  The absorption and variation of the velocity of a surface acoustic wave of
frequency $f$= 30 MHz interacting with two-dimensional electrons are
investigated in GaAs/AlGaAs heterostructures with an electron density $n=(1.3 -
2.8) \times 10^{11} cm^{-2}$ at $T$=1.5 - 4.2 K in magnetic fields up to 7 T.
Characteristic features associated with spin splitting of the Landau level are
observed. The effective g factor and the width of the spin-split Landau bands
are determined: $g^* \simeq 5$ and $A$=0.6 meV. The greater width of the
orbital-split Landau bands (2 meV) relative to the spin-split bands is
attributed to different shielding of the random fluctuation potential of
charged impurities by 2D electrons. The mechanisms of the nonlinearities
manifested in the dependence of the absorption and the velocity increment of
the SAW on the SAW power in the presence of spin splitting of the Landau levels
are investigated.
