We report the observation and micromagnetic analysis of current-driven magnetization switching in nanoscale ring-shaped magnetic tunnel junctions. When the electric current density exceeds a critical value of the order of $6\times 10^{6}$A/cm$^2$, the magnetization of the two magnetic rings can be switched back and forth between parallel and antiparallel onion states. Theoretical analysis and micromagnetic simulation show that the dominant mechanism for the observed current-driven switching is the spin torque rather than the current-induced circular Oersted field
Abstract. Abnormal magnetoresistance behavior is found in superconducting Nb films perforated with rectangular arrays of antidots (holes). Generally magnetoresistance were always found to increase with increasing magnetic field. Here we observed a reversal of this behavior for particular in low temperature or current density. This phenomenon is due to a strong 'caging effect' which interstitial vortices are strongly trapped among pinned multivortices. IntroductionSuperconducting thin films with periodic arrays of pinning sites have received much attention in purposes to enhance critical parameters [1,2,3,4] and artificially control the motion of vortices [5]. When the number of vortices equals to an integer multiple or fractional of the number of pinning centers, dips in resistance or peaks in critical current as a function of the applied magnetic field can be visible, which was known as the commensurate effect or matching effect [1,6]. In Nb thin films with rectangular array of magnetic dots and nonmagnetic dots, interesting phenomena have been revealed, such as the channeling effect [7], anisotropy in critical current [8] and vortex-lattice reconfiguration transition [9,10,11]. When the reconfiguration transition occurred, changes in the shape of the minima and their periodicity in the magnetoresistance curves were found. Two possible models have been proposed to explain this phenomenon: the geometrical reconfiguration model [9], and the multivortex model [11]. However, it's still unclear that the causes of the oscillations in magnetoresistance after the transition. In this paper, to investigate the pinning mechanism in high field regime, we performed transport measurements in rectangular arrays of antidots with different aspect ratios of unit cell. We observed a temperature and current dependent reconfiguration transition. After the transition, a decrease in magnetoresistance was found at high field. This abnormal behavior is accounted for a reduction of the mobility of interstitial vortices by a 'caging effect'. This effect has been predicted by theoretical simulations [12,13].
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