A High-Field Tunneling Magnetoresistive Angle Sensor

Magnetoresistive sensors combined with permanent magnets are becoming increasingly popular for measuring mechanical displacement, velocity, rotation, etc. [1] In the case of magnetic angle sensors, a permanent magnet is attached to a rotating component, such as a shaft, and a magnetoresistive sensor detects the magnetic field change as the permanent magnet rotates with the shaft. [2] The measured magnetic field value is converted into a measure of shaft rotation angle. Because this is a non-contact measurement, it provides an inherently wear-free means of monitoring the rotation of a mechanical component. There are other advantages, for example, unlike optical sensors that require a powered optical source and transparent medium through which the measurement is made, magnetic sensors are low power and can tolerate dirt, dust, and other unavoidable contamination that may occur during its operational lifetime. Anisotropic magnetoresistance (AMR), giant magnetoresistance (GMR), and tunneling magnetoresistance (TMR) sensors are often used in angle sensor applications, but TMR is preferred due to its higher output and full 360 degree measurement capability. [3] Unfortunately, the accuracy and linearity of 360 degree GMR or TMR angle sensors are known to degrade as the applied field from the moving permanent magnet is increased. [4] Figure 1 illustrates the response of a TMR angle sensor as a function of the strength of a rotating magnetic field. It shows nearly ideal sinusoidal behavior at 50 Oe applied field, but the sensor output becomes a triangle wave with undesired harmonics when a field of 400 Oe is used. This is unfortunate, since the ability to tolerate larger magnetic field makes an angle sensor more immune to external field interference and to displacement between the permanent magnet and the sensor. It is thus desirable to find a means for increasing the operating field range. We will show the reason for the change to the triangular waveform, which degrades angle measurement accuracy, is a result of motion of the pinned layer magnetization in response to the rotating magnetic field. In order to overcome this effect, we modeled, designed, fabricated, and tested a novel 360-degree TMR angle sensor which includes a circular attenuator placed above each TMR element. The design concept is illustrated in the inset of Figure 2. These novel TMR angle sensors consist of a conventional stack comprising Seed/PtMn/CoFe/Ru/ CoFe/CoFeB/MgO/CoFeB/NiFe/Cap, which is a bottom-pinned synthetic antiferromagnet pinned layer, MgO tunnel barrier, and simple bilayer freelayer structure. The attenuator is composed of plated permalloy, with thickness of about 5 microns. The attenuation was modeled using finite elements software. Figure 2 compares the angular error measured on an attenuated and a standard TMR angle sensor at various values of applied magnetic field ranging from 50 to 1500 Oe. The angle error is calculated by fitting a sinusoid to the voltage as a function of magnetic field angle curves. Note that the standard TMR angle sensor shows optimal performance over a range of about 25 to 100 Oe. The attenuated angle sensor uses an attenuator with an attenuation ratio of about 12x. As a result, the optimal field range is shifted from less than about 300 to about 800 Oe, but the error remains low, even from 100 to 1400 Oe. This experiment also shows the linearity of the angle sensor seems improved over the standard design. There are likely two main reasons for this. First, the pinning layer is well shielded, which reduces the motion of the pinned layer magnetization. Second, the attenuators may make the field at the location of the TMR elements more uniform. In summary, a 360-degree TMR angle sensor capable of operating with low error and high linearity in magnetic fields ranging from 100 Oe to over 1.4 kOe was designed, fabricated, and tested. The results agree well with modeling, and the resulting sensors are ideally suited for precise angle measurement in magnetically harsh environments.