Magnetic Properties Of Patterned Ferromagnetic Thin Films Investigated By Micromagnetic Simulation

Micromagnetic simulation has long been used in investigation of magnetic properties of ferromagnetic materials on a mesoscope scale. Particularly this method is every predictive and accurate in patterned thin film magnetic devices, such as magnetic sensors, magnetic recording media, and magnetoelectronic devices, etc.. In the past ten years with the rapid development of the experimental techniques, mesoscopic fabrication can be precisely controlled. Magnetic properties are quite sensitive to dimensionality of the elements and stress developed during fabrications on this micrometer/nanometer scale. They are also keys to design and investigation of the micro magnetic devices. It is, therefore, of great importance to know the mesoscopic magnetic processes in these patterned thin films by micromagnetic simulations.In this thesis magnetization, demagnetization and dynamic processes in these patterned magnetic thin film elements are investigated by micromagnetic simulations. Materials specified in these simulations are NiFe, FeCoSiB which are typical materials in magnetic recording and stress sensors. Numerical methods in the open source micromagnetic codes OOMMF are presented in great details. A phenomenological model is established in order to cope the stress effect into the present code. The contents are organized as foliowings:1. Energy terms including exchange, magnetoanisotropic, magnetoelastic, magnetostatic and Zeeman effects are explicitly written out, and Brown equations for static problems and Landau-Lifschitz-Gilbert equations for dynamic problems are deduced. Finite difference methods are used to solve the equations. The static problems are solved by the conjugated gradient method while the dynamic one is solved by the Euler's method. These serve as the basis for further calculations and discussions.2. NiFe elements with diamond shape show potential applications in magnetic recording with unique magnetic reversal characters compared with rectangle shape. Influence of the width and thickness of the bit/word lines, and their distance to the magnetic properties of the elements are investigated. Our results show that the critical length of the short axis is 100 ran, and the easy axis should be in the direction of the long axis. The reversal field increases with the increase of the aspect ratio when the aspect ratio is beyond 2.4. Otherwise, it does not vary noticeably. A minimum switching field is achieved to the thickness of the word lines of 70 nm, while a maximum of 50 nm. The magnetoresistance is mostly sensitive to the distances between the word/bit lines and the elements. The simulation provides a new solution to MRAM.3. An equivalent field model is proposed to include the effect of uniaxial stress. The strength and direction of the equivalent field is worked out. Stress effects in the magnetization and demagnetization characters of the patterned FeCoSiB elements are studied based on the model above. Remanence in larger elements is more sensitive to the stress and this effect also saturates when the stress is increased. The sensitivity can reach as high as 0.9%/Mpa. Coercivity also increases with the stress. This ratio in a wider element is about 40 mT/GPa and 80 mT/GPa in a narrower element when the elements are short. Stress can effectively change the magnetic state when the size of it is properly chosen. The results can be used to envisage new stress/strain sensors.4. Dynamic properties of the patterned NiFe elements in diamond and ellipse are investigated under pulse magnetic field in the x and z directions. We confirm that the vortex center does not move under the pulse in z directions. The excitation modes at low frequency have nodes around the vortex center in elements without rotational symmetry. This is a first step pointing to short writing field design in MRAM within nanosecond.