now has a permanent offset which is linear around the null point. Because of its appearance, this sensor type is called 'barber pole'.

The AMR effect is used in a wide array of sensors for measurement of Earth's magnetic field (electronic compass), for electrical current measuring (by measuring the magnetic field created around the conductor), for traffic detection and for linear position and angle sensing. The biggest AMR sensor manufacturers are Honeywell, NXP Semiconductors, and Sensitec GmbH.

References and notes

1. ^ ^{a b} G Giuliani, (2008). "A general law for electromagnetic induction". EPS 81: 60002. doi:10.1209/0295-5075/81/60002. http://www.iop.org/EJ/article/0295-5075/81/6/60002/epl_81_6_60002.html. 
2. ^ Genish, I. (2004). "Paramagnetic anisotropic magnetoresistance in thin films of SrRuO₃". Journal of Applied Physics 95: 6681–2004. doi:10.1063/1.1676052.  edit
3. ^ P. Wiśniewski, "Giant anisotropic magnetoresistance and magnetothermopower in cubic 3:4 uranium pnictides" (2007) http://dx.doi.org/10.1063/1.2737904

See also

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• Giant magnetoresistance
• Colossal magnetoresistance
• Magnetoresistive random access memory

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