  The Major Atmospheric Gamma Imaging Cerenkov (MAGIC) telescope participated
in three multiwavelength (MWL) campaigns, observing the blazar Markarian (Mkn)
421 during the nights of 2006 April 28, 29, and 2006 June 14. We analyzed the
corresponding MAGIC very-high energy observations during 9 nights from 2006
April 22 to 30 and on 2006 June 14. We inferred light curves with sub-day
resolution and night-by-night energy spectra. A strong gamma-ray signal was
detected from Mkn 421 on all observation nights. The flux (E > 250 GeV) varied
on night-by-night basis between (0.92+-0.11)10^-10 cm^-2 s^-1 (0.57 Crab units)
and (3.21+-0.15)10^-10 cm^-2 s^-1 (2.0 Crab units) in 2006 April. There is a
clear indication for intra-night variability with a doubling time of
36+-10(stat) minutes on the night of 2006 April 29, establishing once more
rapid flux variability for this object. For all individual nights gamma-ray
spectra could be inferred, with power-law indices ranging from 1.66 to 2.47. We
did not find statistically significant correlations between the spectral index
and the flux state for individual nights. During the June 2006 campaign, a flux
substantially lower than the one measured by the Whipple 10-m telescope four
days later was found. Using a log-parabolic power law fit we deduced for some
data sets the location of the spectral peak in the very-high energy regime. Our
results confirm the indications of rising peak energy with increasing flux, as
expected in leptonic acceleration models.
