  We measured transitions between the hyperfine levels of the electronic ground
state of potassium-39 atoms (transition frequency: 460 MHz) as the atoms moved
through a periodic magneto-static field produced above the magnetic-stripe
domains of a magnetic film. The period length of the magnetic field was 3.8 um.
The atoms were incident to the field as an impinging beam with the most
probable velocity of 550 m/s and experienced a peak oscillating field of 20 mT.
Unwanted spin relaxation caused by the collisions of the atoms with the film
surface was suppressed by the paraffin coating on the film. We observed
increasing hyperfine transition probabilities as the frequency of the field
oscillations experienced by the atoms increased from 0 to 140 MHz for the
atomic velocity of 550 m/s, by changing the incident angle of the atomic beam
with respect to the stripe domains. Numerical calculation of the time evolution
of the hyperfine states revealed that the oscillating magnetic field
experienced by the atoms induced the hyperfine transitions, and the main
process was not a single-quantum transition but rather multi-quanta
transitions.
