de Haas—van Alphen measurements in the ferromagnetic compoundsU3P4andU3As4

De Haas—van Alphen measurements are reported for the ferromagnetic actinide compounds ${\mathrm{U}}_3$ ${\mathrm{P}}_4$ and ${\mathrm{U}}_3$ ${\mathrm{As}}_4$. The large effective masses indicate $f$-electron interaction with conduction bands. The data are interpreted in terms of a semimetallic band structure producing a large electron sheet at point $\Gamma$ which is compensated by several smaller hole pockets along the $\Gamma -P$ line. The measured internal field is much smaller than expected on the basis of magnetization measurements. Several discrepancies with other measurements point to additional unobserved pieces of the Fermi surface.