One spin-orbit torque device for O-sense 3D magnetic fields

One spin-orbit torque device for O-sense 3D magnetic fields

Sensors that can detect magnetic fields have many potential applications, such as in the development of modern medical devices and transportation systems. However, most of the 3D magnetic field detection approaches developed so far require multiple sensors, making them cumbersome and difficult to implement on a large scale.

To this end, researchers from the Nanoscale Energy-Efficient Devices and Systems Laboratory (NEEDS) at Huazhong University of Science and Technology in China have begun developing a single spin-orbit device that can individually detect 3D magnetic fields. The device they developed, presented in a paper published in Nature Electronics and inspired by their previous work, is based on the Ta/CoFeB/MgO (tantalum/cobalt-iron-boron/magnesium oxide) heterostructure.

"One of our past papers, published in the IEEE IEDM in 2018, reported that increasing the written current density can gradually reduce the forcing capacity of a magnet until it reaches zero," Long You, one of the researchers who conducted the study, told TechXplore. "Subsequently, in two papers published in AEM and APL, we proposed that the current and in-plane regions have a constant regulation of the resistance of the AHE device through the movement of the domain wall. Based on these previous works, we have begun to detect a three-dimensional (3D) magnetic field with spin-orbit torque (SOT) devices."

A widely used approach to sensing 3D magnetic fields relies on the use of three magnetic sensors, with their sensing directions strategically located along three coordinate axes (x, y, and z). In addition, some researchers have used so-called planar sensors with a magnetic flux guide attached to them.

In your work, you and your colleagues investigated the possibility of detecting a vector magnetic field using a single torque device in orbit. The Ta/CoFeB/MgO heterostructure they designed achieves this by shifting the domain walls in the CoFeB layer, which allows for modulating such anomalous Hall effect resistance.

"According to the various symmetric symbols of the current polarity-dependent dynamics of magnetization or switching, we separate the contributions in-plane (IP) and out-of-plane (OOP) fields and implement 3D magnetic field sensing using a simple method," you said. "For the first time, we created a connection between the resistance of AHE and Nixchigrecczet based on the symmetry symbols of the R-H curves, applying positive and negative currents."

A vector magnetic field is made up of two IP field components (i.e., Hx or Hy) and one OOP field component (Hz). These three elements can lead to different domain wall (DW) motions in the CoFeB layer when positive and negative currents are applied utilizing a spin-orbit torque (SOT), which ultimately leads to the modulation of the associated AHE resistance.

You and his colleagues derived the relationships between the measured AHE resistance and the vector magnetic field's three orthogonal components. Their analyses showed that at certain ranges, these relationships are linear. Subsequently, they used the different symmetrical characters of current/polarity-dependent DW motions to separate the contributions of IP and OOP fields. This ultimately allowed them to achieve 3D magnetic field sensing using a single SOT device.

The sensing device developed by You and his colleague has a linear range between −10 and +10 Oe for the IP field, and between −4 and +4 Oe for the OOP field. Based on the symmetry characters of R-H curves under positive and negative currents, the researchers were able to collect two AHE resistance values under positive and negative current densities in the x-axis, which they called Rxy (+Jx) and Rxy (−Jx)."

Research Group of the Laboratory of Nanoscale Energy-saving Devices and Systems (NEEDS). Photo: Li et al.
Research Group of the Laboratory of Nanoscale Energy-saving Devices and Systems (NEEDS). Photo: Li et al.

"If these two AHE resistance values are processed using a subtraction operation, which allows you to eliminate the Nzet contribution, the net resistance contributed by only the Nix component can be obtained," you explained. "If these two values are processed with the addition operation, the net resistance contributed to only the Nzet component can be obtained. Similarly, we can get the net resistance contributed by only the Nihrek of the component by applying the ±Jhrek. Accordingly, you can find out the magnitude and direction of the vector magnetic field by composing (Nikschigrekchzet)."

You and his colleagues have shown that there is a correlation between the linear displacement of DW and the magnetic field that their device measures using direct current. This specific phenomenon, which has rarely been investigated in the past, has played a key role in their implementation of a single 3D vector magnetic sensor.

"Our device physically separates the contribution of its three components to achieve a single vector magnetic field detection device, so the fact that the three components of the magnetic field being measured is not orthogonal or are not in the same spatial position does not impair its performance," You said. "To our knowledge, this is the first time someone has implemented a 3D vector magnetic sensor using a single device, which has been a challenge faced in both academic settings and the electronics industry."

The simple structure and innovative design proposed by you and his colleagues can have many interesting applications. As traditional transistor technologies approach their physical limits, new technologies such as the device developed by these researchers could make a big difference, as they could open up new opportunities to develop faster and more efficient devices.

"Huge efforts have been made in this area, which has allowed the accumulation of sensors, MEMS, optoelectronics, radio, and mm-wave devices," You said. "Our proposed 3D sensor, based on spintronic technology, can be easily integrated into a Si-based chip, compared to conventional approaches that use three or more devices."

The research could form the basis for the development of new spintronic devices and integrated circuits. In addition, the 3D magnetic field sensor they created can have a wide range of applications, for example, allowing you to create new IoT and GPS devices.

"In the future, we are going to replace the AHE heterostructural structures with MTJ structures," You said. "In addition, we design and build a peripheral circuit system and develop a suitable algorithm so that our device can be used in practical applications such as navigation and positioning or in heterogeneous integration technologies and neural networks."

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