Spin-torque gate (STG) magnetic field sensor

In the last 10 years, spin-orbit torque (SOT) in ferromagnet (FM) / heavy metal (HM) bilayers has been studied intensively as an efficient pathway to manipulate the magnetic state and magnetization dynamics of magnetic materials with perpendicular magnetic anisotropy (PMA), which is crucial for energy-efficient operation of a variety of spintronic devices such as magnetic memory, logic, oscillator, and neuromorphic computing. One of the undesirable requirements for SOT-driven magnetization switching of PMA films is the necessity of a longitudinal assistive field (H_x). While this is indeed undesirable for the aforementioned applications, it is exactly the same requirement that has prompted us to develop a spin torque gate magnetic sensor with extremely simple structure by exploiting the longitudinal field dependence of the spin torque driven magnetization switching. Unlike most magnetoresistance sensors which require a delicate magnetic bias to achieve a linear response to the external field, the spin torque gate sensor can achieve the same without any magnetic bias, which greatly simplifies the sensor structure. Furthermore, by driving the sensor using an ac current, the dc offset is automatically suppressed, which eliminates the need for a bridge or compensation circuit.

Hang Xie, Xin Chen, Ziyan Luo, and Yihong Wu, Phys. Rev. Applied 15, 024041 (2021).

All-in-one spin Hall magnetic sensor

Commercial AMR sensors require delicate transverse bias for output linearization (e.g., barber pole biasing) and set/reset coils for DC offset and noise reduction. For example, the Honeywell HMC1001 AMR sensor (the most widely studied AMR sensor) requires three electric currents for normal operation: i) DC sense current, ii) DC offset compensation current, and iii) pulsed set/reset current (~ 3 A). This does not only increase the manufacturing cost but also results in high energy consumption.

Recently, we have developed an all-in-one spin Hall magnetoresistance (SMR) sensor which featuresAn extremely simple structure (consisting of only an ultrathin NiFe/Pt bilayer) Built-in ac excitation and rectification detection (completely new biasing and detection scheme)Nearly zero dc offset and negligible hysteresis (extremely difficult if not possible to realize in existing magnetoresistance sensors)Low noise and high sensitivity at low-fieldSame angle dependence to external field as giant magnetoresistance (GMR) and tunnelling magnetoresistance (TMR) sensors, though there is only a single magnetic layer in the SMR sensorAll these features are now embodied in the simple NiFe/Pt bilayer in the SMR sensor and what one needs is just an ac current. The realization of built-in ac excitation represents a significant advance in the development of MR sensors because it simultaneously reduces the three undesirable features of any MR sensor: dc offset, hysteresis and low-frequency noise (this was not possible in the last 40 years). With these unprecedented features and elegant and simple design, the SMR sensor developed in this work promises great potential in many applications.

[1] Y.M. Yang et al., “Semitransparent anisotropic and spin Hall magnetoresistance sensor enabled by spin-orbit torque biasing”, Appl. Phys. Lett. 111, 032402 (2017). [2] Y. J., Xu et al., “Macro-spin modeling and experimental study of spin-orbit torque biased magnetic sensors”, J. Appl. Phys. 122, 193904 (2017).

[3] Y.J. Xu et al., Adv. Mater. Technol., “Ultrathin All‐in‐One Spin Hall Magnetic Sensor with Built‐In AC Excitation Enabled by Spin Current”, Adv. Mater. Tech. 3, 1800073 (2018).

Spin-orbit torque based position sensor

Angular position sensor is essential for rotational motion control in various industrial and consumer product applications. Currently, Hall sensors are dominant in automotive applications due to its low cost. However, the drawback of Hall sensor is that it is sensitive to temperature variation as it utilizes semiconductors. In addition, when being used as angular position sensor, it requires four sensors to resolve a full angle range of 360o as the Hall sensor is only sensitive to vertical filed component.

We have proposed a simple scheme for magnetic rotational position sensor based on current-induced spin-orbit torque effect. A full range detection of 360o is realized with a pair of Hall cross made of heavy metal/ferromagnet heterostructures. The Hall crosses are placed with their current axes orthogonal to each other such that when both devices are subject to a rotational in-plane magnetic field, the differential Hall voltage due to current pulses of opposite polarity exhibits a sine and cosine angular dependence, respectively. The rotational angle is then extracted from the two output signals via the arctan2 function. A linear correspondence between calculated and actual angle is obtained in the entire field range, with an average angle error of 0.4 – 0.65o in the field range of 500 – 2000 Oe. The performance is comparable to commercial angle sensors using giant or tunnel magnetoresistance effect, but the structure of the proposed device is significantly simpler and thus can potentially have a much lower cost.

Z.Y. Luo et al, Appl. Phys. Lett. 112, 262405 (2018)