TMR Sensor Magnets
TMR sensor is an epoch-making magnetoresistance effect sensor which has been applied to industrial application in recent years. TMR sensor has a much higher resistance change rate compared to AMR and GMR sensor, thus obtained many merits include low cost and power consumption, high response frequency and sensitivity. SDM has also collected some experience in TMR sensor magnet because of the widely use of TMR sensor.
The TMR sensors are a new type of magnetic sensors utilizing a TMR element, which is a highly-sensitive reproducing element of an HDD head. TMR Sensor Magnets
The reading elements of HDD heads are based on the magnetoresistance effect, which refers to a change in resistance induced by the application of an external magnetic field. Since the 1980’s, they have promoted significant improvement of the recording density of HDDs, while evolving into AMR (anisotropic magnetoresistance effect) elements, GMR (giant magnetoresistance effect) elements, and TMR (tunnel magnetoresistance effect) elements. The basic structure of each element is shown:
The magnetic structure of a TMR element is almost the same as that of a GMR element. However, in a TMR element, the current flows perpendicular to the film surface, while it flows horizontally to the film surface in a GMR element.
A TMR element is a thin-film element with a structure in which a barrier layer made of a thin insulator of 1 to 2nm is sandwiched between two ferromagnetic layers (free layer/pin layer), made using advanced thin-film processing technology. Although the magnetization direction of the pin layer is fixed, the magnetization direction of the free layer changes according to the external magnetic field direction. The electrical resistance of the TMR element changes along with this change in the free layer. The electrical resistance becomes the smallest when the magnetization directions of the pin layer and free layer are parallel, causing a large current to flow into the barrier layer. When the magnetization directions are antiparallel, the resistance becomes extremely large, and almost no current flows into the barrier layer:
Left: when the magnetization directions of the free layer and pin layer are parallel, the resistance becomes small and a large current flows.
Right: when the magnetization directions of the free layer and pin layer are antiparallel, the resistance becomes large and only a weak current flows.
What is Tunneling Magnetoresistance Effect?
The sandwich structure made by two ferromagnetic layers and insulating layer can be called magnetic tunnel junctions (MTJs). Tunneling magnetoresistance of MTJs will be changed when changing the relative orientation of two ferromagnetic layer’s magnetic moment, and this phenomenon is known as tunneling magetoresistance (TMR) effect. The structure of TMR and GMR component is basically the same, and current direction of TMR and GMR component is perpendicular and parallel to the layer respectively. TMR Sensor Magnets
The magnetization direction of the pinned layer is fixed, and magnetization direction of the free layer can be adjusted according to the external magnetic field, therefore, electric resistance of TMR component will also changes. TMR component shows the least resistance once magnetization direction of the free layer is parallel to the pinned layer’s and then high-current flow through the barrier. Instead, resistance will significantly increase when magnetization direction antiparallel, thus little current flow through the barrier.
From the angle of quantum mechanics, when magnetization directions of two ferromagnetic layers are parallel to each other, electrons of majority-spin and minority-spin subband enters into unoccupied states of another ferromagnetic layer’s majority-spin and minority-spin subband respectively, then tunnel current is relatively high and component is in the low resistance state. When magnetization directions are antiparallel to each other, the opposite occurs. The electrons of majority-spin and minority-spin subband enter into unoccupied states of another ferromagnetic layer’s minority-spin and majority-spin subband respectively, then tunnel current is relatively low and component is in the high resistance state.
The magnetization direction of the pin layer is fixed, while the magnetization direction of the free layer follows the external magnetic field direction.
Since the resistance value of the element is proportional to the relative angle between the magnetization directions of the pin layer and free layer, it can perform 360° angle detection as an angle sensor.
The output of TMR sensors is 500 times higher than that of a hall element, and also has low power consumption (5mW/under recommended conditions), which makes them ideal as sensors for automotive applications. For example, they can replace conventional angle sensors using hall elements, as automotive steering angle sensors or EPS (electric power steering) motor angle sensors.
A low temperature drift (changes in output caused by ambient temperature changes) is a basic requirement for sensors. is a graph comparing the temperature dependence of the angle errors of TMR sensor and a conventional AMR sensor. In the conventional AMR sensor, the angle error becomes extremely large on the low or high temperature side. By contrast, TMR sensor maintains a stable angle accuracy in a broad temperature range (angle error of ±0.6° or smaller in the magnetic field range of 20 to 80mT and the temperature range of -40 to 150°C). In addition, low aging deterioration is another notable feature of TDK TMR sensors. Due to this feature, they are expected to be utilized not only in automotive electronic equipment but in various types of industrial equipment.
Contributing to eco-driving as rotation sensors or current sensors in the future
Sensing technologies are greatly contributing to the improvement of the fuel efficiency of automobiles. In an automotive engine, crank angle sensors or cam angle sensors are used to obtain information for calculating the optimal timing and amount of fuel injection in the engine ECU (electronic control unit).
Although there are various types of crank angle or cam angle sensors, non-contact type magnetic sensors have become the mainstream because of their insusceptibility to wear or dust. A toothed gear pulsar (pulsar rotor) made of a magnetic material is attached to a crankshaft or cam shaft, and a magnetic sensor to which a magnetic field applied by a bias magnet is contactlessly placed to face it. When the engine starts and the gear pulsar rotates, the density of the magnetic flux from the magnet changes alternately due to the projections and recesses of the gear teeth. The magnetic sensor extracts this as a pulse signal, and detects the rotation speed based on the number of pulses per unit time. Because of this mechanism, these sensors are also called gear tooth sensors.
Compared to sensors using a hall element, TMR sensors feature extremely high sensitivity and high output, and realize superior sensing capability as wheel velocity sensors for ABS (anti-lock braking) systems. Moreover, they can also be expected to be utilized as current sensors that conserve energy by controlling charging and discharging of batteries. TMR Sensor Magnets
About TMR Sensor Magnets
MPCO MAGNETICS team will help customers to choose suitable TMR sensor according to the detailed technical specifications, and looking forward to cooperating with relevant customers.
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