Studies Of The α-Fe/TbFe₂ Nano-Crystalline Exchange-Coupling Effect And Magnetostriction

Giant magnetostrictive films can be widely used in MEMS and micromagnetic devices, such as micro-pumps, micro-valves, micro-switches. In this dissertation, the nano-crystalline exchange-coupling effect of TbFe₂ magnetostrictive phase andα-Fe soft magnetic phase was utilized to reduce TbFe film saturation field, and then to improve TbFe film low magnetic field magnetostriction. Thus, the design and realization of film magnetostrictive coefficient (λ) measuring system, the preparation of amorphous TbFe films, the establishing ofα-Fe/TbFe₂ nano-crystalline exchange-coupling one dimension model, and the influences of the rapid recurrent thermal annealing on TbFe film magnetostriction were investigated systematically. The main results are as follows:Firstly, based on the material mechanical principles, the magnetostrictive film-substrate cantilever was analysised. The formula of magnetostrictive coefficient and cantilever deflection is deduced. Then, based on the laser cantilever deflection method, the film magnetostrictive coefficient measuring system was designed and realized. By calibration, the error of the measuring system is within 1ppm.Secondly, amorphous TbFe films were prepared by magnetron sputtering. The influences of argon partial pressure and sputtering angles on TbFe film micro-structures, magnetic and magnetostrictive performances were investigated systematically. With the increase of argon partial pressure, the content of Tb in the as-deposited TbFe films increases. With the increase of argon partial pressure, the magnetic easy direction of the films gradually changes from perpendicular direction to parrallel to the film plane. Within a certain argon partial pressure region, the increase of the argon partial pressure can improve the in-plane soft magnetic performances of the samples, and then improve film low magnetic field magnetostriction. The in-plane magnetostriction characteristics at lower magnetic field can be greatly improved by the oblique sputtering technique. With the decrease of deposition angles, the easy magnetization directions of the films can be gradually changed from perpendicular to in the film plane, and the in-plane magnetostriction also increases at lower magnetic field. Thirdly, the one dimension model of a-Fe/TbFe2 nano-crystalline exchange-coupling was established by the free energy functional minimization. Based on the model, the influences ofα-Fe/TbFe₂ nano-crystalline exchange-coupling effect, magnetic anisotropy angles ofα-Fe, magneto-crystalline anisotropy constants of soft magnetic phase and grain sizes on TbFe film magnetic and magnetostrictive performances were simulated. The results show that theα-Fe/TbFe₂ nano-crystalline exchange-coupling effect can improve the TbFe film soft magnetic properties, and thus improve film low field magnetostriction. However, the influences of magnetic anisotropy angles ofα-Fe can be ignored. With the decrease of magneto-crystalline anisotropy constants of soft magnetic phase, the soft magnetic performances of TbFe films are improved, then the low magnetic field magnetostriction is increased. The grain size plays an important role in the influences on the magnetic and magnetostrictive performances of TbFe films. Within the exchange-coupling length, with the increase of grain size of TbFe₂ andα-Fe, the magnetostriction of TbFe films increases at lower magnetic field, however, it decreases at larger magnetic field.Fourthly, the influences of rapid recurrent thermal annealing on TbFe film magnetostriction were investigated systematically. The results show that TbFe₂ andα-Fe nano-crystalline can be formed in TbFe films within a certain annealing temperature and time region. Within the exchange-coupling length, the nano-crystalline exchange-coupling effect of TbFe₂ andα-Fe can improve TbFe film soft magnetic performances, and then improve film low field magnetostriction. The experimental results agree with the micromagnetic simulations very well.The optimal preparing process parameters of TbFe films are as followings: 0.6Pa of argon partial pressure, 80W of sputtering power, 70mm of target-substrate distance, 15°of sputtering angels, 550℃of annealing temperature,240 seconds of annealing time and 50℃/s of heating rate. The magnetostriction of the films prepared by this optimal preparing process can reach 280ppm at the static magnetic field of 16kA/m.