Theoretical Limits on the Minimal Switching Field and the Switching Current in Magnetization Reversal
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1032-I01-05
Theoretical Limits on the Minimal Switching Field and the Switching Current in Magnetization Reversal Xiangrong Wang1, and Zhouzhou Sun2 1 Department of Physics, The Hong Kong University of Science and Technology, Clear Water Bay, Hong Kong SAR, China, People's Republic of 2 Center of Super-Diamond and Advanced Films (COSDAF) & Department of Physics and Materials Science, City University of Hong Kong, Hong Kong SAR, China, People's Republic of ABSTRACT Recent theoretical limits of the minimal switching field and current for uniaxial magnetic nano-structures are reviewed. The results include also the optimal field and current pulses for the fastest magnetization reversal. Contrary to the general belief, the precessional magnetization reversal is not the fastest one, and its critical switching field is neither the lowest one.
Stoner-Wohlfarth (SW) problem [1] is about magnetization reversal of single-domain magnetic particles (Stoner particles) which is relevant to nano-technology and nano-sciences [2] as magnetic memory cell size reaches nano-meter scale and our ability to fabricate magnetic nano-particles [3-5] increases. In general, magnetization can be manipulated by a magnetic field [6-14], or a spin-polarized electric current [15-20], or a laser light [21], but only the reversal by a magnetic field and/or a current is considered here. Challenging issues are how to make switching field/current small and how to reverse a magnetization fast. Many of our understandings on SW problem are from the SW's original seminar paper in 1948 [1]. According to the SW's theory, a large enough field can drive the initial state out of local minimum and at the same time make the target state to be the global minimum. Thus the system can roll down to the target state through ringing effect [7-10]. The field that just destroys the initial local minimum is called SW-limit. Picoseconds magnetization switching has recently been observed in experiments [11, 12] in which magnetic fields are applied in a perpendicular direction such that the magnetization undergoes a precession. This approach has received many theoretical attentions [6, 7, 12]. Numerical investigations [6] showed that the switching time can be substantially reduced because ringing effect can be suppressed so that the magnetization will move along a so-called ballistic trajectory [10, 12]. The precessional magnetization reversal provides not only a shorter time but also lower switching field (below the SW-limit), as predicted by numerical calculations [6]. The questions that we would like to ask here are: Is the critical switching field in the precessional reversal the smallest one? For a given field magnitude, is the precessional reversal the fastest one? Surprisingly, the answers to the both questions are `NO'. In this proceeding, the following recent results are reviewed: If an arbitrary field/current pulse is allowed, there exist theoretical limits of critical switching field/current. Fix the magnitudes of the field or current, but allow the field or the c
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