What’s GMR sensor : “Giant Magnetoresistance”?
The “giant magnetoresistive” (GMR) effect was discovered in the late 1980s by two European scientists working independently: Peter Gruenberg of the KFA research institute in Julich, Germany, and Albert Fert of the University of Paris-Sud .
In 1988, scientists discovered the “Giant Magneto Resistive” effect – a large change in electrical resistance that occurs when thin stacked layers of ferromagnetic and non-magnetic materials are exposed to a magnetic field. Since then, many companies have sought to develop practical applications for this intriguing technology.
Although the term “giant” in giant magnetoresistance (GMR) seems incongruous for a nanotechnology device, it refers to a large change in resistance (typically 10 to 20%) when the devices are subjected to a magnetic field, compared with a maximum sensitivity of a few percent for other types of magnetic sensors.
GMR structures are ferromagnetic alloys sandwiched around an ultrathin nonmagnetic conducting middle layer:
(A) is a conductive, nonmagnetic interlayer. Magnetic moment in alloy (B) layers face opposite directions due to antiferromagnetic coupling. Resistance to current (C) is high. The nonmagnetic conducting layer is often copper. Copper is normally an excellent conductor, but when it is only a few atoms thick, electron scattering causes copper’s resistance to increase significantly. This resistance changes depending on the relative orientation of electron spins surrounding the conducting layer
Applying an external magnetic field (D) overcomes antiferromagnetic coupling, aligning magnetic moments in alloy (B) layers:
GMR With Field Applied
Such exposure changes the device resistance so the structure can be used to sense an external field. Practical devices are often made of multiple layers of alternating magnetic and nonmagnetic layers to improve sensitivity.
The Quantum Mechanics of GMR
To understand how GMR works on the atomic level, consider the following analogies: If a person throws a ball (analogous to a conduction electron) between two sets of rollers turning the same direction (analogous to parallel spin-aligned magnetic layers), the ball tends to go through smoothly. But if the top and bottom rollers turn in opposite directions, the ball tends to bounce and scatter. Alternatively, the GMR effect may be compared to light passing through polarizers. When the polarizers are aligned, light passes through; when their optical axes are rotated with respect to each other, light is blocked.
The resistance of metals depends on the mean free path of their conduction electrons, which, in GMR devices, depends on the spin orientation. In ferromagnetic materials, conduction electrons either spin up when their spin is parallel to the magnetic moment of the ferromagnet, or spin down when they are antiparallel. In nonmagnetic conductors, there are equal numbers of spin-up and spin-down electrons in all energy bands. Because of the ferromagnetic exchange interaction, there is a difference between the number of spin-up and spin-down electrons in the conduction bands. Quantum mechanics dictates that the probability of an electron being scattered when it passes into a ferromagnetic conductor depends on the direction of its spin. In general, electrons with a spin aligned with the majority of spins in the ferromagnets will travel further without being scattered.
In a GMR spintronic device, the first magnetic layer polarizes the electron spins. The second layer scatters the spins strongly if its moment is not aligned with the polarizer’s moment. If the second layer’s moment is aligned, it allows the spins to pass. The resistance therefore changes depending on whether the moments of the magnetic layers are parallel (low resistance) or antiparallel (high resistance).
Optimal layer thicknesses enhance magnetic-layer antiparallel coupling, which is necessary to keep the sensor in the high-resistance state when no field is applied. When an external field overcomes the antiparallel coupling, the moments in the magnetic layers align and reduce the resistance. If the layers are not the proper thickness, however, the coupling mechanism can destroy the GMR effect by causing ferromagnetic coupling between the magnetic layers.
For spin-dependent scattering to be a significant part of the total resistance, the layers must be thinner (to a magnitude of several nanometers) than the mean free path of electrons in most spintronic materials. A typical GMR medical sensor has a conducting layer approximately 3 nm (or one ten-millionth of an inch) thick. For reference, that is less than 10 atomic layers of copper, and less than one ten-thousandth the thickness of a piece of tissue paper.
Spintronic GMR Bridge Sensors
A photomicrograph of a typical GMR magnetic sensor, also known as a magnetometer is shown below:
The thin metal-alloy films are vacuum deposited onto silicon wafers. Other manufacturing steps include thermal annealing, magnetic annealing, and photolithography. GMR resistors are generally patterned into serpentine resistors using photolithography. The serpentine configuration maximizes resistance per unit area. Maximizing resistance minimizes power consumption when the sensor is sampled.
In a typical sensor, four GMR resistors are configured as a Wheatstone bridge. A bridge configuration provides an easy-to-use voltage output that is proportional to the magnetic field applied but insensitive to any variations in the absolute resistance of the GMR device.
Two of the resistors are sensing resistors; the other two are reference resistors. The reference resistors are covered by a nickel-iron magnetic shield that measures 0.0004 in. thick. In response to an external magnetic field, the exposed sensing resistors decrease in electrical resistance while the reference resistors remain unchanged, causing a voltage at the bridge output.
The shield may also serve as a flux concentrator for the sensing resistors, increasing the sensitivity of the device and improving its spatial specificity. Because of the small geometries of spintronic sensors, flux concentration is especially effective and can increase sensitivity up to a factor of 100.
GMR Electrical Characteristics
A typical GMR sensor output is shown below:
When a magnetic field produces no further change in resistance, it is deemed saturated. The change in resistance from no field to saturation, usually expressed as a percentage of saturated resistance, is known as magnetoresistance. Hysteresis is the separation between positive- and negative-going curves.
Although this diagram shows an omnipolar response, meaning it has the same change in resistance for a directionally positive or directionally negative magnetic field, bipolar sensors have recently become available. Bipolar sensors maintain an operate point with the application of a negative (South) magnetic field, and a release point with the application of a positive (North) magnetic field. The part is ideal for use with magnetic encoders that have alternating North/South poles.