A scanning Hall probe microscope is used to study flux pinning in a thin superconducting Pb film covering a square array of single-domain Co dots with in-plane magnetization. We show that single flux quanta of opposite sign thread the superconducting film below T(c) at the opposite poles of these dipoles. Depending on the polarity of the applied field, flux lines are attracted to a specific pole of the dipoles, due to the direct interaction with the vortexlike structures induced by the local stray field.
The spontaneous formation of domains of commensurate vortex patterns near rational fractional matching fields of a periodic pinning array has been investigated with high resolution scanning Hall probe microscopy. We show that domain formation is promoted due to the efficient incorporation of mismatched excess vortices and vacancies at the corners of domain walls, which outweighs the energetic cost of creating them. Molecular dynamics simulations with a generic pinning potential reveal that domains are formed only when vortex-vortex interactions are long range.
We report an observation of magnetostochastic resonance (MSR) in bistable magnetic systems. MSR manifests itself as a peak of a response of such a system to a periodic field as a function of the noise strength. Bi-substituted ferrite-garnet films with uniaxial anisotropy were used as simple examples of bistable systems. Noise was produced by (i) thermal fluctuations of an effective magnetic field and (ii) externally applied ‘‘noiselike’’ magnetic fields. The position of the peak depended dramatically on the magnitude of applied dc fields, so that dc fields of about 1 mOe in the spatial scale of 10 μm were measured. MSR applications to various tunneling phenomena such as tunneling of magnetization in small magnetic particles are discussed.
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