The Properties Of Magnetic Tunnel Junction And Its Application In Linear Displacement Sensor

Magnetic tunnel junction (MTJ) has been studied intensively because of its application in many fields. For the applications of MTJ, a basic requirement is hysteresis-free TMR transfer curves and low noise. So it is meaningful to research on the hysteresis-free characteristics and the noise characteristics of MTJs. In this paper, effects of annealing temperature on magnetic properties in CoFeB/MgO/CoFeB MTJ are analyzed. The noise characteristic in MTJs also is analyzed. On the other hand, a monotonous magnetic field is calculated, and a linear displacement sensor is designed. There are two parts in this paper:Ⅰ. Effects of thickness of CoFeB free layer and annealing temperature on CoFeB/MgO/CoFeB magnetic tunnel junctions are analyzed; Effects of bias current and applied magnetic field on noise in MTJs also are analyzed; Effects of PM magnetic field on hysteresis in MTJ are analyzed:(1) After annealed at240℃, the TMR transfer curves of MTJs with CoFeB free layer thickness15.1A are hysteresis-free. The TMR transfer curves of MTJs with CoFeB free layer thickness16.2A and17.4A are not hysteresis-free until the annealing temperature increases to380℃. This hysteresis-free TMR transfer curves originate from the superparamagnetism in the thin free layer. The samples with hysteresis-free transfer curve are indeed superparamagnetic, this conclusion is proved by the theory of superparamagnetism and the TMR transfer curves;(2) When the bias magnetic field is applied along the perpendicular direction of sensitivity, the TMR hysteresis decreases;(3) In the low frequency range,1/f Noise is dominant in MTJ; With the Bias Current increases, the Noise in MTJ increases. Hooges’constant (αH) is independent of the bias current in MTJ. Magnetic noise is present in MTJ, when the applied magnetic field is large enough to make the magnetic configuration of the free layer stabilized, the magnetic noise can be minimized or eliminate, then the electric noise is dominate in MTJ.Ⅱ. A linear displacement sensor with TMR chip is designed; the performance of the linear displacement sensor is analyzed:(1) A function which can calculate the special magnetic field caused by the rectangular magnet which is unidirectional magnetization has been given;(2) A monotonous magnetic field and a uniform magnetic field have been simulated analyzed;(3) A linear displacement sensor with TMR chip is designed, its linearity is1.6%, and its sensitivity is25.4mV/mm;(4) When the bias magnetic field is applied along the perpendicular direction of sensitivity of TMR, the hysteresis of linear displacement sensor decreases.