Sample quality and the magnetic phase transitions in Nd3Pd20Ge6: neutron diffraction study

For the cubic intermetallic compound Nd3Pd20Ge6 (space group Fm (3) over ba r m) magnetic and transport properties are known to be highly sensitive to sample quality. We have performed powder neutron diffraction experiments do wn to 100 mK to study the magnetic structures of an inhomogeneous Nd3Pd20Ge 6 sample, which is the only known 3-20-6 system with three successive magne tic phase transitions. The onset of magnetic Nd order at the 8c site (simpl e cubic sublattice) is observed in two phase transitions with a dominant co mmensurate antiferromagnetic component (k(1)=[1,1,1] below T-N1=1.75 K) and an additional weak incommensurate perturbation (k(1i)=[tau,tau,tau], tau=0 .79, below T-N1i = 1.40 K). The Nd moments at the 4a site (face-centered cu bic sublattice) remain disordered down to T-N2 = 0.58 K and align perpendic ular to the propagation vector k(2)=[0,0,1]. The phase transition at T-N1i appears because of atomic disorder inside the cubic lattice of the 3-20-6 s tructure. High-quality Nd3Pd20Ge6 samples show only two magnetic phase tran sitions at T-N1 and T-N2. (C) 2000 Elsevier Science S.A. All rights reserve d.