Tunnel magnetoimpedance effects and Kerr-microscopic domain imaging

• Main Authors: Yin-Ming Chang, 張銀銘
• Other Authors: Minn-Tsong Lin
• Format: Others
• Language: en_US
• Online Access: http://ndltd.ncl.edu.tw/handle/24008881558874768946

博士 === 國立臺灣大學 === 物理研究所 === 92 === Layered structure ferromagnet/insulator/ferromagnet (FM/I/FM) constitutes the very basic element of a magnetic tunnel junctions (MTJ) junction, and the tunnel magnetoresistance (TMR) effect in it is strongly influenced by the electronic and magnetic properties at the FM/I interface. With the help of specific structure arrangement, however, the intrinsic spin polarization of FM layers may be modulated. Taking double barrier tunnel junction for example, the spin dependent transport were studied here by means of transfer matrix calculation, and the oscillatory TMR effect was verified with ferromagnet of varying thickness inserted between two tunnel barriers. In this work, pseudo spin-valve (PSV) and spin-valve (SV) magnetic tunnel junctions were fabricated by an particularly designed ultra-high-vacuum (UHV) sputtering system. The TMR values of fabricated PSV and SV MTJs may reach 33% and 42%, respectively. The characteristic spin dependent transport was examined by invoking magneto-optical Kerr effect (MOKE) and 4-terminal magnetoresistance measurements. Moreover, the structural characterization and the monitoring of elements diffusion were accomplished by transmission electron microscopy (TEM) and energy dispersive X-ray spectrometer (EDS), respectively. In addition to the electric response under direct current (DC) source, the tunnel magnetoimpedance (TMI) effects were studied in a series of MTJs with sensing frequency up to 1 MHz as well. The observed real part was found to change sign as the sensing frequency increases and led to dramatic fractional change under sweeping magnetic field. By introducing a frequency dependent weighting factor to the lead resistance, such kind of anomalous behavior is explained in the picture of frequency assisted inhomogeneous current distribution effect. Because of the crucial influence of exchange bias coupling and the artificial ferromagnetic coupling (AFC) in SV and AFC-SV MTJs, the correlation between interfacial magnetic micro-structures and the interlayer magnetic coupling were studied in NiO/Cu/NiFe trilayers. By monitoring the evolution of magnetic domains in the NiFe layer with Kerr microscopy, the stability of the coupling was found to depend both on temperature and the direction of external magnetic field. By taking advantage of the rotational field experiments, the temperature dependent critical angle that defines the irreversible switching of magnetic domains may be obtained. It is believed that not the domain wall itself but the torque exerted by in-plane ferromagnetic moments re-magnetizes the antiferromagnetic NiO layer.