SCIENCE CHINA Physics, Mechanics & Astronomy © Science China Press and Springer-Verlag Berlin Heidelberg 2011 phys.scichina.com www.springerlink.com *Corresponding author (email: xqma@sas.ustb.edu.cn)  Research Paper  July 2011 Vol.54 No.7: 1227–1234 Special Topic: Magnetic Materials at Nano-scale doi: 10.1007/s11433-011-4313-1 Magnetization switching modes in nanopillar spin valve under the external field HUANG HouBing 1 , MA XingQiao 1* , YUE Tao 1 , XIAO ZhiHua 1 , SHI SanQiang 3 & CHEN LongQing 2 1 Department of Physics, University of Science and Technology Beijing, Beijing 100083, China; 2 Department of Materials Science and Engineering, The Pennsylvania State University, University Park, PA 16802, USA; 3 Department of Mechanical Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, China Received December 15, 2010; accepted February 28, 2011; published online May 24, 2011 The current-induced magnetic switching is studied in Co/Cu/Co nanopillar with an in-plane magnetization traversed under the perpendicular-to-plane external field. Magnetization switching is found to take place when the current density exceeds a threshold. By analyzing precessional trajectories, evolutions of domain walls and magnetization switching times under the perpendicular magnetic field, there are two different magnetization switching modes: nucleation and domain wall motion re- versal; uniform magnetization reversal. The first mode occurs at lower current density, which is realized by the formation of the reversal nucleus and domain wall motion; while the second mode occurs through complete magnetization reversal at higher current density. Furthermore, the switching time reduces as the spin-polarized current density increases, which can also be grouped into two reversal modes. current-induced nanopillar, spin transfer torque, magnetization switching, domain wall motion reversal, uniform mag- netization reversal PACS: 72.25.Pn 1 Introduction A torque, called spin transfer torque (STT) [1,2], is exerted on the local magnetic moments of a ferromagnet when a spin-polarized current is injected into a ferromagnetic thin film. The origin of STT is the “s-d” exchange interactions between the spin moments of the conduction electrons and the local magnetic moments of a ferromagnet, which con- sequently leads to a reversal or precession of the magnetic moments. The effects of STT on driving magnetization switching have been verified by experimental and theoreti- cal studies in a variety of nanostructures [3–7]. Depending on the direction of the current, STT can either force the free layer into parallel or antiparallel alignment compared to the fixed layer when STT is strong enough to overcome the coercive field of the free layer. Furthermore, when an ex- ternal magnetic field is applied, the effect of STT leads to the excitation of spin wave precessional modes in the free layer [8–11]. One key to memory applications is the reduc- tion of the current required to reverse the magnetization of the free layer and the shortening of the switching time while maintaining thermal stability. There has been a range of approaches to lower the critical currents [7,12–15]. One of the most crucial requirements for lowering critical currents is the clear description of the process of the magnetization switching. We have investigated the magnetization dynam- ics of Co/Cu/Co nanoscale spin valve at various current densities by micro-magnetic simulations and illustrated the different dynamic modes when zero field is applied [16,17]. Guo et al. [18] reported the magnetization dynamics of a