Spin Doctors

Magnetoresistive sensors that can detect changes in magnetic fields of the order of one tenth the strength of the earth's magnetic field are now a reality as a result of research undertaken by a team headed by Dr Jan Evetts. This opens the way to a new generation of simple, robust magnetic devices and sensors for application in many areas, such as read-heads for disc drives and control circuitry for electrical machinery. Such sensors can also be used to measure changes in rotation of small magnetic fields, very accurately, giving the possibility of precise three dimensional direction finding. A possible application, for instance, would be to track a fire-fighter's movements within a smoke filled building giving his precise location. " It is the robust nature of this type of sensor, as well as its simplicity, making it relatively cheap to produce, that is the exciting feature." explains Dr Jan Evetts. " They can be used for all sorts of applications where a contactless measurement of position and orientation is required, as for instance in cars, where a large number of sensors are employed. Such sensors obviously have great advantages over their counterparts which involve the use of moving parts." A magnetoresistive response is a change in electrical resistance caused by a change in magnetic field. Conventional magnetoresistive elements are based on nickel film or wire, and the changes in resistance observed are rather small, of the order of 1-2%. A breakthrough in magnetic sensor technology occurred about seven years ago when it was discovered that metal multi-layers, typically using copper and cobalt, could induce much larger changes in resistance for a given change in magnetic field. This signalled the discovery of the so-called giant magnetoresistive (GMR) devices. Research carried out by a team headed by Dr Jan Evetts into the mechanism of these devices has lead to the production of "on-chip" GMR devices with a field sensitivity 200 times greater than that achieved by more conventional GMR devices.

The mechanism

Schematic of conduction electrons passing through multi-layer in high and low magnetic fields.

Cobalt is ferromagnetic at room temperatures and the electrons in its band structure can be considered as having two populations, one with spin up and one with spin down. The differences in these populations produces a magnetic moment in a particular direction. Conduction electrons aligned with the magnetic moment cannot find sites to be scattered into, and thus if there is no scattering, there is a low resistance path through the cobalt layer. If, however, a conduction electron is aligned in the opposite direction to the magnetic moment, then it can easily be scattered into available electron states in the cobalt layer, giving a high resistance path. The multi-layer in a GMR device is designed with a carefully chosen copper thickness (typically 1.9 nm) to achieve anti-parallel magnetic coupling between successive cobalt layers in zero applied field. This means that alternate cobalt layers have opposite magnetic moments under zero magnetic field (see accompanying figure). Thus a conduction electron entering the multi-layer in zero field must be scattered in alternate cobalt layers, regardless of its spin direction, thus producing a high resistance. Under the influence of a high magnetic field, however, the anti-ferromagnetic coupling is overcome (see figure) and the magnetic moments of the Co layers become aligned, so that conduction electrons with the appropriate spin direction i.e. parallel to the magnetic moment find a low resistance channel for their movement throughout the multilayer thus giving a low resistance state at high magnetic fields. This is known as a "spin valve" effect, and it is this phenomenon which produces the greatly enhanced change of resistivity with applied magnetic field observed in GMR devices.

A further enhancement was made by Evetts' team, by introducing a soft adjacent layer (SAL) structure (shown in red in figure) in association with the devices. This SAL layer concentrates the magnetic field across the gap producing a further amplification effect which is as large as 200 fold in prototype devices.. The final result of the research programme was to develop the actual devices, and that has now been achieved. "On-chip" GMR devices have been produced which are suitable for a range of sensor applications. For further information please contact Dr Jan Evetts T: 01223 334364 or see the Device Materials Group web site.

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