Anomalous Magnetic and Transport Properties in Ce₂Fe₁₇ and Lu₂Fe₁₇

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Using single phased Ce₂Fe₁₇ and Lu₂Fe₁₇ with the rhombohedral Th₂Zn₁₇-type and Th₂Ni₁₇-type structure, respectively, we performed measurements of magnetization and resistivity under various magnetic field up to 5.5T. Ce₂Fe₁₇ exhibits a simple helical spin structure with a wave vecter of τ= 0.0372 Å^-1 parallel to the c-axis in the temperature range of T_C=125K < T<T_N = 210 K. For T<T_C, it shows a modified helix of τ= 0.0435 Å^-1 parallel to the c-axis with a superstructured magnetic cell, in which the magnetic c-parameter is twice as large as the chemical unit cell. Only below T_C, a giant magnetoresistance is induced by a metamagnetic transition from the helix to ferromagnetic phase in the field of H < 2T. The value of Δρ/ρ(= (ρ_AF-ρ_F )/ρ_AF) reaches 0.85 at 4.2 K. Such GMR may be atributed to the suppression of a super-zone gap induced by a formation of the superstructured magnetic cell below T_C. On the other hand, GMR could not be observed in Lu₂Fe₁₇.