We describe experiments in which ultracold rubidium atoms held by a microscopic magnetic trap near a room temperature metal wire are used to study the magnetic fields above the wire. By studying the density distribution of the atoms we detect a weak static magnetic field component ΔB_z parallel to the wire. This field is proportional to the current in the wire and is approximately periodic along the wire with period λ = 230 μm. We find that the decrease of this field with distance from the centre of the wire is well described by the Bessel function K_1(2πy/λ), as one would expect for the far field of a transversely oscillating current within the wire. We have also studied the lifetime for the atoms to remain in the microtrap over distances down to 27 μm from the surface of the metal. We observe the loss of atoms from the microtrap due to spin flips. These are induced by radio-frequency thermal fluctuations of the magnetic field near the surface, as predicted but not previously observed.
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