Micromagnetic analysis of magnetic noise in ferromagnetic nanowires

We investigate the magnetic thermal noise in magnetic nanowires with and without a domain wall by employing micromagnetic simulations. The magnetic thermal noise due to random thermal fluctuation fields gives important physical quantities related to the magnetic susceptibility. We find that the resonance frequency of a domain wall is distinguishable from that of a magnetic domain itself. For a single domain without a domain wall, the resonance frequency is well described by Kittel's formula considering a ferromagnetic specimen as a simple ellipsoid with demagnetizing factors for various wire widths and thicknesses. However, additional resonance frequencies from the magnetic domain wall show different dependences on the wire width and thickness, which implies that the spins inside the domain wall have different effective fields and spin dynamics.