Micromagnetic simulation of the magnetic switching behaviour of mesoscopic and nanoscopic structures

Magnetic switching of small particles, thin film elements and nano-wires becomes increasingly important in magnetic storage and magneto-electronic devices. The magnetisation reversal processes are studied using a three-dimensional hybrid finite element/boundary element micromagnetic model. Transient magnetization states during switching are investigated numerically in thin Ni80Fe20 and Co nano-elements of square (100×100 nm2), rectangular (100×300 nm2) and circular (100 nm diameter) shapes with a thickness of 20 nm. Switching dynamics are calculated for external fields applied instantaneously and for rotational fields with field strengths in the order of the static critical field (Hext=0.02–0.32Js/μ0). Reversal in the unidirectional field proceeds by the nucleation and propagation of end domains toward the centre of the particle. It is found that the switching time strongly depends on the Gilbert damping parameter α. Small values of α (⩽0.1) lead to shorter switching times at small field strength values, whereas for Hext=0.6–1.0Js/μ0 minimum switching times occur for large damping value α=1.0. Materials with uniaxial magneto-crystalline anisotropy, such as Co, require larger field strengths, but exhibit shorter switching times. Reversal in rotational fields involves inhomogeneous rotation of the end domains toward the rotational field direction. Depending on the damping parameter fast switching times (⩽0.1 ns) are obtained by increasing the field strength to Hext=0.5Js/μ0. Taking into account thermal fluctuations the reversal mechanisms of Co nano-wires were studied, and values for the activation volume and for the domain wall velocity were derived.