The magnetic sensor can be used to sense the magnetic components of an external magnetic field such as the geomagnetic field. The magnetic sensor has been applied to fields such as automobiles, auto-control systems, medical systems, and electronic compasses, in which the electronic compass needs to sense the three magnetic components of the external magnetic field. The smartphone is typically implemented with an electronic compass nowadays. The electronic compass needs to sense the geomagnetic field and thus requires the magnetic sensing capability to sense the three axes of the magnetic field. In addition, high sensitivity is needed for the electronic compass because of the geomagnetic field.
The magnetic sensor may have various designs. The magnetic tunnel junction (MTJ) device has been widely used to sense the horizontal component of an external magnetic field. The magnetic sensing layer of the conventional MTJ device may be formed by in-plane magnetic anisotropy (IMA) materials, which have relatively high sensitivity for the in-plane component of the magnetic field but relatively low sensitivity for the out-of-plane component of the magnetic field. In other words, even if the thickness of the magnetic sensing layer for the conventional MTJ device is controlled at the superparamagnetic condition, the MTJ device is still insensitive to the out-of-plane (Z-axis) component of magnetic field with relatively high sensitivity for the in-plane (X-axis and Y-axis) components of the magnetic field. One reason is that, although the magnetic moments for the superparamagnetic IMA are isotropically distributed, the superparamagnetic IMA maintains the in-plane property. Therefore, the response to the out-of-plane (Z-axis) magnetic field needs to overcome the demagnetization energy in order for the magnetic moments to be perpendicular to the magnetic sensing layer, resulting in insensitivity to the Z-axis magnetic field.
A MTJ device with a simple structure for sensing the out-of-plane (Z-axis) magnetic component of the geomagnetic field is under development.