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Magnetization Process in Vortex-imprinted Ni₈₀Fe₂₀/Ir₂₀Mn₈₀ Square Elements
H. Xu,J. Kolthammer,J. Rudge,E. Girgis,B. C. Choi,Y. K. Hong,G. Abo,Th. Speliotis,D. Niarchos 한국자기학회 2011 Journal of Magnetics Vol.16 No.2
The vortex-driven magnetization process of micron-sized, exchange-coupled square elements with composition of Ni??Fe₂? (12 ㎚)/Ir₂?Mn?? (5 ㎚) is investigated. The exchange-bias is introduced by field-cooling through the blocking temperature (TB) of the system, whereby Landau-shaped vortex states of the Ni??Fe₂? layer are imprinted into the Ir₂?Mn??. In the case of zero-field cooling, the exchange-coupling at the ferromagnetic/antiferromagnetic interface significantly enhances the vortex stability by increasing the nucleation and annihilation fields, while reducing coercivity and remanence. For the field-cooled elements, the hysteresis loops are shifted along the cooling field axis. The loop shift is attributed to the imprinting of displaced vortex state of Ni??Fe₂? into Ir₂?Mn??, which leads to asymmetric effective local pinning fields at the interface. The asymmetry of the hysteresis loop and the strength of the exchange-bias field can be tuned by varying the strength of cooling field. Micromagnetic modeling reproduces the experimentally observed vortex-driven magnetization process if the local pinning fields induced by exchange-coupling of the ferromagnetic and antiferromagnetic layers are taken into account.
B. C. Choi,Y. K. Hong,J. Rudge,G. Donohoe,Q. F. Xiao 한국자기학회 2006 Journal of Magnetics Vol.11 No.2
The physical origin of complex dynamic domain configuration in nonequilibrium magnetic systems with mesoscopic length scales has been studied. An increasing complexity in the spatial feature of the evolution is found to accompany the increasing reversal speed, when a ferromagnetic element is driven by progressively faster switching fields applied antiparallel to the initial magnetization direction. As reversal rates approach the characteristic precession frequencies of spin fluctuations, the thermal energy can boost the magnetization into local configurations which are completely different from those experienced during quasistatic reversal. The sensitive dependence of the spatial pattern on switching speed can be understood in terms of a dynamic exchange interaction of thermally excited spins; the coherent modulation of the spins is strongly dependent on the rise time of switching pulses.