Preparation For Magnetoresistance Sensors And Study On The Noise Characterization

The magnetic tunnel junction(MTJ)sensors have been widely used in weak magnetic field detection,current sensor,angle transducer,biomolecular detection and weaponry for their low saturation field,high sensitivity,small size and low power consumption,but their low-frequency 1/f noise is extremely severe,which greatly limits the improvement of the detectivity.To address this problem,the MgO-based MTJs are designed and fabricated with standard micromachining techniques.We comprehensively analyze the influences of working conditions on the low-frequency noise,which provides a significant reference for studying the noise mechanism of MTJs and optimizing magnetic sensors.The main work is as follows:Before studying the noise properties,the MgO-based MTJ elements are designed and fabricated using the micromachining techniques.Then,the MTJ devices were performed by two steps of annealing process,and the magnetizations of two ferromagnetic layers are set orthogonal,thus yielding a linear response to the field.Based on the magnetic measurement probe station,a magnetoresistance measurement system for MTJs is designed and built.The system uses a four-probe method and DC reversal measurement method to eliminate the contact resistance and thermoelectric voltages,which improves the accuracy of the DC magnetoresistance measurement.At the same time,the system can realize the differential conductance measurement and provide the evidence for the electron tunneling mechanism.Aiming at the need of noise test,an automated noise test system with high precision and strong anti-interference ability is built.The system is optimized using the following three processes.Firstly,the optimal magnification is determined by testing the amplifier noise voltage at different magnifications.Secondly,the two-channel cross-correlation technique is used to eliminate the influence of amplifier noise voltage.Finally,the amplifier noise current is evaluated and eliminated,which further improves the measurement accuracy.Furthermore,a LabVIEW program is designed to control the whole system.Through the metal film resistance calibration,it is verified that the system is performed well and its error is less than 3%.Through the noise test of MgO-based MTJs under different working conditions,the dependence of low-frequency noise on magnetic field and bias current is obtained: 1)the magnetic noise of MTJs is larger when the magnetizations are inverted,but it is obviously reduced under the saturation field;2)the Hooge parameter(?)of electric 1/f noise decreases with the increase of bias current.Therefore,it is possible to separate the magnetic noise from the electrical noise by applying a saturation field,and to suppress the electrical 1/f noise by increasing the bias current.The conclusion has an important reference value for revealing the noise mechanism of MTJs and optimizing the working conditions of magnetic sensors.It's worth noting that increasing bias current will also reduce the TMR ratio,so we need to weigh the influence of bias current on both.