  We report measurements of the spin susceptibility and the electron effective
mass for two-dimensional electrons confined at the interfaces of MgxZn1-xO/ZnO
single heterostructures (x = 0.05, 0.08, and 0.11), grown by molecular-beam
epitaxy on (0001) ZnO substrates. By tuning the built-in polarization through
control of the barrier composition, the electron density was systematically
varied in the range of 5.6 x 10^11 to 1.6 x 10^12 cm^-2, corresponding to a
range of 3.1 < rs < 5.2, where rs is the average electron spacing measured in
units of the effective Bohr radius. We used the coincidence technique, where
crossings of the spin-split Landau levels occur at critical tilt angles of
magnetic field, to evaluate the spin susceptibility. In addition, we determined
the effective mass from the temperature dependence of the Shubnikov-de Haas
oscillations measured at the coincidence conditions. The susceptibility and the
effective mass both gradually increase with decreasing electron density,
reflecting the role of electron-electron interaction.
