A microwave test structure has been designed to measure the high-speed response of giant magnetoresistive (GMR) devices. The test structure uses microwave transmission lines for both writing and sensing the devices. Pseudo-spin-valve devices, with line widths between 0.4 and 0.8 μm, were successfully switched with pulses whose full width at half-maximum was 0.5 ns. For small pulse widths τpw the switching fields are observed to increase linearly with 1/τpw. The increase in switching fields at short pulse widths is characterized by a slope which, for the current devices, varies between 4 and 16 μA s/m (50–200 Oe ns). The magnetoresistive response during rotation and switching was observed. For small rotations (∼45° between layer magnetizations) the GMR response pulses had widths of 0.46 ns, which is at the bandwidth limit of our electronics. For larger rotations (∼90°) the response pulses broadened considerably as the magnetic layers were rotated near the unstable equilibrium point perpendicular to the device axis. Full 180° switches of both soft and hard layers were observed with switching times of approximately 0.5 ns.
We report on magnetotransport measurements of spin valve films that have been fabricated into rectangular stripes with Au current leads. The spin valve films consisted of two magnetic NiFe layers separated by a nonmagnetic Cu layer. The top NiFe layer was magnetically pinned by a FeMn layer with an effective pinning field of 12 kA/m (150 Oe). After device fabrication, the transport properties changed dramatically as the stripe-height of the device was decreased below 1 μm. Internal demagnetizing fields and magnetostatic interactions between the magnetic layers dominated the magnetic response. These interactions change the biasing point and the linearity, and cause a decrease in sensitivity to field changes. We have developed a simple single-domain rotation model that includes magnetostatic, anisotropy, and exchange interactions to describe the magnetic behavior, from which we calculate the transport response.
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