  We study electron tunneling through Co nanoparticles in the presence of
repeated microwave pulses at 4.2K. While individual pulses are too weak to
affect the magnetic switching field, repeated microwave pulses start to reduce
the magnetic switching field at 10{\mu}s spacing. We use I-V curve as a
thermometer to show that the microwave pulses do not heat the sample, showing
that magnetization in Co nanoparticles is directly excited by microwave pulses,
and the relaxation time of the excitation energy is in the range of
microsecond.
