de Haas-van Alphen Effect in the Heavy Fermion Compound with Amplitude-Modulated Magnetic Structure

The experiments of de Haas-van Aiphen (dHvA) effect have been carried out at temperatures down to 40 mK in CeNi_2AI_5 with an incommensurate magnetic structure. A complex phase diagram of the magnetic transition field versus temperature is completed. Many branches of the dHvA frequencies observed in the higher-field regions, II, Ill and IV, have been understood by the branches predicted from the theoretical Fermi surfaces of nonmagnetic reference-sample LaNi_2AI_5 better rather than the theoretical ones in CeNi_2AI_5 with 4f electrons treated as itinerant. Branches observed in the lowest field region I are not clearly analyzed with the theoretical prediction in LaNi_2AI_5 or CeNi_2AI_5, The electrons with heavy cyclotron mass are observed in the region IV ; 12.4 m_0 in eta branch, 11.2 m_0 in zeta branch and 9.6 m_0 in theta branch. Some pairs of parallel branches are observed due to the spin-splitting related to the magnetism, and the exchange energy DELTA E is obtained ; DELTA E=12.9 meV in beta branch and 7.8 meV in delta branch. The relations between the Fermi surfaces and the magnetic structure are discussed.