Pinning Effect Of Various Defects On The Magnetization Reversal Processes In Permally Films In-situ Observed By A Lorentz TEM

Magnetism and magnetic recording materials have been attracted numerous interest since the giant magnetoresistance (GMR) was found in1988and applied in hard disk drive (HDD). The understanding and direct observation of magnetic reversal of magnetic materials manipulated by external magnetic field is significant and important for the design and fabrication of novel magnetic devices. Lorentz TEM has become a powerful tool to realize these studies of magnetic materials due to its advantages, including high spatial resolution and the ability of observation in real-time.In this thesis, the principles and critical operations of a Lorentz TEM will be introduced. Moreover, the distribution of magnetic moments surrounded various defects and the influence of these defects on the magnetic reversal in Permally alloy films were observed by the Lorentz TEM. The holes on holey carbon coated copper grids, nanofibres of M-type barium ferrite, iron oxide particles, nickel nanowires and tin oxide nanoparticles are intended to inert into Permally alloy films as defects, and their influences on the moment distribution and reversal of Permally alloy films were investigated. The following findings and conclusions are achieved:(1) The block defects in Permalloy thin film cause a circle distribution of magnetic moment surrounded these defects, where the local magnetic moments do not align to the direction of main magnetic moments any more. And the moment distribution in a typical range of the block defects forms two symmetrical parabola-like small domain walls.(2) The geometry and magnetic properties of the inserted defects influence the distribution of magnetic moments surrounded these defects.(3) A coherent and continuous reversal process occurs when the Permally alloy film inserted defects was applied an external magnetic field through tilting a small angle of sample holder. Whist, a domain wall motion happens when the sample holder has been tilted a large angle. (4) The domain wall motion has been pinned by not only defects themselves but also the fan-shaped region of magnetic moments formed by the defects.(5) The small domain walls located at the two sides of a main domain wall emerge together in the beginning and then swap each other when the domain walls pass through the inserted defects.