| Summary: | 博士 === 國立彰化師範大學 === 物理學系 === 101 === The influences of direct current on magnetization reversal characteristics of elliptical spin valve devices with current-in-plane configuration are investigated in this dissertation. Various long/short axes of the elliptical devices, 10/1, 6/1, 4/1, and 2/1 μm, are fabricated by using electron beam lithography processes. The magnetization reversal characteristics of these devices are examined with differential magnetoresistance (MR) measurements and are carried out with various direct current strengths that are superimposed onto a low sensing AC current. Through an analysis of the switching behavior in devices with different aspect ratios, the current induced effects are identified as Joule heating, the Oersted field, current induced domain wall motion, and the spin torque transfer effect.
By comparing the MR curves of various direct current strengths to the MR curves taken at various ambient temperatures, Joule heating can be estimated from the resistance as a function of direct current and temperature. In the 4×1μm2 device, direct current can be transformed to temperature as T = 12.03 I2 + 296 and T = 12.33 I2 + 297 at low and high states, respectively. The slope of the T-I2 function is related to the heat capacity of the film material and the resistance of the device. From this relation between T and I2, the temperature increased from direct current can be estimated in the later study. Because the descent rate in the coercive as a function of direct current is larger than in the coercive as a function of temperature, the influences from other current induced effects are stronger than heating effect on magnetization reversal characteristics.
Current induced domain wall motion is observed in the 2×1μm2 device that has a multi-domain magnetization configuration at remanence. Conventional field scanned MR minor loops and subminor loops are measured under different magnetic field scan ranges to verify existing metastable states. In current scanned MR loop under various external fields, the resistance levels which indicate different magnetic domain configurations can be transformed between two resistance levels which are identified in accordance with the field scanned MR loops. While the MR loops are measured with the current below 0.6 mA, the current induced Oersted field affects the domain configuration and reduces the number of metastable states; while the current is during 0.6 and 2.6 mA, the reversal behavior of free layer is related to the current direction because of spin transfer; as the current is above 2.8 mA, the metastable states are increased because of the Joule heating. Hence, Joule heating, Oersted field, current induced domain wall motion, and spin torque transfer effect are observed in the 2×1μm2 device.
The efficiency of the current induced Oersted field is calculated as 10.5 Oe/mA in the10×1μm2 device that is pinned along the short axis, and is determined from the shifted field of the MR curve as a function of direct current. Notice that the Oersted field is generated along the short axis while direct current flows through the long axis. While the direction of pinned layer is aligned with the long axis, the current induced transverse field reduces the switching field. Therefore, the switching field as a function of direct current reveals a symmetrical phase diagram, much like the reversal behavior in the device of 6×1μm2 which is pinned along long axis. Strikingly, when the aspect ratio is increased to 10 and the pinned direction is also aligned in long axis, the switching field is not symmetrical to the current direction, and the current switching curves reveal a current polarity effect in the CIP configuration. After measuring and accounting for all other possible influences, it is deduced that ceterus parabus, the directional spin transfer effect influences magnetization reversal characteristic in this high aspect ratio elliptical device.
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