  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 with the magnetic susceptibility. We find that the resonance
frequency of a domain wall is distinguishable from one of a magnetic domain
itself. For the single domain without a domain wall, the resonance frequency is
well described by the 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 the different dependences of the wire width and thickness. It implies
that the spins inside the domain wall have different effective fields and the
spin dynamics.
