TechArchive

This article was originally written in the period 1995-2000

Many metals exhibit a phenomenon called magnetoresistance (MR). This means that they show slight changes in electrical resistance when placed in a magnetic field. By designing materials made up of very thin layers of metals, it has been found possible to amplify this effect. This is called – would you believe – Giant Magnetoresistance (GMR)!

GMR sensors have greater output than conventional anisotropic magnetorestrictive (AMR) sensors and Hall effect sensors. They are able to operate at magnetic fields well above the range of AMR sensors. In addition, high fields will not “flip” GMR sensors or reverse their output, as is possible with AMR sensors. The output of GMR sensors is frequency insensitive and the sensor produces an output even with a constant field, setting them apart from inductive (variable reluctance) field sensors, which respond only to changes in magnetic field.

While GMR was discovered in 1988, it is now moving out of the lab into product development. In September 1994, Non Volatile Electronics (NVE) announced products using GMR materials – magnetic field sensors which can be used for position, wheel speed and current sensing for the automotive and other industrial sectors. These initial products were bridges without active on-chip circuits. Now, integrated sensors are planned for release this year.

Electrical resistors are designed to occupy a very small amount of silicon area on the wafer and generate a large resistance change when a magnetic field is applied. Multilayer materials show a sheet resistance of about 10e/sq and resistor legs on silicon which are 2aem across are easily fabricated. And the materials used can be tailored during production to sense magnetic field strengths in specific ranges.

NVE has entered into an agency agreement with Rhopoint Components to market its full range of giant magnetoresistance bridge sensors throughout the UK and Europe.

The materials used for the bridge sensors exhibit a much larger magnetoresistive effect than the standard magnetorestrictive anisotropic (AMR) materials and saturate at larger applied fields.

The NVSB sensor comprises four GMR resistors connected in a Wheatstone Bridge configuration. Two are shielded from the applied field with a thick magnetic material and so have a fixed resistance value. This is plated onto the IC after all other processing is finished. The other two resistors change as a function of the applied magnetic field. The resistance change of these two resistors results in a voltage output which varies between 5-6% of the supply voltage.

In addition to the magnetic shielding, the NVSB sensors can utilise the same thick magnetic material to form flux concentrators. These are positioned so that the field applied to the chip is magnified around the two resistors which change in value. The flux concentrators allow the NVSB sensor to saturate at fields smaller than the saturation value of the GMR material itself. The concentration factor can range from 2 to 100.

The bridge output is linear for approximately 70% of the full range and maximum output signal occurs at saturation.

Because many users of magnetic sensors require a fast turnaround, a rapid prototyping service is available. This is centred on a flexible magnetic sensor with on-chip signal processing which can be personalised in six weeks. To achieve this it must be possible to alter quickly both the sensor element and the signal conditioning electronics.

For the electronics, a “masterslice” technique is used where transistor processing (both bipolar and MOS) on a silicon wafer is completed up to first metallisation. With these in inventory, NVE can tailor the analogue and digital circuits using second metal. The sputter-deposited GMR sensor element can also be configured to match the individual application.

So what is magnetoresistance?

Magnetoresistance was discovered by British physicist William Thomson (better known as Lord Kelvin) in 1856. However, it could not be explained until the development of quantum mechanics in the 1920s. Basically, holding a magnet near these certain metals causes their atoms to tilt. The tilted atoms present larger obstacles that untilted ones to passing electrons, thus presenting higher resistance.

The Giant phenomenon stems from the additional fact that electrons can spin up or down. To change the resistance, it is necessary to construct a material in which one of these electron types can pass through more easily, aided by an external magnetic field.

With GMR, the layers are used to form a sandwich of magnetic layers, with a non-magnetic material as the filling. In this way, each successive magnetic layer is naturally magnetised in the opposite direction, in the same way as the poles of bar magnets line up in opposite directions when placed against each other.

When electric current is passed through such a sandwich, both up and down electrons encounter many obstacles. By adding an external magnetic field, all the magnetism in the layers is forced to line up in the same direction and the down electrons in the current avoid the obstacles, creating a short-circuit effect and a large drop in resistance.