NMR Investigation of How Free Composite Fermions Are at [Formula presented]

NMR measurements of the electron spin polarization ([Formula presented]) have been performed on a 2D electron system at and around half-filled lowest Landau level. Comparing the magnetic field and the temperature dependence of [Formula presented] to models of free and interacting composite fermions (CF), the imbalance of spin-up and spin-down CF Fermi seas is mapped as a function of Zeeman energy. Independent measurements of the CF effective mass, [Formula presented] factor, and Fermi energy are obtained from the thermal activation of [Formula presented] in tilted fields. The filling factor dependence of the [Formula presented] for [Formula presented] reveals a broken particle-hole symmetry for the partially polarized CF Fermi sea.