Nanocaps: A numerical study of remanence, quasistatic and dynamic switching

Finite element micromagnetics are used to simulate remanence, hysteresis and dynamic switching in crescent shaped permalloy caps, a geometry that corresponds to either normal or oblique deposition of films on nanosphere arrays. Oblique deposition breaks the symmetry and favours onion states rather than vortices. It introduces in-plane shape anisotropy analogous to that of an elongated island and increases the in-plane coercivity. In isolated caps, the critical diameter above which the vortex state is the minimum energy state, is D = 123 nm for angles below θ = 20 deg. It increases with θ up to a vlaue of D = 157 nm at θ = 45 deg. Even for sizes for which the onion state is the lowest energy state, vortex states can be stabilized in isolated caps but the interparticle magnetostatic interactions tend to destabilize them driving the system to its ground state. The switching field of onion states can be reduced by 36% using microwave resonance at frequencies approaching the resonant (Kittel mode) frequencies.