Antiparallel-Pinned Spin Valves With Modified Artificial Antiferromagnetic Layer for Full-Bridge Magnetic Sensors

Abstract

A full-bridge magnetoresistive spin valve (SV) sensor scheme for applications of linear magnetic measurement is fabricated and analyzed with the sign of magnetoresistance (MR) can be tuned by the modified the artificial antiferromagnetic (AAF) layer. In the last two decades, SV sensors have shown their crucial roles in the life. The common applications of SV sensors are used in position, angle sensors [1], [2], electronic compasses, and contactless current sensors [3]. To build a sensor, especially for resistive sensors, a Wheatstone-bridge is always a good solution for its advantages of the lessening dc-offset, and temperature errors [4]. In the MR sensor fabrication, the bias direction is also the main factor that decides the sensor architectures. For instance, a half-bridge was realized by two active cells, whereas shielding the other two cells [5], [6] or killing their sense by the roughened surfaces [7]. However, the effective sensitivity and linearity of the sensor are reduced [8]. There are several methods to form a full-bridge that provides a maximum output of the sensor. For example, the current strap was directly fabricated on the surface of the device to induce the local bias magnetic field during the cool-field post-annealing process [9]. Using an appropriate location of MR cells within the gap of the flux guide, a full-bridge could be established [10]. An antiparallel alignment of the pinned direction of the two pairs of MR cells was carried out by a microscopic focused laser heating during cool-field treatment [11]. Another method is that the usage of two deposition processes for two pairs of MR cells in the opposite applied magnetic field [12]. In this work, we propose a method to tune the bias direction of two neighboring meanders in the full-bridge by varying the thickness of Ru in AAF layers of patterned SV cells with a single post-field annealing step. In order to implement the proposed design, the two deposition processes were carried out. First one, two normal magnetoresistance SV cells are the structure of (Si/SiO 2 ) /Ta(50A)/NiFe(30A)/Co(15A)/Cu(24A)/CoFe(25A)/ IrMn(100A)/Ta(50A), whereas, the other two cells are the inverse magnetoresistance with an AAF layer of structure of (Si/SiO 2 ) /Ta(50A)/NiFe(30A)/ Co(15A)/Cu(24A)/[CoFe(15A)/Ru(7A)/CoFe(25A)]/IrMn(100A)/Ta(50A). The normal MR and inverse MR curves of the different SV structures are shown in Fig. 1. The SV films were prepared by RF magnetron sputtering. The conventional lift-off method was used to pattern the MR cells. A tiny permanent magnet was used to adjust the sensor operating point. To estimate an appropriate MR ratio for an expectation sensitivity, the hyperbolic tangent model was introduced by author et al. [13], a required MR correspondences a sensitivity and a saturation field range was given by MR =(2.({dV/V/dB}).$B_{s}$)/ (G_{m} -$(dV/V/dB).$B_{s})(1)$ where $MR$ is the magnetoresistance ratio, dV/V/dB is the sensitivity of the bridge (V/V/Oe), $B_{s}$ is the saturation field, and $G_{m}$ is the flux amplification factor. Suppose, $G_{m}$ \quad =1(no gain flux density), and the $B_{s}$ is about 50 Oe, in order to yield a sensitivity of 1 mV/V/Oe, the MR ratio should be at least 10.5 %. The SV cells were patterned to the dimensions of $200 \mu \mathrm{m}\times 2 \mu \mathrm{m}$. The resistance of individual SV cell is about 10 kv Ω. The cut chip dimension is 2 mm \times 1 mm was attached to a printed circuit (PCB) board using the aluminum wire bonding method. The voltage output of the bridge was amplified by a preamplifier using AD620 from Analog Device, Inc., and connected to a data acquisition device (MyDAQ). The data was recorded using a software coded in the LabVIEW. The performance of the SV full-bridge was verified by setting up the sensor in a sweeping magnetic field of \pm 150 Oe induced by a Helmholtz coil. The response curve of the full bridge to the external fields are shown in Fig. 2. The obtained sensitivity is 0.23 mV/V/Oe with $B_{s}$ is about 100 Oe and a patterned MR ratio of 4.5 % is in agreement with the hyperbolic tangent model in Eq. (1) [13]. Further experiments to increase the resistance of GMR elements using the advanced microfabrication methods are still ongoing. The sensitivity of the bridge can be further enhanced by the patterning SV cells in series [14] or the usage of a flux amplification (flux concentrators) [15].