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

(1)LaNi_2Al_5的De-Haas-van Aiphen效应和费米面的研究：用磁阻和De-Haas-van Alphen测量方法研究了致密磁性化合物CeNi_2Al_5的参照物LaNi_2Al_5的费米表面性质。在角分辨dHvA测量中观察到13个频率在1.25×10~(-6)-101×10~(-6)Oe之间的分支，磁阻表明存在开放轨道。用FLAPW能带结构计算得到的费米面很好地解释了这一结果。(2)磁性致密近藤化合物CeNi_2Al_5的De Haas-van Alphen效应和费米面的研究：在无公度磁结构的CeNi_2Al_5中，在40mK以下进行了De Haas-van Aiphen(DHvA)效应的实验。完成了磁相变场随温度变化的复相图。用非磁性参考样品LaNi_2Al_5的费米面理论预测的dHvA频率分支比用4f电子巡游的CeNi_2Al_5样品的理论费米面预测的dHvA频率分支更好地理解了在高场区观察到的dHvA的许多分支。在LaNi_2Al_5和CeNi_2Al_5中观察到的低场I分支与理论预测的结果不一致，在IV区观察到具有较大回旋质量的电子，在ETa支中观测到12.4m0，在zeta支中观测到11.2m0，在theta支中观测到9.6m0。由于与磁性有关的自旋分裂，观察到了几对平行的支链，得到了交换能E，β支链的交换能为12.9 meV，三角洲支链的交换能为7.8 meV。讨论了费米面与磁结构之间的关系。

(1) Studies of de Haas-van Aiphen Effect and Fermi Surfaces of LaNi_2Al_5 :Magnetoresistance and de Haas-van Alphen measurements have been performed to investigate Fermi-surface property of LaNi_2AI_5, which is a reference material of a magnetic dense-Kondo compound CeNi_2AI_5. Thirteen branches with the frequencies from 1.25x10^6 to 101 x10^6 Oe have been observed in the angular-resolved dHvA measurements, and the magnetoresistance shows the existence of open orbits. The results are well explained by the Fermi surfaces obtainedfrom an FLAPW band-structure calculation.(2) Studies of de Haas-van Alphen Effect and Fermi Surfaces of Magnetic Dense-Kondo Compound CeNi_2AI_5 :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.